ML20206E714

Rev 1 to Issue Resolution Status Rept Key Technical Issue: Igneous Activity

ML20206E714
Person / Time
Issue date:	07/30/1998
From:	NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
To:
References
NUDOCS 9905050211
Download: ML20206E714 (170)

Text

_

ISSUE RESOLUTION STATUS REPORT KEY TECHNICAL ISSUE:

IGNEOUS ACTIVITY I

I I

Division of Waste Management g

Office of Nuclear Material Safety and Safeguards U.S. Nuclear Regulatory Commission I

I Revision 1 July 1998 I

E M

,. 9

[

(30 '

OfB -C-I y

EA' 885** 20573, g of 77 g 47

)

f

._ _ -- __ --------- --- ------ - - - - - - - - -~

Change History of *lssue Resolution Status Report, Key TechnicalIssue: Igneous Activity" Sectiort/

Revision #

Paraaraoh Date Modification Rev0 all March 1998 None. Initialissue Revi 2.0/all June 1998 Minor modifications to reflect focus of this version of IRSR is consequences Revi 3.3.3/2 June 1998 Reference correction Revi 3.4/all June 1998 Update section to reflect current dose calculation methodologies for igneous events used in TPA 3.1.3, update Figure 2 for CCDF from TPA 3.1.3 ground-surface dose, address risk methodologies associated with disruptive events and reflect potentialinput from pre-existing eruption deposits Revi 4.1.2.3.3/1 June 1998 Reference correction Revi 4.1.4.3.1/

June 1998 Modification to reflect necessity of evaluation of new 9,10, & 11 information on temporal recurrence rate calculations Revi 4.1.4.4/2 June 1998 Reference correction Revi 4.1.4.4/2 June 1998 Modification to reflect necessity of evaluation of new information on temporal recurrence rato calculations Revi 4.1.6.3.4/1 June 1998 Reference Correction Revi 4.1.6.4/1 June 1998 Modification to reflect recognition of potential effect of new information on probability values Revi 4.2/all June 1998 Extensive revision to present acceptance criteria, review methods and technical basis for consequence ofigneous activity Revi 5.1/1 & 2 June 1998 Modification to recognize potential effect of new information on probability calculations Revi 5.1.4/2 June 1998 Modification to reflect potential effects of new information on recurrence rate values Rev1 5.2/all June 1998 Extensive revision to present status of issue resolution for consequence of igneous activity Revi 6.0/all June 1998 Provide references for Revi

CONTENTS Section Page FIGURES

.. iv ACKNOWLEDGMENTS.....

...... vi QUALITY OF DATA, ANALYSES, AND CODE DEVELOPMENT

.. vi

1.0 INTRODUCTION

1 2.0 KEY TECHNICAL ISSUE AND SUBISSUES..

... 3 3.0 IMPORTANCE OF ISSUE TO REPOSITORY PERFORMANCE.......

.5 E

3.1 RELATIONSHIP AND IMPORTANCE OF SUBISSUE TO TOTAL SYSTEM PERFORMANCE.

1

....... 5 3.2 RELATIONSHIP OF SUBISSUES TO DOE'S REPOSITORY SAFETY STRATEGY..

.8 3.3 CONSIDERATION OF IGNEOUS ACTIVITY IN PREVIOUS PERFORMANCE l

ASSESSMENTS.........

.8 u

3.3.1 Link, et al.,1982

.8 3.3.2 TSPA 91...........

.8 1

3.3.3 TSPA 93..

.9 3.3.4 NRC iterative Performance Assessment Phase 2 - 1995.

.9 3.4 NRC/CNWRA SENSITIVITY STUDIES.............................. 9 4.0 REVIEW METHODS AND ACCEPTANCE CRITERIA.......

. 13 4.1 PROBABILITY..

. 13 4.1.1 Probability Criterion 1.

.. 14 4.1.1.1 Acceptance Criterion

. 14 4.1.1.2 Review Method...

.. 14 4.1.1.3 Technical Basis......

.. 14 1

4.1.1.4 Summary.

. 18 4.1.2 Probability Criterion 2..........

. 18 4.1.2.1 Acceptance Criterion..........

. 18 4.1.2.2 Review Method........

18 I

4.1.2.3 Technical Basis...............

..... 19 4.1.2.3.1 Individual Eruptive Units

.19 4.1.2.3.2 Episodes of Vent or Vent-Alignment I

Formation........

. 20 4.1.2.3.3 Emplacement of an Igneous Intrusion 23 1

4.1.2.3.4 Volcanic Eruptions with Accompanying Dike in

.. 26 Summary.............. jection 4.1.2.4

.... 26 4.1.3 Probability Critenon 3..........

. 26 4.1.3.1 Acceptance Criterion..........

. 26 4.1.3.2 Review Method...

.27 4.1.3.3 Technical Basis....................

. 27 4.1.3.3.1 Shifts in the Location of Basaltic I

e

?

-a

Volcanism

.27 4.1.3.3.2 Vent Clustering.

.28 4.1.3.3.3 Vent Alignments and Correlation of g

Vent Alignments and Faults

. 30 g

4.1.3.4 Summary

.33 4.1.4 Probability Criterion 4

. 33 4.1.4.1 Acceptance Criterion.

. 33 4.1.4.2 Review Method.

.33 4.1.4.3 Technical Basis.

.33 4.1.4.3.1 Temporal Recurrence Rate

... 33 3

4.1.4.3.2 Spatial Recurrence Rate 36 3

4.1.4.3.3 Area Affected by Igneous Events.

.43 4.1.4.4 Summary....

.47 g

4.1.5 Probability Criterion 5

.. 48 g

4.1.5.1 Acceptance Criterion.

... 48 4.1.5.2 Review Method

.. 48 4.1.5.3 Technical Basis

.48 4.1.5.3.1 Regional Tectonic Models..

.49 4.1.5.3.2 Mechanistic Relationship Between Crustal Extension and Magma Generation....

. 53 4.1.5.3.3 Local Structural Controls on Magma Ascent..

. 62 4.1.5.4 Summary...

.63 4.1.6 Probability Criterion 6

. 65 4.1.6.1 Acceptance Criterion...

.65 4.1.6.2 Review Method

... 65 4.1.6.3 Technical Basis.

. 65 4.1.6.3.1 Individual Mappable Eruptive Units and Vents..

.. 66 El 4.1.6.3.2 Vent Alignments

. 67 5

4.1.6.3.3 Vent Alignments with Tectonic Control

.68 g

4.1.6.3.4 Igneous intrusions....

. 71 E-4.1.6.4 Summary..

. 74 4.1.7 Probability Criterion 7

. 77 4.1.7.1 Acceptanco Criterion

. 77 4.1.7.2 Review Method...

.... 77 4.1.7.3 Technical Basis.

.. 77 4.1.7.4 Summary

. 77 l

4.1.8 Probability Criterion 8

.... 78 5:

4.1.8.1 Acceptance Criterion.....

. 78 4.1.8.2 Review Method.

. 78 3

4.1.8.3 Technical Basis

. 78 3

4.1.8.4 Summary......

.79 4.1.9 Probability Criterion 9.

. 80 g

4.1.9.1 Acceptance Criterion.

. 80 g-4.1.9.2 Review Method..

.80 4.1.9.3 Technical Basis

.80 4.1.9.4 Summary

. 81 1"1 I

o 4.2 CONSEQUENCES.......

................81 4.2.1 Consequence Criterion 1..

. 82 4.2.1.1 Acceptance Criterion

... 82 4.2.1.2.

. Review Method....

...... 82 4.2.1.3 Technical Basis......................

.. 83 4.2.1.4 Summary

. 84 4.2.2 Consequence Criterion 2....

.......... 84 4.2.2.1 Acceptance Criterion..........

............ 84 4.2.2.2 Review Method.................

........... 84 4.2.2.3 Technical Basis.............................. 84 4.2.2.4 Summary.....................

. 91

~

4.2.3 Consequence Criterion 3.

.. 91 4.2.3.1

. Acceptance Criterion...

.....................91 4.2.3.2 Review Method............................... 92 4.2.3.3 Technical Basis............................ 92 4.2.3.4 Summary.................................. 92 4.2.4 - Consequence Criterion 4................................. 93 4.2.4.1 Acceptance Criterion......................... 93 4.2.4.2 Review Method......

........................93 4.2.4.3 Technical Basis............................. 93 j

4.2.4.4 Summary...........

.......................94 4.2.5 Consequence Criterion 5..........................

.... 94 4.2.5.1 Acceptance Criterion.......................... 94 4.2.5.2 Review Method...

. 94 4.2.5.3 Technical Basis............................. 95 4.2.5.3.1 Subsurface Disruption.................. 95 4.2.5.3.2 Dose Conversion.......................... 97 4.2.5.4 Sum ma ry.................................. 99 4.2.6 Consequence Criterion 6............

...........99 4.2.6.1 Acceptance Criterion

........................99 4.2.6.2 Review Method

................... 100 4.2.6.3 Technical Basis.

..........................100 4.2.6.4 Su mmary.................................. 100 4.2.7 Consequence Criterion 7.................................. 100 4.2.7.1 Acceptance Criterion....................... 100 4.2.7.2 Review Method............................ 101 4.2.7.3 Technical Basis......................... 101 4.2.7.4 Summa ry.................................. 101 5.0 STATUS OF ISSUE RESOLUTION AT STAFF LEVEL..................... 103 5.1 STATUS OF RESOLUTION OF PROBABILITY ISSUES............... 103 5.1.1 Probability Criterion 1...........

.. 103 5.1.2 Probability Criterion 2........................ -,.

. 104 5.1.3 Probability Criterion 3........

................... 1 04 5.1.4 Probability Criterion 4..........

... 104 5.1.5 = Probability Criterion 5.................................

105 5.1.6 Probability Criterion 6...............................

.. 105 5.1.7.

Probability Criterion 7.................................

105

{

5.1.8 - Probability Criterion 8..........

................. 1 06 i

5.1.9 Probability Criterion 9......................

. 106 til J

5.2 STATUS RESOLUTION OF CONSEQUENCE ISSUES 106 5.2.1 Consequence Criterion 1 107 5.2.2 Consequence Criterion 2.

108 5.2.3 Consequence Criterion 3.

108 5.2.4 Consequence Criterion 4.

.108 5.2.5 Consequence Criterion 5..

109 5.2.6 Consequence Criterion 6...

109 5.2.7 Consequence Criterion 7.

109 5.3 NRC DISPOSITION OF COMMENTS RELATED TO IGNEOUS ACTIVITY,.110

6.0 REFERENCES

117 APPENDIX A: COMPILATION OF DATES FOR BASALTIC ROCKS OF THE YUCCA MOUNTAIN REGION I'

I-I I

I"V

{

FIGURES Figure Page 1

Flowdown diagram for total system performance assessment..

.6 2

CCDF for volcanism disruptive scenario, all parameters samp!ed from ranges, receptor location 20 km south of repository,10,000 yr performance period..

. 10 3

Basaltic volcanic rocks of the Westem Great Basin since about 12 Ma, from Luedke and Smith (1981) and references in Connor and Hill (1994) and Appendix A.........

15 4

Basaltic volcanic rocks of the Yucca Mountain region since about 11 Ma.......... 17 5

Development of multiple vent alignments along a fault is illustrated by the Mesa Butte alignment in the San Francisco volcanic field, Arizona.

....... 21 6

Detailed geochronology shows that the Mesa Butte alignment formed over a period of more than 1 m.y. through several distinct episodes of volcanism......... 22 7

Basaltic volcanic rocks of the Crater Flat area, Nevada..............

. 24 8

Distribution of dikes, breccia zones sills, and vents in the San Rafael volcanic field, Utah..................,.............

... 25 9

Ground magnetic map of Amrgosa Aeromagnetic Anomaly A showing three aligned anomalies

.......... 29 10 Ground magnetic map of the Northem Cone area, Crater Flat, Nevada.......... 32 11 Comparison of observed fraction of volcanoes within a given distance of their I

nearest-neighbor volcano with Gaussian kemel models calculated using h = 3 km, S km, a n d 7 km..............................................

. 40 12 Comparison of observed fraction of volcanic events within a given distance of 1

their nearest-neighbor volcanic event with Gaussian kemel models calculated using h = 5 km and h = 7 km............

. 41 13 Comparison of observed fraction of volcanoes within a given distance of their nearest-neighbor volcano with Epanechnikov kemel models calculated using h = 5 km,10 km, and 18 km.

. 42 14 Distribution of Plio-Quatemary vents by vent alignment half-length...

.45 15 Distribution of the orientation of fault segments with respect to north

. 46 16 Simplified geologic map of the area around Yucca Mountain showing major geologic units, including Plio-Quatemary volcanoes and faults.

50 v

r

17 Two balanced cross sections across Bare Mountain, Crater Flat, and Yucca Mountain (from Ferrill, et al.,1996b).

. 51 18 Comparison of density profiles beneath Bare Mountain (BM) and Crater Flat (CF)

. 54 19 Conceptual model of melt generation in response to crustal extension 56 20 Bouguer gravity anomaly map of the Yucca Mountain region 57 21 Apparent density variation across the Yucca Mountain region, d rived from gravity e

data.......

.60 22 Schmidt plot of fault dilation tendency for Yucca Mountain region stresses.

. 63 23 Annual probability of volcanic eruptions within the repository boundary. Igneous

~

events are defined as individual mappable eruptive units and vents.

. 67 24 Annual probability of volcanic eruptions within the repository boundary. Igneous events are defined as vents and vent alignments.

. 69 25 The weighting function, fr(x,y), is derived from changes in average crustal densities under the locations of Plio-Quatemary YMR volcanoes...

. 70 26 The spatial recurrence rate (v/km ) is contoured in the area of Yucca Mountain, 2

using the Gaussian kemel function (Eq. 35).

.. 72 27 The spatial recurrence rate (v/km ) is contoured in the area of Yucca Mountain, 2

using the modified Gaussian kernel function (Eqs. 37 to 39) to incorporate tectonic control on the probability estimate...

. 73 28 Annual probability of volcanic eruptions within the repository boundary using a -

modified Gaussian kemel. Igneous events are defined as vents and vent alignments.....

. 74 29 Annual probabiliiy of volcanic eruptions within the repository boundary using regional recurrence rates of 4 = 1 x 10~',2 x 10-e 3 x 10-5,4 x 10-8,and 5 x 10-8/yr.......

75 30 Ash columns on erupting cinder cones vary from strong vertical columns with sustained gas-thrust regions and little deflection by the wind (e.g.,1947 Paricutine (McGregor and Abston,1992),1975 Tolbachick, and 1968 Cerro Negro), to weaker plumes with little or no gas-thrust region above the vent and than bend easily in the wind (e.g.,1995 Cerro Negro)...

. 86 31 (a) Vertical velocity of particles in the volcanic column and (b) change in column radius as a function of height for a violent strombolian eruption, based on l

parameters in Table 1......

. 90 I

It l

I

Tables Table Page 1

Example initial conditions and constants for eruption column model.

. 89 2

Volumes of historically active basaltic volcanoes used to estimate fall-deposit volumes for YMR Quatemary volcanoes.....

.... 96 3

Summary of eruption parameters with calculated column heights and eruption power for historically active basaltic volcanoes reasonably analogous to YMR volcanoes.... 97 I

I I

I I

I

[

[

[

[

\\11 d

I ACKNOWLEDGMENTS This report was prepared jointly by the U.S. Nuclear Regulatory Commission and the Center for Nuclear Waste Regulatory Analyses (CNWRA) staffs. Primary authors of the report are Charles B. Connor (CNWRA), Brittain E. Hill (CNWRA), and John S. Trapp (NRC). The authors E

thank David Ferrill (CNWRA), F. Michael Conway (CNWRA), Goodluck Ofoegbu (CNWRA),

R Philip Justus (NRC), and Mark Jarzemba (CNWRA) for their assistance on the discussions and interpretations of the models presented herein; John Stamatakos (CNWRA),

g H. Lawrence McKague (CNWRA), Budhi Sagar (CNWRA), N. King Stablein (NRC) and g

David J. Brooks (NRC) for their review of this report; Ron Martin (CNWRA), Peter La Femina (CNWRA), and Sammantha Magsino (CNWRA) for their expert technical assistance; and Annette Mandujano (CNWRA) and Carrie Crawford (NRC) for their assistance in preparing this report.

QUALITY OF DATA, ANALYSIS, AND CODE DEVELOPMENT DATA: CNWRA-generated data contained within this report meet quality assurance g

requirements described in the CNWRA Quality Assurance Manual. Sources for other data 3

should be consulted for determining the level of quality for those data.

ANALYSIS AND CODES: Probability models that form the basis of this report have been tested for accuracy. The calculations were checked as required by QAP-014, Documentation and Verification of Scientific and Engineering Calculations, and recorded in a scientific notebook.

I I'

I e

g I

l

1.0 INTRODUCTION

One of the primary objectives of the U.S. Nuclear Regulatory Commission's (NRC) refocused prelicensing program is to direct all activities towards resolving 10 key technical issues (KTis) considered most important to repository performance. This approach is summarized in Chapter 1 of the staffs fiscal year (FY) 1996 Annual Progress Report (Sagar,1997). Other chapters address each of the 10 KTis by describing the scope of the issue and subissues, path to resolution, and progress achieved during FY96. For the purposes of this report, " staff" shall refer to NRC and Center for Nuclear Waste Regulatory Analyses (CNWRA) staff.

Consistent with NRC regulations on prelicensing consultations and a 1992 agreement with the U.S. Department of Energy (DOE), staff-levelissue resolution can be achieved during the prelicensing consultation period; however, such resolution at the staff level would not. preclude the issue being raised and considered during licensing proceedings. Msue resolution at the staff level during prelicensing is achieved when the staff has no further questions or comments (i.e.,

open items), at a point in time, regarding how the DOE program is addressing an issue. There I

may be some cases where resolution at the staff level may be limited to documenting a common understanding regarding differences in the NRC and the DOE technical positions. Pertinent, additionalinformation could raise new questions or comments regarding a previously-resolved issue.

An important step in the staffs approach to issue resolution is to provide DOE with feedback regarding issue resolution before viability assessment. Issue Resolution Status Reports (IRSRs)

I are the primary mechanism that NRC and CNWRA staff will use to provide DOE with feedback on KTl subissues. IRSRs focus on: (i) acceptance criteria for issue resolution; and (ii) the status of resolution, including areas of agreement or when the staff currently has comments or i

questions. Feedback is also contained in the staffs Annual Progress Report, which summarizes the significant technical work toward resolution of all KTis during the preceding fiscal year.

Finally, open meetings and technical exchanges with DOE provide opportunities to discuss issue I

resolution, identify areas of agreement and disagreement, and develop plans to resolve such disagreements.

in addition to providing feedback, the IRSRs will guide the staffs review of information in the I

DOE viability assessment. The staff also plans to use the IRSRs in the future to develop the Yucca Mountain Review Plan for the repository license application.

I Each IRSR contains five sections. This introduction is Section 1.0. Section 2.0 defines the KTI, all the related subissues, and the scope of the particular subissue that is the subject of the IRSR.

Section 3.0 discusses the importance of the subissue to repository performance including: (i)

I qualitative descriptions; (ii) reference to total system performance (TSP); (iii) results of available sensitivity analyses; and (iv) relationship to the DOE Repository Safety Strategy (RSS), that is, it's approach to the viability assessment. Section 4.0 provides the staffs review methods and I

acceptance criteria, which indicate the technical basis for resolution of the subissue and that will be used by the staff in subsequent reviews of DOE submittals. These acceptance criteria are guidance for the staff and, indirectly, for DOE as well. The staffs technical basis for the acceptance criteria is also explained in detail to further document the rationale for staff decisions.

Section 5.0 concludes the IRSR with the status of resolution, indicating those items resolved at

[

the staff level or those items remaining open. These open items will be tracked by the staff, and resolution will be documented in future IRSRs.

[

~

1

lI I

I, THIS PAGE INTENTIONALLY BLANK I

I Il I

I Il I

I-I I

I I

I 2

'I

R 2.0 KEY TECHNICAL ISSUE AND SUBISSUES The Igneous Activity KTl (IA KTl) has been defined by the NRC as " predicting the consequence and probability of igneous activity affecting the repository in relationship to the overall system performance objective." igneous activity is the process of the formation of igneous rocks from molten or partially-molten material (magma). Igneous processes are normally divided into two classes; intrusive activity, whereby magma is emplaced into preexisting rocks, and extrusive or volcanic activity, whereby magma and its associated materials rise into the crust and are deposited on the earth's surface. The dividing line between intrusive and extrusive processes and events is at times indistinct. Dikes, which are by definition intrusive features, can break through to the earth's surface and are responsible for many lava flows. In addition, many volcanoes first start as a dike in which flow becomes constricted to a certain location, the volcanic vent. For purposes of this 1RSR, volcanic activity is restricted to mean only those features and processes associated with the volcano and volcanic vent itself.

The main objective of work within the lA KTl is to evaluate the significance of igneous activity to I

repository performance by reviewing and independently confirming critical data, and evaluating and developing attemative conceptual models for estimating the probability and consequence of igneous activity at the proposed repository site. The scope of work includes reviewing various DOE documents, as well as applicable documents in the open literature, participating in meetings with DOE to discuss issues related to the KTl, observing of Quality Assurance (QA) audits of DOE, conducting independent technical investigations, and performing sensitivity studies related to igneous activity and TSP.

The lA KTl has been factored by NRC into two subissues, which contain specific technical l

components. The first subissue, probability, focuses on: (i) definition of igneous events; (ii) 3 determination of recurrence rates; and (iii) examination of geologic factors that control the timing and location of igneous activity. Under this subissue, nine acceptance criteria have been developed that relate to these areas of focus and use this information to develop probability I

values. The second subissue considers the consequences of igneous activity within the repository setting. The primary topics addressed for the second subissue are: (i) definition of the physical characteristics ofigneous events; (ii) determination of the eruption characteristics for I

modem and ancient basaltic igneous features in the Yucca Mountain Region (YMR) and analogous geologic settings; (iii) models of the effect of the geologic repository setting on igneous processes; (iv) evaluation of magma-waste package / waste form interactions; and (v)

I determination of volcanic deposit characteristics relevant to the consequences of igneous activity. Revision 0 of this IRSR addressed Subissue 1 (probability) with specific emphasis on the probability of volcanic activity disrupting the repository. This version of the IRSR (Rev1)is intended to specifically address Subissue 2,'as well as provide some updates on subissue 1.

Issue resolution regarding probability has been achieved by gaining agreement on reasonable mechanisms and realistic ranges of the critical parameters necessary to evaluate the likelihood I

and character of future igneous activity at or near the proposed repository site. This required an evaluation of existing data and models from DOE, the CNWRA, and others to arrive at a reasonably conservative value for the probability of future igneous activity at the proposed I

repository site. Probability models w!" need to reflect the limitations of YMR characterization activities along with the uncertainties associated with understanding igneous processes.

Reasonably conservative values are needed due to the limitations and uncertainty in our understanding of igneous processes in general, and within the YMR in particular. Further, there I

must be confidence that the values used do not underestimate possible effects of igneous I

3 u

1 I

activity on the proposed repository site.

Issue resolution for the consequences of igneous activity will be achieved through comparison of B

results from independent estimates of the staff with those from the DOE program and through g

agreement on reasonable or bounding mechanisms and realistic or bounding ranges of parameters necessary to evaluate the potential effects of igneous activity on repository performance. This will require an evaluation of direct and indirect effects of both the intrusive and extrusive aspects of igneous disruption of a waste repository, to include the physical, j

chemical, and thermal effects of magma on engineered systems. Critical to this resolution is the building of confidence in the consequence models by testing some of their components (e.g., ash E

dispersal) against known data from analogous basaltic volcanoes.

3 I

Il I

I I I

I I

I I

l 3.0 IMPORTANCE OF ISSUE TO REPOSITORY PERFORMANCE Basaltic igneous activity has been characteristic of the region around the proposed repository site since cessation of caldera magmatism at about 11 Ma. Prior to this time, silicic volcanic activity had been characteristic of the YMR, however, since about 10 Ma, such activity has been absent from the YMR. As a result, during the Technical Exchange between NRC and DOE on February 25-26,1997, NRC and DOE agreed that silicic activity need not be considered in performance assessment of the Yucca Mountain site. Since about 1 Ma, five basaltic volcanoes have erupted within 20 km of the proposed repository site. Based on the long record of scattered, smallwolume basaltic volcanism in the region and large time-scales of geological processes that liAely control the production and distribution of basaltic volcanism, the YMR has the potential for a basaltic volcano to erupt during the next 10,000 to 1,000,000 years. Staff from I

NRC, CNWRA, DOE, and the State of Nevada have conducted numerous investigations regarding the probability and likely consequences of repository disruption by basaltic igneous activity. Results of these investigations demonstrate that the probability and likely consequences of future igneous activity are sufficiently large, such that basaltic igneous activity needs to be I

considered in repository performance assessments.

3.1 RELATIONSHIP AND IMPORTANCE OF SUBISSUES TO TOTAL SYSTEM l

PERFORMANCE The staff is developing a strategy for evaluating the performance of a proposed repository at Yucca Mountain. As is currently visualized by the staff, key elements of this strategy are defined by those elements necessary for DOE to demonstrate repository performance. These elements are illustrated in Figure 1. Figure 1 is a simplified illustration of the key elements of system and I

subsystem abstraction that are needed for input into the performance assessment mod'sts. The probability of volcanism, the focus of this IRSR, is a key element for evaluating the significance of direct release on repository performance.

If igneous activity were to resume in the YMR, there are four possible outcomes: (i) the activity would not intersect the repository and would have no effect on repository performance; (ii) such activity would result in features and processes that would not directly intersect the repository, but I

would have indirect effects on the repository; (iii) the igneous features would directly intersect the repository, have direct and indirect effects on the repository, but a volcano would not form within the repository boundary; and (iv) basaltic volcanic activity would directly intersect the repository, and both directly and indirectly affect repository performance.

The most probable outcome of basaltic igneous activity in the YMR would be outcome (i); there I

would be no effect on repository performance, it is believed that as a result of outcome (ii), the results of such features as dikes modifying the groundwater flow system (modification of both unsaturated (UZ) and saturated (SZ) flow, Figure 1), or dikes and/or sills changing the thermal regime and possibly resulting in release of volcanic gases that could result in degradation of the I

waste package / waste form and modification of the geochemistry of the system (affects on waste package corrosion, Figure 1) would have relatively minor consequences. Outcome (iii) could occur either solely through intrusive activity developing features that directly intersect the repository causing thermal, mechanical, and chemical changes to the repository, waste package, and waste form or by development of a volcano outside the repository boundary with the associated dikes intersecting the repository causing the same effects. Such an outcome could increase waste package / waste form degradation by corrosion, mechanical disruption or changing J

M

)g e

n 0 )r l

d p r e ty i

s n

r h

oium nr k

d ou ie hp M

ag p rc u

r r

eo d c E

n f ne fdr nr E

g ohp a

R o

oiw l

s c

i n e(

nu e na

_l E

ta odr on e oc ii H

mEe tuir i

h tai ot t

l M

r t

u h c c c P

e c i 2 a dd u rD r (c

o. f a

a S

s t

l t

O oa DC IB r

(

M n

s o e s

g t

yka d

mc s

.a fe n

dp od at k

r c e et i

r c eo nr n ou t

r sp a a opn t

c so w

bni cas l

een o

r d

B R p t

M f

ia rl a Vo r

y_

i e E

C h

)

M YN E

7 n

m

)

o n

RA k

R s

E dr u

t h

oi c

i v OMli H

no u

nco t

ap nd iun T

aR q P

io nda l

M s

er IRu S

wn o

p iod t

rn t

a SOiodr O

gl r r s d

oa r -

apa t

wt OFv E

gFT e e rrs oa n ae e n

ie t t G

r n

e l

o ea PRds

(

Fwz Rwz m

o M

EEno R P (I D ss e

7 s

s dr t

a la e

no w

e ap rw s

e o

at c

wn nt ma s

po c

n n

e o inhz a

ef ft t

e Zoa t o o

n nd m

n U F 'n dn ne oie l

t r

t s t o

no oex M

ae a iwi dru f

ai r

r t

r tu et ta rut r

la he taw at a e

itm s

t c s r

e an p

t p

sx RRu i u m

s Df h

l M

e D

ts E

d y

R e

s E

e s

t. t **

t.d r

l m

y=

a EM y

a' r

r t

e e

• D.

th d

o i

l r

ni

• hm t

NE n

".m s

i r i )e uW n

t IT gr m

as r

GS na nenw

'r.

ha e rs o

t o

t NY EB oha

"*a e=

d er f

ht k i c. r 8 *e d t

s r

g s

e.*

ury m

e ca

( ES e

o 1

n f

M r ip otm*

nee a

4-m

• *= n.e r

e b' osg r

o e

r ab c

e s

e i

n r e u g

him a h. h t

d t

5M l

al Pd eo a

"h.

Rrr i

n e

d TE S

)

= *

(

fi(

i e

s M

NT n

c n n M

w o

a E S E

om i

ht N Y o

t T

us r

d 5

M_

w M

S crb O B l or 5

Y cup P U T

E C at e o

L 5

eb l

M MS N

T F

E l

i U

dol OF E

S A t r e A T B

st v n

n CO M

YRS e

1 T S S

e c 5TN e

nym YE B5 O u

OY r

ELFUBI e ee kt t

TS nf s

g I

y EEOSAT (oc iF M

M

the quantity and chemistry of water contacting the waste packages and waste form allowing more rapid release of radionuclides, and could locally modify the UZ and SZ flow systems (Figure 1). Waste material could be brought to the surface by a dike, but in this case, the material would I

be incorporated into the lava, and although it would be available for leaching and erosion, for all practical purposes, the waste material would be immobilized, and the only potentially significant health effect would be through direct exposure to an individualin the vicinity of the dike or associated flows. Scoping calculations are planned to evaluate outcomes (ii) and (iii).

The fourth outcome, intersection of the repository by a volcano with resulting direct release of the I

material to the accessible environment, is the scenario that NRC has spent the most time analyzing, as it appears to be the most significant from a dose-or risk-based perspective. This is illustrated by the Direct Release path on Figure 1. The emplacement of basaltic magma (i.e.,

molten rock) in or through a high-level radioactive waste repository introduces thermal, I

mechanical, and chemicalloads on engineered and surrounding geologic systems that are difficult to evaluate. Basaltic magma can be generally described as a material with a temperature of about 1,100 *C, density around 2,600 kg m-8, viscosities between about I

10-100 Pa s, and a chemical composition that produces acidic gases in addition to very low oxygen fugacities (roughly 10 log units below atmospheric conditions). During initial dike ascent or lava flow through open drifts, magma velocities will be on the order of 1 m s". Flow velocities I

in the magma conduit, however, can reach 100 m s once a volcano is established at the d

surface. There has been little quantitative evaluation of the effect of these thermal, mechanical, and chemicalloads on the engineered repository systems. All previous total system performance I

assessments (TSPA) sponsored by DOE (Link, et al.,1982; Bamard, et al.,1992; Barr, et al.,

1993; Wilson, et al.,1994) have assumed that a waste package fails upon contact with basaltic magma. This assumption appears reasonable, based on current information, but will be re-evaluated as new data er models become available.

I The consequences of the extrusive component of basaltic volcanic activity are govemed by two primary processes and associated assumptions. First, radioactive waste is incorporated into the I

ascending magma under the physical conditions outlined in the preceding paragraph. Particle diameter is a critical parameter because of the difficulty in transporting dense spent-fuel (about 10,000 kg m'*) in viscous, ascending 2,600-kg m-3 basaltic magma. Although spent-fuel pellets I

are originally 1 cm diameter, they are highly fractured and have average particle diameters on the order of 1 mm (Jarzemba and LaPlante,1996). These spent-fuel grains are relatively friable and under tests in which spent fuel is subject to simple physical disruption (impact from a steel plate) degrade to an average particle diameter of 0.001-0.01 mm. during disruptive events (Ayer, et al.,

I 1988), in addition, during heating, oxidation of the fuel proceeds rapidly to U 0s, producing 3

micron size particles (Einziger, et al.,1992). Under the thermal and physical conditions of a volcanic event, it appears reasonable to assume that the spent fuel can degrade to the millimeter I

to micron size. The NRC model, discussed in Section 3.4, assumes that small waste particles will be incorporated into larger ash particles (Jarzemba, et al.,1997). The second primary process concems transport and dispersal of contaminated tephra to subaerial locations. Only traces remain of the distributed tephra erupted from YMR volcanoes, requiring comparison with analog volcanoes to determine suitable dispersal characteristics (e.g., Connor,1993).

Historically active basaltic volcanoes are capable of dispersing tephra particles >0.1-mm diameter at least 30 km from the vent, resulting in 1-to 100 mm thick deposits (Hill, et al.,1996).

Based on the highly-fragmented character of some Quatemary YMR volcanoes (Crowe and

[

Perry,1991; Crowe, et al.,1995; Hill,1996), YMR volcanoes were potentially capable of t

transporting material these distances. A repository-disrupting volcano would likely be capable of directly transporting some amount of high-level waste at least 30-km downwind. Basaltic

{

7

r volcanism, thus, appears capable of breaching waste canisters, incorporating some finite amount of spent fuel, and potentially transporting some portion of the incorporated spent-fuel to likely inhabited regions (e.g., Link, et al.,1982).

3.2 Relationship of Subissues to DOE'S Repository Safety Strategy The lA KTl has been defined by NRC as " predicting the consequence and probability of igneous activity affecting the repository in relationship to the overall system performance objective." This j

definition is a comparable but broader definition than the hypothesis evaluated in the DOE RSS that " volcanic events within the controlled area will be rare and the dose consequences of volcanism will be too small to significantly affect waste isolation." As the majority of the NRC effort has been directed toward understanding the effects of volcanic activity, the differences in the focus of the two programs has been minor. The probability and consequence subissues of E),

the overallissue are directly incorporated in both the NRC issue and the DOE RSS.

N 3.3 Consideration of Igneous Activity in Fre Acus Performance Assessments 3.3.1 Link, et al.,1982 Link, et al., (1982) provides the most detailed analysis of the effects of igneous activity on a site in the YMR. This repart considered thermal effects from a dike, the effects of dispersion of radioactive waste parte,les, and carried the analysis through ingestio.. exposure and dot e.

While the input values, assumptions, and methodology are outdated, the report does provide a good first approximation of the relative contributions of the various possible exposure scenarios on overall dose. Volcamsm was assumed to occur through development of a dike that localized into a volcanic vent within the repository The probability assumed in the report for disruption of j

the repository by the dike was 2.9 X 10 / year.

3.3.2 TSPA 91 in TSPA 91 (Bamard, et al.,1992), the effects of igneous activity were modeled as a dike, which localized into a volcano, intersecting the repository. The probability of the dike intersection was 4

a 2.4 X 10 in 10,000 years. This probability is within the low end of the general range that NRC E'

considers representative for a volcano erupting through a repository. However, this analysis was based on the remanded EPA standard. Therefore, the consequence analysis was only concemed with transporting waste to the " accessible environment" ( the ground surface), rather than to a " critical group." Nevertheless, the values used to represent the incorporation or the i

entrainment of waste moved to the surface (i.e. the relative relationship of waste volume to magma volume) did not reflect values that are representative of the expected conditions in the YMR.

For example, the TSPA analysis assumed a mean entrainment factor of 0.03%. Since there is g

no actual data on " waste" entrainment in magma, a practical approach is to assume that the g

entrainment of lithic fragments in magma in the YMR provides a reasonable approximation.

Crowe, et al., (1986) performed such an analysis with respect to materials from Lathrop Wells.

That study found that for " normal" eruptive sequences, an average value for the entrainment of l

lithic fragments was approximately 1%, and that values for *hydrovolcanic" sequences reached 17%. While other values for other regions can be found in the literature, the work by Crowe, et al., (1986) is the only published report with data for volcanoes in the YMR. Consequently, the E

use of.03% in TSPA 91 instead of the values reported by Crowe, et al., (1986) appears to 5

substantially underestimate entrainment, and if so, could result in underestimating 8

I

consequences by approximately two orders of magnitude.

3.3.3 TSPA 93 in TSPA 93 (Wilson, et al.,1994), there was no new analysis beyond what was performed in TSPA 91 with respect to the effects of a direct volcanic eruption through a repository. TSPA 93 concentrated on the indirect effects caused by the intrusion of a dike into the repository. The analysis was such that the dike was constrained from intersecting waste canisters. Therefore, the consequence analysis was only concemed with the effects of temperature and magmatic gases on repository performance. In the analysis, probability values from 1.0 to 1.8 X 10 per d

10,000 years were used. These values are at the extreme low end of those values that NRC considers reasonable for direct volcanic disruption.

i As discussed in Section 4.1.6.3, the probability of indirect disruption by the intrusion of a dike into the repository is necessarily higher than the probability of direct volcanic disruption. Therefore, the probability values used in TSPA 93 are low. By analogy with the San Rafael volcanic field I

(Delaney and Gartner,1997), the probability of dike intrusion into the repository is two to five times the probability of direct volcanic eruption through a repository. While NRC considers this probability value low, the results of TSPA 93 suggest that the releases due to these indirect effects would be much less than releases from direct effects.

3.3.4 NRC lterative Performance Assessment Phase 2 - 1995 in the NRC lierative Performance Assessment Phase 2 (IPA Phase 2), volcanism was modeled as a dike intersecting the repository. To simulate the effects of direct disruption,4 percent of the materialintersected by a dike was assumed to be released to the atmosphere. While the methodology was extremely simplistic, the results did suggest that: (i) the effects of direct release through volcanism could make a discemable difference in the expected releases in the tails (low probability) portion of the distribution function, and (ii) the effects of igneous processes other than direct volcanic release will probably have a very minor effect on overall dose or risk.

These calculations have been used to revise NRC's TSPA code (i.e., TPA-3, Manteufel, et al.,

1997) and evaluate dose sensitivity to variations in key igneous activity parameters.

3.4 NRC/CNWRA SENSITIVITY STUDIES Initial dose calet.!ations for basaltic volcanic activity have been completed using NRC's Total-system Performance Assessment code (TPA) Version 3.1.3. These calculations build on y

previous calculations used to evaluate the possible impacts on repository performance l

associated with basaltic volcanism (Jarzemba and LaPlante,1996; Jarzemba, et al.,1997; Manteufel, et al.,1997; Hill,1997). The results of the current calculations are presented in Figure 2, which shows doses calculated to result from eruptions that are assumed to occur and I

disrupt the repository (i.e., they are not weighted based on the probability of occurrence of such 4

events, i.e.,10 per year). These calculations had the following basic assumptions:

• Cntical group located 20 km from repository along the axis of the main contaminant plume.

[

• Volcanic eruption occurs through repository between +1 and +10,000 yr post-closure.

[

9 W

I I

I 10-3 i

i i

i e

l E 104 -

.O CU

.O O

u e

G)

OC tG e

O O

10 '

~

d i

q,

-4)

!i I

10-8 i

"i O.1 1

10 100 Peak Annual Ground-Surface TEDE (rem)

I I

Figure 2. CCDF for volcanism disruptive scenario, all parameters sampled from ranges, g!

receptor location 20 km from repository,10,000-yr performance period.

g 10 Il

I

{

. One to ten canisters fail and all waste (10-100 MTU) is available for magmatic transport.

= NRC TPA 3.1.3 approach used for dose calculations.

= waste-particle diameters are between 1-100 pm.

Eruption power and duration (i.e., column height, eruption rate, mass), time of eruption, wind speed, and tephra diameter are uncorrelated random variables.

Using these parameters and 400 simulations, mean annual peak dose 20 km south of the proposed repository site due solely to serial dispersion of high-level waste (HLW) through

[

basaltic volcanism is 8 rem yr" with a standard error on the mean of 2 rem yr (See Figure 2).

d In addition, the contaminated ash-deposit will remain on the surface for thousands of years following the eruption. The annual dose from the residual ash-deposit will be less than the peak

(

dose acquired during the first year of the eruption, due to elutriation of fine-ash particles, deposit i

erosion, and radioactive decay. Preliminary analysis suggests, however, that dose from the residual ash-depocit is significant and needs to be accounted for in overall annual risk calculations.

. Although this analysis is ongoing, initial studies suggest that dose from volcanic disruption is most sensitive to ash-particle diameter, wind direction, conduit diameter, and wind speed. One

[

of the more interesting results of this analysis is that, within the range of eruptive energies considered reasonable for the YMR, relatively low energy eruptions result in higher doses to a critical group 20 km from the repository than more energetic eruptions. Although more energetic

(

eruptions may result in release of more HLW, the wider dispersion of the HLW effectively dilutes i

the dose to a critical group. Largest doses are realized for a relatively small-volume eruption that occurs shortly after repository closure, during a period of high annual' wind speed. Another result is that the amount of contaminated ash resuspended from the fall deposit is very important to calculating volcanism dose, as there is a nearly linear relationship between dose and airt>ome mass-loading factor. In addition, about 90 percent of the dose from volcanism is caused by inhalation of contaminated ash. Parameters used to support these sensitivity analyses are examined in Section 4.2.5.3 of this IRSR. The additional effects of intrusion on repository performance have not been evaluated in detail. These effects, however, will likely result in early failure of some' fraction of the waste package inventory (e.g., Wilson, et al.,1994). Dose to

(~

affected individuals from intrusive events will occur through hydrologic flow and transport processes, in contrast to direct dispersal of HLW into the accessible environment through a volcanic eruption. Preliminary volcanism consequence calculations only consider doses produced through the air transport pathway. These calculations do not include any contribution r

i from the groundwater pathway as a result of igneous activity failing some additional portion of waste packages, or other potential thermal, mechanical, and chemical effects on the repository, in addition, contributions to dose from leaching of radionuclides that were brought to tlie surface

[

through volcanic activity and then leached to the groundwater have not been incYded. The total dose that would occur during a volcanism scenario would, thus, be the summatwo of direct airbome dose, secondary contributions from such things as leaching of surface materials, and

{

dose expected via groundwater pathways.

Methods for assessing risk from disruptive events are discussed in greater detail in the TSPA IRSR (Rev0, Section 4.1.3.1), in the lA IRSR, risk associated with basaltic volcanic activity r

i during the proposed 10,000-yr post-closure period is calculated by weighing the conditional cumulative distribution function of dose by the occurrence probability. Using this methodology

(

lI r

F

with a 10-8 probability of volcanic disruption over 10,000 yr, the mean peak annual risk from basaltic volcanic activity from this preiiminary calculation can be estimated at 012 mrem. This calculation represents dose incurred during the first year of the eruption (i.e., peak dose).

To date, NRC has only analyzed the probability of direct (i.e., extrusive) volcanic activity affecting the repository. As is stated in the following (for example, Section 4.1.6.3.3), available information a

allows only an approximation of probability values for the other igneous scenarios. Als -

E preliminary consequence analysis has been done only for the tephra-fall portion of this scenario.

During FY99, the staff will continue to work on the consequences of direct volcanic disruption of the repository. Effort will also be directed to determining the consequences of an intrusion in the repository, which will evaluate if these effects are indeed minor as indicated by preliminary calculations. If so, NRC will conclude that additional effort on refining the probability of intrusion is unnecessary. As discussed in Section 4.1.6.3, NRC will be using an interim probab!!ity value of 2 to 5 times the probability of volcanic disruption (Delaney and Gartner,1997) to evaluate intrusive effects at the repository.

I I

Ii I

I 12 I

4.0 REVIEW METHODS AND ACCEPTANCE CRITERIA Review methods and acceptance criteria are listed for the probability and consequence subissues. Detailed technical bases are presented in subsequent subsections to support these acceptance criteria and review methods. These technical bases address the most significant topics for resolution of these subissues, including reviews of relevant work and newly-developed models that provide an independent technical basis for resolution.

4.1 PROBABILITY DOE will need to estimate the probability of future volcanic eruptions and igneous intrusions affecting the performance of the proposed repository. Staff will review DOE assumptions made in estimation of the probability of volcanic eruptions and igneous intrusions for consistency with known past igneous activity in the YMR and to determine if the analysis and assumptions do not underestimate effects. The following nine acceptance criteria apply to the probabilistic assessment of igneous hazards.

Estimates of the probability of future igneous activity in the YMR will be acceptable provided that:

(1)

The estimates are based on past pattems of igneous activity in the YMR, (2)

The definitions ofigneous events are used consistent;y. Intrusive and extrusive events should be distinguished and their probabilities estimated separately.

(3)

The models are consistent with observed pattems of volcanic vents and related igneous features in the YMR.

(4)

Parameters used in probabilistic volcanic hazard assessments, related to recurrence rate of igneous activity in the YMR, spatial variation in frequency of igneous events, and area affected by igneous events are technicallyjustified and documented by DOE.

(5)

The models are consistent with tectonic models proposed by NRC and DOE for the YMR.

(6)

The probability values used by DOE in performance assessments reflect the uncertainty in DOE's probabilistic volcanic hazard estimates.

(7)

The values used (single values, distributions, or bounds on probabilities) are technically justified and account for uncertainties in probability estimates.

(8)

If used, expert elicitations were conducted and documented, using the guidance in the Branch Technical Position on Expert Elicitation (NRC,1996), or other acceptable approaches.

I I

(9)

The collection, documentation, and development of data and models have been performed under acceptable QA procedures, or if data was not collected under an established QA program, it has been qualified under appropriate QA procedures.

I 13

The following sections present the review methods, technical basis, and status of resolution of these criteria. These technical bases represent a summary of relevant information used to evaluate the status of the subissue.

I 4.1.1 Probability Criterion 1 4.1.1.1 Acceptance Criterion Estimates of the probability of future igneous activity in the YMR will be acceptable provided that:

- The estimates are based on past pattems of Igneous Activity in the YMR.

4.1.1.2 Review Method During its review, staff should ascertain the adequacy and sufficiency of DOE characterization and documentation of past igneous activity, including the remaining uncertainties about the distribution, timing and nature of this past igneous activity. At a minimum, documentation of past volcanic activity should encompass the Yucca Mountain-Death Valley isotopic province of Yogodzinski and Smith (1995) since the cessation of large-volume silicic volcanism in the region at approximately 11 Ma. Particular attention should be given to assuring that the locations, ages, volumes, geochemistry, and geologic settings of <6-Ma basaltic igneous features, such as cinder cones, lava flows, igneous dikes, and sills, are adequately documented. Staff should determine that DOE used geological and geophysical information relevant to past volcanic activity contained in the literature (e.g., references in Appendix A).

4.1.1.3 Technical Basis Acceptable probability models use past pattems of YMR igneous activity to estimat6 probabilities of future igneous events. Current models in the available literature for the spatial and temporal recurrence of basaltic volcanism rely on probebilistic methods (e.g., Ho,1991; Kuntz, et al.,

1986; Mc8imey,1992; Wadge, et al.,1994; Connor and Hill,1995). In these models, pattems of future activity are primarily estimated from pattems of past volcanic activity, including eruption location, frequency, volume, and chemistry, in addition, geologic processes, particularly structural deformation, have been investigated as partially controlling the distribution and timing of volcanism (Bacon,1982; Parsons and Thompson,1991; Connor, et al.,1992; Lutz and Gutmann,1995; Conway, et al.,1997). Probabilistic models of volcanism at the proposed g

repository site should be consistent with rates and timing of past volcanism and with g

observations made in the YMR and other volcanic fields, regarding the relationship between igneous activity and other tectonic processes.

Il Basaltic igneous activity has been a characteristic of the Westem Great Basin (WGB) in Nevada and Califomia since about 12 Ma (e.g., Luedke and Smith,1981). Although much of this activity has occurred near the boundaries of the WGB since 10 Ma (Figure 3), distributed volcanism E

between Death Valley, Yucca Mountain, and the Reveille Range is a well-recognized feature of M

the WGB (e.g., Carr,1982). Basaltic volcanism, however, is localized in specific areas of the WGB and often shows regular spatial shifts through time (Connor and Hill,1994). Many of the E:

WGB basaltic volcanic fields exhibit clear spatial and temporal boundaries to igneous activity. In g

contrast, diffuse basaltic volcanism in the YMR is distributed over a relatively large area with often ambiguous spatial and temporal bounds (Figure 3).

j 14

s.

e s,

.c

'[;['; Crater

' Lunar

', yag9

,4 - cw i

s

_f 4200000

_ (.;. },

s

~. -

A h'

s f e,,y i

.*L.Long s

l

',4,1;

Valley V',g.

,"A Caldera /y g

• :.y l

i r

Q.

i

~..

l

,,, ~.,

g i

4100000 fet 3 _ _, _ %;

W g l

,4 Big'(

C ' *'"',N g. f g?l Pine; '

i

's

\\'5"9 YUCCA i r

's 4

.,> MOUNTAIN

.s

~.N m.a g.

g g

4.. 00..0. 00.. 0

.g M

..7.;,g...

.,..3.......

. I' "

N,[

i s, l \\.,

$ltz's l

',\\

3900000 i

'y'

. * ', x..

'y.

1 N

I t

i

\\

A l

Quatemary N

i I

l WGB

,4 b + 'l N

Volcanism s J' '

s.y l Amboy

)

I 6

<2 Ma Data sources in Luedke 1 "'

"~~.,"

2 6 Ma and Srnnh (1981) and

• 3 s

6-12 Ma Connor and Hill (1994).

i I

e km 25 50 75 100 p.

rne 25 50 75 100 T

~

200000 300000,.,

!400'6 @

d-500000 600000 I

Figure 3. Basaltic volcanic rocks of the Western Great Basin since about 12 Ma, from Luedke and Smith (1981) and references in Connor and Hill (1994) and Appendix A. Dashed area represents zone used to calculate background Quaternary volcano recurrence rates, which does not encompass the Long Valley Caldera magmatic system. Coordinates in Universal Transverse Mercator meters, Zone 11, NAD27.

[

15 W

W

I Defining the spatial and temporal extent of the YMR magma system is the first step in quantifying patterns of igneous activity for use in probability models. Quantitative criteria, however, do not clearly define the extent of the YMR basaltic volcanic system in space and time. For example, to a

date, petrogenetic relationships between <6-Ma and 6-11-Ma basalts are ambiguous, as similar g

composition basalts occur within each interval of time. Isotopic geochemical characteristics are distinct for s 6-Ma basalts located within 40 km of the proposed repository site, which is a distance that encompasses the main YMR system. Some s 6-Ma basalts within 90 km south and west of the proposed repository site, however, have the same distinct compositional characteristics and, thus, may be part of the YMR volcanic system.

Numerous attempts to define the extent of the YMR basaltic volcanic system have been based on qualitative to semi-quantitative criteria. Early workers (Vaniman, et al.,1982; Crowe, et al.,

1982) concluded that basalts younger than about 9 Ma were petrologically distinct from 9-to 11-gl Ma basalts and, thus, constitute the igneous system of interest. Subsequent work (Crowe, et al.-

E 1983; 1986) generally confirmed this interpretation; however, many analyzed Plio-Quaternary basalts have petrogenetic characteristics similar to some 9-to 11-Ma basalts (i.e., Crowe, et al.,

1986). Crowe and Perry (1989) used similar petrogenetic arguments to define the Crater Flat Volcanic Zone (CFVZ), which is a northwest-trending zone based on the occurrence of <5-Ma volcanoes between Sleeping Butte and buried volcanoes in the Amargosa Desert (Figure 4).

Smith, et al., (1990) expanded the CFVZ to include Buckboard Mesa. Numerous other E

subdivisions are possible, based on the pattem of <5-Ma basaltic volcanoes (e.g., Crowe, et al.,

E, 1995; Geomatrix,1996).

isotopic geochemical characteristics commonly are used to define the extent of basaltic igneous systems (e.g., Leeman,1970; Farmer, et al.,1989). Isotopes of Sr and Nd are distinct for s 6-Ma basalts located within 50 km of the proposed repository site (Farmer, et al.,1989; Yogodzinski and Smith,1995; Hill, et al.,1996). In addition, Pliocene basalts in the Grapevine Mountains, Funeral Formation, and southem Death Valley (Figure 4) also share these distinctive j

isotopic characteristics. These more distal basalts, however, are located in significantly different tectonic regimes than the YMR. Crustal tectonics likely influence magma ascent and eruption El rates (e.g., McKenzie and Bickle,1988). Although the distal basalts may have originated from a El compositionally-similar mantle, differences in tectonic history or crustal lithologies may have resulted in spatial and temporal controls on basaltic volcanism that are significantly different from the YMR. Figure 4 shows the extent of basalts that are potentially part of the YMR igneous system, based on temporal, spatial, and geochemical affinities. Although a range of geochronological techniqu. s has been utilized in the YMR to date Quatemary basaltic features, e

most basalts older than about 1 Ma have been dated using standard K-Ar and *Ar/"Ar methods (Hill, et al.,1993). These data are compiled in Appendix A and are used in subsequent probability analyses. The extent of the YMR magmatic system was also considered during the DOE-sponsored formal expert elicitation (Geomatrix,1996). This report utilized areas that generally encompassed about the same general region as that shown on Figure 4. However, more extensive regions were often included in the background or regional recurrence rate estimates. In general, the report concluded that the <5 Ma basalts were most important to define g

temporal recurrence rates for the YMR. However, it appears from Geomatrix,1996, that 3

petrographic data and models were not used to define spatial pattems or process models. It also is not clear why the 5-11 MA volcanics were not considered by all experts to define spatial a

pattems or derive process models. As a result, the areas used for the regional recurrence rate g

estimates do not appear to be well supported by the petrographic data and models.

The significance of basaltic centers >40 km from the site to probability issues depends on the 16

8 3r -

• .. M..f

• T s,.

e e

e e

M

• m N

\\

.= *9 **...

Stonewali Mith b

3 7.5-9 N

's

,[Y Pahute de's! *.,

g

,.a h. h-( _,E -(

,,V

~

y' I

.o

. -. N

,8

\\d.

s a,

i s

6D

.'t I i v (

, # 's 3

8 8

8 g

's a - ~ *,. f s r/

8

\\,

9 7,012g.

s,j s\\

Sleepen B #f..';

Rocket.'Buc r?[6 f.s

• b 035 sob 5 1

8 2 01 1 Wash 4e

-,'f8204--. )

I, -

Ng'r g (,

+g,*g',,

Thirsty utn l*sifi"'** U, *.

., # ap ;,

s

• .g;,y...

) (1'q

• l '

g,=ili i N

g a3r e s.

e e

t "h

B'estry 3[\\ Q s,,, M, #,e -

e.3 -;

e

~,e c,n,

0 7.1e0.1 y Ubehe p

. r 0.017 N.;

4.x. sw

'g','

f..

'g-i 10.5*0.4 c.

10.0*0 4 \\

l e

og e

k )Oe0.3 Crater l'iat Frenchman

- } Kiwi Mesa 10.2s0.2 Fiat 8 5*0.1 1.1*01 Jackass r

%. 113*04 Fiat 9 6 0.4

/

3

k. 7.520.3 4 %. 6.3*0.8

- a

/*

4 67,

y g

1 8

s-

.s

=

%.N/eM Crater Flat-3.8241 l

I L Skus MtN ~ Sk".230.5,, * *-

I' MI" 10 920.1-0 VH1 11.3:0 4 - glathro Wells

,/

84:04.

VH2 <12 8 0.13.02 e,, +

g o

1 Funeral.

g O

/

i

.s

~ tm N.

Ama,oo.a e

O 4 120.1

NE Amargop j

I

-Nova s

s.,

7

".,e

,i I

or ~ O.m w.

V_o e

se0 SE Amargosa e/

o r

e e e e

g ti:

3'

• g 96 0.1

/

?

g j

~,

• I'L'*.

/

I s

< va-

~

QPlateau 9

y

'g l

e

• '. 4

'.p**

I fd h

~

'h FuneralFm

. g h.

,=.-e

s

.\\ s s *'

l b

m af, e N.

e e

• %l

<2 Ma Ages in rnilhons of years, i

ga l 2-6 Ma queried when unartain.

(

I e-12 Ma

- G/eenwater, 1,

Data sources m Qhoshod

• Aeromagnetic Appendix A, northam volcanics,

anomahes part of map pnmarily from I

/r o'

e Dris holes t) '

Coso

' kd

.e

-," Caldera boundary (

oo VF 8 8

8

%,)8 g.g J,./

A Pumteous 3r n

alluvium 200 m contourintersal a.

I h

'b

. i 5. reline Bu sho

+

35rw A

01 tir w.

m.

5 10 15 20 2s iir I

Figure 4. Basaltic volcanic rocks of the Yucca Mountain region since about 11 Ma. Data sources listed in Appendix A. Dotted line represents the extent of basaltic volcanic rocks that potentially constitute the Yucca Mountain region maCma system.

17

model being evaluated. Probability models that depend heavily on the timing of past events (e g., Ho,1992) are strongly affected by inclusion of these centers in the YMR system.

Depending on the time used to calculate future volcano recurrence rates, inclusion of the distal centers may substantially elevate or decrease the probability of future eruptions at the proposed I

repository site. In contrast, models that spatially define the extent of the system and evaluate the area of the system to the area of the proposed repository (e.g., Crowe, et al.,1982; Geomatrix, 3

1996) may exhibit a marked decrease in probability at the site due to expansion of the YMR j

system to accommodate distal volcanoes. Finally, the presence of the distal volcanic centers has little effect on spatio-temporal recurrence models (e g., Connor and Hill,1995), as distal centers are too old and too far away from the proposed repository site to strongly influence the locus of volcanism in Crater Flat basin.

4.1.1.4 Summary Sufficient information exists on the spatial and temporal extent of the YMR basaltic system to support spatio-temporal probability models (e.g., Connor and Hill,1995). Evaluation and acceptance of other models, however, requires assessment of the petrogenesis of 0.1-11-Ma basalt of the YMR. A reasonably-conservative, working hypothesis for these assessments is that all s 6-Ma basalt within the dashed boundaries of Figure 4 is part of the YMR igneous system.

Relevant data for these volcanic centers are summarized in Appendix A. In addition, some 6 I Ma basalt within these boundaries has the same petrogenesis as s 6-Ma basalt and, thus, may be part of the YMR igneous system of interest.

All current probability estimates for future igneous activity at the proposed repository site are based on past pattems of igneous activity in the YMR. These models are, thus, acceptable to NRC. Some parameter values or ranges used in these probability models, however, are I

dependent on definitions of the spatial or temporal extent of the YMR igneous system. Models that may be developed by DOE subsequent to those discussed in this report will need to be evaluated independently by NRC to assure that the parameters and definitions are intemally consistent.

4.1.2 Probability Criterion 2 4.1.2.1 Acceptance Criterion Estimates of the probability of future igneous activity in the YMR will be acceptable provided that:

-The definitions of igneous events are used consistently. Intrusive and extrusive events should be distinguished and their probabilities estirnated separately.

4.1.2.2 Review Method Staff should determine that igneous events are defined consistently by DOE and that probabilities ofintrusive and extrusive igneous events are calculated separately. Definitions in current use for extrusive volcanic events include formation of a new volcano (Crowe, et al.,1982; Connor and Hill,1995); an episode of eruptive activity at a new or existing volcano following an extended period of quiescence (Ho, et al.,1991; Bradshaw and Smith,1994); and mappable eruptive units, each being an assemblage of volcanic products with intemal stratigraphic features that indicate a cogenetic origin and eruption from a common vent (Condit and Connor,1996).

Definitions of intrusive events include injection of single, igneous dikes and formation of dike is I

swarms (Delaney and Gartner,1995).

l 4.1.2.3 Technical Basis Although all volcanic events are associated with an intrusive event, basaltic intrusions may reach subsurface depths of less than 300 m without forming a volcano (Gudmundsson,1984, Carter Krogh and Valentine,1995, Ratcliff, et al.,1994) Therefore, probability calculations must distinguish between volcanic (i.e., extrusive) and intrusive events in order to be applicable in repository performance and risk assessment models.

Because recurrence rates used in many probability models are sensitive to the size, duration, and area affected by igneous events, igneous event definitions must be used consistently throughout an acceptable analysis. Furthermore, differences in igneous event definitions must be considered when comparing the results of different probabilistic hazard analyses (See Criterion 6). In addition, the method used to count igneous events affects the outcome of the probability analysis. Definitions of.t %anic and intrusive igneous events commonly found in the geologic literature include:

Individual, mappable eruptive units Episodes of vent or vent-alignment formation Emplacement of an igneous intrusion -

Volcanic eruption and accompanying dike injection As discussed in the following section, igneous activity in the YMR can be categorized, using each of these definitions with varying degrees of confidence.-

4.1.2.3.1 Individual Eruptive Units Definitions of volcanic events very widely in the literature (Condit, et al.,1989; Bemis and Smith, 1993; Delaney and Gartner,1995; Lutz and Gutmann,1995; Connor and Hill,1995). Ideally, volcanic events would correspond to eruptions. Unfortunately, subsequent geologic processes often obliterate evidence of previous eruptions from the geologic record (e.g., Walker,1993).

Consequently, volcanic events often have been defined as mappable eruptive units, each unit being an assemblage of volcanic products having intemal stratigraphic features that indicate a cogenetic origin and eruption from a common vent (Condit and Connor,1996). A simple definition that can be applied to young cinder cones, spatter mounds, and maars is based on morphology; an individual edifice represents an individual volcanic event (Connor and Hill, 1995). In older, eroded systems, such as Pliocene Crater Flat, evidence of vent occurrence, such as near-vent breccias or radial dikes, is required. One important advantage of this definition of volcanic events is its reliance on geological and geophysical mapping, with no requirement for geochronological data. Therefore, this definition can be applied with greater confidence than the other definitions, which require relatively precise geochronological data.

Volcanic hazard analyses using the individual vent definition for volcanic events assume all mapped volcanic units occur as independent events. The resulting probability estimate is for direct disruption of the proposed repository by a single vent-forming volcanic eruption (e.g.,

Connor and Hill,1995).

19

However, it should be noted that several edifices can form during an essentially-continuous basaltic, eruptive episode. For example, three closely-spaced cinder cones formed during the 1975 Tolbachik eruption (Tokarev,1983; Magus' kin, et al.,1983). In this case, the three cinder cones represent a single, eruptive event that is distributed over a larger area than represented by a single cinder cone. The three 1975 Tolbachik cinder cones have very different morphologies and erupted adjacent to three cider (Holocene) cinder cones (Braytseva, et al.,1983). Together, this group of six cinder cones forms a 5-km-long, north-trending alignment. Without observing the formation of this alignment, it likely would be difficult to resolve the number of volcanic events represented by these six cinder cones if the number of volcanic events was defined as the number of eruptions. This type of eruptive activity raises uncertainties about how a number of volcanic events represented by individual volcanoes should be assessed, even where these volcanoes are well-preserved.

Geochemical and apparent geochronological variations present at some YMR Quatemary volcanoes have been interpreted as reactivation of individual volcanoes after more than 10,000-yr quiescence (Wells, et al.,1990; Crowe, et al.,1992; Bradshaw and Smith,1994). Results from paleomagnetic studies, however, appear to contradict this interpretation (Champion,1991; Turrin, et al.,1991) and cast doubt on the likelihood that cinder cones in the YMR have reactivated long after their original formation (Whitney and Shroba,1991; Wells, et al.,1990, 1992; Turrin, et al.,1992; Geomatrix,1996). Given the possibility of cinder cone reactivation, the number of volcanoes present in the YMR may underestimate the rate of future YMR volcanic eruptions. In the context of volcanic hazards for the proposed repository, however, the spatially-dispersec character of volcanism is extremely important in calculating the probability of M

occurrence, whereas the reactivation of an existing cinder cone is more important in determining g

consequence of the activity. Thus, reactivation of cinder cones is interesting as a gauge of overall activity in the volcanic system, but, is not easily related to rates of new volcano formation.

4.1.2.3.2 Episodes of Vent or Vent-Alignment Formation Additional investigations in other volcanic fields have demonstrated that some cinder cone E

alignments develop over long periods of time during multiple episodes of volcanic eruption 5

(Connor, et al.,1992; Conway, et al.,1997), particularly where a large fault controls the locations of basaltic vents. For example, Conway, et al. (1997) found that the northem segment of the E

Mesa Butte fault zone in the San Francisco volcanic field, Arizona, repeatedly served as a g

pathway for magma ascent for at least 1 m.y. and formed a 20-km-long cinder cone alignment (Figure 5). Isochron dates reported in Conway, et al. (1997) indicate volcanism along the northem Mesa Butte fault was episodic, and successive episodes were separated in time by as much as 400 k.y. (Figure 6). Spatial pattems of volcanism along the Mesa Butte alignment apparently were indep9ndent of field-wide trends, indicated by the large lateral shifts in volcanic loci between successive episodes (Conway, et al.,1997). These observations help clarify trends E

observed in the development of young, potentially active volcanic alignments. For example, the 5

largely Holocene Craters of the Moon volcanic field, Idaho, shows similar eruption pattems characterized by multiple episodes of magmatism and frequently-shifting loci of volcanism along the Great Rift (Kuntz, et al.,1986), albeit on a time scale of thousands of years. This behavior contrasts sharply with eruption pattems of other short-lived fissure eruptions, such as the Laki fissure eruption (Thordarson and Self,1993) or the Tolbachik eruption of 1975 (Tokarev,1983).

Evidence of episodic volcanism along the Mesa Butte fault indicates independent magmatic episodes may recur along geologic structures even following periods of quiescence lasting 100 k.y. or more. Volcano alignments in the YMR, such as the Amargosa Aeromagnetic Anomaly A alignment (Connor, et al.,1997), thus, may constitute multiple volcanic events.

20

\\

FV1 900i80

\\

-830180 MB-06

\\800180k 880 MB-03 MB-04 MB-01 Vent 66

.\\

-8501100 760i50

\\

MB-07 Mesa B ' "' [4 ** 3 -

\\

g Con i g

~820i60 Vent 6618

i. , s

4 MB-05 s

• ), _j 9

v' 1020i30 '

\\ Cedar Ranchh850i60 MB-02 km fault MB-08 0

5 Figure 5. Development of multiple vent alignments along a fault is illustrated by the Mesa Butte alignment in the San Francisco volcanic field, Arizona. Dated cones and associated lava flows show that this 20-km-long alignment developed by repeated injection of magma along the Mesa Butte fault. Figure from Conway, et al. (1997).

21

I I

1600 g

n

, h ChapelMountain I

1400 j',

(sig j

g g

\\

j 3

Y

\\d M

7 1200 E

~

Cedar Ranch

,s CD

/

O

\\

~__ __ __ _ T {1__

g c

N x

E E

g 1000

}

LR '

Mesa BuV; 1

~~

e o

E

[R

~

o 4i R

E 800

_o__

j o

E s

w N

RedMountain 3

~

600 is 2L j

2 400

~

Shadow Mountain

~

4I' N I

q, T

1 l\\

200 w

~

c

~

m m

x m

w n

v

[E$b$d5$$$$$$5$

Not to scale SW 4 NE I

Figure 6. Detailed geochronology shows that the Mesa Butte alignment formed over a period of more than 1 m.y. through several distinct episodes of volcanism. Figure from Conway, et al. (1997).

22

['

Palsomagnetic (Champion,1991) and radiometric dating (Appendix A) of the Quatemary Crater

{

' Flat cinder cones (Figure 7) suggests these cinder cones may have formed during a relatively brief period of time (<100,000 yr) and, therefore, may represent a single, eruptive event like the Tolbachik alignment. Evidence from aeromagnetic and ground magnetic surveys (Langenheim,

' et al.,1993, Connor, et al.,1997) suggests that older, buried volcanoes exist in southem Crater r.

[

Flat along this alignment. Therefore, the alignment may have reactivated through time, in a manner similar to the Mesa Butte volcano alignment.

[

Defining aligned volcanoes of similar ages as single volcanic events effectively reduces both the total number of volcanic events in the region and the regional recurrence rate. The area affected by the entire cone alignment, however, is much greater than the areas impacted by individual

{

cinder cones. This variation in disruption area must be propagated through the volcanic hazard analysis.

Hazard analyses defining vents and vent alignments as volcanic events are used to estimate the probability of direct disruption of the proposed repor,itory. Primary uncertainties in those probability estimates result from uncertainty in the number and distribution of volcanic vents along alignments.

4.1.2.3.3 Emplacement of an igneous intrusion

('

igneous events are a broader class than volcanic events in that igneous events must encompass the intrusive and extrusive components of igneous activity. The number of mapped, igneous dikes generally is not considered a reasonable definition of an igneous event because multiple

{

dikes often are injected into the shallow crust during single episodes of igneous intrusion.

L Furthermore, individual dikes frequently coalesce at lower stratigraphic levels. As a result, several mapped dikes may represent a single igneous event. For example, Delaney and Gartner (1995) mapped approximately 1,700 individual dikes in the Pliocene San Rafael volcanic field, r

[

Utah (Figure 8). These dikes are associated with approximately 60 breccia zones and volcanic buds, which are interpreted as the roots of eroded, volcanic vents. Based on their mapping, Delaney and Gartner (1995) suggested that approximately 175 episodes of intrusion resulted in

(.

the emplacement of the 1,700 dikes and 60 volcanic vents, but also indicated that this grouping of mapped units was a subjective process.

[

in the YMR, the number of Plio-Quatemary igneous events is unknown. Based on analogy with the San Rafael volcanic field, YMR intrusive events may be a factor of two or more greater than the number of volcanic events (Delaney and Gartner,1997). Studies in the YMR by Ratcliff, et

[-

al. (1994) and Carter Krough and Valentine (1995) have demonstrated that some Miocene basaltic, igneous intrusions stagnated within several hundred meters of the surface without erupting. These basaltic dikes and silis are mapped in Miocene tuffs, similar in character and composition to those underlying Yucca Mountain. Thus, probability estimates based on the

[

number of igneous events characterized by this approach would encompass both direct disruption of the repository with transport of waste into the accessible environment during a volcanic eruption and the indirect effects, such as canister failure during dike or sill intrusion.

(

Additional complications arise with this definition based on the limited ability of a shallow dike to laterally transport entrained material into the volcanic conduit (e.g., Spence and Turcotte,1985).

A volcano may form outside of the repository boundary, with an associated subsurface dike that

[

penetrates the repository directly. Although an intrusive, igneous event definition would indicate i

disruption of the repository, tfm ability of the waste to be transported laterally by the dike and dispersed into the accessible environment by the volcano would be extremely limited. The 23 I

L

,m I

yg dj '-

<g$ _.

l pW'y

' % +% Q$f, % l y%p l

\\ ~N N

.N'

~/

l litario-

\\

0.0t0.4 l Northern I.

1.1510.07 8 g,

,? Repository

. Site l

Black

. 03

$0.95i0.03 g

a i0.07 OL 1%

Little

6.3i0.8 D. g0.06 )s!-

5 5

,gmy, 10.9i0.3 'k

• 3.Bi0.1 y

w.

h Lathrop Wells W

0.1310.02 I

O s, +.

q{

C$ \\,,

Amargosa d

4

~~

\\,

g 4.110.1

.A j

<2 Ma

~~ss; 2-6 Ma

$P km 5

10 6-12 Ma l

g

\\, b mi 10 EB Figure 7. Basaltic volcanic rocks of the Crater Flat area, Nevada. Data sources listed in Appendix A. Extent of Amargosa Desert basalt interpreted from aeromagnetic data in g

Langenheim, et al. (1993) and Connor, et al. (1997).

3 24 I

I

\\

l i

~

4290o%

\\

t Y

l 1280ow

\\4 q

\\

\\

i I

\\s ([c!n 1

./

l 327o*o p

PK 3

s y

A ' ('

l t

~

I

%r g /j/

1

)

b

\\*

g il <

= '\\ \\

d' s

s

'i

,d. ;.

\\

/

b' $ 'j' 00 4250 0 A vent l \\l B Brecciazona Geology from 8 8

/

}l l Dike Gartner and M Sill Delaney (1988)

\\

i km i

l t \\

s.

~

g 0

10 480 @l 1

470000 1 0

0 490 @ ;

Figure 8. Distribution of dikes, breccia zones, sills, and vents in the San Rafael volcanic field, Utah. Figure from map by Gartner and Delaney (1988).

25

definition of an igneous event as encompassing both volcanic and intrusive components, while strictly correct from a geologic perspective, is unsuitable for application in risk assessments because of the dramatically-different consequences of intrusive and extrusive igneous activity.

m Therefore, it is best to consider only the intrusive component of igneous events under this g

definition, reserving extrusive components for definitions based on vents and vent alignments.

4.1.2.3.4 Volcanic Eruptions with Accompanying Dike Injection An igneous event can be similarly defined in terms of the subsurface area disrupted by the intrusion of magma during a volcanic event. For example, numerous dikes in the San Rafael E

volcanic field were injected laterally through the shallow subsurface for hundreds of meters away E

from volcanic vents during volcanic eruptions (Delaney and Gartner,1995). Uncertainties resulting from this definition of an igneous event include estimates of probable lengths and widths of dike zones associated with the formation of vents and the locations of vents along these dike zones (e.g., Hill,1996). The effects of these laterally-injected dikes on performance, however, are substantially less than the direct effects of vent formation, because of the limited ability of the waste to be directly transported to the surface along nearly the length of the dikes when compared to the transportation ability of the volcanic vent itself.

)

4.1.2.4 Summary There is no one generally-accepted criterion to singularly define an igneous event. Probability j

models are acceptable, provided igneous events are explicitly defined and the definition is 3l applied consistently throughout the model. Therefore, all the above definitions can be El considered acceptable. Repository performance considerations, however, require that the probability of volcanic disruption is calculated discretely from the probability of intrusive disruption. All volcanic events that may penetrate the proposed repository are accompanied by a subsurface intrusion. However, intrusive events may occur without direct volcanic disruption, either because a volcano does not form at the surface or the location of the volcano is at a distance greater than the lateral transport ability of a shallow dike. Therefore, the probability of intrusive, igneous events affecting the proposed repository is at least as large as, and could be significantly larger than, the probability of volcanic disruption.

Potentialintrusive and extrusive events must be considered separately because the effects on repository performance are significantly different for extrusive and intrusive processes. A volcanic, igneous event that penetrates the repository has the potential to entrain, fragment, and transport radioactive material into the subaerial accessible environment. In contrast, an intrusive, igneous event that penetrates the repository would produce thermal, mechanical, and chemicalloads on engineered systems, which could impact waste-package degradation.

Radioactive release associated with intrusive, igneous events is through hydrologic flow and E

transport, rather than through direct transport by volcanic processes. Therefore, probability Bl calculations must distinguish between volcanic and intrusive, igneous events in order to be applicable in repository performance and risk assessment models.

4.1.3 Probability Criterion 3 4.1.3.1 Acceptance Criterion Estimates of the probability of future igneous activity in the YMR will be acceptable provided that:

26 I

n The models are consistent with observed pattems of volcanic vents and related igneous features in the YMR.

4.1.3.2 Review Method Staff should determine if DOE probability models are consistent with known Pliocene and Quatemary igneous events in the Yucca Mountain-Death Valley magmatic system and that the proposed probability models are consistent with pattoms of igneous activity in other, comparable V:lcanic fields. Current interpretations indicate these pattems include a tendency for basaltic v:lcanic vents to cluster and form northeast-trending vent alignments in the YMR. Structural control of the locations of individual volcanoes by faults also is prevalent. Other interpretations that lead to reasonably-conservative estimates of probability will be acceptable.

4.1.3.3 Technical Basis Previous studies of volcanism in the YMR, and elsewhere, cumulatively indicate that models describing the recurrence rate or probability of basaltic volcanism should reflect the clustered n:ture of basaltic volcanism and shifts in the locus of basaltic volcanism through time. Models clso should be amenable to comparison with basic geological data, such as fault pattoms and neotectonic stress information, that affect vent distributions on a comparatively more detailed scale. The models used to estimate future igneous activity in the YMR should either explicitly

)

cccount for the following or obtain bounding estimates:

Shifts in the locus of volcanic activity through time I

Vent clusters

)

Vent alignments and correlation of vents and faults j

Data from other basaltic volcanic fields may be used to test the models. Each of these spatial pattoms is reviewed in this section, with emphasis on the nature of these spatial pattoms in the YMR and how these compare with spatial pattoms in cinder cone volcanism observed in other basaltic volcanic fields. This comparison is followed by a discussion in Section 4.1.4.3 of how these spatial pattoms in volcanic activity can be used to calibrate and test probabilistic volcanic hazard models for disruption of the proposed repository.

4.1.3.3.1 Shifts in the Location of Basaltic Volcanism Spatial variation in recurrence rate of volcanism in the YMR has been suggested based on cpparent shifts in the locus of basaltic volcanism from east-to-west since the cessation of caldera-forming volcanism in the Miocene Southem Nevada Volcanic Field (Crowe and Perry, 1989). Well-defined shifts in volcanism have occurred in many other basaltic volcanic fields. In the Coso volcanic field, Califomia, Duffield, et al., (1980) found that basaltic volcanism occurred in essentially two stages. Eruption of basalts occurred over a broad area in what is now the northem and westem portions of the Coso volcanic field from approximately 4 to 2.5 Ma. In the Quatemary, the locus of volcanism shifted to the southem portion of the Coso volcanic field.

Condit, et al., (1989) noted the tendency for basaltic volcanism to gradually migrate from west to cast in the Springerville volcanic field between 2.5 and 0.3 Ms. Other examples of continental brsaltic volcanic fields in which the location of cinder cone volcanism has migrated include the S:n Francisco volcanic field, Arizona, (Tanaka, et al.,1986), the Lunar Crater volcanic field, 27

I Nevada, (Foland and Bergman,1992), the Michoacsn-Guanajuato volcanic field, Mexico, (Hasenaka and Carmichael,1985), and the Cima volcanic field, California, (Dohrenwend, et al.,

1984; Turrin, et al.,1985). In some areas, such as the San Francisco and Springerville volcanic fields, migration is readily explained by plate movement (Tanaka, et al.,1986; Condit, et al.,

1989; Connor, et al.,1992). In other areas, the direction of migration or shifts in the locus of volcanism does not correlate with the direction of plate movement. In either case, models developed to describe recurrence rate of volcanism or to predict the locations of future eruptions in volcanic fields need to be sensitive to these shifts in the location of volcanic activity.

Sensitivity to shifts in the locus of volcanism can be accomplished by weighing more recent (e.g.,

g Pliocene and Quaternary) volcanic events more heavily than older (e.g., Miocene) volcanic 3

events. Shifts in the locus of volcanism, however, also introduce uncertainty into the probabilistic hazard assessment. For example, in the Cima volcanic field, <1.2-Ma basaltic vents are located g;I south of significantly older volcanic vents (Dohrenwend, et al.,1984; Turrin, et al.,1985). This E

suggests that probability models based on the distribution of older vents would not have forecast i

the location of subsequent (<1.2 Ma) eruptions adequately. In the Springerville volcanic field, large-scale shifts in the locus of volcanism accompanied a major geochemical change in the basalts from tholeiitic to more alkalic, suggesting that a fundamental change in petrogenesis may have affected shifts in the locus of volcanism (Condit and Connor,1996).

As the period required for large-scale shifts in the locus of volcanism is much greater than the period of performance of a repository, the effects of these shifts can be effectively mitigated in the probability models by simply applying a more heavy weight to the distribution of Quatemary a

volcanic events than older volcanic events in the probability analysis.

g 4.1.3.3.2 Vent Clustering Crowe, et al. (1992) and Sheridan (1992) noted that basaltic vents appear to cluster in the YMR.

Connor and Hill (1995) performed a series of analyses of volcano distribution that yielded several useful observations about the nature of volcano clustering in the region. First, vents form El statistically-significant clusters in the YMR. Spatially, volcanoes less than 5 Ma form four El clusters: Sleeping Butte, Crater Flat, Amargosa Desert, and Buckboard Mesa. The Crater Flat and Amargosa Desert Clusters overlap somewhat due to the position of Lathrop Wells volcano EI and the three Amargosa Aeromagnetic Anomaly A vents (Figure 9). Second, a volcanic event E'

located at the repository would be spatially part of, albeit near the edge of, the Crater Flat Cluster, rather than forming between or far from clusters in the YMR. Third, three of the four clusters reactivated in the Quatemary, indicating these clusters are long-lived and, thus, provide i

some constraints on the areas of future volcanism.

Cinder cones are known to cluster within many volcanic fields (Heming,1980; Hasenaka and E

Carmichael,1985; Tanaka, et al.,1986; Condit and Connor,1996). Spatial clustering can be E

recognized through field observation or through the use of exploratory data analysis or cluster analysis techniques (Connor,1990). Clusters identified using the latter approach in the g

Michoackn-Guanajuato and the Springerville volcanic fields were found to consist of 10 to 100 g

individual cinder cones. Clusters in these fields are roughly circular to elongate in shape with diameters of 10 to 50 km. The simplest explanation for the occurrence, size, and geochemical differences between many of these clusters is that these areas have higher magma supply rates from the mantle. Factors affecting magma pathways through the upper crust, such as fault distribution, appear to have little influence on cluster formation (Connor,1990; Condit and Connor,1996). In some volcanic fields, such as Coso, the presence of silicic magma bodies in 28

I I

I PREDECISIONAL I

4056000 l.

nT

-80

-100 I

4055000 -

-12 0

-140

-160 I

-18 0

-200

-220 4054000 l

-240

-260

-280 I

-300

-320 4053000 -

-340

-360 I

-380

-400

-420 I

4052000 -

-440

-460 g

-480

-500 4051000 -

i

?

?

?

P""8

-l i

i i

544000 545000 546000 547000 Figure 9. Ground magnetic map of Amargosa Aeromagnetic Anomaly A showing three aligned anomalies, interpreted to be produced by a buried alignment of three basaltic volcanoes. Contour interval is 10 nT. Figure from Connor, et al. (1997).

29 I

I

the crust may influence cinder cone distribution by impeding the rise of denser mafic magma (Eichelberger and Gooley,1977; Bacon,1982), resulting in the formation of mafic volcano clusters peripheral to the silicic magma bodies.

Basaltic vent clustering has a profound effect on estimates of recurrence rate of basaltic volcanism. For example, Condit and Connor (1996) found that recurrence rate varies by more

. than two orders of magnitude across the Springerville volcanic field due to spatio-temporal clustering of volcanic eruptions. In the YMR, Connor and Hill (1995) identified variations in recurrence rate of more than one order of magnitude from the Amargosa Desert to southem

' Crater Flat due to the clustering Quatemary volcanism. In contrast, probability models based on a homogeneous Poisson density distribution that ignores clustering will overestimate the likelihood of future igneous activity in parts of the YMR far from Quatemary centers and underestimate the likelihood of future igneous activity within and close to Quatemary volcano clusters.

4.1.3.3.3 Vent Ali9nments and Correlation of Vent Alignments and Faults Tectonic setting, strain-rate, and fault distribution all may influence the distribution of basaltic vents within clusters, and sometimes across whole volcanic fields (Nakamura,1977; Smith, et cl.,1990; Parsons and Thompson,1991; Takada,1994). Kear (1964) discussed local vent clignments, in which vents are the same age and easily explained by a single episode of dike injection, and regional alignments, in which vents of varying age and composition are aligned ever distances of 20 to 50 km or more. For example, by Kear's (1964) definition, the Mesa Butte clignment (Figure 5) would be a regional alignment that is more likely to reactivate after a long period of quiescence than a local alignment. Thus, this distinction between local and regional clignmer,ts can potentially alter probability estimates.

Numerous mathematical techniques have been developed to identify and map vent alignments on different scales, including the Hough transform (Wadge and Cross,1988), two-point azimuth cnalysis (Lutz,1986), frequency-domain map filtering techniques (Connor,1990), and application of kamel functions (Lutz and Gutmann,1995). Regional alignments identified using these techniques are commonly colinear or parallel to mapped regional structures. For example, Draper, et al., (1994) and Conway, et al. (1997) mapped vent alignments in the San Francisco volcanic field that are parallel to, or colinear with, segments of major fault systems in the area.

About 30 percent of the cinder cones and maars in the San Francisco volcanic field are located along these regional alignments (Draper, et al.,1994). Lutz and Gutmann (1995) identified similar pattems in the Panicked volcanic field, Mexico. Although alignments clearly can form as a result of single episodes of dike injection (Nakamura,1977) and, therefore, are sensitive to stress orientation (Zoback,1989), there are also examples of injection along pre-existing faults (s.g., Kear,1964; Draper, et al.,1994; Conway, et al.,1997). Therefore, stress orientation in the crust and orientations of faults are indicators of possible vent-alignment orientations.

In the YMR, Smith, et al., (1990) and Ho (1992) define north-nor1heast-trending zones within which average recurrence rates exceed that of the surrounding region. The trend of these z:nes corresponds to cinder cone alignment orientations, including Quatemary Crater Flat and Sleeping Butte, that Smith, et al., (1990) and Ho (1992) hypothesize may occur as a result of structural control. Recent geophysical surveys of Armagosa Aeromagnetic Anomaly A provide further evidence of the significance of northeast-trending alignments in the YMR (Connor, et al.,

1997). The ground magnetic map of data collected over Amargosa Aeromagnetic Anomaly A d:lineates three separate anomalies associated with shallowly-buried basalt with a strong 30

I

~

,)

i I

I I

I I

I I

I I

reversed polarity remanent magnetization (Figure 9). These anomalies are distributed over 4.5 km on a northeast trend, each having an amplitude of 70-150 nT. Although these features can be partially resolved with aeromagnetic data (Langenheim, et al.,1993), trenchant details emerge I

from the ground magnetic survey that are important to probabilistic volcanic hazard analyses and tectonic studies of the region. The southemmost anomaly, which has a smaller amplitude than those to the north but is nonetheless distinctive, and the northeast-trending structure within the negative portion of the central anomaly, which mimics the overall trend of the alignment (Figure 9), are important characteristics. The ground magnetic data also enhance the small positive anomalies north of each of the three larger amplitude, negative anomalies, reinforcing the interpretation that Amargosa Aeromagnetic Anomaly A is produced by coherent basaltic vents

'with strongly-reversed remanent magnetizations.

A key result of this ground magnetic surveyis identification of the northeast trend of the anomalies, which is quite similar to the alignment of five Quaternary cinder cones in Crater Flat (Figure 7) and to the Sleeping Butte cinder cones, a Quatemary vent alignment 40 km to the northwest of Crater Flat. Although the age of the Amargosa ^r 7 magnetic Anomaly A alignment is at present uncertain, it suggests that development of northeast-trending cone alignments is a pattem of volcanism that has persisted through time in the YMR and supports the idea that future volcanism may exhibit a similar pattem (Smith, et al.,1990).

Other ground magnetic surveys provide further evidence of cinder cone localization along faults (Stamatakos, et al.,1997a; Connor, et al.,1997). Northem Cone is located approximately 8 km from the repository site in Crater Flat and is the closest Quatemary volcano to Yucca Mountain.

Its proximity to the site of the proposed repository makes the structural setting of Northem Cone of particular interest to volcanic hazard assessment. Northem Cone consists of approximately 2

0.4 km of highly magnetized (10-20 A m") lava flows, near-vent agglutinato, and scoria aprons I

resting on a thin alluvial fan. Large-amplitude, short-wavelength magnetic anomalies were observed over the lavas. No evidence of northeast-trending structures was discovered that could directly relate Northem Cone to the rest of the Quatemary Crater Flat cinder cone I

alignment. Instead, prominent linear anomalies surrounding Northem Cone trend nearly north-

{

south and have amplitudes of up to 400 nT (Figure 10). These anomalies likely result from offsets in underlying tuff across faults extending beneath the alluvium (cf. Faulds, et al.,1994).

The relationship between faults and Northem Cone is clarified when the ground magnetic map is compared with topographic and fault maps (Frizzell and Schulters,1990; Faulds, et al.,1994).

The north-trending anomalies at Northem Cone roughly coincide with mapped faults immediately I

north of the survey area that have topographic expression resulting from large vertical displacements. These mapped faults and faults inferred from the magnetic map are all oriented north to north-northeast, which are trends favorable for dilation and dike injection in the current stress state of the crust (e.g., Morris, et al.,1996). Thus, the Northem Cone magnetic survey provides further support for the concept that volcanism on the eastem margin of Crater Flat was localized along faults.

Thus, there is ample evidence to suggest pattems in YMR basaltic volcanic activity are influenced by the stress state of the crust and by fault pattems. This influence includes the development of northeast-trending volcanic alignments and the localization of vents along faults.

Smith, et al., (1990) noted that the occurrence of northeast trending alignments is particularly important because much of the Quatemary volcanic activity in the region has occurred southwest of the proposed repository site. Furthermore, faults that bound and penetrate the repository block have a map pattem similar to those faults that have hosted volcanism at Northem Cone 31 I

I

r I

PREDECISIONAL I

/ 'fs' 5 g y.

,i y,

L'[

~f_()k,

-f 1,

C 1

N I

~%

)

r 4081000

('

f sj t

"[

50

%l l-0 y

4 1 /(

Of

.g'i,

-50 4oooooo

,100

[

-150 l

4079000

-200 t

~f

-225

)

-250 4078000

-350 M

1;?

f 1

i 539000 540000 541000 542000 0

1 2km I

Figure 10. Ground magnetic map of the Northern Cone area, Crater Flat, Nevada. Northern Cone is located in the central part of the map, as indicated by high-amplitude, short-wavelength anomalies. North-south trending anomalies are interpreted to be produced by faults that displace tuff beneath the thin alluvial cover. Figure from Connor, et al.,1997.

n I

II l

and Lathrop Wells. Given these observations, probability models forigneous disruption of the proposed repository need to account for these trends because they tend to increase the probability of igneous activity at the site relative to spatially-homogenous models.

4.1.3.4 Summary Good agreement exists on the basic pattems of basaltic volcanism in the YMR. These pattems include changes in the locus of volcanism with time, recurring volcanic activity within vent clusters, formation of vent alignments, and structural controls on the locations of cinder cones.

Each of these pattems in vent distribution has an important impact on volcanic probability models and is considered in current NRC, DOE, and State of Nevada probability models.

4.1.4 Probability Criterion 4 4.1.4.1 Acceptance Criterion l

Estimates of the probability of future igneous activity in the YMR will be acceptable provided that:

I The parameters used in probabilistic volcanic hazard assessments, related to recurrence rate of igneous activity in the YMR, spatial variation in frequency of l

igneous events, and area affected by igneous events, are technicallyjustified and documented by DOE.

J 4.1.4,2 Review Method i

I Staff should ascertain whether parameters used in volcanic hazard assessments are reasonable, based on data from the YMR and comparable volcanic systems.

4.1.4.3 Technical Basis Models to estimate the probability of volcanic disruption of the proposed repository are likely to rely on a set of parameters. Use of values or ranges for these parameters must be justified using geologic data and analyses, in the following, current understanding of parameters related to:

Temporal recurrence rate of volcanism Spatial recurrence rate of volcanism Area affected by volcanic and igneous events

+

are discussed and evaluated.

I 4.1.4.3.1 Temporal Recurrence Rate Probability models use estimates of the expected regional recurrence rate of volcanism in the l

YMR in order to calculate the probability of future disruptive volcanic activity. Previous est; mates have relied on past recurrence rates of volcanism as a guide to future rates of volcanic activity.

This approach has yielded estimates of regional recurrence rate between 1 and 12 volcanic 33 I

lI k.

events per million years (v/m.y.) (e.g., Ho,1991; Ho, et al.,1991; Crowe, et al.,1992; Margulies, et al.,1992; Connor and Hill,1995), with the various definitions of what constitutes a volcanic event accounting for at least part of this range.

The simplest approach to estimate regional recurrence rate is to average the number of volcanic events that have occurred during some time period of arbitrary length. For instance, Ho, et al.

(1991) average the number of volcanoes that have formed during the Quatemary (1.6 m.y.) to calculate recurrence rate. Through this approach, they estimate an expected recurrence rate of 5 v/m.y. Crowe, et al., (1982) averaged the number of new volcanoes over a 1.8-m.y. period.

Crowe, et al., (1992) considered the two Little Cones to represent a single volcanic event, and, therefore, concluded that there are seven Quatemary volcanic events in the YMR. This lowers the estimated recurrence rate to approximately 4 v/m.y. The probability of a new volcano forming I

in the YMR during the next 10,000 yr is 4-5 percent, assuming a recurrence rate of between 4 E

and 5 v/m.y.

E An attemative approach is the repose time method (Ho, et al.,1991). In this method, a recurrence rate is defined using a maximum likelihood estimator (Hogg and Tanis,1988) that averages events over a specific period of volcanic activity:

(T

)

(I) where Nis the number of events, T is the age of the first event, T is the age of the most recent y

event, and A, is the estimated recurrence rate. Using eight Quatemary volcanoes as the number of events, N, and 0.1 Ma for the formation of Lathrop Wells (Appendix A), the estimated recurrence rate depends on the age of the first Quatemary volcanic eruption in Crater Flat. Using a mean age of 1.0 Ma (Appendix A) yields an expected recurrence rate of approximately 8 v/m.y.

The ages of Crater Flat volcanoes, however, are currently estimated at approximately 1.0

• 0.2 Ma (Appendix A). Within the limits of this uncertainty, the expected recurrence rate is between E

approximately 7 and 10 v/m.y. Of course, using different definitions of volcanic events leads to E,

different estimates of recurrence rate. For example, using the formation of vents and vent alignments during the Quatemary, N = 3 and the recurrence rate is 2-3 v/m.y. The repose-time method has distinct advantages over techniques that average over an arbitrary period of time because it restricts the analysis to a time period that is meaningfulin terms of volcanic activity.

In this sense, it is similar to methods applied previously to estimate time-dependent relationships in active volcanic fields (Kuntz, et al.,1986). Application of these methods has shown that steady-state recurrence rates characterize many basaltic, volcanic fields.

Ho (1991) applied a Weibull-Poisson technique (Crowe,1982) to estimate the recurrence rate of g

new volcano formation in the YMR as a function of time. Ho (1991) estimates A(t) as:

E r

r i p.g A(t) =

(2) where t is the total time interval under consideration (such as the Quatemary), and p and 0 are g

intensity parameters in the Weibull distribution that depend on the frequency of new volcano g

formation within the time period, t. In a time-truncated series, p and 8 are estimated from the distribution of past events. In this case, there are N = 8 new volcanoes formed in the YMR during the Quatemary. p and 0 are given by (Ho,1991):

34

i t

p.

i a

f,

E in (3)

s. n In and 8

yi, (4) where t, refers to the time of the P volcanic event. If p is approximately equal to unity, there is little or no change in the recurrence rate as a function of time, and a homogeneous nonstationary Poisson model would provide an estimate of regional recurrence rate quite similar to the nonhomogeneous Weibull-Poisson model. If p>1, then a temporal trend exists in the recurrence rate and the system is waxing; new volcanoes form more frequently with time. If p<1, new volcanoes form less frequently over time, and the magmatic system may be waning.

Where few data are available, such as in analysis of volcanism in the YMR, the value of $ can be strongly dependent on the period I and the timing of individual eruptions. This independence strongly reduces the confidence with which p can be determined. Ho (1991) analyzed volcanism from 6 Ma, 3.7 Ma, and 1.6 Ma to the present and concluded that volcanism is developing in the YMR on time scales of I = 6 Ma and 3.7 Ma, and has been relatively steady, p = 1.1, during the Quatemary.

Uncertainty in the ages of Quatemary volcanoes has a strong impact on recurrence rate estimates calculated using a Weibull-Poisson model. For example, if mean ages of Quatemary volcanoes are used (Appendix A) and t = 1.6 Ma then, as Ho (1991) calculated, p = 1.1, and the probability of a new volcano forming in the region within the next 10,000 yr is approximately 5

{

percent. This agrees well with recurrence rate calculations based on simple averaging of the number of new volcanoes that have formed since 1.6 Ma.

i I

Crowe, et al., (1995), however, concluded that the Weibull-Poisson model is strongly dependent on the value of I and suggested that t should be limited to the time since the initiation of a particular episode of volcanic activity. This has an important effect on Weibull-Poisson probability medels. If mean ages of Quatemary volcanoes are used and t = 1.2 Ma, the probability of a new volcano forming in the next 10,000 yr drops from 5 percent to 2 percent, and p<1, indicating waning activity. Altematively, if volcanism was initiated along the alignment i

cpproximately 1.2 Ma but continued through 0.8 Ma, the expected recurrence rate is again close to 5 v/m.y., and the probability of new volcanism in the YMR within the next 10,000 yr is about 5 percent (t = 1.2 Ma). The confidence intervals calculated on A(t) are quite large in all of these cxamples due to the few volcanic events (N = 8) on which the calculations are based (Connor cnd Hill,1993).

Cumulatively, these analyses indicate that a broad range of recurrence rates should be considered, this range varying with the definition of igneous event used. Many recurrence rate models depend on additionalinformation to estimate recurrence rates of volcanism. Bacon l

(1982) observed that cumulative-erupted volume in the Coso volcanic field since about 0.4 Ma is remarkably linear in time. Successive eruptions occur at time intervals that depend on the cumulative volume of the previous eruptions. This linear relationship was used by Bacon (1982) to forecast future eruptions and to speculate about processes, such as strain rate, that may i

35

govern magma supply and output in the Coso volcanic field. Kuntz, et al., (1986) successfully applied a volume-predictable model to several areas on the Snake River Plain, where recurrence rates of late Quaternary volcanism are much higher than in the Coso volcanic field, but the g

cumulative volumetric rate of basaltic magmatism is, nonetheless, linear in time. Condit and l

Connor (1996) discovered volume eruption rates were relatively constant in the Springerville volcanic field between 1.2 and 0.3 Ma, but the number of cone-forming eruptions varied in time, in conjunction with changes in petrogenesis. These relationships between eruption volume, petrogenesis, strain rate, and frequency of volcanic events observed in other volcanic fields suggest that recurrence rate estimates in the YMR can be further refined by considering fault location, magma generation, and strain rate.

A recent paper by Wemicke, et al., (1998) has suggested that the atrain rates in the Yucca Mountain area are at least an order of magnitude higher than would be predicted from the j

Quatemary volcanic and tectonic history of the area. Wemicke, et al., (1998) further suggest that i

because of what they consider anomalous strain in the Yucca Mountain area, the current

(

probabilities of future magmatic and tectonic events may be underestimated by an order of I

g magnitude. It is the NRC's understanding that DOE will be funding studies to determine if the I

strain rates observed by Wemicke, et al., (1998) can be verified.

Subsequent to the release of the paper by Wemicke, et al., (1998) NRC received a copy of a study by Earthfield Technology, Inc., (Earthfield,1995) from DOE that provides processing and interpretation of the available regional gravity and aeromagnetic data. Appendix 11 of Earthfield, (1995) contains a map that shows the locations of approximately 40 seromagnetic anomalies that are interpreted as buried intrusions in the Yucca Mountain area. These anomalies cannot be correlated with known volcanic centers. Three of these interpreted intrusions coincide with locations where the CNWRA (Magsino, et al.,1998) have performed ground magnetic surveys, a

and preliminary interpretations would suggest that two of the survey locations mapped buried g

basalts, while the other location appears to be located over a faulted tuff block. Magsino, et al.,

(1998) also report on 10 other survey locations (not correlated with the Earthfield locations) of which 3 appear to contain buried basalts. Interpretation of the results of the magnetic surveys of Magsino, et al., (1998) has not been completed.

While NRC has no basis for disagreeing with the strain pattems presented by Wemicke, et al.,

I (1998), it recognizes that other interpretations of these results can be made that do not require 3

any change in the volcanic hazard assessment for Yucca Mountain. The results of Earthfield, (1995) and Magsino, et al., (1998) could, however, be used to support the arguments of Wemicke, et al., (1998). In addition, even excluding the arguments by Wemicke, et al., (1998),

the presence of the potential buried basaltic bodies could require a change in the interpreted temporal recurrence rate of basaltic igneous activity. NRC is presently evaluating this information, and future versions of this IRSR will address the effects of this information.

4.1.4.3.2 Spatial Recurrence Rate Early models assessing the probability of future volcanism in the YMR and the likelihood of a repository-disrupting igneous event relied on the assumption that Plio-Quatemary basaltic volcanoes are distributed in a spatia!!y-uniform, random manner over some bounded area (e.g.,

Crowe, et al.,1982; Crowe, et al.,1992; Ho, et al.,1991; Margulies, et al.,1992). However, as discussed in Section 4.1.4, pattems in the distribution and age of basaltic volcanoes in the YMR make the choice of these bounded areas subjective. For example, Smith, et al., (1990) and Ho (1992) define north-northeast-trending zones within which average recurrence rates exceed that 36 I

of the surrounding region. These zones correspond to cinder cone alignment orientations that Smith, et al., (1990) and Ho (1992) hypothesize may result from structural control. These narrow zones lead to comparatively high estimates of spatial recurrence rate and probability of volcanic disruption of the proposed repository site. Utilizing bounded areas that are large compared to the current distributions of cinder cone clusters, however, results in relatively low estimates of spatial recurrence rate. Ho (1992) argued that, under these circumstances, using narrow bounding areas that include the proposed repository gives conservative estimates of probability of volcanic disruption.

Altematively, spatial recurrence rate can be estimated using models that explicitly account for volcano clustering (Connor and Hill,1995). This approach features several characteristics of nearest-neighbor methods that make them amenable to volcano distribution studies and hazard analysis in areal volcanic fields. First, volcanic eruptions, such as the formation of a new cinder cone, are discrete in time and space. Using nearest-neighbor methods, the probability surface is estimated directly from the location and timing of these past, discrete volcanic events. As a result, nearest-neighbor models are sensitive to pattems generally recognized in cinder cone distributions. Resulting probability surfaces also are continuous, rather than consisting of abrupt changes in probability that must be introduced in spatially-homogeneous models. Continuous probability surfaces can be readily compared to other geologic data, such as fault locations, that may influence volcano distribution. Nearest-neighbor methods also eliminate the need to define areas or zones of volcanic activity, as is required by all spatially homogeneous Poisson models.

]

Past volcanic activity can be used to estimate parameters used in these spatially i

nonhomogeneous Poisson probability models for disruption of the proposed repository. This is particular1y important in modeling the distribution of volcanism in the YMR because of vent clustering. As discussed previously (Section 4.1.2.3), vent clustering results in dramatic changes in spatial recurrence rate across the YMR. In order to model clustering and use these models in PVHA, it is necessary to estimate parameters used in the models. One approach to parameter sstimation is to use observed volcano distributions as a basis for comparison. This parameter 4

cstimation can be done formally, if appropriate models are used.

One estimation method for the spatial recurrence rate of volcanic events in the YMR and the probability of future volcanic events uses kemel functions (Silverman,1986; Lutz and Gutmann, 1995; Connor and Hill,1995; Condit and Connor,1996), in volcanic hazard analysis, the kemel function must be estimated and used to deduce a probability density function for spatial recurrence rate of volcanism. Several types of kemels, including Gaussian and Epanechnikov kemels, are discussed by Silverman (1986). All multivariate kemels have the property:

{K(x) dx = 1 (5) where K(x)is the kamel function, and x is an n-dimensional vector in real space R. A Gaussian kemel function for 2D spatial data is:

K(xy)=

exp -

(r - x,f + (y - y,[

(6) where the kemel is calculated for a point x, y and the center of the kemel, in this case the volcano location, is x,, y,. If the kemelis normalized using the smoothing parameter, h, then the kamel function is a Gaussian function, and h is equivalent to the standard deviation of the 37

distribution:

1 1

' r - x

I y"

K(xy) =

exp '

(7)

+

2nh2 2

h h

r if x and y locations are on a rectangular grid, the probability density function based on the distribution of N volcanoes is:

f(xy)=

N,fK(xy)

(8)

.s where Ax and by are grid spacing in the x and y directions, respectively. The above equations a

can be used to estimate spatial recurrence rate of volcanism, or the probability of volcanic g

disruption of the proposed repository site, given a volcanic eruption in the region. The results of this probability estimate depend on h. The approach to bounding uncertainty in the probability estimates resulting from this calculation is to evaluate probability using a wide range of h (Connor and Hill,1995). Attematively, the effectiveness of the kemel model and optimal values of h can be deduced from the distribution of nearest-neighbor distances between existing volcanoes. For example, the 2D-Gaussian kemel model can be compared with the distribution of nearest-neighbor distances between existing volcanoes by recasting the kemel function (Eq. 7) in polar coordinates:

2 1[ r

E 2

K(r,0) -

h(2n)12exp 2( h 2,,

(9) m where r, 6 is distance and direction from the nearest-neighbor volcanic event. The cumulative probability density function then becomes 2nR

/ 2

E FIR) = ((h(2n)12exp2 ; h 2,,

( 10 )

E drd0

,o where 8(R) is the expected fraction of volcanic events within a distance R of their nearest-neighbor volcanic event.

Distance to nearest-neighbor volcanic event in the YMR varies, depending on the definition used for a volcanic event. Treating all vents as individual volcanic events, the mean distance to nearest-neighbor volcanic event is 3.8 km with a standard deviation of 5.8 km. Some vents, such as southwest and northeast Little Cones, however, are quite closely spaced and may be treated as single volcanic events. Treating vents spaced more closely than 1 km as single volcanic events, the mean distance to nearest-neighbor volcanic event increases to 5.0 km and the standard deviation to 5.9 km. Attematively, volcanic events can be defined in terms of vents and vent alignments. In this definition, Quatemary Crater Flat volcanoes are taken as a single event, as is Pliocene Crater Flat. Using this definition, mean distance to nearest-neighbor volcanic event increases to 7.0 km with a standard deviation of 6.4 km.

The observed fraction of volcanoes erupted at a given nearest-neighbor distance or less is compared with a Gaussian kemel model with standard deviations of 3-7 km in Figure 11. A 3

i Gaussian kemvi model with h = 5 km reasonably describes the expected distance to nearest-E 38

c.

neighbor volcano, particularly between 5 and 10 km. Smaller values, such as h = 3 km, model

' the distribution of individual vents at distances less than 4 km, but do not compare well with vent distributions at distances greater than 4 km. For instance, the h = 3 km model predicts that 95 percent of all volcanoes will be located at nearest-neighbor distances less than 6 km, but actually 15 to 40 percent of all volcanoes in the YMR are located at greater distances than this, depending on the definition of volcanic events used. The h = 7 km model tends to slightly overestimate the number of volcanoes at nearest-neighbor distances greater than 8 km. Thus, the h = 5 km model best describes the overall distribution of YMR vents and vent pairs for use in

)

j evaluation of hazards at the repository, located approximately 8 km from the nearest Quatemary i

volcano. This is slightly less than the standard deviation of the observed distribution, because Buckboard Mesa, located 25 km from its nearest-neighbor, is an outlier in the observed volcano distribution and increases the variance.

i Vents and vent alignments have.%wer nearest-neighbors than expected at distances less 4 km if j

this distribution is modeled using a '3aut,ian kamel (Figure 11). Rather, this distribution can be j

modeled using a simple modification of the Gaussian kemel to account for a mean offset of the probability density function from zero:

2 flR)=

jxp dede

( 33 )

o o h(2n)2 4-where F is the mean offset. Incorporating a mean offset of 5-7 km and h = 3 km results in an improved fit between the observed distribution of distance to nearest-neighbor volcanic events and the Gaussian kamel model (Figure 12). The need for this mean offset arises because vent alignments are more widely spaced than individual vents. Variance does not increase significantly as a result of this increased spacing, however, when vent alignments are considered as single volcanic events. This comparatively low variance suggests there is a characteristic nearest-neighbor distance of 5-10 km in the YMR for volcanic events defined as vents or vent i

alignments.

This analysis indicates volcanic event distribution can be modeled using a Gaussian kemel with h a 5 km provided volcanic events are defined as individual vents or vent pairs. When vent alignments are considered as individual volcanic events, the value of h must increase to h 2 7 km or the Gaussian kemel needs to be modified to include an offset distance. Thus, model j

testing indicates that the types of kemels and parameters used within each kemel to evaluate probability should vary with the definition of volcanic event.

The Epanechnikov kemel function is widely used to estimate spatial recurrence rate in basaltic volcanic fields (Lutz and Gutmann,1995; Connor and Hill,1995; Condit and Connor,1996) and may be tested in a similar manner as the Gaussian kemel function. The Epanechnikov kemel in 2D-Cartesian coordinates is:

.I-N 2

r - I, K'(ry) =

3_

( 12 )

nh h

h r

r 39

]

f where

&-x] + {v-yf s h ll otherwise K,(xy) = 0 in polar coordinates this kemel function becomes e

3 p-K,(r,0) =

1-L

,rsh

( 13 )

Anh

, h 2,,

where ris distance from the volcano and 0 is direction. The cumulative probability density I

i i

i i

i i

l m;

p.

C f~

3 km 0.8 c/

BB -> 25 km 5 km g

0.6

/

El g

7 km gl o

Distance from center o

of proposed repository c

0.4 to nearest Quaternary volcano 0.2 u

l I

~

O t i

i i

i i

i i

i i

l 0

5000 Distance to Nearest-Neighbor Figure 11. Comparison of observed fraction of volcanoes within a given distance of their nearest-neighbor volcano with Gaussian kernel models calculated using h = 3 km,5 km, and 7 km. Observed curves include all vents (open squares), all vents or vent pairs more l

closely spaced than 1 km (solid circles), and vents and vent alignments (open circles).

Buckboard Mesa (BB)is an outlier in the distribution as it is approximately 25 km from its nearest neighbor. The center of the repository site is located 8.2 km from Northern Cone, j

the nearest Quaternary volcano.

40

function is then:

2t R

/

2

$(R) = ((

l-drdo, R s h

( 14 )

h,.

oo r

As was accomplished with the Gaussian kemel, the cumulative probability density function for the Epanechnikov kemel can be compared with the observed fraction of volcanoes erupted at a given nearest-neighbor distance or less for various values of h (Figure 13). This comparison indicates an Epanechnikov kemel function with h = 10 km best models the distribution of distance to nearest-neighbor volcanic events, if volcanic events are defined as vents or vent pairs. If volcanic events are defined as vents or vent alignments,15 km < h<18 km better 1

i i

i j

i i

i 0.8 Y

Sk y

0.6

/

{

~

g 7 km

}

Distance from center of proposed repository

.o 0.4 to nearest Quaternary l

c volcano oj e

u-0.2 l

l 0e O

5000 10000 15000 Distance to Nearest-Neighbor Volcanic Event (m)

Figure 12. Comparison of observed fraction of volcanic events within a given distance of their nearest-neighbor volcanic event with Gaussian kernel models calculated using h = 5 km and h = 7 km. Observed curves include vents and vent alignments (open circles) as single volcanic events, calculated from the center of the vent alignment. Buckboard Mesa is an outlier in the distribution as it is approximately 25 km from its nearest neighbor.

41

approximates the distribution of distances to nearest-neighbor volcanic events, given the distribution of YMR volcanoes. Comparison of the Epanechnikov and Gaussian kernel models suggests the Gaussian kernel models better fit the observed volcano distribution than Epanechnikov distributions, particularly at nearest-neighbor distances greater than 6 km. The difficulty fitting the observed distributions with the Epanechnikov kemel function results from truncation of this distribution at distances greater than h.

Testing models against observed distributions also leads to a natural definition of conservatism.

For example, the distance between the proposed repository and its nearest-neighbor Quatemary volcano is 8.2 km. A Gaussian kemel function with h 2 7 km clearly is conservative because a greater fraction of volcanic events occur at nearest-neighbor distances less than 8.2 km than 1

i i

i j

ijg, y

i i

i i

i 5 km

[

0.8 I

E I

c 88+25 km s

[

10 km

[

.2 0.6

/

E Distance from center E

j f

of proposed repository _

E

/

to nearest Quaternary g

0.4 I

[

l 1

/

E i

[

0.2 I

l I

l 0m g

0 5000 10000 15000 Distance to Nearest-Neighbor li Volcanic Event (m)

Figure 13. Comparison of observed fraction of volcanoes within a given distance of their nearest-neighbor volcano with Epanechnikov kernel models calculated using h = 5 km,10 km, and 18 km. Observed curves include all vents (open squares), all vents or vent pairs more closely spaced than 1 km (solid circles), and vents and vent alignments (open circles). Buckboard Mesa (BB) is an outlier in the distribution as it is approximately 25 km from its nearest neighbor. The center of the repository site is located 8.2 km from Northern Cone, the nearest Quaternary volcano.

I 42 I

e b

predicted by the model, whereas a Gaussian kemel function with h = 3 km is not conservative (Figure 11). Similarly, probability models based on Epanechnikov kemel functions and h 210 km are conservative where volcanic events are defined as vents and vent pairs, and h 218 km where volcanic events are defined as vents and vent alignments.

4.1.4.3.3 Area Affected by igneous Events The area affected by igneous events varies with the definition of igneous event (Section 4.1.2).

Where igneous events are defined in terms of individual, mappable eruptive units, the resulting probability estimate is for direct disruption of the proposed repository and release of waste into the accessible environment. The probability of a volcanic event disrupting the repository depends on the repository area potentially disrupted by flow of magma through the subsurface l

conduit of the volcano as the eruption develops. Observations at cinder cones in the process of i

l formation (e.g., Luhr and Simkin,1993; Fedotov,1983; Doubik, et al.,1995) are that these eruptions initiate by dike injection at comparatively-low ascent velocities, on the order of 1 m s",

I which can deform an area of the ground surface several hundred meters in length. Basaltic eruptions, however, quickly localize into vent areas as the eruption progresses and magma flow velocities increase to around 100 m s. Hill (1996) reviewed literature on subsurface areas d

disrupted by basaltic volcanoes analogous to past volcanic eruptions in the YMR. Based on this review and data collected at Tolbachik volcano, Russia, Hill (1996) concluded that typical subsurface conduit diameters are between 1 m and 50 m at likely repository depths of about 300 m. Vent conduits exposed in the San Rafael volcanic field (Delaney and Gartner,1995),

however, often have diameters on the order of 100 m. Therefore, areas disrupted by vent i

formation, potentially leading to the release of waste into the accessible environment, are on the i

order of 0.01 km or less. Conservatively, such a volcanic event, centered within 50 m of the 2

repository boundary, may result in transport of waste to the surface.

I Using this approach, the probability of a volcanic eruption through the repository, given an eruption, can be approximated as:

{

P[ eruption through repositoryieruption centered at ry]

( 15 )

l i

where the effective area, A,, is the area of the repository and the region about the repository within one conduit radius of the repository boundary (Geomatrix,1996).

Other definitions of igneous events result in the neeo for more complex analyses of area affected because these events have length and orientation (Sheridan,1992; Geomatrix,1996). In these cases, probability density functions must be estimated for both the length and orientation of igneous events. Geometrix (1996) gave the probability of an intrusive, igneous event centered on a given location intersecting the repository, which can be expressed as:

.e l

1p21,, $3s@s@2) " l I /t(l) f,($) d$dl

( 16 )

c e, i

where dis the azimuth of the igneous event with respect to north, with $, and $2 representing the range of azimuths that would result in intersection with the repository, given an igneous event centered on x,y, a distance I, from the repository boundary. The probability that the igneous 43 L

Ii event of half-length, L, will exceed /, at an azimuth between $, and $2 depends on the probability EI density functions f (I) and f,($) for igneous event half-length and azimuth, respectively.

t This characterization of area affected by igneous events must be modified further depending on the type of event considered. Defining igneous events as volcanic vents or vent alignments may result in a probability estimate for volcanic disruption of the repository, if the frequency of vent E

formation along the alignment is included in the calculation. The length of the vent alignment is taken as the distance between the centers of the first and last volcanoes in the alignment. For E

example, the length of the Amargosa Aeromagnetic Anomaly A alignment of three vents is 4.0 km (Figure 9). The length of the Quatemary Crater Flat alignment of five vents is 11.2 km, based on the distance between southwest Little Cone and Northem Cone (Figure 7). Six vents occur along the 3.6-km Pliocene Crater Flat alignment. Average vent density along these alignments is on the order of 0.5-2.0 vents per km. This vent density suggests that, if an alignment defined by the distance between the first and last vents in the alignment intersects the repository, a vent will likely form within the repository boundary as a result of this intersection.

Uncertainty increases considerably when the functions f (I) and I,($) are introduced because t

these functions must be estimated from limited YMR geologic data. If the igneous event is defined as the development of a vent or vent alignment, mapped vent locations are useful in constraining the functions f,($) and f (I). Considering Plio-Quatemary volcanism in the YMR, six t

igneous events consist of the formation of isolated vents, and four igneous events resulted in the formation of vent alignments (Figure 14). Of these four vent alignments, two are less than 4 km long, the Pliocene Crater Flat vents and the Sleeping Butte vent pair. The Amargosa l

Aeromagnetic Anomaly A alignment is slightly longer than 4 km. The Quatemary Crater Flat 5

alignment, one of the youngest and most important volcanic events in the YMR, is also the longest alignment, approximately 11 km long. Although these data provide an idea of the range 3

of alignment lengths possible in the YMR, they are not sufficient to estimate a probability E

distribution for vent alignment lengths, f (I).

t In order to compensate for the lack of data within the YMR, analog information can be used.

Draper, et al., (1994) note that approximately 30 percent of the vents in the San Francisco volcanic field form alignments. The remaining vents are isolated and appear to have formed during independent episodes of volcanic activity. This value appears comparable to the ratio of vent alignments to individual vents in the YMR. Data on vent alignment lengths from other volcanic fields suggests vent alignments may be considerably longer than the Quatemary Crater Flat alignment. For example, Connor, et al., (1992) identified vent alignments >20-km long in the E

Springerville volcanic field, Arizona. Vent alignments of comparable or greaterlength have been E

identified in the Michoackn-Guanajuato volcanic field, Mexico (Wadge and Cross,1988; Connor, 1990), and the Panicked volcanic field, Mexico (Lutz and Gutmann,1995). Smith, et al., (1990) g suggested alignments may be up to 20 km long, with a lower probavility of 40-km-long g

alignments, based on mapping in the Lunar Crater, Reveille Range, and San Francisco volcanic fields. None of these authors, however, developed distributions for vent alignment lengths in these areas. Furthermore, it is not clear that the conditions for vent alignment formation and factors controlling vent alignment length are directly comparable between these different regions and the YMR. As a result, estimation of the distribution function for f (I) for YMR vents and vent t

alignment formation is extremely uncertain.

1 However, given these caveats, the probability density function for event length can be expressed as 44

P

,/=0 ft(l) = <

( 17 )

U[lg, l ] j,

2 By this definition,50 percent of igneous events have zero length and only disrupt the repository if they fall within the effective area of the repository. The remaining 50 percent of igneous events form alignments that affect areas up to a distance /. from the point x,y. This percentage assigned to zero-length igneous events is a source of uncertainty in probability estimates and is

' not well constrained by available data. The probability density function is construed to be a uniform random distribution between /,,n and I because the distribution of alignment lengths is so pooriy known.

Using this definition of f (/), probability estimates of intersectiori of the repository, given an event t

at x,y, will not be strongly dependent on /,,n compared to l.. The value of I. can be chosen as 7

6 2

Sleeping Butte and 5

Pliocene Crater Flat

$ 4 Amargosa Anomaly "A" o

2 2

E3 e

1 2 Quaternary Crater :

Flat i

1 k

f 0

2'

'f'

0 1

2 3

4 5

6 Alignment Half-Length (km)

' Figure 14. Distribution of Plio-Quaternary vents by vent alignment half-length. Most vents in the Yucca Mountain region occur as isolated vents. The youngest and longest vent alignment in the Yucca Mountain region, the Quaternary Crater Flat alignment, is also closest to the repository site.

i 45 l

l

1 5.6 km, taking the Quatemary Crater Flat alignment as the maximum alignment half-length.

Given observations in other volcanic fields, nowever, I, may be 10 km or more.

m The distribution function for azimuth of alignments or dike zones, //$), is better constrained by the data on vent alignments, regional stress distribution, and the orientations of high-dilation tendency faults. Three of the alignments in the YMR trend 020 to 030", perpendicular to the least principle horizontal compressional stress in the region,028: (e.g., Morris, et al.,1996).

Under these circumstances, f/$) may vary over a narrow range. For example, f,($) = U [020', 035 l

( 18 )

l Alternatively, fg$) near the repository may respond to the distribution of fault orientations (Figure

15) if ascending magmas tend to exploit faults as low-energy pathways to the surface (Conway, et al.,1997; Jolly and Sanderson,1997).

Other definitions of igneous events attempt to capture the probability of igneous intrusions intersecting the repository boundary (Sheridan,1992; Geomatrix,1996). Igneous intrusions commonly form anastomosing networks at shallow levels in the crust, forming multiple dike 0.08 gi,,,,;,,,,,i,i,,,,,,,i,i,j,,ii.,,,,,,,j,j,i,i,,,i,,,,;,i,,,i,j,j,i, g) 0.07 0.06

~

g 5

0.05

~

l 3

j g

0.04 i

gj z

4 E

0.03

~

Z E

[

[

l 0.02 0.01 h

d 5

'l'

' l' I' l' l' I' l' l' l' I' l' I' l' I' I' l 0

-90

-65

-40

-15 10 35 60 85 Angle from North (degrees)

Figure 15. Distribution of the orientation of fault segments with respect to north. This distribution is weighted by fault segment length. Near the repository, f/ ) may vary as a function of this distribution of fault orientations if ascending magmas follow fault planes to the surface.

46 Il

it segments at a given structural level (e.g., Gartner and Delaney,1988). Consequently, a term may be added to Eq. (16) to account for the width of igneous events, such as the width of the dike swarm formed during igneous intrusion:

. 6,

$21,, 4 s@s42 W2"r) " [ [ [ft(l) ' fe(&) ' /dw) dwd4dl

( 19 )

3 where fdw) is a probability density function describing the half-width of the igneous event, which may be a significant fraction of the half-length, and w,is the shortest distance to the repository boundary perpendicular to the event azimuth, for a given azimuth and event length. Numerous individual dikes, dike segments, and sills may be located within this zone. Little is known about the distribution fdw). In Pliocene Crater Flat, the half-width of the dike swarm appears to be on the order of 200 m. In contrast, Gartner and Delaney (1988) mapped dike zones up to 5 km wide (W= 2.5 km) in the San Rafael volcanic field (Figure 8).

Given the spatial density of these igneous features, it is conservative to consider intersection of the area defined by Eq. (19) with the effective repository area as resulting in igneous disruption of the site. This definition of an igneous event, however, does not necessarily result in direct transport of radioactive waste to the surface by erupting magma.

4.1.4.4 Summary All probability models for volcanic disruption of the proposed repository rely on estimation of.

parameters to bound the temporal and spatial recurrence rates and magnitudes of igneous events. Ranges of these parameters adopted in the volcanic hazard analysis must be justified

{

using geologic data and models. Estimation of the temporal recurrence rate relies on the i

frequency of past volcanic events in the YMR. These past recurrence rates indicate volcanism has persisted throughout the Pliocene and Quatemary at a low recurrence rate compared to many other Basin and Range volcanic fields. Therefore, such low temporal recurrence rates should be used to model probabilities. No evidence exists to indicate that basaltic volcanism has ceased in the YMR. Because the time elapsed since past volcanic eruptions within the YMR is short compared to common repose periods, the YMR should be considered a geologically-active basaltic volcanic field, with recurrence rates greater than zero. Conversely, recurrence rates in the YMR are not as large as those in many other WGB volcanic fields, such as the Cima volcanic field where at least 30 volcanic eruptions have occurred since 1.2 Ma. Current evidence suggests that such an intense episode of volcanism is not likely in the YMR during the next 10,000 yr.

The temporal recurrence rate must be specified based on the definitions of igneous events. The current staff estimates for these recurrence rates are 2-12 v/m y. for igneous events defined as individual mappable units or vents and 1-5 v/m.y. for vents and vent alignments. The staff will evaluate the new information (i.e., Wemicke, et al.,1998, Magsino, et al.,1998, Earthfield,1995) to determine the effects that this information may have on temporal recurrence rates. Temporal recurrence rate for igneous intrusions without volcanic eruptions is not estimated because data is not available to support such estimates. Based on analog data (Delaney and Gartner,1997) a factor of two or greater is probably reasonable.

Spatial recurrence rate varies across the YMR because of vent clustering and the tendency for 47

l volcanism to recur within these clusters. For example, all Quaternary volcanism in the YMR occurs in proximity to Pliocene volcanoes. Estimations of spatial recurrence rate then must rely on patterns in past volcanic activity, which is done using kernel models. Spatial recurrence rates of igneous events at the repository or elsewhere on Yucca Mountain that are assumed to be at or near zero are not supported by existing data. Yet, spatial recurrence rates of zero or a slightly larger than zero regional background value are assumed at the repository in some models presented in Geomatrix,1996. Staff conclude that the distribution of sparse events does not provide an accurate basis to conclude that spatial recurrence rate within the repository boundary 3

is zero or a low background value. Spatial analyses (e.g., Connor and Hill,1995) indicate that E

the repository site is close to the edge of the Crater Flat cluster, within which most YMR Quatemary basaltic volcanism has occurred. A reasonably-conservative model would, therefore, indicate that the spatial recurrence rate at the repository is greater than median spatial recurrence rates across the YMR.

Similarly, areas affected by igneous events must be described using parameter estimation, which will vary with the definition of igneous events. If igneous events are defined as individual mappable units and vents, then only those that erupt within the effective area of the repository significantly affect performance. Vent alignment lengths and orientations must be considered if igneous events are defined as vents and vent alignments. Vent alignment length is poorly constrained by available data, but its effect on probability is readily assessed using sensitivity studies. Alignment orientation is well constrained by the correlation between existint, vent alignments and crustal stresses. Areas affected by igneous intrusions must be larger than areas affected by individual alignments, but the parameter distributions are poorly constrained.

4.1.5 Probability Criterion 5 j

1 4.1.5.1 Acceptance Criterion Estimates of the probability of future igneous activity in the YMR will be acceptable provided that:

The models are consistent with tectonic models proposed by NRC and DOE for the YMR.

4.1.5.2 Review Method NRC staff should determine whether features of proposed probability models, such as boundaries of volcanic source-zones, patterns of vent distribution, and recurrence rate of igneous activity are consistent with tectonic models. It will be acceptable to use more than one tectonic model (consistent with the available data) to obtain an upper bound on probability. At a minimum, NRC staff should determine whether volcanic probability models are consistent with the range of tectonic models discussed in the Structural Deformation and Seismicity (SDS) KTl E

and used in resolution of other KTis to assess phenomena such as seismic source 5

characterization and patterns of groundwater flow.

4.1.5.3 Technical Basis Probability models need to be consistent with tectonic models proposed for the YMR. Tectonic processes affect igneous processes across a large range of scales. Low recurrence-rate basaltic volcanic activity in the Basin and Range may occur where magmas are generated by decompression of fertile mantle during crustal extension (e.g., Bacon,1982; McKenzie and Bickle,1988). Magma ascent through the crust is enhanced by crustal structures produced by extension, leading to correlation between basaltic volcanism and structure across a range of 48

scales, from the superposition of individual faults and vents to the occurrence of entire volcanic fields at the margins of extensional basins (Connor,1990; Parsons and Thompson,1991; Conway, et al.,1997). Volcar'ic hazard analysis of the proposed repository must quantify these often complex geological relationships.

The relationship between structure and volcanism has been used to suggest both higher and lower probabilities of volcanic disruption of the repository than are predicted using spatio-temporal pattoms in vent distribution alone (Connor and Hill,1995). Smith, et al., (1990) suggested a narrow northeast-trending, structurally-controlled source-zone of potential volcanism extends through the repository site, resulting in comparatively high probabilities of volcanic disruption. Altematively, structure models that exclude the repository from volcanic source-zones result in comparably low probabilities. For example, Crowe and Perry (1989) proposed the north-northwest-trending CFVZ, with an eastem boundary located west of the repository site, effectively isolating the proposed repository. Thus, wide variation in probability estimates is a j

direct result of the varying ways in which these source zones have been drawn. Part of this i

dichotomy may be resolved if the relationships between volcanism and structure are considered mechanistically and in light of mapped YMR structural features. In the following, current understanding of these relationships is discussed in terms of:

Regional tectonic models of Yucca Mountain and surrounding geologic features

=

j Mechanistic relationships between crustal extension and magma generation Local structural controls on magma ascent a

4.1.5.3.1 Regional Tectonic Modela Yucca Mountain lies within the Basin and Range Province of the westem North American

I Cordillera; a province characterized by spatially-segregated regions of east-west extension between zones of northwest-trending, dextral strike-slip or oblique strike-slip faults. Coupled with the overall pattom of crustal extension and transtensicq are numerous small-volume volcanic fields (Figure 16). Within this tectonic framework, five v!able tectonic models that describe the pattom of regional and local deformation around Yucca Mountain emerge from all those that have been proposed in the geologic literature over the past two decades (Stamatakos, et al.,

l 1997b). These five models are:

.. Half-graben with deep detachment fault f

Half-graben with moderate depth detachment fault Elastic-viscous crust with planar faults with intemal block deformation and ductile flow f

of middle crust Pull-apart basin (rhombochasm or sphenochasm)

Amargosa shear or Amargosa Desert fault system in a broad sense, these five models can be considered in two general categories of deformation.

The first three are dominantly related to extensional deformation, and the latter are dominantly related to strike-slip deformation. Moreover, the five models are not mutually exclusive. Locally 49

M N

N 9'.

7 6

6 3

3 k ]#

/

M

!g-Pl M

(

g 9

io N

ff g

un y'

J n

k i W

Sta i

e n d

4 u3 u

3 f

lt iio$

t 6

II Ln l

1 c

1 8

m n

5 i

M' t

g s

in u

c

~

ig o

l J

^

4 oe g

r

)

4 o

j m

a f

k l

m s

a gn

- Q, b -

F i

k?

}y w

o h

i I

yg s

jb i

bg

@E

/,-

in 6 e jj

g

{

t a

n

/

~

u f

"s o

p*

M a

j y k'

b

.. i 1

r 6

.d u9 g

Y9 y$

g d1 hid n

M

\\f ul a o

4 rt y 'V ae 4

al l ei M

r r

r a e 1

9 eF

., !a iat ms h

n a

t et f

m t

r u

a n u a u t

u a r

n BoF nh

.i i

f o aT or a

M P

ra pF a

.s its ml t

s t

s o

ua c u ic p

ta t

gf a

n e

F i a D

a l

S u

od r

s t

c a

n lo ic n i o n r

l

,i V i a T

a e a t

e d

ir gu e

S t

X n

n u

gs m

go c

a de m

l s c M

iu io e o lu d

ic e

i n P

v r

n P f

h lA a

lo a

a i

n a

lpl c d c

n m

v s,

mo y

i i

c i

e io iv A

i n

D a

- e o s ;

Sy z

NSM t

.r c

e a

A ut 6 a o

la ae V

P Fo 1 n W

E

[E@E(s r

k N

f e e 5

J r

7 t

M u a 6

gu 1

1 iFQ M

M

extensional-dominated deformation, within Crater Flat for example, can exist within a larger region of transtensional deformation related to a pull-apart basin.

In the deep detachment fault model (e.g., Ferrill, et al.,1995b), the Crater Flat-Yucca Mountain faults are envisioned as soling into the Bare Mountain fault at the base of the seismogenic crust, at approximately 15 km depth (Figure 17a). The faults at Yucca Mountain accommodate strain within the hanging wall of the Bare Mountain fault. This modelis dominantly extensional and compatible with a regional strike-slip system in which the Crater Flat-Yucca Mountain domain has A

Bare Crater

\\

Mounta n Flat i

Yucca Fak WNW ESE Mountain Mio9ene volcanics

_g a.

'a

//

-5 Precambrian aleozoic Clastics Carbonates and Clastics? -10 km i

B Bare Mountain WNW Fault Crater Yucca ESE N

Flat Mountain a

5 Precambrian Clastics Paleozoic 7

Carbonates and Clastics 10km

&&&M557~WW'~

~:":'

~

Figure 17. Two balanced cross sections across Bare Mountain, Crater Flat, and Yucca Mountain (from Ferrill, et al.,1996b). The cross sections differ in the depth of the detachment fault. High-angle normal faults at Yucca Mountain intersect this detachment at depths between 5 km (b) and 10 km (a). These high dilation-tendency faults may serve as pathways for ascending magmas.

51

I largely dip-slip faulting, similar to a pull-apart basin. In addition, the model respects the geologic constraints on the timing of deformation (i.e., variable dips of fault blocks with growth of tuff strata across faults that were active during tuff deposition), as well as rollover in fault blocks.

Restored cross-sections, however, are more difficult to balance than with a moderate-depth detachment fault.

The moderate-depth detachment fault model(Young, et al.,1992; Ferrill, et al.,1995; Ofoegbu and Ferrill 1995)is similar to the deep de'achment model, but the Crater Flat-Yucca Mountain faults sole into a detachment fault at 5-10 km depth (Figure 17b). The detachment then terminates against the deeper, larger Bare Mountain fault. The geometry of this modelis the most reasonable for obtaining a balanced, restored cross-section of the upper crustal section.

Both shallow and moderately deep detachment models may influence basaltic magmatic activity B

in two ways. First, faults that sole into the detachment may serve as conduits for magma ascent g

in the shallow crust, if these faults provide relatively low-energy pathways to the surface (McDuffie, et al.,1994; Jolly and Sanderson,1997). Second, dominantly-extensional models result in large-scale density contrasts in the shallow crust. Relatively-dense, Precambrian and Paleozoic rocks dominate the upper crustal section west of the Bare Mountain fault. East of the Bare Mountain fault, extension results in the formation of a half-graben and the upper crustal section is dominated by less-dense tuffs and alluvium. This broad, density contrast may g

influence rates of partial melting, a topic discussed in Section 4.1.5.3.2.

E Altematively, Crater Flat-Yucca Mountain faults have been interpreted as planar to the ductile g

middle crust (Fridrich,1997). This is an extension-dominant model; fault dips do not become g

more shallow with depth. This model, which serves as the conceptual basis for the United States Geological Survey boundary element model (Stamatakos, et al.,1997b), assumes the surface geometry of faults and fault blocks cannot be used to constrain deformation at depth.

Intemal fault-block deformation and ductile flow (and perhaps magma intrusion) at depth are assumed to compensate for variable fault-block dips, which would otherwise produce large triangular-shaped gaps in the subsurface.

The pull-apart basin model envisions Crater Flat as a pull-apart basin that formed in a releasing bend of a north-northwest-trending, regional strike-slip system (Fridrich,1997). The pull-apart basin is a half-graben with a well-defined westem edge in the Bare Mountain fault, the diffuse set of Crater Flat-Yucca Mountain faults to the east, and an eastem edge in westem Jackass Flats.

The regional strike-slip system remains hypothetical, presumably buried beneath Amargosa Desert alluvium southeast of the southem end of the Bare Mountain fault. The pull-apart model explains the vertical axis rotation of the southem reaches of Crater Flat-Yucca Mountain (e.g.,

Hudson, et al.,1994) as crustal-scale block rotations within overall regional dextral shear. This shear is related to diffuse boundary interactions between the North American and Pacific plates.

E The model explains the north-northeast arcuate trend of Quatemary volcanic centers of Crater E

Flat as an alignment along a Reidel shear within the basin.

Fridrich (1997) has proposed two versions of this model. In the rhombochasm version of the pull-apart model, the basin-bounding, strike-slip fault trends north-northwest out of Crater Flat and is concealed beneath the Timber Mountain-Oasis Valley calderas. In the sphenochasm version, the northem extent of the bounding strike-slip fault is pinned at the northem end of Crater Flat. Strike-slip deformation increases south and east from the pin point. In response, the basin fans open to the south, and extension on basin bounding normal faults like the Bare Mountain fault increases southward (Scott,1990; Stamatakos, et al.,1997a).

52 i

The Amargosa shear modelis similar to the rhombochasm model, with Crater Flat representing a diffuse dextral shear-zone along a major north-northwest-trending crustal shear (e.g.,

. Schweickert and Lahren,1997). The shear zone extends northward along a hypothetical strike-slip fault extending north-northwest from Crater Flat beneath the Timber Mountain and Oasis Valley calderas. The lack of offset of these calderas is explained as diffuse detachment of the tuffs from underlying crust, in which offset is absorbed by horizontal faults within the tuff layers (Hardyman and Oldow,1991). The southem extension of the shear links with the Stewart Valley-State Line fault. Totallength of the fault and shear zones is greater than 250 km.

The Crater Flat shear zone includes the motion on faults within westem Bare Mountain, the vertical axis rotation within southem Yucca Mountain, and the sites of volcanic activity in Crater Flat. The Quatemary cone alignment is believed to represent a Reidel shear oblique to the main shear axis. Based on a palinspastic reconstruction between southem Bare Mountain and the Striped Hills, this model calls for >30 km of right-lateral offset along the southem extension of this shear since 11.5 Ma (Schweickert and Lahren,1997). This aspect of the model is suspect tiecause of disparate exhumation ages for Bare Mountain and the Striped Hills, based on fission-track ages (Ferrill, et al.,1997) and paleomagnetic results (Stamatakos, et al.,1997c).

Strike-slip-dominated models have been used to infer an entirely different basis for distribution of volcanoes in the YMR other than purely-extensional models. For example, Schweickert and Lahren (1997) envision a relatively-uniform melt generation region beneath the YMR. In these circumstances, crustal structures such as Reidel shears in puli-apart basins allow magmas to ascend to the surface. Fridrich (1997) also proposed that tensional structures control the ascent of magma through the crust and that volcanism will be limitea to areas where these tensional structures exist. Some source-zone probability models (e.g., Crowe and Perry,1989) propose that Yucca Mountain lies outside of pull-apart basins, and, therefore, the probability of volcanism at Yucca Mountain is extremely low, compared with Crater Flat. As noted above, however, the strike-slip fault on the eastem edge of the pull-apart has not been mapped or identified. This lack of direct geologic evidence for a bounding fault on the east side of Crater Flat basin greatly reduces the confidence with which such source zones for basaltic volcanism can be drawn.

Elements of the above tectonic models are not mutually exclusive. For example, predominately-strike-slip deformation may have given way to predominantly-extensional deformation as regional shear resulted in rotation of the direction of maximum horizontal compressional stress relative to fault planes. In light of these models, it is appropriate to consider mechanistic relationships between crustal extension in the YMR and basaltic magma generation. These relationships rely on a physical link between regional extension of the brittle crust and magma production deeper in the lithosphere.

4.1.5.3.2 Mechanistic Relationships Between Crustal Extension and Magma Generation Crustal extension controls or strongly influences basaltic magmatism in the WGB (e.g., Leeman and Fitton,1989; Lachenbruch and Morgan,1990; Pedersen and Ro,1992). Magmas that originate in WGB lithospheric mantle, including those of the YMR, were likely produced through decompression melting associated with extension (Farmer, et al.,1989; Hawkesworth, et al.,

1995). Decompression melting is favored in zones of mantle lithosphere that have been previously enriched in incompatible elements, which enables melt formation at lower temperatures (e.g., McKenzie and Bickle,1988). Based on mineralogical phase relationships cnd geochemical studies, decompression-induced lithospheric melting likely occurs at depths between 40-80 km (Takahashi and Kushiro,1983; Rogers, et al.,1995). Extension and 53 I

i Il associated crustal deformation will produce local changes in lithostatic pressure at the base of the crust. Variations in lithostatic pressure produced through this extension may decompress enriched zones in lithospheric mantle sufficiently to partially melt and produce basaltic magma.

Thus, lateral changes in lithostatic pressure across the YMR may control areas of future igneous activity.

i l

Crustal extension has resulted in large density differences in the upper 5-6 km of the crust in the I1 YMR due to the displacement of Paleozoic and Precambrian rocks across the Bare Mountain fault, the formation of the Crater Flat basin, and subsequent deposition of tuff and alluvium in Crater Flat (Figure 18). The average density of a 5.6-km column of rock beneath Crater Flat and gl Bare Mountain can be calculated from this cross-section using average rock densities for the 5

region (McKague,1980; Howard,1985). This difference in average density is 280 kg m-Beneath this 5.6-km column, little density difference is expected because any faulting that occurs g

below 5.6 km does not juxtapose rocks of significantly different densities.

g 1

Given lithostatic pressure as I

BM CF

['.,:.i::.) Alluvium 1800 kg/m3

@ Tuff 2100 kg/m ggo 3

.v.

n

. a m

M Paleozoic 2650 kg/m

~l 3

Precambrian 2750 kg/m3

^:^: a" B

El

.ve "

WNW ESE

' * ^a

~

-2 o

BM CF

^^^

^O^-

.g j

m g f.

^

-3 s

.b 9

m'.

..v:: :.:

.v.v:.v.v: ::.v:

' Q i. =4..: Q : Q. y y

,.y,4:Q:Q:.y,4:Q*Q:

Q:.: -4 9

..., v,....

m.

m.v~5

^^^

..A

~ 7%e

^^}

,.,.,. -5 E

,7

-6 -10 A

A' Figure 18. Comparison of density profiles beneath Bare Mountain (BM) and Crater Flat (CF).

Profiles are constructed using a balanced cross-section (Ferrill et al.,1996b) and density values from McKague (1980) and Howard (1985). Density differences are assumed to be negligible beneath 5.6 km.

l 54 I

l 2

f = fp(z)g d t

(20) 0 2

where g is gravity (9.8 m/s ), p(:) s rock density at a given depth z, and z is the total depth (5.6 km), this density difference in the upper crust produces a lithostatic pressure difference between Bare Mountain and Crater Flat of approximately 15 MPa at a depth equivalent to the base of the Paleozoic section in Crater Flat. This lithostatic pressure estimate excludes topographic effects, because these effects attenuate rapidly with depth (Anderson,1989).

Lateral changes in density at the surface, such as those produced by topographic variations or the development of a basin, attenuate with depth because of changes in the magnitudes of horizontal stresses relative to vertical stress as a function of depth. In this case, lithostatic pressure is best estimated as:

P = - fg + o,, + %)

( 21 )

where o, o,,, and o are the orthogonal riormal stresses.

y y

Because of this attenuation, comparatively large-scale density variations are required to create lateral pressure changes in the mantle. Furthermore, lateral density contrast in the crust will I

cause lateral pressure changes in the mantle only if the Moho discontinuity is not deflected as a result of isostatic compensation (Figure 19). Isostatic compensation is not likely because the scale of features like Bare Mountain and Crater Flat are small compared to the scale of features normally compensated for by isostasy (Anderson,1989). Existing geophysical data (Brocher, et I

al.,1996) support a flat Moho discontinuity in the YMR.

Bouguer gravity anomalies indicate that large-scale crustal density variations necessary to I

produce pressure variation in the mantle at >40 km occur in the YMR (Figure 20). The gravity map is dominated by large, negative anomalies produced by Timber Mountain-Oasis Valley calderas and a positive gravity anomaly associated with the Funeral Mountains. A north-trending area of largely-negative gravity anomalies extends through Crater Flat and the Amargosa Desert.

These gravity data can be used to create an apparent crustal density map, following the methods of Gupta and Grant (1984), and to infer changes in apparent lithostatic pressure, AP, at t

comparatively-shallow depths. Construction of the apparent density, or AP, map from the t

gravity data requires several assumptions:

I The gravity data must be on a regular grid. In this case, the gravity data were interpolated to a regular grid using a minimum tension bicubic-spline gridding algorithm.

All density variation occurs due to lateral density variation between grid points.

Density is taken to be constant between the surface and a depth, Z, within each grid cell. Density variations in the Earth below Z are not considered to contribute to the gravity anomalies.

55

^

~

P2 Pt

--S.

x l

Moho-b Depth Po (not to

.-- - ww% - -

P 1

scale)

' ' ' ';??%-.

a :a w <.n u,l d,a a n ;,,,,;. d, h mantle near solidus I

~

50 km

=

2800 kg/m3 l

2550 kg/m3 2900 kg/m3 u = 0.25 15 km 3300 kg/m3 u = 0.45 1200

-100 km 1

I I

I I

I I

I I

1195

-h

}

P-

}

o

{ 1190 p3 g

E 1185 5.

n.

j l

1180 1

I I

I I

I I

I I

0 20 40 60 80 100 Distance (km)

Figure 19. Conceptual model of melt generation in response to crustal extension.

(a) [ upper Figure) Extension results in lateral density contrast in the crust that deflects iso-pressure surfaces downward to P, from their initial depth P This local decrease in pressure results in the partial melting of near-solidus mantle. A simple finite element model (b)[ lower Figure) indicates that pressure changes of 7 MPa are expected at depths of 40 km in response to large density variations in the upper 5 km of the crust, using the bulk densities and values of Poisson's ratio, u, indicated.

56

iM (Q99

'N (y

b)_'['

\\_ s J

'(' \\QA, 1

,+

e f

'N

/ /

9

/

(

/

QN g-

'N,

v' '

/

C somL(h's\\\\L k&

G A

4

' \\\\QQ D

N')

E,fQQ

^

o f

^ -150

\\

J

/

'O

%V N

x

/

MQQQDx_

t.

]

?. ' / 4 r,

x q

/

ha j sJ J

w h

059 6 ) \\ ]e [/

Q

@)

G 1 N

/]

g 4

1 00 6240'00 Sb000 /

544 0mb 000 C d64000 574000 Figure 20. Douguer gravity anomaly map of the Yucca Mountain region. Data compiled from numerous sources and obtained from the Geophysics Data Repository at Lawrence-Berkeley Laboratory.

57

The method assumes a horizontal ground surface. The YMR gravity data have been reduced to a Bouguer anomaly, meaning density variations produced by topography and altitude effects have been removed from the gravity map. Using this data set results in lower density variation than expected, if topography is factored into the calculation. However, topographic effects have relatively-short wavelengths, do not produce significant pressure differences at depths of magma generation, and, therefore, may be neglected.

Using the notation of Gupta and Grant (1984), the gravity anomaly at a point, Ag(ry), at the surface due to density variation at a point, Ap((,71,() beneath the surface, is:

B f f

( Ap((.q)d(dyd!

Ag(ry,0) - G 22 )

" -. -- o /x-()2 (y_q)2.(7_g)2 a,9 where G is the universal gravitational constant. Note that, in this formulation, density does not vary as a function of depth. All density variation is lateral, and the amplitude of the gravity anomaly changes with depth of the anomalous mass only because of the change in distance from the mass anomaly to the gravity meter. Only the vertical component of the gravity anomaly is considered because this is measured by the gravity meter. Differentiating with respect to z gives

..z

-Ap((,qk/ dad (

og(ry,0) = G [ [ [kr-()2.(p_q)2 g2p2

-. -. o

( 23 )

then integrating across depth Ap((,qhhl (

A#,W d

g(rA ) = G O

_g

( 24 )

-. -- /@-()2.(p.q)2

_,,, g(y_g)2 (p_q)2 7 which expresses the change in gravity in terms of the horizontal distance between the gravity meter and the density anomaly, and the average anomalous density averaged between the surface and depth Z. Because all gravity variations are assumed to result from lateral variations in density, the relationship between gravity anomalies and apparent density anomalies can be El E

expressed using a 2D Fourier transform of the gravity data. The 2D Fourier transform of the

)

gravity field is given by:

j bg(u,v) = { {Ag(ry,0) exp "'

• d dydr K

( 25 )

where u and v are wave numbers. Gupta and Grant (1984) developed a simple filter to relate density and gravity in the wave number domain, based on the wavelengths of anomalies:

Ag(.>. ; g.,

A,(.>

( 2e )

58 I!

[

where

(

ta=/u2.y2

( 27 )

The inverse Fourier transform then yields apparent density in the spatial domain:

(

I D

Ap(ry) = 2nG [ [t-exp* g(u,v) dvdu A

( 28 )

The change in lithostatic pressure across the map region is then AP(ry) = Ap(ry)g2 1

( 29 )

(

where g is now the average gravitational acceleration, 9.8 m s', and Zis the thickness of the crust within which all density changes are assumed to have occurred. Again, no significant density changes, in terms of overall change in lithostatic pressure, are assumed to occur at depths greater than Z.

[

For Z = 5000 m, Ap(ry) varies from approximately -100 to +240 kg m across the YMR (Figure 8

21). The apparent density contrasts across the Bare Mountain fault in southem Crater Flat of

(

240-280 kg m-3 are in agreement with density contrasts obtained from the balanced cross-section and measured density values in the region (Figure 18). The most prominent feature of this map is the abrupt change in apparent density from high values west of the Bare Mountain

{

fault to low values east of the Bare Mountain fault. Although this change is most abrupt adjacent to the Crater Flat basin, the apparent density map also reveals that this change persists south of Bare Mountain into the Amargosa Desert, and north of Bare Mountain. The apparent density

[

map also shows that this change in density across the Bare Mountain fault is a long-wavelength feature. Apparent density values remain low east of the Bare Mountain fault for at least 50 km and high west of the Bare Mountain fault to the edge of the gravity map (Figure 21).

[

Because the magnitude of lateral pressure change will attenuate as a function of depth, only long-wavelength density variations in the crust will produce pressure changes in the mantle at depths of 40-80 km, the probable depth of magma generation in the YMR. The magnitude of

{

pressure variations resulting from crustal density contrasts calculated across the Bare Mountain fault can be explored using finite element analysis. Based on a simplified geometric representation of the development of the basin, lateral pressure variations on the order of 7 MPa

[

are expected to occur at depths of 40 km (Figure 19), attenuating to 2 MPa at a depth of 80 km, and < 1 MPa at 100 km. Mantle rocks at depths of 40-100 km are under average lithostatic pressures of 1000-3000 MPa. Thus, a change of 2-7 MPa across the density discontinuity represents a small fraction of the total pressure at that depth. This small difference reinforces

(

the idea that extension and deformation of the magnitude observed in the YMR can only result in renewed magmatism if mantle rocks are already near their solidus (Figure 19).

{

Observations of the distribution of volcanoes in the YMR suggest that these small, lithostatic pressure differences are sufficient to generate basaltic melt. Plio-Quatemary volcanoes lie in the lower AP (ry) areas east of the Bare Mountain fault, as expected if decreases in lithostatic t

(

pressure result in production of partial melts in the YMR. Nearly all of these volcanoes occur within the gravity low, which, in part, defines the Amargosa Gravity Trough (O' Leary,1996) 59

r I

I I

I

\\

I

.L..

.w y

( /'[s c...

y 2

s. 4 m ) \\

g t

\\

N'gp'-

'g\\ '

)

' & '\\'x O <-e x

\\j

,,. s/ nT'OT6x L. %V'_ J i y x 40f 900 'N j-g' m \\ Y f& Y / Qt \\1 [ 0 %Q}Le \\ 4 g x \\ -O [ m ')')Xn m \\\\ g \\ ll ) /; ()g l p, D. /\\ /' Ng' 0 ) // V g, @k l[ ^# / 'I' 54000 C N 64000s. / 574000 \\ i 514000 \\ 5240 64 544000' 5 I Figure 21. Apparent density variation across the Yucca Mountain region, derived from gravity data. Change from the mean apparent density in the map area is contoured in kg m. Volcanoes tend to occur in areas of relatively low average density, east of 'he Bare Mountain fault. 60 I I l (Figure 20). Topographically, Lathrop Wells cinder cone lies outside Crater Flat but, based on gravity data, is within the larger north-trending basin and at the margin of the prominent basement low in southemmost Crater Flat. Aeromagnetic anomalies (Langenheim, et al.,1993) in the Amargosa Desert produced by buried Pliocene (?) basalts also lie within or at the margins of the southem extension of this basin. The eastemmost of these buried basalts lies close to the north-trending gravity anomaly demarcating the eastem edge of the Amargosa Desert alluvial basin in this area. ( These YMR volcanoes erupted in areas of lower AP (ry) than expected if eruptions occurred t randomly throughout the map area. In fact, only one Plio-Quatemary volcano erupted where f. AP (ry) >+2 MPa, and this volcano, Aeromagnetic Anomaly E (Appendix A), erupted in a high, t gravity-gradient area along the southem projection of the Bare Mountain fault. These observations suggest that long-wavelength density differences in the YMR, dominated by displacement across the Bare Mountain fault and its apparent extension south into the Amargosa ( Desert, are sufficient to produce the pressure changes in the mantle that cause partial melting and volcanism. { This lithostatic pressure model suggests a correlation between the timing of extension and the timing of volcanism. Magma generated in response to extension, resulting in Quatemary volcanism within vent clusters formed by Miocene and Pliocene basaltic volcanism, occurred [ because mantle rocks beneath these regions were near their solidus and partially melted when i ' comparatively small amounts of extension took place. A given rate of extension will result in the greatest rate of change in mantle pressure directly beneath the lateral change in crustal density, such as at the Bare Mountain fault, Thus, with continuing extension, mantle in the region of this ( inflection has the greatest opportunity of producing partial melts as a result of a given amount of crustal extension. Episodes of. extension and basaltic volcanism may correlate temporally, because pressure variations in the mantle will likely equilibrate due to ductile flow over time. In ( other words, pressure changes in the mantle that result from crustal extension will be transitory. Change in lithostatic pressure also affects magmatism, because magmas ascend by buoyant [ rise. The buoyancy forces acting on the magma are equivalent to the hydrostatic pressure t gradient, given by Lister and Kerr (1991) as: Z [ Pg (pg(r) - p,)g dz ( 30) ( where p, and p, are densities of rock and magma, respectively, g is gravitational acceleration, and Zis the depth of magma generation. Rock density varies as a function of depth, most dramatically at the Moho. Because the density of magma is typically less than that { of mantle, but greater than most crustal rocks, a level of neutral magma buoyancy exists in the crust. An isolated pod of magma above the level of neutral buoyancy sinks and a pod below the level of neutral buoyancy rises. Magmas fed by conduits respond to the integrated hydrostatic r pressure along the conduit but also have flow characteristics that respond to the local hydrostatic l pressure. Thus, dikes propagate laterally above the level of neutral buoyancy (Lister and Kerr, 1991). The level of neutral buoyancy is deeper in the crust beneath basins than beneath mountains. As Quatemary basalts in the YMR demonstrate, basalts do not stagnate in the { alluvial basins as they rise through them because hydrostatic pressure is integrated over the depth from origination of the melt. Longer dikes and dike swarms, however, preferably form in 61 L ? these alluvial basins because of the basins' comparatively low lithostatic pressure. Thus, from j the perspective of volcanic hazards analysis, understanding changes in lithostatic pressure i across the region constrains areas of likely melt generation and areas of likely dike propagation i above the level of neutral buoyancy. 4.1.5.3.3 Local Structural Controls on Magma Ascent Observations in the YMR indicate a strong correlation between structure and volcanism. These observations include vent alignments (Smith, et al.,1990; Connor, et al.,1997) and cinder cones along faults (Section 4.1.4 and Connor, et al.,1997). These observations suggest that structural influences should be considered in PVHA of the proposed repository. Basaltic magmas are transported from the mantle to higher levels in the crust or to the surface by 3 igneous dikes. Propagating dikes, like other hydraulic fractures, typically form perpendicular to g the least principal stress and parallel to the principal horizontal stress in extensional terrains (Stevens,1911; Anderson,1938). Under some conditions, pre existing faults or extension fractures serve as pathways for magma instead of propagating a new dike-fracture. Assuming that a pre-existing fault or extension fracture has no tensile strength, pre-existing fractures dilate (i.e., capture magma) if the fluid pressure exceeds the normal stress resolved on that fracture (Delaney, et al.,1986; Reches and Fink,1988; Jolly and Sanderson,1997). The likelihood of dilation and capture is controlled by the magnitude of the three principal stresses (o,,0. 0 ), fluid pressure, and orientations of pre-2 3 existing fractures in the in situ stress field. The ability of any fault or fracture to dilate during magma injection is directly related to the normal stress acting across the fracture. Assuming cohesionless faults, the relative tendency for a fault of a given orientation to dilate in a given stress state (i.e., dilation tendency) can be expressed by comparing the normal stress acting across the fault with the differential stress (e.g., Morris, et al., 1996). Dilation tendency of the fault is expressed as: (o,- o,,) ( (o - 0 ) I ) i 3 where o, and a are the maximum and minimum compressional stresses, respectively, and o is s n the normal stress acting across the fracture. Faults with T, greater than some threshold value, such as 0.8, are considered to have a high-dilation tendency (Morris, et al.,1996). A Schmidt plot of dilation tendency and fault poles indicates that, in the YMR region, faults oriented 355'-085 with dips >50 have a high dilatio'n-tendency (Figure 22). In the YMR region, o, is vertical, c is horizontal and oriented 028, and 0 is horizontal and 2 3 oriented 298' (Morris, et al.,1996). The relative magnitudes of ci:0 :0 are estimated to be 2 3 90:65:25. As a result of this stress pattem, steeply-dipping, north-northeast-trending faults have a greater dilation tendency than faults of other orientations. Areas with higher concentrations of g high dilation-tendency faults, therefore, are more likely to be the areas of volcanic activity. E' Cinder cone alignments form over prolonged periods of time if high dilation-tendency faults repeatedly serve as conduits for magma ascent (e.g., Conway, et al.,1997). I-62 I 1 McDuffie, et al., (1994) provide analytical results that show that the ability of a fault or fault zone to redirect ascending magma depends on the depth at which the dike intersects the fault and the dip of the fault zone. Only high-angle faults with dips greater than 40-50' are capable of dike capture at depths below 1 km. At depths of 10 km, faults dipping at angles less than 70 do not provide low-energy pathways to the surface. compared to vertical dike propagation. Steeply-dipping, high-dilation-tendency faults in the YMP. include many faults that bound the Yucca Mountain block, such as the Solitario Canyon and Ghost Dance faults. The Solitario Canyon fault adjacent to the repository site hosted dike injection at approximately 10.9 Ma. Moreover, the Solitario Canyon fault extends to the detachment fault at depths of 5-10 km (Figure 16). The distribution of faults with relatively high potentials for acting as magma conduits can be inferred from geologic mapping. In areas of alluvial cover, gravity and magnetic data provide the best indication of the distribution of these faults (e.g., Connor, et al.,1996). 4.1.5.4 Summary Tectonic setting is important to consider in volcanic hazard analyses at several scales. On regional scales, crustal extension results in changes in pressure in the mantle and gives rise to Dilation Tendency = (01 U )/(U1 0) 3 n N 8 U2 e e.. 9, *.,..- e e, *. *

• e.,..-

. -l.. e,*.- Wosse e ..eseoE 0.5 e * * *. ' U1 /'.****e ,..,e f . ',.. *, 8, 03 ...*e S 0 Figure 22. Schmidt plot of fault dilation tendency for Yucca Mountain region stresses. High dilation-tendency faults are oriented 355'-085* with dips greater than 50' (cf. Figure 15). 63 r I: partial melting. Extension also results in the formation of dip-slip fault systems, which serve as conduits for magma rise. On local scales and at shallow depths, individual dikes may propagate along faults that have high dilation-tendencies and dike lengths may be controlled in part by local 3i lithostatic pressure. Field investigations in the YMR have shown that all of these factors operate E in the YMR, partially controlling the distribution and timing of basaltic volcanism. Sufficient evidence exists to indicate basaltic volcanism in the WGB is linked to crustal deformation. Currently, several tectonic models are in use for the YMR, including detachment fault, simple horst and graben, Amargosa shear, and pull-apart models. Some commonality exists among these models with regard to basaltic volcanism. In particular, all of these models evaluate Crater Flat as an extensional half-graben, bounded on its westem margin by the Bare Mountain fault. This structural basin appears to localize volcanism. Detachment fault, pull-apart, and Amargosa shear models all characterize the Bare Mountain fault as a major structure, E transecting the brittle crust. The occurrence of the Bare Mountain fault can impact basaltic 5 volcanism at several scales. On a regional scale, the Bare Mountain fault creates a substantial density contrast in the brittle crust. This density contrast causes changes in lithostatic pressure g in the mantle that may induce partial melting. The Bare Mountain fault also may serve as a g conduit for magma ascent through the brittle crust. The planar fault modelis closer to a classical Basin and Range model of horst and graben formation (e.g., Stewart,1971) than other tectonic models proposed for the YMR. However, this model shares elements with the other tectonic models in that the Bare Mountain fault is a major structure and Crater Flat basin is formed by extension. All of the tectonic models proposed to date include Yucca Mountain in the same structural domain as Crater Flat (O' Leary,1996; Fridrich, et al.,1997; Schweikert and Lahren, 1997; Young, et al.,1992; Ferrill, et al.,1995; Ofoegbu and Ferrill,1995; Stamatakos, et al., 1997a). Results of a number of analyses indicate that incorporation of tectonic models into probability studies increases the probability of volcanic disruption of the proposed repository site compared I to models that do not account for the tectonic setting of the site explicitly (Connor, et al.,1996; g Hill, et al.,1996). This result primarily reflects the fact that Yucca Mountain is structurally part of g the Crater Flat basin, with high dilation-tendency faults bounding and penetrating Yucca Mountain itself. Because of the presence of these structures, the lowerlimit on probability is represented by the nonhomogeneous Poisson models that do not incorporate structure. Probability models that incorporate tectonic features (e.g., the modified kemel model) are similar to some source-zone models in that the probability surface is elongate in a north-northwest { direction, similar to the CFVZ proposed by Crowe and Peny (1989). The same tectonic features that enhance the probability of volcanism in Crater Flat, however, increase the probability of volcanism at Yucca Mountain, albeit to a lesser degree. f On local scales and at shallow depths, individual basaltic dikes may propagate along faults that have high-dilation tendencies. Dike lengths may be in part controlled by local hydrostatic pressure. Field investigations in the YMR have shown that all of these factors may operate in the YMR, partially controlling the distribution and, possibly, the timing of basaltic volcanism. There is general agreement that volcano distribution is affected by local structural control. Dikes and vent alignments tend to be oriented northeast throughout the region in response to horizontal stresses in the crust. Northeast trends have been accounted for in most analyses (e.g., Geomatrix,1996; E Smith, et al.,1990; Connor, et al.,1997). 5 I ( u E

4.1.6 Probability Criterion 6 4.1.6.1 Acceptance Criterion Estimates of the probability of future igneous activity in the YMR will be acceptable provided that:

The probability values used by DOE in performance assessments reflect the uncertainty in DOE's probabilistic volcanic hazard estimates. 4.1.6.2 Review Method NRC staff should review these probability values in light of the range of values used in the literature for the YMR and comparable volcanic fields. At a minimum, NRC staff should evaluate probability models by testing their sensitivity to uncertainties about the past distribution of volcanic vents, the recurrence rate of volcanism, and the relationship between igneous activity 3 and tectonism. Probability models must be sufficiently robust to reasonably approximate the current distribution of volcanoes. Probability values need to have estimates of the uncertainties associated with calculated values in order to be acceptable. Uncertainty for reported probability values needs to incorporate both the precision of the probability model (e.g., influence of parameter uncertainty on the range of model results) and accuracy of the probability model (e.g., how well does the model predict the locations of volcanoes). Also, if a conservative value of probability is used in performance assessment then the reasons why this value is considered to be conservative should be clear and transparent. 4.1.6.3 Technical Bases One of the difficulties inherent in the PVHA of the proposed repository is that the small number of volcanoes in the YMR makes it difficult to evaluate models quantitatively. Application of-probability models in other volcanic fields (e.g., Condit and Connor,1996) provides one method of evaluating probability models applied to the YMR. A second, equally important approach to model evaluation is to apply a range of models to estimate the probability of igneous events affecting the proposed repository and evaluate the sensitivity of probability estimates to bound the range of models. in the following, such a sensitivity analysis is performed for a range of models. The models differ primarily in how igneous events are defined and how more realistic, but often less well-constrained, geologic processes are included in the analysis. These probability models are based on: I Individual mappable eruptive units and vents Vents and vent alignments Vents and vent alignments with regional tectonic control Igneous intrusions in the following, annual probabilities of igneous events are calculated and compared using these models and a range of parameters for recurrence rate and area affected by volcanism. i 65 4.1.6.3.1 Individual Mappable Eruptive Units and Vents Individual mappable eruptive units and vents were used by Connor and Hill (1993,1995) to g estimate the probability of volcanic eruptions at the site. This definition of igneous events involves the fewest assumptions about volcanism, resulting in a straightforward sensitivity g analysis. Assuming that the probability of more than one event in a given year is small, the annual probability of volcanic eruptions within the repository boundary is given by: P[ volcanic eruptions within repository boundary] = l-exp[-A, A, A,] ( 32 ) where A,is the annual regional recurrence rate of volcanic vent formation, A,is the effective g repository area (Geomatrix,1996), and A,is the spatial recurrence rate of volcanic eruptions at g the repository, given a volcanic event in the region. Using a Gaussian kemel: 1 1 I -I' ' y y ( A,(ry) = 2nh2N v[.: exp' + i ( 33 ) 2 h, h,j 3 where x,y is a Cartesian coordinate within the repository boundary, x,,y,is the coordinate of the center of an igneous event, Nis the number of such igneous events, h is a smoothing parameter (Section 4.1.4.3). For the following calculations, x,y is 548500, 4078500 and x,,y, are in g Universal Transverse Mercator coordinates (Appendix A). Based on the analysis in 5 Section 4.1.4.3, a smoothing parameter, h 2 5 km, is appropriate for the Gaussian kemel. An effective repository area of 5.49 km is used in this analysis, based on the current repository g design (Figure 7) and a 50 m buffer zone about the repository perimeter. The number of igneous g events, N, depends on whether Pliocene and Quatemary or only Quatemary volcanoes are considered in the probability estimate. I Eight igneous events have occurred in the YMR during the Quatemary, if these events are defined as individual mappable eruptive units and vents. Connor and Hill (1995) used this definition for igneous events and varied recurrence rates between 5-10 v/m.y. Here, we model a E range of 2-12 v/m.y. A recurrence rate >12 v/m.y. would signal a marked increase in activity 5 compared to other WGB volcanic fields. Recurrence rates in the Cima volcanic field, Califomia, which is one of the most active basaltic volcanic fields in the WGB, are on the order of 30 v/m.y. (Turrin, et al.,1985). Comparable rates of basaltic volcanism have not occurred during the Plio-Quatemary in the YMR, with the possible exception of in the Funeral Formation. Rates of less than 2 v/m.y. would signal a marked decrease in magmatism in the YMR. No evidence currently a available suggests such a decrease is likely. Therefore, the assumption that such a decrease in g regional recurrence rate will occur can not be supported for the volcanic hazard analysis. Estimated probabilities using this model are sensitive to temporal recurrence rate of igneous events in the YMR, A,, and choice of h in the calculation of A,(x,y)(Figure 23). Based on these parameters, the annual probability of volcanic eruptions within the repository boundary is between 0.5 x 10 8 and 3.5 x 10' Probabilities are slightly higher if the distribution of Quatemary volcanoes is considered in estimation of A,, rather than the distribution of Plio-Quatemary volcanoes, because Quatemary volcanoes are, on average, located closer to the repository site. These values are quite close to those calculated by Connor and Hill (1995) using Epanechnikov kemel and nearest-neighbor estimators of spatial and spatio-temporal recurrence 66 rate. Connor and Hill (1995) used A, = 8 km and estimated annual probabilities of volcanic 2 disruption of the site between 1 x 10 e and 5 x 10'8 4.1.6.3.2 Vent Alignments if igneous events are defined as vents and vent alignments, probability of volcanic eruptions within the repository boundary incorporates distance and direction of an igneous event centered at a point, x,y, from the repository boundary. The probability of an igneous event centered at x,y ( is given by: Fy [ igneous event at xy )=1-exp(-A,1,ArAy) ( 34 ) where A,is the regional recurrence rate and A,is the spatial recurrence rate at point x,y, E

3. 5 i

i i i i i i i i i i i i i i i i_ s.- 3.0 h i [ m3 $ ( 2.5 h i si 5 2.0 I-12 _= oy y E 1.5 ~ 0 T E8 [ n a. t j j 1.0 Ec 2 EC. = ~ I l [ 0 5 10 15 20 i i i i i i i i i i h (km) [ Figure 23. Annual probability of volcanic eruptions within the repository boundary, r Igneous events are defined as individual mappable eruptive units and vents. A Gaussian i kernel is used with smoothing parameter, h, varying from 0-20 km. Curves are shown for various regional recurrence rates of volcanic vent formation (A, = 2 x 10~' v/yr,8 x 10 ' v/yr,12 x 10-* v/yr), based on the distribution of Quaternary volcanoes (heavy lines) and f Plio-Quaternary volcanoes (light lines). The effective repository area, A,, is 5.49 km. 2 67 N calculated using the Gaussian kemel[Eq. (33)). In practice, A,is calculated on a grid of points with map extent X, Y and grid spacing A, dy. This probability is then weighted by the probability that an igneous event centered at x,y, or occurring within A, dy will result in a volcanic eruption E within the repository boundary. For vent alignments in the YMR, the spacing of vents along the RI alignments is small compared to the size of the repository (Section 4.1.3.2). Vent alignment length is defined as the distance between the centers of the first and last vents on the alignment. Therefore, the probability that an igneous event centered at x,y will result in vent alignment intersection with the repository boundary and subsequent volcanic eruption within the repository boundary is: P,,[ volcanic eruptions within repository boundary l igneous event at ry] 1, x, YEA, 1 I - 1, ( 35 ) mo l s I, s l_ =< a 2 l_ - Is 0, l, > I,, where I, and I, are the minimum and maximum alignment half-lengths, respectively, and I,is a the distance from x,y to the nearest repository boundary along the direction of the alignment. For g this analysis, vent alignments are assumed to be oriented 028, perpendicular to the direction of minimum compressional stress in the YMR. Experimentation indicates that choosing a range of values of alignment orientation between 020* and 035' has a negligible effect on probabilities of volcanic eruptions within the repository boundary. Probabilities are sensitive to /, which is varied over a range of values in the following analysis, but are not sensitive to the selection of 1,, which for the following calculations is 100 m. As indicated in Eq. (35),50 percent of all l igneous vents are not part of vent alignments in this model. The probability of volcanic eruptions a within the repository boundary is then: P[ volcanic eruptions within the repository boundary] X Y = E EP,,(x,y,) P,,(x,y,) ( 36 ) s.n j.s where x,, yj are on a rectangular grid of extent X,Yand grid spacing h, dy. Annual probability of volcanic eruptions within the repository boundary were calculated using 5200 m s /,, s 10,200 m, and h = 5 and 7 km (Figure 24). Based on nearest-neighbor vent and vent alignment distances in the YMR, h 2 7 km is reasonably conservative (Figure 11). Using three Quatemary igneous events (Lathrop Wells, Quatemary Crater Flat, Sleeping Butte), results E in annual probabilities of volcanic eruptions within the repository boundary between 1 x 10-e and E 3 x 10-', assuming a regional recurrence rate of 3 v/m.y. A rate of 5 v/m.y. results in annual probabilities of 6 x 10-e, 4.1.6.3.3 Vent Alignments With Tectonic Control For a more complete analysis, the above probability estimates should be modified to incorporate additional geologic contro's on volcanism. Tectonism in the YMR has led to regional variations in crustal density that may cause variation in rates of partial melting across the YMR (Section 4.1.4.3). These variations are most apparent across the Bare Mountain fault. Plio-Quatemary basaltic volcanism clusters east of this fault, in areas of anomalously low crustal density. In 68 I contrast, basaltic volcanism since the mid-Miocene is apparently absent west of the Bare Mountain fault and its southern extension into the Amargosa Desert. Standard Gaussian kemel functions do not take into account these geologic details. As a result, the standard Gaussian kernel [i.e, Eq. (33)]is too simple and overestimates probabilities #cf volcanic eruptions in some areas, for example on Bare Mountain, and underestimates probabilities elsewhere in the YMR. The standard Gaussian kemel model developed above was modified by developing a weighting function that accounts for crustal density. The model for basaltic volcanism in extensional environments developed in Section 4.1.5.3 relates lithostatic pressure gradients in the mantle to regional changes in crustal density caused by extension. As illustrated in Figure 19, partial melting occurs where partial melting had occurred previously and close to active graben-bounding faults where slip in the crust causes the greatest pressure change in the mantle. Pressure change in the mantle is inferred conceptually from simple numerical models of mantle stresses (Figure 19). The weighting function can be estimated from the frequency of volcanic e 25 3 c ~ / ~ o r Eo 2 2.5 mx 2& 7 E4 j E >g 2 5 E ^ b >0 xx 5g ,g g. 1. 5 em n. q E 1 ''I'l''''l 4 5000 6000 7000 8000 9000 10000 11000 /, (m) Figure 24. Annual probability of volcanic eruptions within the repository boundary. Igneous events are defined as vents and vent alignments. A Gaussian kernel is used with smoothing parameter, h, of 5 and 7 km (labeled lines) and is based on the distribution of three Quaternary igneous events. Vent alignment half-length, l,,, varied between 5,200 and 10,200 m, roughly changing probability estimates by a factor of two. Probabilities are calculated using A, = 3 x 10~'/yr. 69 L eruptions as a function of crustal density. The distribution of this function, fr(X,Y). was defined based on average crustal densities in the upper 5 km of the crust at the locations of existing volcanoes, derived from application of the density filter to the gravity data set (Figure 25). The g Gaussian kernel was then modified to estimate the recurrence rate of volcanism at x,y. g K,(x,, y,) = exp'[- t 12 r 12 6~#v ( 37 ) + l 1 X Y l E E K,(x,35) G, " x (38) r E E fy(x, y,) K,(x, y)) a= 1 j= 1 1 A,(ry) = 2nh2g E,G,fy(xy) K,(ry) ( 39 ) Introduction of the ratio Q, assures that the integral of the modified Gaussian kemel for a single volcano over a large map extent X,Y relative to the smoothing parameter, h, will be unity [Eq. (5)). The probabilities, however, are redistributed based on crustal density variations in the g vicinity of the volcano. 3 I ^ ^ i i i i i i i iiii j l iiijiiiil iiiil i ~ 0.8 o o ~ g 0.6 g h %~ 0.4 l j 5 volcanoes [ 0.2 I' ~ l'l'' d ': 0 'l -10 -5 0 5 10 15 20 Change in Pressure (MPa) Figure 25. The weighting function, fr(x,y), is derived from changes in average crustal ] densities under the locations of Plio-Quaternary Yucca Mountain region Volcanoes. l l 70 ) I! I Comparison of the modified and standard kernels was made by contouring A,(x,y) across the YMR, using the distribution of Quatemary vents and vent alignments and h = 9,000 m. As previously, N = 3 in this model, defined by Quatemary Crater Flat, Lathrop Wells, and Sleeping Butte as the three Quatemaryigneous events. In Figure 26, A,(x,y)is contoured across the map region using Eq. (33). Given an igneous event in the region, there is a 68-percent chance that the igneous event will occur within this map area. The Sleeping Butte alignment lies north-northwest of the mapped region (see Figure 4). Larger values of A,(x,y) indicate areas where igneous events are most likely centered. The largest values occur in southem Crater Flat because of the proximity of Lathrop Wells and the Quatemary Crater Flat alignment. In this area, A,(x,y) varies between 8 x 10'd volcanic events per square kilometer (v/km ) and 2 x 10-d v/km, 2 2 Figure 27 is based on the modified kemel [Eqs. (37) to (39)] using the same parameters as used in the standard kemel calculation (N = 3, h = 9,000 m), but weighting the kemel using crustal densities derived using Eqs. (22) to (29). Use of the modified kemel reduces the area of the A,(x,y) surface at, for example, the 2 x 10'd v/km contour, and increases the amplitude of the 2 surfacei The A,(x,y) surface also becomes asymmetric as a result of application of the modified kemel function. Values of A,(x,y) are greatest in southem Crater Flat, exceeding 1.2 x 10' 2 v/km, and decrease abruptly near the Bare Mountain fault. Probability values decrease less abruptly on the eastem boundary of Crater Flat because crustal densitias change less rapidly on the eastem edge of the basin; This more gradual change in A,(x,y) on the eastem edge of the basin is consistent with the proposed model linking crustal extension and basaltic volcanism (Figure 19). The annual probability of volcanic eruptions within the repository boundary increases when the modified kemel function is used. Annual probability of volcanic eruptions within the repository boundary was calculated using 5,200 m s 1,,, s 10,200 m, and h = 7 km (Figure 28). Using the three Quatemary igneous events (Lathrop Wells, Quatemary Crater Flat, Sleeping Butte) results in annual probabilities of volcanic eruptions within the repository boundary between 3 x 10-e and 5.5 x 10-8, assuming a regional recurrence rate of 3 v/m.y. Including Pliocene volcanoes in the estimation of A,(x,y) decreases the annual probability at the repository because many Pliocene volcanoes are located in the Amargosa Desert. Annual probabilities based.on the modified kemel distribution and Plio-Quatemary volcanoes vary between 1.5 x 10-e and 3 x 10-e, comparable to the annual probabilities estimated using the standard kemel and the distribution of Quatemary vents and vent alignments. The regional recurrence rate of vent and vent alignment formation is poorly constrained in the YMR. Varying regional recurrence rate of igneous events between 1 and 5 v/m.y. results in nearly one order of magnitude variation in the annual probability of volcanic eruptions within the repository boundary. Using the modified kemel model, h = 7 km, and 5,200 m s I,n, s 10,200 m, annual probability' of volcanic eruptions within the repository varies between 1 x 10-e and 9 x 10-e (Figure 29). 4.1.6.3.4 Igneous intrusions The probability of igneous intrusions, such as dike swarms, intersecting the repository is greater than the probability of volcanic eruptions within the repository, because igneous intrusions must have greater areas than vent alignments and most likely occur with greater frequency. All alignments have associated intrusions but not allintrusions produce vent alignments. The recurrence rate of igneous intrusions and their geometry, however, are so poorly constrained by available data that these parameters are not estimated. Based on analogy with the San Rafael volcanic field (Delaney and Gartner,1997), probabilities of igneous intrusion into the repository 71 I. I Il 4039000- / WR .0 / r %~ i-p' 5 b G 4089000 4 7 N s t' R f? R / i% / \\ 40.Z9000 / Y ex g' \\ I / N 8 6 4 2 4 *4 K}\\ 4[ m 4069000 a i1 m /' O l \\ th( ~- 4059000 ) I \\\\ (-g//- I ,i 3 514000 524000 $34000 644000 Ti54000 664000 674000 I Figure 26. The spatial recurrence rate (v/km ) is contoured in the area of Yucca 2 Mountain, using the Gaussian kernel function (Eq. 35). In this model, h = 9,000 m and N = 3, based on the number of igneous events. The contour intervalis 2 x 10d v/km, 2 Other symbols are as in Figure 7. 72 I 4099000 \\ /5 (y %, Q ge[?f 4089000 x_ \\ 4 ,,k, 4f 'l ~ \\ 6 4029000 10 g W/ 12 '4p 3 s 14 f o j 16 4 ( r 18 fb f / forf 'gyW ^ 4 Oy ) /g *. 4059000. k k 9t. V V Am ) ~. 514000 524000' 534000 #544000 .n5 000 564000 574000 Figure 27. The spatial recurrence rate (v/km )is contoured in the area of Yucca 2 Mountain, using the modified Gaussian kernel function (Eq. 37 to 39) to incorporate tectonic control on the probability estimate. In this model, h = 9,000 m and N = 3, based on the number of igneous events. The contourintervalis 2 x 10 v/km. Other symbols d 2 are as in Figure 7. 73 I boundary may be two to five times the probability of volcanic eruptions within the repository boundary. While such a value is speculative it does provide a basis for development of an interim probability value for igneous intrusion intersecting the repository. 4.1.6.4 Summary Annual probability of volcanic eruptions within the repository boundary varies between 10-e to 4 10 based on a range of models. This range accounts for varying definitions of igneous events and uncertainty in parameter distributions used to estimate probability. This range does not account for potential modifications to the temporal recurrence rate that may be necessary pending analysis of the new information presented in Wemicke, et al., (1998), Magsino, et al., (1998), and Earthfield (1995), which could increase temporal rate values and thus increase overall probability values. Annual probabilities are generally between 1 x 10-8 and 3 x 10-s for E igneous events defined as individual mappable units and vents. This definition of igneous events E cj 5.5 m I c 5 Z g S $,_ E o. E 4.5 Y tu u $k4 _= Quaternary Alignments _ 2 gy with modified Gaussian ga kernel > g 3.5 oy i >, o 3 ) E! 2: Quaternary Alignments i '5 E with standard Gaussian .g g. 2.5 kernel ll $"2 E \\ \\ E 1 Plio-Quaternary Alignments 3I m with modified Gaussian kernel-3- E 1.5 l'l'l ''''i'- 4 5000 8000 7000 8000 9000 10000 11000 g I, (m) Figure 28. Annual probability of volcanic eruptions within the repository boundary using a modified Gaussian kernel. Igneous events are defined as vents and vent alignments. A modified Gaussian kernelis used with a smoothing parameter, h = 7 km, based on the distribution of three Quaternary igneous events. Vent alignment half-length, l, varied E between 5,200 and 10,200 m, roughly changing probability estimates by a factor of two. E Probabilities are calculated using A,= 3 x 10~'lyr. Curves are shown calculated using Plio-Quaternary events (N = 12) and the modified Gaussian kernel, and Quaternary events (N = g

3) and the standard Gaussian kernel for comparison.

3 74 requires the fewest assumptions about underlying parameter distributions but also neglects some features of vent distribution that are important in the YMR. In particular, the formation of vent alignments is not accounted for in this model. Defining igneous events as vents and vent alignments results in a similar range of probability estimates for the annual probability of volcanic eruptions within the repository boundary,1 x 10' to 6 x 10'8 Although recurrence rates are lower using this definition of igneous events, the area affected by individual events is greater. The distribution of alignment length and regional recurrence rate of these igneous events introduces the greatest uncertainties into these probability models. Incorporating regional crustal density variation into this model results in a model more closely linked to geologic processes. Based on the crustal density models and similar models presented previously (Hill, et al.,1996; I Connor, et al.,1996), '.he annual probability of volcanic eruptions within the repository boundary is between 1 x 10~8 and 9 x 10~8. Probabilities of intersection of igneous intrusions with the repository are likely higher, but cannot be confidently estimated from available geologic data. As I a value is needed for use in performance assessment, the NRC will assume the rate is a factor of between 2 to 5 higher than for volcanic disruption. Finally, it is noted that this range of probability values,10 to 10~7, arises from the application of a variety of models and a range of I ej 9 _i,iiii,,iiiiiiiie iiiiiiil ' i i i I } c 8 5) T O m l 5 2 7 mx $I6 4 i I e3 2c $j 5 } l 7:i 3 y bo 4 b i l 3~ 2 te E X-i i 2 T 5 A-5 e ,,,,,,I,,,,IiiiiIiie iliie 3 - r,, i M 5000 6000 7000 8000 9000 10000 11000 / (m) ma l l Figure 29. Annual probability of volcanic eruptions within the repository boundary using I regional recurrence rates of A, = 1 x 10-8,2 x 10-8,3 x 10-8,4 x 10-', and 5 x 10lyr. Igneous events are defined as vents and vent alignments. A modified Gaussian kernelis used with smoothing parameter, h = 7 km, based on the distribution of three Quaternary igneous events. 75 1 M parameter distributions. Nothing in the above analysis suggests that this range of probabilities has central tendency, that the mean or median of this range of probabilities is significant, or that high or low values in this range are more or less likely. This situation arises because, at least at E the current time, it is not feasible to develop an objective basis for assigning likelihood to 3 individual models, due to both lack of data and uncertainty in our understanding of the process. For the purpose of performance assessment the NRC will assume the value of 10. As the NRC g recognizes the potential effect on probabilities that the new information discussed above could g produce, based on the models used in this report the NRC see no present basis for changing this value, and consider that the new information further justifies the use of the 10 value. The WGB, which includes Yucca Mountain, is a magmatic province characterized by Quatemary basaltic volcanism (Fitton, et al.,1991). At least 211 basaltic volcanoes <2 Ma occur in the 2 82,000 km region defined by Amboy volcano, the Big Pine volcanic field, and the Lunar Crater g volcanic field (Figure 3; Luedke and Smith,1981; Connor and Hill,1994). Assuming that 3 volcanism is randomly distributed throughout this source-zone (cf. Crowe, et al.,1995; Geomatrix,1996), volcano recurrence rates are 1.3 x 10 yr km-2 The annual probability of g 4 d 2 volcanic disruption of any 5-km area (i.e., repository area) in this source zone is thus 6 x 10. g 4 This analysis overlooks the fact that volcanoes cluster within the WGB (Figure 3). The YMR, however, constitutes one of the volcano clusters within the WGB (Connor and Hill,1995), within which probability should be higher than expected, based on a uniform random model. An annual 4 probability of 6 x 10 appears a reasonable and general measure of background volcano occurrence for any 5-km area within the WGB, including the Yucca Mountain repository site. Models that propose an annual probability of volcano formation at the proposed repository site of E 4 less than 6 x 10, thus, do not appear to be reasonable, based on geologic data. 5 The likely regional background rate for basaltic intrusions is necessarily higher than that of single g volcanoes, due to the larger area affected by a shallow basaltic dike. Using conditions 3 appropriate for the Yucca Mountain repository site, the regional probability of a shallow basaltic intrusion can be assessed by sampling a uniform random distribution of dike half-length between 0.1-4 km and trending 28" from north. The annual probability of igneous disruption of any 5-km2 area in the WGB is then 1.7 x 10-e This simple calculation does not consider the possibility of unmapped shallow dikes that were emplaced without an associated volcanic eruption, or the presence of misdated Quatemary cinder cones in the WGB. Models that propose an annual g probability of igneous dike intersection with the proposed repository site of less than 1.7 x 10-e do E not appear to be reasonably supported. Uncertainty associated with any probability model consists of two components that measure precision and accuracy. Precision is also referred to as " parameter uncertainty," whereas accuracy often reflects "model uncertainty"(Performance Assessment Working Group,1997). Of the range of probability models proposed for the YMR, only the spatio-temporal nonhomogeneous models of Connor and Hill (1995) have been evaluated for model accuracy (Condit and Connor,1996). This initial evaluation demonstrates that these probability models reasonably estimate the locations of basaltic volcanoes in the Springerville volcanic field when basalt petrogenesis remains relatively constant. These models are unsuccessfulin estimating the future locations of basaltic volcanoes when the magmatic system undergoes abrupt and large shifts in petrogenesis (Condit and Connor,1996). The YMR has not undergone similar-g magnitude petrogenetic shifts since about 5 Ma (e.g., Crowe, et al.,1986), thus, these probability 3 models should be reasonably accurate when applied to the YMR system. 76 i - 4.1.7 Probability Criterion 7 4.1.7.1 Acceptance Criterion Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: - The values used (single values, distributions, or bounds on probabilities) are technically justified and account for uncertainties in probability estimates. 4.1.7.2 Review Method The NRC considers a range of different approaches for evaluating uncertainty in performance models used in licensing nuclear facilities. The end-members of the uncertainty analysis are represented by a deterministic bound on the upper limits of dose and a probabilistic approach that represents a distribution of model results (Performance Assessment Working Group,1997). Regardless of the method used, the rationale used in making the analysis and a reasonable and comprehensive understanding of the system being modeled are required for acceptance. Staff should confirm whether the probability values were directly incorporated or models were appropriately abstracted for use in assessments of repository performance, taking into consideration uncertainties in these estimates. 4.1.7.3 Technical Basis ' A deterministic approach evaluates uncertainty by bounding model parameters. Parameter values are generally selected such that overall risk is not underestimated. This approach results in a single, straightforward value that bounds performance but does not provide any quantitative information on the uncertainty associated with this value (Performance Assessment Working Group,1997). Detailed documentation and justification for parameter values used in this approach are required in order to determine the appropriate level of conservatism needed to represent the range cf data. l A probabilistic approach provides a distribution of model results, which, in tum, provides a quantitative measure of uncertainty. This approach is more objective than a deterministic approach in that a level of conservatism is not implicitly required. The range of parameter values i must be reasonable, and appropriate sampling methods must be used in the analysis ] (Performance Assessment Working Group,1997). The mean value of a probabilistic analysis is generally used to determine compliance with the performance objective (Performance Assessment Working Group,1997). For low-level waste licensing, NRC staff also recommended that the 95* percentile of the performance distribution be less than a given value to demonstrate compliance (Performance Assessment Working Group,1997). As NRC is using a single value in performance assessment for volcanic probability, it is furtherjustification of the use of the value of 10. 4.1.7.4 Summary Based on the range of work currently available, the probability of igneous events at the proposed repository site can be described by single values, mean values of various distributions, entire probability distributions, or bounds on probability distributions. Any of these approaches may be used, based on current NRC regulations. Regardless of the value(s) used, the methods used to 77 r ~ I derive the values must be justified, and the data used to derive the values must be clearly presented. l 4.1.8 Probability Criterion 8 4.1.8.1 Acceptance Criterion Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: If used, expert elicitations were conducted and documented using the guidance in the Branch Technical Position on Expert Elicitation (NRC, 1996), or other acceptable approaches. 4.1.8.2 Review Method if DOE uses expert elicitation in developing estimates of the probability of future volcanism and igneous intrusion, staff will review the documentation to assure that the expert elicitation: (i) followed the procedure described in the Branch Technical Position on Expert Elicitation (NRC, 1996); (ii) considered the range of models in the relevant literature; (iii) considered the range of data in the relevant literature; and (iv) is consistent with available information, including information developed subsequent to the elicitation. 4.1.8.3. Technical Basis As summarized in NRC (1996), the NRC expects that subjective judgments of groups of experts will be used by DOE to assess issues related to overall performance of the proposed high-level radioactive waste repository site at Yucca Mountain. NRC has traditionally accepted expert judgment as part of a license application to supplement other sources of scientific and technical data. Expert elicitation is commonly used when I! Empirical data are not reasonably obtainable or analyses are not practical to perform. Uncertainties are large and significant to a demonstration of compliance. More than one conceptual model can explain, and be consistent with, the available data. Technicaljudgments are required to assess whether bounding assumptions or calculations are appropriately conservative. NRC(1996) also summarize a series of technical positions and procedures conceming the use of expert elicitation in demonstrating compliance with geologic repository disposal regulations. These procedures emphasize the need for detailed documentation during the elicitation and for transparency in the aggregation of multiple expert's judgments. An elicitailon also should provide a means to evaluate new data that may arise between completion of the elicitation and submittal of licensing documents (NRC,1996). DOE used expert judgement to arrive at a probability value for igneous activity at the repository site (Geomatrix,1996). Although the report generally followed the NRC Branch Technical Position (BTP) regarding expert elicitation (NRC,1996), several areas of weakness in the 78 elicitation procedure were noted in the September,1996 Appendix 7 meeting with DOE: Criteria and procedures for incorporating new data into the existing elicitation need to be established and published. Central issues need to be deconvoluted as much as possible, so that standard definitions of terms can be used consistently throughout the elicitation. Greater balance is needed on the panel to encompass a wider range of viewpoints, along with more thorough documentation of the selection processes and potential conflicts of interest for panel members. Intermediate judgments of the experts after the elicitation and any changes of rationales need to be documented. Following the Appendix 7 meeting, NRC concluded that the elicitation (Geomatrix,1996) is generally consistent with the BTP regarding the conduct of an expert elicitation. NRC will, thus, give the elicitation the appropriate level of consideration in the review of licensing documents (Bell,1997). Staff have performed a technical review of the PVHA elicitation report (Geomatrix,1996) and, as explained in previous sections of this report, have several technical concems regarding the PVHA results and their application in the Yucca Mountain program. The most significant concem is that many of the models in the PVHA are critically dependent on the definition of volcanic source-zones. Many of the source-zone models bypass the proposed repository site due to a lack of previou.s igneous activity at the site (Geomatrix,1996). Although some geological data appear to suggest such division, critical analyses reveal that these apparent divisions are only manifestations of surficial features and not important to deeper structural control of volcanism (e.g., Stamatakos, et al.,1997b). In addition, larger-scale geologic features that commonly affect the localization of basaltic igneous activity are remarkably similar between the proposed repository site and the locations of past igneous activity. Based on these geologic relationships, staff conclude that volcanic source-zones that fail to include the proposed repository site are not reasonably conservative. According to Geomatrix (1996) mean annual probability of repository disruption is 1.5 x 10-e r 4 l yr. This is, however, a combined probability for both volcanic and igneous events. Utilizing the source zone models that preclude volcanoes from forming at the repository site, as was done repeatedly in Geomatrix,1996, requires that the actual probability of volcanic disruption based on this methodology is necessarily lower than 1.5 x 10.a yr1. A rough estimate is that the mean PVHA probability for volcanic disruption may be an order of magnitude lower than the combined probability for all classes of igneous events. In order to use probability estimates in performance assessment they must, in some way, be separated into volcanic and intrusive events. DOE is planning additional analysis in this area (TRW,1997). 4.1.8.4 Summary There are no generally-accepted methodologies for calculating the probabilities of future igneous activity in distributed volcanic fields over periods of 10,000 yr. In addition, more than one h conceptual model can be applied to this problem, resulting in a wide range of probability values. DOE is using expert elicitation (Geomatrix,1996) to evaluate a range of probability models, 79 i II estimate uncertainties in moael results due to reasonable variations in model parameters, and determine a probability distribution for use in performance assessment models. 4.1.9 Probability Criterion 9 4.1.9.1 Acceptance Critorion Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: The collection, documentation, and development of data and models has g been performed under acceptable QA procedures, or if data was not 3 collected under an established QA program, it has been qualified under appropriate QA procedures. 4.1.9.2 Review Method NRC will attend, as observers, DOE-conducted QA audits of program participants who are involved in technicalinvestigations related to igneous activity. NRC will also track the progress made in resolving deficiencies and nonconformities in the program that arise from QA audits and g independent review of DOE products. E 4.1.9.3 Technical Basis 4 Both DOE and NRC have approved QA programs for technical investigations conducted by their respective agencies and contractors, and NRC has stated that the quality of data will be acceptable if the c'ata are developed under an approved QA program (Nuclear Regulatory E Commission,1997). These QA programs detail the procedures necessary to collect, document, E and develop data and models in an acceptable manner. Periodic technical audits are conducted by DOE and NRC to ensure that appropriate QA procedures are implemented in technical 3 investigations. These audits are usually attended by observers from each agency, who provide g an independent assessment of audit effectiveness and conclusions. Independent, technical evaluation is still warranted for many data collected under an approved QA program. One common area of concem is reconciling new data with previously-published data, which is a problem that may not be apparent during a routine QA audit. For example, many of the He dates in Crowe, et al. (1995) were intemally inconsistent, in addition to contradicting, previously-published values for the same geological units (Hill,1995). Similar discrepancies were noted in the September 1996 Los Alamos National Laboratories (LANL) QA audit (e.g., Austin,1996). Discrepancies between new and previously-published data will need to be reconciled in order to provide a solid technical basis for evaluating licensing documents. Site characterization activities have produced an abundance of data on YMR basaltic volcanoes. In addition to DOE, the State of Nevada, the U.S. Geological Survey, the CNWRA and NRC have conducted independent geologicalinvestigations in the YMR. Each of these organizations operate under different QA programs. Other researchers associated with universities and national labs also conduct high-quality investigations in the YMR, with varying degrees of formal QA programs. Many of these data are clearly important to licensing issues and must be considered during review of DOE licensing documents. As part of the license application, DOE willlikely qualify many of these extemally-produced data. Qualification procedures for these extemally-produced data include production under a QA program equivalent to U.S. Nuclear 80 l [ I Regulatory Commission (1997), publication through the peer-review process. independent corroboration, or confirmatory testing. 4.1.9.4 Summary ' Staff have participated in recent QA audits of DOE and its contractors (e.g., Austin,1996) and provided numerous reviews of DOE study plans and contractor reports (e.g., Connor, et al., 1993). NRC staff continue to monitor DOE QA activities related to the IA KTl and disposition of { QA deficiencies from the September,1996, LANL QA audit. Staff have concluded that the LANL QA performance was " marginally effective" and will continue to monitor the DOE /LANL QA program (Austin,1996). As a result of this audit, the DOE audit team concluded that the LANL's ( - QA performance was " marginally effective." The NRC agreed with this finding, and the NRC L concems identified during this audit were deferred to the appropriate DOE deficiency reports (YM-96-D 105 to 108). NRC recently reviewed the remedial actions proposed for these. deficiencies and determined that the proposed actions appeared appropriate. Review of the ( associated Volcanism Synthesis Report is needed to determine if these actions have been effectively carried out, and if the concems have been resolved. [- Staff have conducted independent technical investigations in igneous activity to: (i) evaluate DOE data and models likely contained in licensing documents; (ii) develop and test attemative hypotheses to those proposed by DOE; (iii) evaluate relevant data and models proposed by other [ agencies, such as the State of Nevada; and (iv) reduce uncertainties in models of repository performance. The results of these investigations have been presented in numerous CNWRA reports and peer-reviewed joumal articles, many of which are cited in preceding sections of this report. As part of these investigations, staff have compiled all relevant data on the age and location of YMR basaltic igneous features younger than about 11 Ma (Appendix A). These data form the basis for probability models and review of appropriate DOE licensing documents. Staff will continue to evaluate data in the peer-reviewed literature and products from other agencies, in [ addition to data produced by DOE and its contractors. Although the DOE Volcanism Synthesis Report was not issued in FY97, data presented in Appendix A are used in this IRSR to resolve the igneous activity probability subissue. 4.2 CONSEQUENCES The DOE will need to estimate the dose consequences ofigneous activity affecting the performance of the proposed repository. Basaltic igneous systems exhibit a wide range of physical characteristics that must be interpreted from sparse, often poorly-preserved geologic features in the YMR. In addition, the interactions of basaltic magma with the geologic repository h system have no known analog. Dose calculations will require significant extrapolation of igneous process models to the disturbed geologic setting of the repository and to potential interactions with the engineered barrier systems. Staff will review DOE assumptions and models used to [_- estimate the effects of volcanic eruptions and igneous intrusions for consistency with past igneous activity in the YMR and with pmcesses observed at historically-active volcanoes analogous to those in the YMR. Statt also will determine if the dose analyses have been {' performed in a way such that the effects of igneous activity have not been underestimated. The following seven acceptance criteria apply directly to assessing the consequences of igneous hazards. h Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive warte repository will be acceptable provided that: 81 (1) The models are consistent with the geologic record of basaltic igneous activity within the YMR. (2) The models are verified against igneous processes observed at active or recently-active analog igneous systems and reflect the fundamental details of ash-plume dynamics. (3) The models adequately account for changes in magma ascent characteristics and magma / rock interactions brought about by repository construction. (4) The models account for the interactions of basaltic magma with engineered barriers and waste forms. (5) The parameters are constrained by data from YMR igneous features and from appropriate analog systems such that the effects of igneous activity on waste containment and isolation are not underestimated. (6) If used, expert elicitations were conducted and documented, using the guidance in the Branch Techn;:al Position on Expert Elicitation (NRC, 19%), or other acceptable approaches. (7) The collection, documentation, and development of data and models have been performed under acceptable QA procedures, or if data was not collected under an established QA program, it has been qualified under appropriate QA procedures. These criteria address: (1) the characteristics of basaltic volcanic eruptions that would be expected in the YMR; (2) the dynamics of the eruptive column; (3) the effects of the repository on the eruption characteristics; (4) waste package / waste form-magma interactions; and (5) 3' important parameters necessary to allow reasonable dose conversion models to be E' implemented, along with necessary programmatic concems (6 and 7). 4.2.1 Consequences Criterion 1 4.2.1.1 Acceptance Criterion I Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: The models are consistent with the geologic record of basaltic igneous activity within the YMR. 4.2.1.2 Review Methods Staff should determine the adequacy and sufficiency of DOE characterization and documentation g of past YMR igneous activity, including uncertainties about the interpreted characteristics of past g activity, such that reasonable projections can be made of the expected characteristics of potential future eruptions in the YMR. Particular emphasis will be placed on igneous processes that directly affect the ability of igneous events to disrupt and transport HLW into the accessible 82 [ environment. Models will need to address the apparent changes in disruption potential for YMR igneous events since approximately 4-5 Ma. 4.2.1.3 Technical Basis This criterion outlines staff's current understanding of the range of physical processes represented by the basaltic igneous systems in the YMR. Because most of these basaltic systems are poorly preserved or exposed in the YMR, igneous processes important to performance must be interpreted from sparse data. Within these limitations, however, staff conclude that the character of past YMR igneous activity represents the most conservative bounds on future YMR activity, in order to test performance models for consistency with past YMR basaltic igneous activity, staff must develop an independent technical evaluation of the range of important processes represented by existing YMR basaltic igneous systems. A detailed - technical basis for this criterion will be developed in subsequent revisions. ( Basaltic igneous activity in the YMR since around 8 Ma has encompassed a wide range of processes that effect different implications for repository performance. Many of these processes are interpreted from a sparse, poorly-preserved geologic record, especially for basaltic centers [ older than about 4 Ma. Observations at some older YMR centers, in addition to historically-active basaltic volcanoes, indicate that low-energy, low-dispersivity eruptions have limited potential to disperse HLW to critical group locations. Such volcanoes commonly are referred to as hawaiian [ orlow-energy strombolian style and are characterized by small volumes of subsurface disruption, low eruption velocities, and limited dispersal of tephra (e.g., Walker,1993). The youngest YMR volcanoes and many analogous historically-active cinder cones, however, clearly had relatively high-energy, high-dispersivity eruptions with the potential to disperse HLW to proposed critical group locations (Connor,1993; Hill and Connor,1995; Hill et al.,1995; Hill,1996). These eruptions are commonly referred to as violent strombolian style and are characterized by relatively large volumes of subsurface disruption, high eruption velocities, and extensive dispersal of tephra (e.g., Blackbum, et al.,1976; Walker,1993). Acceptable consequence models will examine in detail the characteristics of violent strombolian basaltic volcanoes, as these eruption styles present the greatest potential hazard to inhabitants located tens of { kilometers away from the proposed site. For example, several features at Lathrop Wells and Little Black Peak volcanoes indicate a violent strombolian eruption style. First, these volcanoes have unusually high subsurface rock-fragment [. abundances relative to other Quatemary YMR volcanoes and other basaltic volcanoes in the westem Basin and Range. Rock fragments <1 mm average around 1 volume percent at Lathrop Wells (Crowe, et al.,1986). As explained in Section 4.2.3.5.1, millimeter-to-decimeter diameter xenoliths at Lathrop Wells average 0.9 volume percent. Larger rock fragments also appear to be about 0.5 percent at Little Black Peak. In contrast, other typical Basin and Range basaltic volcanoes have less than 0.01 volume percent rock fragments (e.g., Valentine and Groves, 1996). Second, juvenile cone scoria at Lathrop Wells and, to a lesser extent, Little Black Peak consists of angular, broken pieces of larger fragments that were cool on impact with the cone slope. Typically, cinder cone eruptions do not eject material high enough to cool sufficiently to permit brittle fragmentation (e.g., Walker and Croasdale,1972) whereas violent strombolian eruptions do. Finally, a common strombolian cinder cone feature is beds of agglutinated tephra that accumulated at temperatures high enough to deform plastically and form highly cohesive beds (e.g., Walker,1993). Lathrop Wells and, to a lesser extent, Little Black Peak consist of loose, nonagglutinated tephra, indicating that these eruptions were more explosive than typical strombolian basaltic volcanoes. Relative to other Quatemary YMR volcanoes, Hidden Cone and 83 G I! the Little Cones also show scoria fragmentation and agglutination characteristics representative of periodically-sustained eruption columns and may have had periods of violent strombolian activity. Lathrop Wells was an unusually explosive basaltic volcanic eruption, as evidenced by anomalously high rock-fragment abundances and loose accumulations of broken tephra. To a lesser degree, Little Black Peak also was more explosive than typical strombolian cinder cone eruptions. Remnants of the latest, most potentially-disruptive stage of these eruptions, however, are only preserved on the cone flanks. Erosion has removed the upper several meters of the Lathrop Wells tephra-fall deposits (e.g., Crowe, et al.,1995), whereas fall deposits have been completely eroded at Little Black Peak. As documented in Hill (1996), xenolith breccias indicated that late-stage disruption events likely occurred at Lathrop Wells volcano and possibly at Little Black Peak, analogous to those that occurred during the 1975 Tolbachik eruption (Budnikov, et g al.,1983; Doubik,1997). As outlined in Section 4.2.3.5.1 and in Hill (1996), these late-stage ' g events are a previously undocumented feature of violent strombolian eruptions and have the potential to widen the subsurface conduit to many tens of meters in diameter. 4.2.1.4 Summary ] The physical volcanology of YMR basaltic volcanoes is varied but indicates that violent strombolian activity was common and appears characteristic of the most recent eruptions. Thus, models of volcanic eruption through the proposed repository need to encompass dose-estimates j resulting from this style of volcanic activity. 4.2.2 Consequences Criterion 2 4.2.2.1 Acceptance Criterion Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: j The models are verified against igneous processes observed at active or recently active analog igneous systems and reflect the fundamental details of ash-plume dynamics. I 4.2.2.2 Review Methods Because many of the igneous processes important for consequence evaluation are not preserved in the YMR geologic record, proposed process-level consequence models should be verified with data from reasonably analogous small-volume basaltic volcanic systems in order to be acceptable. Staff will evaluate the effectiveness of proposed eruption and dispersion models in quantifying transport mechanics at basaltic violent-strombolian volcanoes. Staff will compare proposed models with igneous processes and deposits documented for reasonably analogous eruptions, including but not limited to the 1975 Tolbachik, Russia, 1943-52 Paricutin, Mexico, E and 1850-1995 Cerro Negro, Nicaragua, violent strombolian eruptions. Staff also will evaluate 5 the effectiveness of proposed models in quantifying HLW transport based on the physics of proposed models. 4.2.2.3 Technical Basis Acceptable estimates of radiological dose and risk associated with volcanic eruptions through the 84 YM repository depend on numerical models of HLW transport upward in a volcanic tephra column, advection and dispersion of HLW with volcanic ash in the atmosphere, and deposition of HLW in the tephra deposit at a critical group location. The accuracy of these estimates depends on capturing fundamental details of volcanic ash-plume dynamics (e.g., Sparks,1986; Sparks, et al.,1997), of which there are numerous historical examples from basaltic cinder cone eruptions (Figure 30). Models of volcanic tephra eruptions range from simplistic models that can capture the general pattem of tephra dispersion without attempting to portray the physics of volcanic columns accurately (e.g., Suzuki,1983), to thermo-fluid-dynamic models of eruption columns and particle advection and dispersion (e.g., Woods and Bursik,1991; Sparks, et al.,1992; Woods,1993; 1995; Sparks, et al.,1997). These latter models make a convincing case that accurate, quantitative descriptions of tephra deposition at the ground surface result from application of physically accurate models. Thus, although computationally complex, these models can likely provide insight into the behavior of HLW in the eruption column despite the very different physical properties of HLW relative to basaltic tephra. These same arguments for physical detail extend to the sedimentation of tephra and HLW out of l the atmosphere. For example, Bonadonna, et al., (1998) have shown that particle Reynolds l number plays a critical role in partic!e settling velocity and, as a result, the particle-size density distributions in the resulting tephra deposit. One of the first attempts to quantify the dispersion of tephra in volcanic eruptions was by Suzuki (1983). Suzuki's model has been modified and I applied to volcanic eruptions by Glaze and Self (1991) and Hill, et al. (1998), and applied to the transport of HLW during volcanic eruptions by Jarzemba (1997). In the Suzuki model, the erupting column is treated as a line source reaching some maximum height governed by the I enerpy and mass flow of the eruption. A linear decrease in the upward velocity of particles is assumed, resulting in segregation of tephra or tephra and waste particles in the ascending column by settling velocity, which is a function of particle size, shape, and density. Particles are I removed from the column based on their settling velocity, the upward decrease in velocity of the column as a function of height, and a probability density function that attempts to capture some of the natural variations in the parameters goveming particle diffusion out of the column. Dispersion of the ash diffused out of the column is modeled for a uniform wind-field and is I govemed by the diffusion-advection equation with vertical settling. The Suzuki (1983) model does not attempt to quantify the thermo-fluid-dynamics of volcanic I eruptions. The more recent class of models, pioneered by Woods (1988), concentrates on the bulk thermophysical properties of the column, defining a gas thrust region near the vent and a convective region above, within which the thermal contrast between the atmosphere and the I rising column results in the entrainment of air and buoyancy forces loft particles upward. In contrast to Suzuki (1983), this class of models results in a highly nonlinear velocity profile within the ascending column. This difference can have a profound effect on the ascent height of HLW particles in an ascending eruption column and ensuing dispersion in the accessible environment. I Woods (1988; 1995) developed the following method of modeling the physical state for the eruption column. Vertical flux of materialin the rising column is given by nt'up, where u, L, and are column velocity, column radius, and column bulk density, respectively. Air is entrained in the column based on an entrainment coefficient, c (typically equal to 0.1), and the surface area of the column. In the steady-state, conservation of mass in the gas-thrust region of the eruption column is given by d(ul2p) "7 [ { 8 (40) 85 W Q I 2-k:' ~ h. l s. 4 s I 1947 Paricutin, Mexico wwyn 1975 Tolbachik, Russia g: 3 ? a. _p _.A n c sa. t., .g A '"w. g+'y.;

(

x Y >:5,, ,M k I .... ' ~. _,..<lin : yy h% y I 1968 Cerro Negro 1995 Cerro Negro, Nicaragua Figure 30. Ash columns on erupting cinder cones vary from strong vertical columns with sustained gas-thrust regions and little deflection by the wind (e.g.,1947 Paricutin (McGregor and Abston,1992),1975 Tolbachik, and 1968 Cerro Negro), to weaker plumes with little or no gas-thrust region above the vent and that bend easily in the wind (e.g., 1995 Cerro Negro). Models that estimate the consequences of eruptions through the proposed repository need to quantify these varying styles of activity. 86 II where a is the ambient air density and z is vertical distance above the vent. In the convective region of the eruption column, conservation of mass is expressed as:

0) = 2 caul (41)

This formulation does not account for the loss of large particles from the plume that have settling velocities greater than the upward velocity of the plume or are ejected as projectiles from the margins of the column. Woods (1988,1995) casts the conservation of momentum equation for buoyantly rising volcanic columns as: ) = (a - p)gL 2 (42) where g is gravitational acceleration, and conservation of energy as: !(C,0puL2) = C,T!(puL ) + $!(puL ) - aul g 2& (43) where: C, = C, + (C,, - C,) I _ (44) ~ ") Tis air temperature, C,is the bulk specific heat of the gas column (magmatic gas + entrained air + pyroclasts), 6is the temperature of the column, C, is the specific heat of air, C,,is the specific heat of the magma, n is the gas mass.. fraction in the column, and n,is the gas mass-fraction in the column at the vent. Bulk density of the ascending column is: 1 nR 0 - = (1 - n) 1 + (45) p a P where ois the pyroclast density, Pis atmospheric pressure, and R,is the molecular weight of the bulk gas in the eruption column multiplied by the gas constant. The gas mass fraction is in tum given by n = 1 + (n, - 1) (L,2,opo) (46) L'up where L, u,, and #, are the initial vent radius, velocity, and bulk column density at the vent and - ~" R, = R, + (R,, - R,) (47) 1 - n,, n __where R,, and R, are the products of the gas constant and the molecular weight of gas in the gruption column at the vent and air, respectively. These equations can be recast in terms of three variables, here called M1, M2, and M3, and the three coupled differential equations can be solved numerically for a given set of initial conditions. In the gas thrust region: M1 = ul2 (48) 87 L 2 2 M2 = u 7 p (49) M3 = C,0puL 2 (50) I M (51) = i dW2 = (a - p)gL 2 g (52) i "I C,T + " - aul'g (53) = d: 2, d: where: M2 u= (54) y and M3 l 0=& (55) P n = 1 + (n, - 1 ) (L,'u,p,) (56) M1 in the convective region of the column: i dW1 = 2 caul (57) d: and the other equations remain unchanged. As an examp!e using the initial conditions and constants from Table 1, the gas-thrust region extends to approximately 150 m above the vent. At this point, 6= 921 *K, n = 0.31, u = 58.7 m s, and L = 75 m. The plume then becomes buoyant above 150 m and rises to a column height of approximately 4.5 km. At about 4 km, the radius of the eruption column begins to increase rapidly to L > 1 km, and the upward velocity of the column begins to decrease rapidly (Figure 31) as the column reaches neutral buoyancy. Thus, these initial conditions and g parameter distributions yield a column height appropriate for the sustained column during a E violent strombolian eruption (Figure 30). Total rise time in the plume (Rr) is calculated as: 88 Table 1. Example initial conditions and constants for eruption column model. Initial Conditions Example Value Units Explanation n, 0.01 dimensionless mass fraction of gas at vent L, 10 m vent radius u, 50 ms' velocity at vent 6, 1100 K temperature of the column at vent C, 1617 J kg-' K-' heat capacity of column at vent R, 462 J K kg-' molecular weight of gas in eruption column at vent x gas constant Constants ) a 1000 kg m-3 density of solid pyroclasts P 10000 Pa air pressure t 293 K air temperature C, 998 J kg-' K-heat capacity of air (D<a) (u(r)-0) I I fU f R= d: + d: {58) 7 pe(:) Uw,,,(Z ) m

20 ~} _~ E ca k 10 ; gas thrust region p 0 ~ >l ''I il ~ 0 1000 2000 3000 4000 Height (m) l; i 1000 (b) 800 E 600 v u> .2 V 400 ce ~ 200 l

• I

l 'i'l 0 O 1000 2000 3000 4000 Height (m) Figure 31. (a) Vertical velocity of particles in the volcanic column and (b) change in the column radius as a function of height for a violent strombolian eruption, based on the g parameters in Table 1. E 90 result than predicted from Suzuki (1983), who estimates the height at which material diffuses out of the column as a simple function of the particle settling velocity. Hence, the Suzuki (1983) model predicts that dense HLW particles will tend to be " released" from the eruption column at comparatively low altitudes, resulting in comparatively lower dispersion. In contrast, the thermo-fluid-dynamic model tends to transport HLW and HLW-laden particles to higher altitudes, resulting in wider dispersion of this material. The difference between these models will become more pronounced at higher eruption velocities. Furthermore, parameters like bulk density (Eq.

45) of the column can be modified to specifically examine the dispersion of HLW. Differences between these models may significantly affect dose calculated at critical group locations 20 km from the proposed repository site.

i Less energetic stages of a cinder cone-forming eruption produce weak plurnes that bend over.as ) they rise due to advoction by wind (Figure 30). Sparks, et al., (1997) note that these weak plumes can remain highly organized as they are advected downwind. Such plumes can form convection cells or retain a puffy character with little entrainment and mixing with air. Thus, j sedimentation out of these plumes may be slower than expected using the diffusion-advection cquation. For example, although the 1995 eruption of Cerro Negro (Figure 30) produced a relatively small volume of tephra (3x10' m ) in a column that rose to only 2-2.5 km, ash-fall j 3 deposits 20 km downwind were 0.5 cm (Hill, et al.,1998). Eruptions of this magnitude are capable of effectig peak annual TEDE's on the order of roms for critical groups located 20 km from a repository-penetrating volcanic eruption. Clearty, reasonably-conservative consequence cnalyses will need to evaluate dose from large, convective eruptions that ascend to atmospheric levels of neutral buoyancy as well as smaller eruptions with column ascent limited by prevailing winds. 4.2.2.4 Summary Basaltic eruptions that build cinder cones evince dramatic variations in energy, duration, and style. Numerical models that quantify the physics of these eruptions have reached a stage of development that allows exploration of the parameters goveming this variation. Thus, many of the nuances of observed eruption columns and their deposits can now be understood in terms of fundamental physical processes (e.g., Sparks, et al.,1997). Such an understanding is critical for volcanic risk assessment related to the proposed YM repository because there are no observations of the behavior of very dense HLW particles in eruption columns. There also is considerable uncertainty in how to simulate the entrainment and dispersal of HLW in these columns. Physically-accurate eruption column models provide an opportunity to extend our understanding of tephra plumes to encompass the distribution and deposition of dense HLW particles in tephra deposits. In these circumstances, application of physically-accurate models is a fundamental step in stochastic modeling of dose and risk to a critical group. 4.2.3 Consequences Criterion 3 4.2.3.1 Acceptance Criterion Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: The models adequately account for changes in magma ascent characteristics and magma / rock interactions brought about by repository construction. 91 ) 4.2.3.2 Review Methods Staff will determine if DOE has accounted for significant changes in igneous processes that are effected by construction of the subsurface geologic repository and emplacement of HLW. As there are no known analogs to ascending basaltic magma intersecting a relatively large tunnel network at 300-m below the surface, acceptance will necessarily be evaluated through review of physical process models. Acceptable models will evaluate the influences of rock-stress redistribution induced by drift emplacement on the ascent characteristics of basaltic magma. Acceptable models also will evaluate decompression effects on magma encountering the relatively free-surface of the drifts while under 300-m lithostatic confining pressures. As rock-strength characteristics can change with temperature, acceptrble models also may need to evaluate how igneous processes may change if significant wall-rock heating is induced through HLW emplacement. 4.2.3.3 Technical Basis This criterion outlines how repository construction can potentially interact with and modify the characteristics of the volcanic eruption. Construction and the effects of the repository will cause stress redistribution associated with drift free-surface effects and possibly thermal effects on rock strength associated with waste emplacement. Detailed technical bases for this enterion will be I developed in subsequent IRSR revisions. m Basaltic intrusion propagation is largely controlled by the distribution of stress in the shallow (i.e., g <10 km) crust (e.g., Delaney, et al.,1986). The emplacement of 5-to 10-m diameter drifts at g 300-m depths represents a free surface that will likely affect the distribution of crustal stress for some distance around the drifts. The upward ascent of basaltic magma may be affected by this stress redistribution, resulting in ascent characteristics that are not reasonably analogous with magma ascent in undisturbed geologic settings. Lateralintrusion propagation also may be l affected by this stress redistribution, which affects the area disrupted by an igneous event. In addition to stress redistribution, the repository drifts represent free surfaces where lithostatic confining pressure is zero. Ascending basaltic magma, which contains dissolved volatiles, will i be under roughly 100 kPa lithostatic confining pressure when it encounters the drifts. Nonequilibrium decompression will ensue, resulting in rapid volatile exsolution (e.g., Connor and Hill,1993b). Although the magnitude and consequences of this rapid exsolution have not yet been modeled, volatile expansion and magma fragmentation are often rela'.ed to conduit erosion and wall-rock entrainment (i.e., Macedonio, et al,1994; Valentine and Groves,1996). At present it is uncertain as to how the ascending magma will respond to the changes in stress characteristics brought about by construction of the repository. This is an area where additional studies are needed. 4.2.3.4 Summary Emplacement of the repository potentially affects the shallow subsurface ascent of magma. These effects include change in the depth of volatile exsolution, resulting in potential changes in B eruption style, and changes in intrusion geometry. A technical evaluation of these effects has g not yet been developed. I 92 I 4.2.4 Consequences Criterion 4 4.2.4.1 Acceptance Criterion Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: i The models account for the interactions of basaltic magma with engineered barriers and waste forms. i 4.2.4.2 Review Methods i Staff will evaluate models that account for the behavior of engineered barriers and HLW under basaltic magmatic conditions. These models will need to represent physical conditions that generally exceed the design conditions for these systems. Models that credit engineered barriers with HLW containment during igneous disruption will need to be supported by analyses that cxplicitly consider the physical conditions associated with basaltic igneous activity characteristic cf the YMR. Although no known physical analogs for these models exist, acceptable models j need to address the physical behavior of geologic materials and man-made materials during basaltic igneous events. Staff also will need to evaluate models of HLW behavior during igneous j svents, for which there is little information available. Acceptable models will address how the i thermal, physical, and chemical effects of igneous activity will likely affect HLW form. i 4.2.4.3 Technical Basis DOE performance assessments have all assumed that the waste package fails upon contact with basaltic magma (Link, et al.,1982; Bamard, et al.,1992; Barr, et al.,1993; Wilson, et al.,1994). The general physical characteristics of basaltic magma exceed the design criteria commonly cpplied to HLW emplacement canisters, such that canister failure appears to be a reasonable, though conservative, initial assumption. For example, basaltic magma in the YMR has an initial I temperature of around 1100 *C (i.e., Vaniman, et al.,1982; Knutson and Green,1975). I Assuming no extemal stress, such as that induced by magma flow,2.5Cr-1Mo steel will fail through intergranular creep rupture alone at these temperatures at time scales equivalent to the duration of historical basaltic volcanic eruptions (Fields, et al.,1980; Viswanathan,1989). Ascending basaltic magma also has a nonvesiculated density around 2600 kg m-3 and likely impacts the HLW canister between 1-100 m s", creating significant extemal stress that will cnhance failure through ductile fracturing (e.g., Ashby, et al.,1979). In addition, basaltic magmatic oxygen fugacities commonly are 10 log units below atmospheric conditions (e.g., Carmichael and Ghiorso,1990), which may affect Fe.2 p 3 and Ni/NiO phase relationships in the f canister. In addition, basaltic magmas may contain around 0.1 weight percent sulfur, which is readily degassed from the magma at low pressures (e.g., Carroll and Webster,1994) and likely will affect nickel and chrome alloy phase relationships. A HLW canister failure thus appears reasonably likely for canisters directly intersected by a volcanic conduit. Canisters in contact with basaltic magma introduced through dikes and intradrift lavas may also fail, although thermal and mechanical loads are much lower than those encountered in the volcanic conduit area. In addition to affecting the emplacement canister, the physical conditions associated with . cscending basaltic magma will likely affect HLW form. This is important because particle size will directly affect how HLW is incorporated and dispersed during a volcanic event. Particle size clso will determine the dosimetry effected through inhalation of contaminated tephra and discrete 93 HLW partic!es. The high temperature, reducing conditions associated with basaltic magma will likely result in a reduction in spent fuel particle-size through fracturing along grain boundaries and transgranular fracturing (e.g., Ayer, et al.,1988; Einzinger,1994; Einzinger and Buchanen,1988). As magma fragments during ascent, particle size will be decreased further through shear induced by conduit flow and volatile expansion. Cooling and atmospheric mixing will occur rapidly in the column (e.g., Thomas and Sparks,1992), inducing additional thermal and chemical stress on the waste particles. These rapid and relatively large changes in temperature and oxygen fugacity also will likely affect the oxidation state of the HLW, which can affect the mobility of actinide elements at surficial conditions. Process models that calculate the dose consequences of igneous activity will need to account for how the physical conditions of a volcanic eruption affect HLW form. During the 1960s, the U.S. govemment developed nuclear-power rocket engines that operated at temperatures comparable to basaltic igneous events (500-1500 C). Literature from this program was reviewed to determine if there was reasonable analogy with potential HLW behavior during igneous events. The nuclear rocket engines used a reactor core consisting of hollow hexagonal tubes made from 1-7 percent UO -Y O -ZrO fuel in a ceramicized BeO matrix 2 2 3 2 (Cahoon, et al.,1962). Although these tubes were stable at pressures of 342 psi and temperatures of 1454 C (Lorence,1973), they do not appear chemically or mechanically analogous to HLW potentially exposed to basaltic magma. Spent reactor-fuel pellets consist of Bl 100 percent UO and associated fission products and are formed from pressed powders having 3l 2 initial particle sizes around 1 pm. They lack a BeO matrix and are not ceramicized, both of which will enhance high-temperature stability significantly. Behavior of nuclear-rocket fuel during g engine operation, thus, does not appear reasonably analogous to behavior of HLW during g igneous disruptive events. While the staff is seeking other potentially-analogous information, satisfactory resolution of this criterion may depend totally on modeling. 4.2.4.4 Summary Preliminary information suggests that the waste package will not be an effective deterrent to the E' transport and dispersion of HLW during volcanic eruptions. Additional analyses of waste E package behavior at high temperature and high mechanical loads may provide new insights, however, a reasonably-conservative interpretation of available data is that the waste package 3' fails during a volcanic eruption. Current analysis also suggests that HLW particle fragmentation g will occur during a volcanic eruption, further reducing the average HLW particle size. 4.2.5 Consequences Criterion 5 4.2.5.1 Acceptance Criterion Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: The parameters are constrained by data from YMR igneous features and from appropriate analog systems such that the effects of igneous activity on waste containment are not underestimated. 4.2.5.2 Review Method Staff will review parameters used in DOE performance models for consistency with the range of 94 { characteristics interpreted for YMR basaltic igneous systems. Because many important parameters cannot be derived directly from YMR igneous systems, staff also will compare r- { proposed parameters values with parameters measured directly at reasonably analogous, ' historically active basaltic igneous systems. 4.2.5.3 Technical Basis Basaltic igneous activity encompasses a wide range of characteristics that effect significantly different dose consequences. Low-energy basaltic eruptions, such as those characteristic of oceanic island volcanism, have limited ability to entrain subsurface material or transport entrained material more than several kilometers from the vent. High-energy basaltic eruptions [ characteristic of are volcanism, however, are capable of entraining and transporting significant 1 amounts of subsurface material. Eroded basaltic volcanoes in the YMR are interpreted to represent a wide range of eruption styles (e.g., Crowe, et al., 1983,1995; Hill and Connor,1995; Hill,1996). As outlined in Section 4.2.1.3, however, the youngest YMR volcanoes have deposits characteristic of highly dispersive, violent strombolian eruptions. This eruption style provides a reasonably-conservative estimate of the type of eruption most likely to occur during potential future periods of YMR basaltic volcanic activity. Igneous features of the YMR provide the best possible basis to derive parameters used to calculate the dose consequences of igneous activity on repository performance. Many important model parameters, however, can only be derived accurately from erupting or recently-erupted basaltic volcanoes. For these parameters, data from reasonably analogous basaltic igneous systems will provide an acceptable basis to derive parameter values. The dose consequences of igneous activity currently are evaluated using the NRC TPA code version 3.1.3, as outlined in Section 3.4 of this IRSR. For the TPA code, parameters can be generally classified as those affecting subsurface disruption, waste transport, and dose [ conversion. Key parameters for TPA 3.1.3 and the current technical basis for these parameters are discussed in the following sections. { 4.2.5.3.1 Subsurface Disruption The diameter of the volcanic conduit controls the amount of HLW available for transport. {. Conduits for <5 Ma YMR volcanoes are only exposed to depths of several dekameters, which will not accurately represent conduit diameters at 300-m depths. Conduit diameters can be estimated, however, through the volume of shallow wall-rock xenoliths erupted. Xenoliths <0.7 mm in diameter at Lathrop Wells volcano average around 1 volume percent for ( nonhydromagmatic facies (Crowe, et al.,1986). Staff recently evaluated millimeter-to-decimeter diameter xenolith abundances at Lathrop Wells volcano using image analysis methods. For 17 exposures, each encompassing about 1 m, millimeter-to-decimeter diameter xenoliths at 2 [ Lathrop Wells average 0.9*0.6 volume percent. Most of these xenoliths are derived from Miocene tuffs, which have an estimated thickness of around 500 m beneath Lathrop Wells volcano (Swedley and Carr,1987). The Lathrop Wells volcano also is characterized by relatively [ fragmented cone scoria and lacks significant agglutinate beds, indicating a relatively high-energy t eruption (e.g., Hill,1996). Historically-active basaltic volcanoes with cone and tephra-fall characteristics similar to Lathrop Wells have tephra-fall deposits roughly twice the volume of the cone (Segerstrom,1950; Booth, et al.,1978; Budnikov, et al.,1983; Amos,1986; Hill, et al., h 1998). By analogy, tephra-fall deposits at Lathrop Wells volcano were likely twice the cone volume. Lathrop Wells volcano, thus, produced around 7.2x10 m' d k@m @W $ M We 7 95 + I i percent was likely composed of tuffaceous xenoliths. Assuming the conduit was cylindrical and Ii the xenoliths were derived from s500 m, this volume corresponds to a 40-m diameter conduit beneath Lathrop Wells volcano. In comparison,1975 Tolbachik Cone 1 produced a 49i7-m 3 diameter conduit during late-stage disruption (Hill,1996). 5 Only sparse and incomplete exposures of tephra-fall remain for Lathrop Wells volcano, which is g! the youngest and best-preserved YMR volcano (Hill, et al.,1995). With the exception of eroded 5' tephra-fall remnants that occasionally crop out beneath Pliocene lavas and in fault trenches located in and around Crater Flat, tephra-fall deposits have been eroded from other Miocene and younger YMR volcanoes. Tephra-fall volumes for Quatemary YMR volcanoes, however, can be estimated by comparison with fall: cone and cone: lava volume-ratios for well-preserved young basaltic volcanoes. These data are summarized in Table 2. Violent strombolian volcanoes have tephra-fall deposit volumes roughly twice that of the volcanic cone, whereas less energetic E strombolian cones have roughly equivalent tephra-fall and cone volumes. These relationships E are used to estimate fall volumes for Quatemary YMR volcanoes (Table 2). Note that cone: lava ratios for YMR Quatemary volcanoes also encompass the same range as historically-active g analog volcanoes (Table 2). Using an estimated DRE tephra-fall volume of 2.2x10' m' for g Lathrop Wells, an average mass-flow rate of 25 m sd (Table 3), and the relationships in Wilson, et al., (1978) and Walker, et al. (1984), the main tephra-producing phase of the Lathrop Wells eruption lasted roughly 10 days and produced a 3.8-km high column. Table 2. Volumes of historically active basaltic volcanoes used to estimate fall-deposit volumes for YMR Quaternary volcanoes. e Cone Lavas Falls Fall Fall Cone I Volcano Age (km') (km*) (km*) cono lava lava Tolbachik Cone 1 1975 A.D. 0.093 0.025 0.122 1.3 4.8 3.6 Tolbachik Cone 2 1975 A.D. 0.098 0.242 0.099 1.0 0.4 0.4 Sunset Crater 1200 A.D. 0.284 0.150 0.440 1.6 2.9 1.9 Paricutin 1943-1951 A.D. 0.069 0.700 0.410 5.9 0.6 0.1 Heimaey 1973 A.D. 0.015 0.180 0.012 0.8 0.1 0.1 Serra Gorda <5 ka 0.030 0.015 0.042 1.4 2.8 2.0 Cerro Negro 1850-1995 A.D. 0.080 0.043 0.132 1.7 3.1 1.8 g3i Lathrop Wells 0.13*0.01 Ma 0.024 0.038 0.048 2 n/a 0.6 Hidden Cone 0.38i0.02 Ma 0.019 0.009 0.038 2 n/a 2.0 Little Black Peak 0.31i0.02 Ma 0.006 0.007 0.012 2 n/a 0.9 SW Little Cone 0.90f0.02 Ma 0.002* 0.022 0.004 2 n/a 0.1 Red Cone 1.0110.04 Ma 0.0056 0.089 0.005 1 n/a 0.1 Black Cone 0.94i0.03 Ma 0.011" 0.065 0.011 1 n/a 0.2 Note: (a) Cone volume corrected for 50% erosion, (b) cone volume corrected for 33% erosion. Data sources: Tolbachik (Budnikov, et al.,1983); Sunset Crater (Amos,1986); Paricutin (Segerstrom,1950); Heimaey (Self, et al.,1974); Serra Gorda (Booth, et al.,1978); Cerro Negro (Hill, et al.,1998). YMR volcanoes from USGS 7.5' topographic map data. l 96 I Table 3. Summary of eruption parameters with calculated column heights and eruption powers for historically-active basaltic volcanoes reasonably analogous to YMR volcanoes. DRE is dense rock equivalent (i.e., nonvesiculated). W// son refers to the method of Wilson, et al., (1978), where magma density is 2600 kg m-8, specific heat is 1100 J kg" "K", a 1055 *K temperature change, and thermal efficiency of 0.7. Walkerrefers to the method of Walker, et al., (1984).

Wilson, Walker, Column Eruption DRE column

Wilson, column height duration volume height power height Volcano (km)

(s) (m ) (km) (W) (km) 8 Heimaey 1973 2 2.2 x 10' 5.2x10e 2.2 4.9x 10 2.1 8 Paricutin 1943 4-6 7.3x10 1.9x10s 4.0 5.6 x 10' 3.9 8 Tolbachik Cone 1 1975 6-10 1.2x10' 6.0x10 4.7 1.0 x 10" 4.5 7 Tolbachik Cone 2 1975 2-3 3.3x10 4.6x10 3.4 3.0x 10'0 3.3 8 7 Cerro Negro 1947 4-6.5 6.6x10 1.1 x10 6.3 3.5x10" 6.2 d 7 Cerro Negro 1968 1-1.5 3.6x10 4.5x10 1.9 2.6x10 1.8 8 8 8 Cerro Negro 1971 6 6.0x10 1.4x10 3.9 4.9 x 10' 3.8 5 7 I Cerro Negro 1992 3-7 6.4x10 1.1 x10 6.4 3.6 x10" 6.2 d 7 Cerro Negro 1995 2-2.5 3.5x10 1.3x10 2.4 7.9x10 2.4 5 8 8 Data derived from: Heimaey (Self, et al.,1974); Paricutin (Segerstrom,1950); Tolbachik (Budnikov, et al.,1983; Hill, unpub. res.); Cerro Negro (Hill, et al.,1998). Wind velocity is the final parameter that significantly affects tephra dispersion from basaltic volcanoes (e.g., Hill, et al.,1998). The column from the next YMR eruption willlikely reach altitudes of 2-6 km above ground level, as is observed for most violent-strombolian basaltic I eruptions (e.g., Table 3). Although near ground-surface wind data are available for the proposed repository site, low-altitude winds will be affected significantly by surface topographic effects and, thus, have little relevance to modeling dispersal from 2-6-km-high eruption columns (e.g., U.S. I Department of Energy,1997). The nearest available high-altitude wind data are from the Desert Rock airstrip, which is located about 50 km southeast of Yucca Mountain. Based on data in U.S. Department of Energy (1997), average wind speeds at about 2 km above ground level (i.e., 1 700 mbar) are 6 m s". These average wind speeds increase to about 12 m s at altitudes of 4 about 4 km above ground level (i.e.,500 mbar). Staff conclude that an average wind speed of 12 m s4 provides a reasonably-conservative basis to model aerial tephra dispersal from the proposed repository site. 4.2.5.3.2 Dose Conversion Individuals located 20 km downwind from a repository-penetrating volcanic eruption would receive a radiological dose primarily through inhalation of contaminated ash particles. Particles <200 pm in diameter are resuspended through wind-shear, saltation, and mechanical disturbance of the deposit (e.g., Watson,1989). A mass-loading model describes the amount of contaminated ash in airbome suspension and is controlled by two critical parameters, (i) airbome 97 4 mass load, and (ii) thickness of the surficial deposit capable of eolian entrainment. Mass load is defined as the airbome mass of particulates per unit volume of air and consists of two primary components; (i) airbome mass composed of particles less than 10 pm in diameter, which can be inhaled directly into the pulmonary regions of the lung (i.e., respirable fraction), and (ii) airbome mass composed of particles10-200 pm in diameter, which are deposited in the naso-pharynx and tracheal-bronchial regions of the respiratory tract upon inhalation. Mass-El loading factors can range from 10 to 10" g m-3 for tropical to temperate climates (e.g., Tegen E; 4 4 and Fung,1994) and 10 to 10" g m'8 for more arid climates (e.g., Sehmel,1977; Anspaugh, et al.,1975). Intemal dosimetry of the inhaled particles depends on depositional site within the g respiratory tract. While no direct mass load information is available for fresh to 10,000-yr-old 3 basaltic volcanic fall deposits, studies of non-basaltic eruptions indicate that inhalation of fine-grained particles may represent a significant health risk (e.g., Baxter, et al.,1998). This suggests that basaltic eruptions could result in a relatively large opportunity for inhalation doses. Mass-loading factors available in the literature are derived from geological deposits that have limited applicability to basaltic tephra-fall deposits. In addition, little informaticn is presented in most of the relevant literature to discem particle size-distributions for suspended and surficial deposits, degree of soil development or soil type, vegetative cover, wind conditions, or soil moisture content. This information is necessary to address the suitability of published mass-E loading factors in evaluating inhalation dose for volcanic deposits. Based on general soil 3j characteristics from the studied environments, however, these soils likely contain significantly lower abundances of suspendable fine particulates than occur in basaltic volcanic fall deposits. g These nonvolcanic deposits appear depleted in suspendable fine-grained particulates, represent g evolved soil-types [and occur in significantly vegetated areas. Based on these characteristics, mass-loading factors for these deposits may significantly underestimate the amount of suspendable fine particulates, and thus the inhalation dose associated with basaltic volcanic fall deposits. There are some nonvegetated soils and deposits that occur, which in some arid environments El han general grain-size characteristics that can be compared with the volcanic fall deposits. 5 Dune sands, for example, commonly have average grain-sizes comparable to volcanic falls (i.e., 150-300 pm); however, the amount of particles <60 pm is often <1 weight percent (e.g., Watson, 1989), much lower then expected from basaltic fall deposits. To better understand the characteristics of basaltic fall deposits, fresh basaltic volcanic fall deposits were collected 21 km from the vent during the 1995 Cerro Negro, Nicaragua eruption. Preliminary analysis of the Cerro Negro fall deposits indicates that about 2 weight percent of the deposit consists of particles less than 10 pm in diameter, with particles <60 pm constituting about 10 weight percent of the deposit and those <200 pm constituting 50 weight percent of the E deposit. Other fall deposits from larger basaltic cinder cone eruptions may contain 2-5 weight E, percent with diameters <10 pm at 20 km distances (Segerstrom,1950; Budnikov, et al.,1983; Amos,1986). Basaltic volcanoes may also produce unusually fine-grained deposits late in the 3 eruption during subsurface brecciation events (Hill,1996). These tres of deposits from the 3 1975 Tolbachik eruption have more than 40 percent of the assof %d particles smaller than 60 pm (Doubik,1997). Similar late-stage, conduit-widening eveme nikely occurred at the youngest YMR volcanoes (Hill,1996). The largest amount of HLW entrainment would probably occur during this type of event, when the subsurface conduit expanded to dekameters in diameter. Thus, a reasonably conservative risk assessment needs to consider the mass-loading factors associated with tephra-fall deposits arising from these conduit widening events, in i t 1Il t addition to normal violent-strombolian tephra-fall deposits. Using data from the most reasonably-analogous deposits in the available literature (Anspaugh, et al.,1975; Tegen and Fung,1994), and comparing it to the information above on basaltic fall deposits, the staff have determined the ' a mass-loading factor of 10" to 10-2 ~ can be used gm initially to describe the amount of resuspended particles above a fresh basaltic tephra-fall. Fall-deposit characteristics will change with time as the deposit is exposed to subaerial environmental conditions. The amount of resuspendable ash particles will decrease through time by wind elutriation and rainwater infiltration. In addition, the fall deposit will be eroded through sheet-wash and channelized surficial flow. Erosion, however, will expose deeper layers of the deposit that likely contain initial abundances of resuspendable ash particles. The final stage of deposit erosion will expose a basal layer that has likely been enriched in ash particulates through rainwater infiltration. These significant changes in tephra-fall deposit morphology and granulometry through time are very poorly constrained. Erosion of basaltic tephra-fall deposits through time can be constrained initially through examination of reasonably analogous deposits. Only trace amounts of tephra-fall deposit remain within 3 km of the roughly 100 ka Lathrop Wells volcano. ExcluN deposits preserved in irregularities on associated lava flows, fall deposits have been comt ' f eroded from other YMR volcanoes. In contrast, fall deposits are significantly intac. 0 km from the vent at the 1065 A.D. Sunset Crater, Arizona (Amos,1986), I and the 2 ka Xitle volcano near Mexico City (Delgado-Granados, et al.,1998), both of which are located in areas that receive 3-4 times YMR average rainfall. Although fall deposits are eroded within decades from areas with steep topographic gradients, deposits on relatively flat-lying areas I are resistant to erosion (Segerstrom,1960; Malin, et al.,1983; inbar, et al.,1994). Based on comparison with these young analog deposits, staff conclude that tephra-fall deposits willlikely be present up to 10,000 yr after deposition in the semi-arid environment 20 km from the E proposed repository site. The correct mass loading factor to apply to these older deposits is 3 presently unknown, but it is assumed that the factor is significantly lower than for fresh deposits. 4.2.5.4 Summary While significant uncertainy exists, many of the parameters necessary for calculating the dose consequences of volcanic disruption of the proposed repository can be bounded through planned I modeling and observations at historical eruptions. Other parameters, primarily related to interactions between basaltic magma and engineered barrier systems, are difficult to constrain. A joint effort exists between the lA KTl and the Container Life and Source Term KTl to address this concem in FY99. 4.2.6 Consequence Criterion 6 4.2.6.1 Acceptance Criterion Estimates of the dose consequences ofigneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: If used, expert elicitations were conducted and documented using the guidance in the Branch Technical Position on Expert Elicitation (NRC, 1996), or other acceptable approaches. 99 -n.# 4,2.6.2 Review Method if DOE uses expert elicitation in developing estimates of the consequences of future volcanism and igneous intrusion, staff will review the documentation to assure that the expert elicitation: (i) followed the procedure described in the Branch Technical Position on Expert Elicitation (NRC, 1996); (ii) considered the range of models in the relevant literature; (iii) considered the range of j data in the relevant literature; and (iv) is consistent with available information, including information developed subsequent to the elicitation. 4.2.6.3 Technical Basis As summarized in NRC (1996), NRC expects that subjective judgments of groups of experts will be used by the DOE to assess issues related to overall performance of the proposed high-level radioactive waste repository site at Yucca Mountain. NRC has traditionally accepted expert judgment as part of a license application to supplement other sources of scientific and technical data. Expert elicitation is commonly used when: Empirical data are not reasonably obtainable or analyses are not practical to perform. + Uncertainties are large and significant to a demonstration of compliance. More than one conceptual model can explain, and be consistent with, the available data. i Technicaljudgments are required to assess whether bounding assumptions or I calculations are appropriately conservative. I NRC (1996) also summarize a series of technica! positions and procedures conceming the use of expert elicitation in demonstrating compliance with geologic repository disposal regulations. These procedures emphasize the need for detailed documentation during the elicitation and for transparency in the aggregation of multiple expert's judgments. An elicitation also should provide a means to evaluate new data that may arise between completion of the elicitation and submittal of licensing documents (NRC,1996). 1 The staffis not aware of any plans by DOE for the use of expert elicitation in evaluating the consequences of igneous activity. In addition, DOE has committed to following the NRC BTP guidance on all future elicitations (Brocoum,1997). 4.2.6.4 Summary As there are no plans for expert elicitation in this area, and DOE has committed to performing f future elicitations in accordance with NRC policy, this issue is considered resolved. 4.2.7 Consequence Criterion 3 l 4.2.7.1 Acceptance Criterion j Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: l l 100 l The collection, documentation, and development of data and models has been performed under acceptable QA procedures, or if data was not { collected under an established QA program, it has been qualified under appropriate QA procedures. 4.2.7.2 Review Method NRC will attend, as observers, DOE-conducted QA audits of program participants who are f involved in technical investigations related to igneous activity. NRC will also track the progress made in resolving deficiencies and nonconformities in the program that arise from QA audits and independent review of DOE products. 4.2.7.3 Technical Basis See Section 4.1.9.3 4.2.7.4 Summary f See Section 4.1.9.4 { f ( l I ( 101 I' I' THIS PAGE INTENTIONALLY BLANK I! I 102 I 5.0 STATUS OF ISSUE RESOLUTION AT STAFF LEVEL 5.1 STATUS OF RESOLUTION OF PROBABILITY ISSUES Based on available information, staff conclude that a range in annual probabilities of from 10'7 to I 10 bounds the range of credible models on the annual probability of futuro volcanic activity 4 intersecting the proposed repository site. Although a probability distribution can be constructed to evaluate uncertainty due to parameter variations, this uncertainty is small relative to variations in conceptual models used (i.e., Geomatrix,1996) or to uncertainties associated with model I accuracies. As there is no basis for distinguishing between values in this range, the staff will use an annual probability value of 10in performance assessment. The staff will evaluate the new information referred to in Section 4.1.4.3.1 of this report to determine if this value needs to be modified, and future versions of this IRSR will provide this evaluation. The staff does not believe that a meaningful probability for igneous intrusion can be determined with the present data base. Based on field studies at analog sites, the number ofintrusive events may be a factor of two or I more greater than the number of volcanic events, and the area affected by intrusive events may be orders of magnitude greater than the area effected by volcanic events alone. Based on the analog studies, as interim measure, the staff will assume that the probability of an igneous intrusive event is a factor of 2 to 5 higher than that of a volcanic event. DOE and NRC have not yet reached agreement on the appropriate range of volcanic and intrusive probability estimates to use in performance assessment. As stated above, the staff I conclude that a 1 x 10-7 annual probability of volcanic disruption provides a reasonably-conservative value for use in performance assessment. During the DOE /NRC Technical Exchange of February 25-26,1997, DOE agreed that the probability distribution function from I their expert elicitation had an upper bound frequency of 10-7; therefore, there is agreement between NRC and DOE on this value. Further analysis of the probability of volcanic disruption of the site is not warranted until completion of consequence analyses, risk assessment, and the I effects of new information of recurrence rate calculations has been completed. Also as stated above, as an interim measure, the staff will use a factor of 2-5 of the volcanic probability for the probability of igneous intrusion. The staff is not sure what value DOE will be using for intrusive events and awaits the results of the analysis delineated in TRW,1997. As with the volcanic I disruption probability, the need to determine a defendable value for igneous intrusion is also dependent on evaluating the health effects of this phenomenon; therefore, the staff plans little work on the probability of igneous intrusion until the consequences of igneous intrusion have been evaluated. 5.1.1 Probability Criterion 1 Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: The estimates are based on past pattems of igneous activity in the YMR. Sufficient information exists to define the extent of the YMR igneous system based on past pattems of igneous activity in the YMR. Probability estimates from most models are insensitive to volcanoes older than about 6 Ma or located more than about 30 km from the proposed p repository site. Some probability models using nonstationary Poisson or spatially-homogeneous L Poisson methods, however, are relatively sensitive to spatial and temporal definitions of the YMR igneous system, and probability estimates derived from these methods will need to be supported [ 103 I' with clear definitions of the YMR igneous system. 5.1.2 Probability Criterion 2 Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: The definitions of igneous events are used consistently. Intrusive and extrusive events should be distinguished and their probabilities estimated separately. Sufficient information exists to calculate the probability of volcanic disruption for the proposed g repository site, when the event is defined as an individual mappable eruptive unit, or as episodes 3 of vent or vent-alignment formation (e.g., Connor and Hill,1995; Condit and Connor,1996). The staff does not consider, however, that there is presently enough information to rigorously define the probability of igneous activity, or the related probability of intrusive activity affecting the repository. Based on preliminary estimates, it appears that the effects from intrusions intersecting the repository without volcanic eruption may be significantly less than the effects of volcanic disruption. Further work to rigorously define a probability of igneous intrusion is not warranted until completion of the consequence analysis. 5.1.3 Probability Criterion 3 Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: The models are consistent with observed pattems of volcanic vents and related igneous features in the YMR. Good agreement exists on the basic pattems of basaltic volcanism in the YMR. These pattems include changes in the locus of volcanism with time, recurring volcanic activity within vent clusters, formation of vent alignments, and structural controls on the locations of cinder cones. Each of these pattems in vent distribution has an important impact on volcanic probability models ll and is considered in current probability models. E' 5.1.4 Probability Criterion 4 Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: - Parameters used in probabilistic volcanic hazard assessments, related to recurrence rate of igneous activity in the YMR, spatial variation in frequency of igneous events, and area affected by igneous events, are technically justified and documented by DOE. While sufficient evidence exists to technicallyjustify parameters related to the recurrence rate of igneous activity in the YMR, spatial variation in frequency of igneous events, and area affected by igneous events, recently-available information brings to question the validity of the recurrence rate values used. Staff have conducted independent technical investigations in igneous activity to: (i) evaluate DOE data and models; (ii) develop and test altemative hypotheses; and (iii) E reduce uncertainties in models of repository performance. The results of these investigations g have been presented in numerous CNWRA reports and peer-reviewed joumal articles. As part of these investigations, staff have compiled all relevant data on the age and location of YMR basaltic igneous features younger than about 11 Ma (Appendix A). These data form the basis for 104 probability models and review of appropriate DOE licensing documents. The staff will be reviewing the new information to determine if modification of the recurrence rates are warranted and will report on their conclusions in subsequent issues of this IRSR. 5.1.5 Probability Criterion 5. Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: The models are consistent with tectonic models proposed by NRC and DOE for the YMR. All currently proposed tectonic models indicate that the proposed repository site and the locations of <5-Ma YMR volcanoes are in the same tectonic regime, However, most DOE models propose some type of boundary between Crater Flat and Yucca Mountain. This results in much lower probabilities at the site than for those areas just adjoining in Crater Flat. In most cases, the models will not allow, or severely constrain, the probability of a volcanic event forming at Yucca Mountain, while allowing dikes from such features to propagate to the site. Although some geologic data appear to suggest such a division, critical analyses reveal that these apparent divisions are only manifestations of surficial features and not important to deeper structural control of volcanism (e.g., Stamatakos, et al.,1997b). Therefore, reasonably conservative probability analyses must be based on source zones that include the proposed repository. The models considered within this IRSR satisfy this requirement. 5.1.6 Probability Criterion 6 ' Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: - The probability values used by DOE in performance assessments reflect the uncertainty in DOE's probabilistic volcanic hazard estimates. Uncertainty associated with any probability model consists of two components that measure precision and accuracy. Precision is also referred to as " parameter uncertainty," whereas accuracy often reflects "model uncertainty" (Performance Assessment Working Group,1997). Of the range of probability models proposed for the YMR, only the spatio-temporal nonhomogeneous models of Connor and Hill (1995) have been evaluated for model accuracy (Condit and Connor,1996). This evaluation demonstrates that these probability models reasonably estimate the locations of basaltic volcanoes in the Springerville volcanic field when basalt petrogenesis remains relatively constant. These models are unsuccessful in estimating the future locations of basaltic volcanoes when the magmatic system undergoes abrupt and large shifts in petrogenesis (Condit and Connor,1996). The YMR has not undergone similar-magnitude petrogenetic shifts since about 5 Ma (e.g., Crowe, et al.,1986), thus, these probability models should be reasonably accurate when applied to the YMR system. 5.1.7 Probability Criterion 7 Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: - The values used (single values, distributions, or bounds on probabilities) are technically justified and account for uncertainties in probability estimates. 105 DOE has stated that they will use the probability distribution derived from the PHA elicitation (Geomatrix,1996) to conduct sensitivity studies regarding igneous activity (Brocoum,1997). In addition, DOE will use a series of alternative approaches to evaluate model sensitivities to probability values or distributions (Brocoum,1997). This approach is generally acceptable to the NRC staff (Stablein,1997). 5.1.8 Probability Criterion 8 Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: If used, expert elicitations were conducted and documented using the guidance in the Branch Technical Position on Expert Elicitation (NRC,1996), or other acceptable approaches. E DOE used expert judgment to arrive at a probability hazard assessment for the proposed g, repository site (Geomatrix,1996). While there were areas of weakness, the probability hazard assessment elicitation (Geomatrix,1996) is generally consistent with the BTP regarding the conduct of an expert eficitation. NRC will, thus, give the probability hazard assessment elicitation the appropriate level of consideration in review of licensing documents (Bell,1997). 5.1.9 Probability Criterion 9 Estimates of the probability of future igneous activity in the YMR will be acceptable provided that: ~ The collection, documentation, and development of data and models has been performed under acceptable QA procedures, or if data was not collected under an established QA program, it has been qualified under appropriate QA procedures. The most recent audit that NRC observed related to the Igneous Activity KTl was in September, 1996, when DOE audited LANL. During this audit, the NRC staff observers r.oted several discrepancies and inconsistencies in the reported data both within the Draft Volcanism Synthesis Report and between the draft report and data presented in other reports and joumal articles (Austin,1996). As a result of this audit the DOE audit team concluded that the LANL's QA 3, performance was " marginally effective." NRC agreed with this finding, and NRC concems 3 identified during this audit were deferred to the appropriate DOE deficiency reports (YM-96 D-105 to 108). NRC recently reviewed the remedial actions proposed for these deficiencies and determined that the proposed actions appeared appropriate. Review of the associated Volcanism Synthesis Report is needed to determine if these actions have been effectively carried out and if the concems have been resolved. 5.2 STATUS OF RESOLUTION OF CONSEQUENCES ISSUES Based on available information, staff conclude that basaltic volcanic eruptions characteristic of the YMR are capable of disrupting HLW canisters, entraining fragmented HLW, and dispersing this waste to distances of 20 km or greater downwind. There is considerable uncertainty in applying volcanological data and process models derived from undisturbed geologic settings to g the engineered systems located in the disturbed geologic setting of the proposed repository site. E Directed technical investigations still are needed to evaluate the entrainment and dispersal of HLW during volcanic eruptions, to examine granulometric characteristics of basaltic tephra-fall deposits through time, and to quantify interactions between basaltic magma, HLW, and waste 106 I canisters. Staff conclude, however, that conservative assumptions on available data provide a reasonable basis to conduct initial assessments of volcanic consequences on repository performance, with the understanding that these assessments may change substantially as new information becomes available. Effects of igneous intrusions on repository performance have not been evaluated in detail. I Although intrusions into the repository will likely enhance HLW canister failure, subsequent radiological releases will be through hydrologic flow and transport. The significance of igneous intrusions to overall system performance remains difficult to evaluate. If repository performance relies on highly corrosion-resistant canisters, then disruptive events, such as igneous intrusion, I may be the only significant canister-failure mechanism during a 10,000-yr performance period. In this event, the effects of igneous intrusions will need to be examined in detail to provide a reasonably-conservative evaluation of long-term repository safety. If, however, some HLW I canisters fail during the first 10,000 yr through mechanisms other than disruptive events, then the effects of igneous intrusions can be reasonably evaluated through bounding analyses. The approach to issue resolution for igneous intrusions should be clear after staff evaluation of the DOE TSPA-VA in FY99. As outlined in Section 3 of the IRSR, previous DOE TSPAs have evaluated a limited range of i effects from volcanic disruption of the proposed repository. DOE has indicated during several j informal exchanges in FY97 and FY98 that analyses in support of TSPA-VA will examine a greater range of igneous processes than previous TSPAs. DOE also has indicated it will adopt the modified tephra-dispersion model used by NRC (i.e., Jarzemba, et al.,1997) in TSPA-VA analyses. During review of the DOE VA documentation, NRC will evaluate the DOE data, models, and assumptions used to assess the consequences of igneous activity to determine if the DOE methodology is acceptable. 5.2.1 Consequences Criterion 1 Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: - Models used in these calculations are consistent with the geologic record of basaltic igneous activity within the Yucca Mountain Region. Basaltic volcanoes in the YMR record a range of eruption characteristics that are significant to performance. The youngest and best preserved of these volcanoes indicate an eruption style that is commonly referred to as violent strombolian and characterized by sustained eruption ( columns that can transport tephra tens of kilometers downwind in addition, these eruptions can have late-stage disruption events that widen conduit diameters to tens of meters. Although deposits are very poorly preserved at YMR volcanos older than 1 Ma, these older volcanoes appear to have eruption styles that were significantly less disruptive and dispersive than many Quatemary YMR volcanoes. Initial staff analyses conclude that acceptable performance models will be based on a violent strombolian eruption style, as this style presents the greatest credible risk of HLW transport to critical groups located 20 km from the vent and is the most likely style of any future YMR eruption. The staff will evaluate DOE's analysis in the viability assessment to determine what eruption characteristics are being used in the analysis and from that determine the areas of agreement and disagreement on this issue. 107 5.2.2 Consequences Criterion 2 Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: - Process-level models are verified against igneous processes observed at active or recently-active analog igneous systems. Many of the igneous processes necessary for performance modeling are not preserved in the YMR geologic record and can best be derived through study of reasonably-analogous basaltic g igneous systems. Staff conclude that the modified tephra-dispersal model of Suzuki (1983) g provides an acceptable approach to calculating tephra-fall deposits from violent strombolian volcanoes and would appear to provide an acceptable approach to calculating HLW contaminated tephra fall deposits. Significant uncertainty remains, however, and the staff will evaluate attemative modeling approaches to HLW dispersal during FY99. 5.2.3 Consequences Criterion 3 i Estimates of the dose consequences ofigneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: - The models adequately account for changes in magma ascent characteristics and magma / rock interactions brought about by repository construction. Cunently-available models and data are derived from igneous systems that have not encountered a large tunnel network 200--300 m below the surface. The redistribution of crustal stress around the repository drifts will likely affect how ascending basaltic magma interacts with the engineered barrier systems. Although modeling has not been accomplished to evaluate the magnitude of these effects, acceptable performance models will need to address how the presence of the repository system may affect igneous processes significant to safety. These E; processes include, but are not limited to, extent of conduit diameter, magma fragmentation E! induced by rapid decompression, formation of lavas in drifts, and intrusion geometries through drift networks. Current and previous NRC and DOE models have not addressed these effects, thus, staff cannot evaluate the current level of resolution on this criterion. The staff will begin evaluation of these effects in FY99. 5.2.4 Consequences Criterion 4 Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: - Models used in perforrrance calculations account for the interactions of basaltic magma with engineered barriers and waste forms. Erupting basaltic magmas exert large physical, chemical, and thermal loads on HLW and associated waste canisters. These loads commonly are beyond the limits of available experimental studies. Models that propose canister-or HLW-resiliency during igneous events will need to be supported by data that explicitly considers the physical, chemical, and thermal characteristics of basaltic igneous events in order to be acceptable. Extrapolations from low temperature, low strain-rate data, for example, will need to evaluate changes in failure 108 mechanism produced by high temperature, high strain-rate igneous events. Staff conclude that canister failure during an igneous event is a reasonably-conservative assumption for performance assessment calculations. In addition, HLW reasonably can be expected to fragment during volcanic disruption events. Although DOE has not presented models for interactions of basaltic magma with engineered barriers and waste forms, informal f communication suggests that DOE is placing confidence in the " survivability" of the canister during igneous activity. NRC will review the documentation provided with the DOE Viability Assessment to determine areas of agreement and disagreement between NRC and DOE. 5.2.5 Consequences Criterion 5 Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: { - Parameters used in performance calculations are constrained by data from YMR igneous features and from appropriate analog systems such that the effects of igneous activity on waste containment and isolation are not underestimated. Sufficient data exists to technicallyjustify key parameters used by NRC to evaluate the dose consequences of basaltic volcanic activity. These data are derived from YMR basaltic volcanoes and reasonably-analogous, historically-active basaltic volcanoes. The main areas of volcanic parameter uncertainty are associated with the characteristics of the contaminated tephra fall deposit following the eruption. Significant uncertainty also is associated with parameters related to behavior of the waste package and HLW form during igneous disruptive events. Although the effects of igneous intrusions have not been investigated in detail, key parameters for intrusive processes can be reasonably constrained by available data from igneous features in the YMR and basaltic analog systems. The staff will be reviewing the documentation provided with the DOE Viability Assessment to determine areas of agreement and disagreement between NRC { and DO E. 5.2.6 Consequence Criterion 6 Estimates of the dose consequences of igneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: if used, expert elicitations were conducted and documented using the guidance in the Branch Technical Position on Expert Elicitation (NRC,1996), or other acceptable approaches. As DOE is not expected to conduct expert elicitations in this area, and as they have agreed to follow NRC procedures (Brocoum,1997), this issue is considered resolved. 5.2.7 Conseqeunce Criterion 7 { Estimates of the dose consequences ofigneous activity on the proposed Yucca Mountain high-level radioactive waste repository will be acceptable provided that: The collection, documentation, and development of data and models has been i performed under acceptable QA procedures, or if data was not collected under an established QA program, it has been qualified under appropriate QA procedures. 109 i I The most recent audit which the NRC observed related to the Igneous Activity KTl was in September,1996, when DOE audited LANL. During this audit the NRC staff observers noted several discrepancies and inconsistencies in the reported data both within the Draft Volcanism Synthesis Report, and between the draft report and data presented in other reports and joumal articles (Austin,1996). As a result of this audit the DOE audit team concluded that the LANL's QA performance was " marginally effective." The NRC agreed with this finding, and the NRC concems identified during this audit were deferred to the appropriate DOE deficiency reports E (YM-96-D-105 to 108). The NRC recently reviewed the remedial actions proposed for these 5 deficiencies and determined that the proposed actions appeared appropriate. Review of the associated Volcanism Synthesis Report is needed to determine if these actions have been effectively carried out and if the concerns have been resolved. 5.3 NRC DISPOSITION OF COMMENTS RELATED TO IGNEOUS ACTIVITY During review of the DOE Site Characterization Plan, and Study Plans 8.3.1.8.1.1,8.3.1.8.1.2, and 8.3.1.8.5.1, the NRC developed 57 comments and questions related to igneous activity. The change in the overall DOE program has resulted in some of the comments losing validity, and additionalinformation both from DOE and from ongoing work by NRC and CNWRA staff has become available to close many others. As a result,34 of these comments and questions had been closed prior to the development of this IRSR. NRC disposition of the remaining comments i and questions is listed below. SCA Comment 45: Reliance on volcanic rate calculations that are developed largely independent of consideration of the underlying volcanic-tectonic processes appears likely to underestimate potentialimpacts on the performance of the repository. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. The underlying basis of this comment was a concem as to whether rates of volcanic activity should be considered to be increasing, decreasing, or remaining essentially the same for the period of performance. It must be recognized that, at this time, the under1ying processes responsible for volcano formation are very poorly understood. Recent work by both the State of Nevada (Yogodzinski and Smith,1995) and the CNWRA (Hill and Connor,1996) indicates the YMR can be described as lying in a geochemical province that extends south to the Death Valley region and includes the area of the Funeral Formation in the g Greenwater Range. The geochemical similarity of various volcanic units indicates that, although g not understood, comparable geological processes have acted on all these units. When j considering the entire YMR, including the area of the Funeral Formation, the rate of volcanic vent formation has remained relatively constant through the Quatemary and into the Pliocene. The geologic evidence, therefore, suggests that a relatively steady state of volcanic vent formation has occurred for millions of years in the YMR. The NRC staff, therefore, considers that there is no basis for assuming either an increasing or i decreasing rate of volcanic vent formation during the period of repository performance. In review of DOE probability values, and in development of independent probability values, NRC will use g. recurrence rates that reflect a relatively steady state of volcanism. g] I, 110 I> I l SCA Comment 51: Geophysical survey programs as identified in the SCP may not be sufficient to identify and characterize both the deep crustal and shallow geologic features and their interrelationship. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. See Response to Comment 9, Study Plan 8.3.1.8.1.1. SCA Comment 52: No specific geophysical program appears to be planned to identify volcanic origneous features and their extent under or close to the site. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. See Response to Comment 9, Study Plan 8.3.1.8.1.1. SCA Question 12: Why has the Lunar Crater area not been included as a possible analog for detailed study of the processes related to basaltic volcanism in the Death Valley-Pancake Range Volcanic Belt? NRC DISPOSITION OF COMMENT: NRC considers the question resolved. NRC will use its I analog studies, primarily at Tolbachik and Cerro Negro, to evaluate volcanic processes and DOE assumptions about volcanic processes. STUDY PLAN 8.3.1.8.1.1 Comment 1: The use of the term " event"in this study plan appears to I be limited to cone formation, and, therefore, provides an incomplete description of magmatic processes and events, and the requirement to determine consequence of the resultant activity. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. As discussed under Probability Criterion 2, the use of the proper definition of event and the ability to carry this I definition through the calculations can have a large effect on the resultant probabilities. NRC has carried the calculations through to the probability and will assure that the analyses of DOE also reflect the event definitions. STUDY PLAN 8.3.1.8.1.1 Comment 2: Use of surface extrusion rates to approximate magma production rates could underestimate the effects of the magmatic process on repository performance. NRC DISPOSITION OF COMMENT: NRC considers the comment resolved. Recent work on the size of Little Cones (Stamatakos, et al.,1997a) and the Amargosa magnetic anomalies (Connor, I et al.,1997) shows that additional volumes of material have been erupted and were not considered in the various volume predictive calculations presented in the probabilistic hazard assessment report or DOE status reports (e.g., Crowe, et al.,1995). Examination of Geomatrix,1996, shows that this approach had a negligible effect on any probability values reported, and the overall effect of this approach, when averaged over the results of the entire panel, appears negligible (Brocoum,1997). [ Although NRC has concems related to a volume predictive approach, it appears that these concems do not need to be resolved in evaluating DOE probability models. NRC-preferred probability values do not rely on eruption volumes. I11 a I STUDY PLAN 8.3.1.8.1.1, Comment 3: The evaluation of the presence of crustal magma bodies in the vicinity of Yucca Mountain must consider the requirements of 10 CFR Part 60.122(a)(2). I NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. See response to Study Plan 8.3.1.8.1.1, Comment 9. STUDY PLAN 8.3.1.8.1.1 Comment 4: One of the main activities within this study plan, as stated on page 8, is to estimate the probability of future magmatic disruption of the Yucca Mountain site; however, the probability calculations that this study plan is intended to produce appear too limited to resolve the geologic and regulatory concems. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. The IRSR provides a. an acceptable probability for volcanic disruption of the repository and has an interim method for determining the probability of igneous intrusion. If the results of consequence analysis show that g-a more refined probability for igneous intrusion is necessary, an appropriate concem will be raised. STUDY PLAN 8.3.1.8.1.1 Comment 5: it is unclear how a volcanic recurrence model can be constructed without knowledge of magmatic events of a size less than that needed to produce a cone. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. See response to Study Plan 8.3.1.8.1.1., Comment 2. STUDY PLAN 8.3.1.8.1.1 Comment 6: This study plan does not appear to be calculating a " recurrence rate," but rather the average recurrence rate for the sampled population. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. The methodology utilized by DOE in the probabilistic hazard assessment report (Geomatrix,1996) and by the CNWRA in developing the NRC-preferred numbers, alleviates this concem. The IRSR provides an acceptable probability for volcanic disruption of the repository and has an interim method for determining the probability of igneous intrusion. STUDY PLAN 8.3.1.8.1.1 Comment 7: The study plan does not appear to adequately consider models that assume volcanism is a non-Poissonian process. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. The methodology utilized in the probabilistic hazard assessment report (Geomatrix,1996) and in developing the NRC-preferred probability numbers in this IRSR alleviates this concem, as models other than simple homogeneous Poisson processes were considered. STUDY PLAN 8.3.1.8.1.1 Comment 9: The geophysical program described in the SCP and E referred to in this study plan appears too limited to provide the information necessary to 5 develop reasonable probability models. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. SCA 51, and 52, Study Plan 8.3.1.8.1.1, Comments 3 and 9; and Study Plan 8.3.1.8.5.1, Comments 1 and 5; and Question 3 all dealin some way with NRC concems related to DOE program of geophysics as it relates to volcanism. I12 I Due to redirection of the DOE program, many planned activities were curtailed. The DOE intemal report, " Synthesis of Borehole and Geophysical Studies at Yucca Mountain, Nevada and Vicinity," provides a summary of the DOE program as it stands at present. Even prior to the DOE program redirection, due to the concems of NRC with the limitations of the program, NRC authorized the CNWRA to initiate a program of ground magnetics evaluation of several of the known and suspected igneous features in the area of Yucca Mountain to determine if geophysics could cheaply and efficiently resolve many of the basic questions about these features. The ground magnetic work of the CNWRA (Stamatakos, et al.,1997a; Connor, et al.,1997) has demonstrated that there are more buried volcanic bodies in the vicinity of Yucca Mountain than had been suspected during the elicitation of the probabilistic hazard analysis panel (Geomatrix,1996). These buried features, in general, also lie outside the boundaries of the high-probability zones defined in the Geomatrix report. In addition, the characteristics of these features, such as the size of Little Cones, were not as assumed in Geomatrix,1996, and there appears to be a strong association of volcanoes with faulting. Based on evaluation of the results of the DOE program and the work of the CNWRA, the following steps have been taken to address these concems:

1. As additional buried volcanoes have been detected, some in areas outside locations I

previously known to contain buried volcanoes, it must be assumed that even more buried volcanoes are present, both within and outside the locations known from surface work. As a result, the total number of volcanic events utilized in the probability assessments must reflect both the increase in the total number of events I and the uncertainty in this number.

2. The subsurface characteristics of known igneous bodies in the vicinity of Yucca Mountain are presently poorly defined. Therefore, conservative assumptions about these features are being used in both probability and consequence analyses.

I

3. The characteristics and location of smaller igneous intrusive bodies, such as dikes and sills, is poorly known in the YMR. Therefore, undetected dikes and sills must be assumed to exist in the area of Yucca Mountain.

4. Information at present is insufficient to resolve concems related to potential occurrence of crustal magma bodies in the vicinity of Yucca Mountain. Therefore, it is assumed that a source for magma generation is present in the Yucca Mountain area.

5. There are many open questions regarding the interrelationship of structure and I

igneous features. The ground magnetics of the CNWRA (Stamatakos, et al.,1997a; Connor, et al.,1997) strongly suggest that there is a strong relationship in the YMR. Therefore, a conservative series of assumptions is being made regarding this kind of interrelationship.

6. The present geologic and geophysical data provides no geologic basis for structural separation of Yucca Mountain and Crater Flat. Rather, the data drives one to the I

conclusion that Yucca Mountain is part of the Crater Flat basin. Therefore, it is assumed that the Yucca Mountain site lies within the same source region as the basaltic volcanoes of Crater Flat. l 113 - - - - - - - ~ ~ It is the staff's opinion that use of the above assumptions can compensate for the limitations of the geophysical program as implemented. These assumptions have been utilized in the j development of the probability values contained within the IRSR. Therefore, the concerns can be resolved as the IRSR provides an acceptable probability for volcanic disruption of the repository and has an interim method for determining the probability of igneous intrusion. In evaluation of Yucca Mountain site performance and evaluation of programmatic documents. EI such as the Viability Assessment, NRC will utilize the value of 10'7 for direct volcanic disruption of 5 the repository itself.. STUDY PLAN 8.3.1.8.1.1 Comment 10: The MODEL 1 methodology for calculating the probability for repository disruption presented in Section 3.2.2.2 appears to be incorrect. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. The methodology of conem was not used in the development of the probabilistic hazard assessment report or in the development of the NRC-preferred probability numbers by the CNWRA. STUDY PLAN 8.3.1.8.1.2 Comment 2: The study plan does not address how volatile contents of basaltic eruptions will be described and assessed. NRC DISPOSITION OF COMMENT: NRC considers this comment open, pending receipt and review of the Volcanism Synthesis report. This comment will be revisited during revisions of the IRSR dealing with volcanic consequences. STUDY PLAN 8.3.1.8.1.2 Comment 9: The proposed studies for wall-rock fragmentation and subsurface effects do not appear to account for the modification in lithostatic pressures that will occur due to repository construction and operation. NRC DISPOSITION OF COMMENT: NRC considers this comment open, pending receipt and review of the Volcanism Synthesis report. This comment will be revisited during revisions of the IRSR dealing with volcanic consequences. -l STUDY PLAN 8.3.1.8.5.1, RO Question 3: Have additional analog studies, aside from those presented in this activity, been considered by DOE? NRC DISPOSITION OF QUESTION: NRC considers this question resolved. The primary concem remaining on this question was the DOE assumption of waning patterns of volcanism. As is stated in the response to SCA Comment 45, NRC considers that the geologic evidence supports a relatively steady state of YMR volcanism from the Pliocene into the Quaternary. Therefore, recurrence rates used in ph..dy calculations will be evaluated utilizing this assumption. STUDY PLAN 8.3.1.8.5.1 R1 Comment 1: The aeromagnetic data described in Section 2.1.1 g may not be sufficient to detect and resolve magnetic anomalies associated with small 5 intrusions that are of regulatory concem. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. See response to Comment 9, Study Plan 8.3.1.8.1.1. I 114 STUDY PLAN 8.3.1.8.5.1 R1 Comment 4: It is unclear how the volume of eruptive basalt is being calculated. NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. As was shown in - the response to Comment 2, Study Plan 8.3.1.8.1.1, this concem has been somewhat alleviated due to the fact the probability calculations have, in general, not used the volume predictive approach. The concem is applicable, however, when considering the volume of material that must be considered in evaluating consequence of igneous events for performance assessment. The remaining concem in this comment will be addressed primarily by using independent calculations of volume and values obtained by analogs in determining consequences of igneous events. STUDY PLAN 8.3.1.8.5.1 R1 Comment 5: it is unclear how the model that assumes northwest-trending structures provide deep-seated control on magma pathways will be tested. NRC DISPOSITION OF COMMENT: NRC considers this cornment resolved. While NRC has remaining concems as to the geologic basis of the various volcanic zones proposed by DOE, such as the northwest-trending CFVZ, NRC will rely on independent methods of calculating probability that do not rely on volcanic zone definition, such as the methods of Connor and Hill (1995). STUDY PLAN 8.3.1.8.5.1 R1 Comment 7: it is unclear how the research discussed in this study plan will resolve attemative petrogenic models. NRC DISPOSITION OF COMMENT: NRC considers this comment open, pending submittal and review of the Volcanism Synthesis report. This comment will be revisited during revisions of the IRSR dealing with volcanic consequences. STUDY PLAN 8.3.1.8.5.1 R1 Question 3: How are the intrusive geometries associated with the development of the Crater Flat alignment to be characterized? NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. See response to Study Plan 8.3.1.8.1.1, Comment 9. STUDY PLAN 8.3.1.8.5.1 R1 Question 5: If the theory of polycyclic volcanism is correct for the volcanoes in the region of Yucca Mountain, how will it be assured that the age determinations accurately represent the age cf the various cones? NRC DISPOSITION OF COMMENT: NRC considers this comment resolved. The results of recent dating and trenching studies & Lathrop Wells volcano have shown that there has been significant erosion of the cone and that no significant age difference between eruptive units can be demonstrated (see Site Characterization Progress Report #15). As these were two of the main basis points for the theory of polycyclic volcanism, NRC considered that this theory has been refuted and no longer deserves consideration. This theory also was given little weight during the probabilistic hazard assessment (Geomatrix,1996). STUDY PLAN 8.3.1.8.5.1 R1 Question 8: How will volumetric relationships from the different systems in westem North America be used to develop specific, time-dependent, volume-predictive models for the Crater Flat system? I15 I NRC DISPOSITION OF COMMENT: NRC considers the comment resolved. The use of volume-predictive methods for developing volcanic probabilities does not appear to be utilized by DOE, therefore, this question is no longer of concern. See also response to Comment 2 Study Plan 8.3.1.8.1.1. I I I I I I I-I 116 I

6.0 REFERENCES

Amos, R.C., Sunset Crater, Arizona: Evidence fora Large Magnitude Strombolian Eruption.

M S. Thesis, Tempe, AZ: Arizona State University,1986.

Anderson, E.M., The dynamics of sheetintrusion, Proceedinos of the Royal Society of Edinburah 58: 242-251,1938.

Anderson, D.L., Theory of the Earth, London: Blackwell Scientific Publications.1989.

Anspaugh, L.R., J.H. Shinn, P.L. Phelps, N.C. Kennedy, Resuspension and redistribution of l

plutoniumin soils, Health Physics 29: 571-582,1975.

Ashby, M.F., C. Gandhi, and D.M.R. Taplin, Fracture-mechanism maps and theirconstruction for l

F.C.C. metals and alloys, Acta Metalluroica 27: 699-729,1979.

Austin, J.H., Letter (October 24) to R.A. Milner, U.S. Department of Energy, Washington, DC: U.S. Nuclear Regulatory Commission,1996.

Ayer, J.E., A.T. Clark, P. Loysen, M.Y. Ballinger, J. Mishima, P.C. Owczarski, W.S. Gregory, and B.D. Nichols, Nuclear Fuel Cycle Facility Accident Analysis Handbook. NUREG-1320.

Washington, DC, U.S. Nuclear Regulatory Commission,1988.

Baca, R.G., and M.S. Jarzemba, Detailed Review of Selected Aspects of Total System Performance Assessment-1995, IM 5708-761-710. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1997.

Bacon, C.R., Time-predictable bimodal volcanism in the Coso Range, Califomia, Geoloav 10:65-69,1982.

Bamard, R.W., M.L. Wilson, H.A. Dockery, J.H. Gauthier, P.G. Kaplan, R.R. Eaton, F.W.

Bingham, and T.H. Robey, TSPA 1991: An initial Total-System Performance Assessment for Yucca Mountain, Sandia National Laboratories Report SAND 91-2795. Albuquerque, NM: Sandia National Laboratories,1992.

Barr, G.E., E. Dunn, H. Dockery, R. Bamard, G. Valentine, and B. Crowe, Scenarios Constructed forBasaltic Igneous Activity at Yucca Mountain and Vicinity, Sandia National

{

Laboratory ReDort SAND 91-1653. Albuquerque, NM: Sandia National Laboratory,1993.

Baxter, P.J., C. Bonadonna, R. Dupree, V.L. Hards, S.C. Kohn, M.D. Murphy, A. Nichols, R.A. Nicholson, G. Norton, A. Seart, R.S.J. Sparks, B.P. Vickers, Cristobalite in volcanic ash of the Soufriere Hills volcano, Montserrat: Hazardsimplications, Nature. In press,1998.

Bell, M.J., Letter (June 25) to S.J. Brocoum, U.S. Department of Energy, Washington, DC:

t l

U.S. Nuclear Regulatory Commission,1997.

Bemis, K.G., and D.K. Smith, Pmduction of small volcanoes in the Superswell region of the South Pacific, Earth and Planetary Science Letters 118: 251-262,1993.

117

I Blackbum, E.A., L. Wilson, R.S.J. Sparks, Mechanisms and dynamics of strombolian activity, Journal of the Geoloaical Society of London 132: 429-440,1976.

Bonadonna, C., G.G.J. Emst, R.S.J. Sparks, Thickness vanations and volume estimates of tephra falldeposits: The importance of particle Reynolds number, Joumal of Volcanoloav and Geothermal Research. In press,1998.

Booth, B., R. Croasdale, G.P.L. Walker, A quantitative study of five thousand years of volcanism on Sao Miguel, Azores, Philosophical Transactions of the Royal Society. London 288: 271-319, 1978.-

Bradshaw, T.K., and E.I. Smith, Polygenetic Quatemary volcanism at Crater Flat, Nevada, Joumal of Volcanoloav and Geothermal Research 63: 165-182,1994.

Braytseva, O.A., l.V. Melekestev, and V.V. Ponomsreva, Age divisions of the Holocene volcanic formations of the Tolbachik Valley, The Great Tolbachik Fissure Eruption. S.A. Fedotov e

and Y.K. Markhinin, eds. New York, NY: Cambridge University Press: 83-95,1983.

g Brocher, T.M., P.E. Hart, W.C. Hunter, and V.E. Langenheim, Hybrid-Source Reflection Profiling Across Yucca Mountain, Nevada: Regional Lines 2 and 3, U.S. Geological Survey Open-File Report 96-28, Reston, VA: U.S. Geological Survey,1996.

Brocoum, S.J., Letter (April 6) to M. J. Bell, Resolution of NRC Site Characterization Analysis, 3

Comment 3, and Other Comments Related To Use of Expert Elicitations in High-Level Waste E

Program, U.S. Nuclear Regulatory Commission, Washington, DC,1997.

Brocoum, S.J., Letter (June 4) to J.T. Greeves, U.S. Nuclear Regulatory Commission, Washington, DC: U.S. Department of Energy,1997.

Budnikov, V.A., Ye.K. Markhinin, A.A. Ovsyannikov, The quantity, distribution and petrochemical features ofpyroclastics of the great Tolbachik fissure eruption. Fedotov, S.A., and Ye.K.

Markhinin, eds. The Great Tolbachik Fissure Eruption. New York, NY: Cambridge University Press: 41-56,1983.

Cahoon, J.B., C.L. Hoenig, A.J. Rothman, FuelElements forPluto, UCRL-687. Livermore, CA:

Lawrence Livermore National Laboratory,1962.

Carmichael, l.S.E., M.S. Ghiorso, Controls on oxidation-reduction relations in magmas, Modem Methods ofIgneous Petrology: Understanding Magmatic Processes, J. Nicholls and J.K.

Russell, eds. Reviews in Mineraloav Vol. 24: Washington, DC: Mineralogical Society of America: 191-212,1990.

Carr, W.J., Volcano-Tectonic History of Crater Flat, Southwestem Nevada, as Suggested by New Evidence from Drill Hole USU-VH-1 and Vicinity, U S. Geoloaical Survey Open-File Report 82-457. Reston, VA: U.S. Geological Survey,1982.

Carroll, M.R., J.D. Webster, Solubilities of Sulfur, Noble Gases, Nitrogen, Chlorine, and Fluorine in Magmas. Reviews in Mineraloav. Volume 30: Volatiles in Maomas, M.R. Carroll and J.R.

Holloway, eds. Washington, DC: Mineralogical Society of America: 231-280,1994.

I18 I

Carter Krogh, K.E., G.A. Valentine, Dike and sill emplacement in a shallow extensional setting, Paiute Ridge, NV. EOS, Transactions of the American Geoohvsical Union. 76(46): F691, 1995.

Champion, D.E., Volcanic episodes near Yucca Mountain as determined by paleomagnetic studies at Lathrop Wells, CraterFlat, and Sleeping Butte, Nevada, Proceedinas of the Second Annual intomational Conference on Hioh-Level Radioactive Waste Manaaement, La Grange j

Park, IL: American Nuclear Society: 61-67,1991.

. Condit, C.D., and C.B. Connor, Recurrence rates of volcanism in basaltic volcanic fields:

j An example from the Springerville volcanic field, Arizona, Geoloaical Society of America Bulletin 108: 1,225-1,241, 1996.

Condit, C.D., L.S. Crumpler, J.C. Aubele, and W.E. Elston, Pattoms of volcanism along the southem margin of the Colorado Plateau: The Springerville Field, Joumal of Geoohvsical Research 94(B6): 7,975-7,986,1989.

Connor, C.B., Cinder cone clustering in the TransMexican volcanic belt: Structural and

{

petrologic implications, Joumal of Geophysical Research 95: 19,395-19,405,1990, i

Connor, C.B., Technical and Regulatory Basis for the Study of Recently Active Cinder Cones, IM-20-5704-141-001, San Antonio, TX: Center for Nuclear Waste Regulatory Analyses, 1993.

1 Connor, C.B., and B.E. Hill, Volcanism Research, NRC High-LevelRadioactive Weste Research

\\

at CNWRA, July-December 1992, CNWRA 92-02S. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses, 10-1-10-31,1993b.'

Connor, C.B., and B.E. Hill, Estimating the probability of volcanic disruption of the carniodate Yucca Mountain repository using spatially and temporally nonhomogeneous Poisson models, Proceedings, American Nucisar Socety Focus '93 Meeting, La Grange Park, IL:

American Nuclear Society: 174-181,1993.

Connor, C.B., and B.E. Hill, The CNWRA Volcanism Geographic Information System Database, CNWRA 94-004, San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,

.1994.-

Connor, C.B., and B.E. Hill, Three nonhomogeneous Poisson models forthe probability of basaltic volcanism: ' Application to the Yucca Mountain region, Nevada, U.S.A. Joumal of Geoohvsical Research 100(B6): 10,107-10,125,1995.

Connor, C.B., C.D. Condit, L.S. Crumpler, J.C. Aubele, Evidence of regional structural controls on vent distribution: Springerville volcanic held, Arizona, Joumal of Geoohysical Research 97:

12,349L-12,359,1992.

Connor, C.B., B.E. Hill, B.W. Leslie, C. Lin, J.F. Luhr, K.D. Mahrer, G.L Stirewalt, S.R. Young, Review of Preliminary Dralt: Status of Volcanic Hazard Studies for the Yucca Mountain Site i

Characterization Project, Dated February 1993, San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1993.

1 119 1

b-

._.m_..

Connor, C.B., J. Stamatakos, D. Ferrill, B E. Hill, S.B.L. Magsino, P. La Femina, and R.H. Martin, Integrating structural models into probabilistic volcanic hazard analyses: An example from Yucca Mountain, NV, EOS, Transactions of the American Geophysical Union,77(46): F21,1996.

Connor, C.B., S. Lane-Magsino, J.A. Stamatakos, R.H. Martin, P.C. La Femina, B.E. Hill, S. Lieber, Magnetic surveys help reassess volcanic hazards at Yucca Mountain, Nevada, EOS, Transactions of the American Geophysical Union 78(7): 73-78,1997.

Conway, M.F., D.A. Ferrill, C.M. Hall, A.P. Morris, J.A. Stamatakos, C.B. Connor, A.N. Halliday, C. Condit, Timing of basaltic volcanism along the Mesa Butte Fault in the San Francisco volcanic l

field, Arizona, from *ArPArdates: Implications forlongevity of cinder cone alignments, Joumal E

of Geophysical Research 102: 815-824,1997.

Crow, L.H., Confidence intervalprocedures forthe Weibullprocess with applications to reliability growth, Technometrics 24: 67-71,1982.

Crowe, B.M., and F.V. Perry, Volcanic probability calculations forthe Yucca Mountain site:

Estimation of volcanic rates, Proceedinas Nuclear Waste Isolation in the Unsaturated Zone.

Focus '89. La Grange Park, IL: American Nuclear Society: 326-334,1989.

j Crowe, B., and F. Perry, Preliminary Geologic Map of the Sleeping Butte Volcanic Centers, Los Alamos National Laboratorv Report LA-12101-MS. Los Alamos, NM: Los Alamos National Laboratory,1991.

Crowe, B.M., M.E. Johnson, and R.J. Beckman, Calculation of the probability of volcanic disruption of a high-level nuclear waste repository within southem Nevada, USA, Radioactive Waste Manaaement and the Nuclear Fuel Cycle 3: 167-190,1982.

Crowe, B.M., D.T. Vaniman, and W.J. Carr, Status of Volcanic Hazard Studies for the Nevada Nuclear Waste Storage Investigations, Los Alamos National Laboratory Report LA-9325-MS, Los Alamos, NM: Los Alamos National Laboratory,1983.

Crowe, B.M., K.H. Wohletz, D.T. Vaniman, E. Gladney, and N. Bower, Status of Volcanic Hazard Studies for the Nevada Nuclear Waste Storage Investigations, Los Alamos National Laboratory Report LA-9325-MS. Vol. II. Los Alamos, NM: Los Alamos National Laboratory, 1986.

Crowe, B., R. Morley, S. Wells, J. Geissman, E. Mcdonald, L. McFadden, F. Perry, M. Murrell, J. Poths, and S. Forman, The Lathrop Wells volcanic center Status of field and geochronology g

studies, Proceedinas of the Third Intemational Conference on Hiah-Level Radioactive Waste l

Manaaement. La Grange Park, IL: American Nuclear Society: 1,997-2,013,1992.

Crowe, B.M., F.V. Perry, J. Geissman, L. McFadden, S. Wells, M. Murrell, J. Poths, G.A.

Valentine, L. Bowker, and K. Finnegan, Status of Volcanic Hazard Studies for the Yucca Mountain Site Characten' ration Project, Los Alamos National Laboratory Report LA-12908-MS.

Los Alamos, NM: Los Alamos National Laboratory,1995.

Delaney, P.T., A.E Gartner, Physicalprocesses of shallow mafic dike emplacement nearthe San Rafael Swell, Utah, Geoloaical Society of America Bulletin 109:1,177-1,192,1997.

120

L-l

. Delaney, P.T., A.E. Gartner, Physical Processes of Shallow Mafic Dike Emplacement Near the San Rafael Swell, Utah, U.S. Geoloaical Survey Open-file Report 95-491. Reston, VA: U.S.

- Geological Survey,1995.

Delaney, P.T., D.D. Pollard, J.l. Ziony, E.H. McKee, Field relations between dikes andjoints:

Emplacement processos and paleostress analysis, Journal of Geophysical Research 91(BS):

4,920-4,938,1986.

Delgado-Granados, H., P. Cervantes-Laing, R. Molinero-Molinero, J. Nieto-Obregon, H.L. Macias-Gonzalex, C. Mendoza-Rosales, G. Silvia-Romo, A. Pastrana, Geology ofXitle volcano (southem Mexico City): I. Stratigraphy and age, Colima Volcano Sixth Intemational Meetina. Colima, Mexico: University of Colima: 114,1998.

Dohrenwend, J.C., L.D. McFadden, B.D. Turrin, S.G. Wells, K-Ar dating of the Cima Volcanic Field, eastem Mojave Desert, Califomia: Late Cenozoic volcanic history andlandscape evolution, Geoloav 12: 163-167,1984.

Doubik, P., Studies of the Eruptive Products from Three Volcanoes in Kamchatka: Insights into Shallow Magma Evolution and Dynamics, Ph.D. Thesis. Buffalo, NY: State University of New York,1997.

Doubik, Y., A.A. Ovsyannikov, C. Connor, R. Martin, and P. Doubik, Development of the 1975 Tolbachik cinder cone alignment-nature of areal and lateral basaltic volcanism, EOS, Transactions of the American Geoohvsica! Union 76(46): F540,1995.

Draper, G., Z. Chen, M. Conway, C.B. Connor, C.D. Condit, Structural control of magma pathwaysin the uppercrust: Insights from the San Francisco volcanic field, Arizona, Geoloaical Society of America Abstracts with Proarams. 26(7): A-115,1994.

Duffield, W.A., C.R. Bacon, G.B. Dalrymple, Late Cenozoic volcanism, geochronology, and structure of the Coso Range, Inyo County, Califomia, Joumal of Geoohysical Research 85(B5):

2,381-2,404,1980.

Earthfield Technology, Inc., Summary Report Magnetic and Gravity Study of the Yucca Mountain Area Nevada, prepared for Lawrence Berkley National Laboratories,1995.

Eichelberger, J.C., R. Gooley, Evolution of silicic magma chambers and their relationship to basaltic volcanism, The Earth's Crust. Geophysical Monooraoh Series. Volume 20.

J.G. Heacock, ed. Washington, DC: American Geophysical Union: 57-77,1977.

Einziger, R.E., L.E. Thomas, H.C. Buchanan, R.B. Scott, Oxidation of Spent Fuel in Air at 175 to 195 C, Joumal of Nuclear Material 190: 53,1992.

Einzinger, R.E., Preliminary spent LWR fuel oxidation source term model, Proceedinas of the

+

Fourth Annual Intemational Hiah-Level Radioactive Waste Manaaement Conference. La Grange Park, IL: American Nuclear Society: 554-559,1994.

Einzinger, R., H. Buchanen, Long-Term, Low-Temperature Oxidation of PWR Spent Fuel, WHC-EP-0070. Richland, WA: Westinghouse Hanford Company,1S88.

121

)

Farmer, G.L., F.V. Perry, S. Semken, B. Crowe, D. Curtis, D.J. DePaolo, Isotopic evidence on

)

the structure and ongin of subcontinentallithospheric mantle in southem Nevada, Joumal of

\\

Geophysical Research 94: 7,885-7,898,1989.

Faulds, J.E., J.W. Bell, D.L. Feuerbach, A.R. Ramelli, Geologic Map of the Crater Flat Area, Nevada, Nevada Bureau of Mines and Geoloov Map 101. Reno, NV: Nevada Bureau of Mines and Geology,1994.

Fedotov, S.A., Chronology and features of the southem breakout of the Great Tolbachik fissure eruption, 1975-1976, The Great Tolbachik Fissure Eruption. Geolooical and Geophysical Data 1975-1976, S.A Fedotov and Y. K. Markhinin, eds. Cambridge, MA: Cambridge University Press: 11-26,1983.

Ferrill, D.A., J.A. Stamatakos, K.H. Spivey, and A.P. Morris, Tectonic processes in the centralBasin and Range region, NRC Hiah-Level Radioactive Waste Research at CNWRA.

Julv-December 1995. B. Sagar, ed. CNWRA 95-01S, San Antcnio, TX: Center for Nuclear Waste Regulatory Analyses: 6-1 to 6-27,1995.

Ferrill, D.A., J.A. Stamatakos, S.M. Jones, B. Rahe, H.L. McKague, R.H. Martin, and A.P. Morris, Quatemary slip history of the Bare Mountain fault (Nevada) from the morphology and distribution of alluvialfan deposits, Geoloav 24(6): 55 Fields, R.J., T. Weerasooriya, M.F. Ashby, Fracture mechanisms in pure iron, two austenitic l

steels, and one ferritic steel. Metalluraical Transactions 11(A): 333-347.62,1996a,1980.

5 Ferrill, D.A., G.L. Stirewalt, D.B. Henderson., J.A. Stamatakos, K.H. Spivey, B.P. Wemicke, Faulting in the Yucca Mountain Region, CNWRA 95-017. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1996b.

Ferrill, D.A., J.A. Stamatakos, A.P. Morris, R.A. Donetick, A.E. Blythe, S.M. Jones, K. Spivey, Geometric, Thermal, and Temporal Constraints on the Tectonic Evolution of Bare Mountain, Nevada, IM 20-5708-471-731. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses, 1997.

Foiand, K.A., S.C. Bergman, Temporal and spatial distribution of basaltic volcanism in the Pancake and Reveille Ranges north of Yucca Mountain, Proceedinas of the Third Intemational g

Conference on Hich-Level Radioactive Waste Manaaement. La Grange Park, IL: American 3.

Nuclear Society: 2,366-2,371,1992.

Fridrich, C.J., Tectonic evolution of the Crater Flat Basin, Yucca Mountain region, Nevada, Cenozoic Basins of the Death Valley Region, L. Wright and B. Troxel, eds. Geological Society of America Special Paper, Boulder, CO: Geological Society of America, in press,1997.

Il Fridrich, C.J., J.W. Whitney, M.R. Hudson, B.M. Crowe, Late Cenozoic extension, vertical-axis rotation, and volcanism in the CraterFlat basin, southwest Nevada, Cenozoic Basins of the Death Valley Region, L. Wright and B. Troxel, eds. Geolooical Society of America Special Paper.

l Boulder, CO: Geological Society of America, in press,1997.

Frizzell, V.A., Jr., J. Shulters, Geologic Map of the Nevada Test Site, Southem Nevada, Miscellaneous investications Map I-2046. Reston, VA: U.S. Geological Survey,1990.

122 0

Gartner, A.E., P.T. Delaney, Geologic Map Showing a Late Cenozoic Basaltic Intrusive Complex, Emery, Sevier, and Wayne Counties, Utah, Miscellaneous Field Studies Mao MF-2052.

Reston, VA: U.S. Geological Survey,1988.

I Geomatrix, Probabilistic Volcanic Hazards Analysis for Yucca Mountain, Nevada, Report BA0000000-1717-2200-00082. Rev. O. San Francisco, CA: Geomatrix Consultants,1996.

Glaze, L.S., S. Self, Ashfall dispersal for the 16 September 1986, eruption of Lascar, Chile, calculated by a turbulent diffusion model, Geophysical Research Letters 18(7): 1,237-1240, 1991.

J Gudmundsson, A., Formation of dykes, feeder-dykes, and the intrusion of dykes from magma chambers, Bulletin of Volcanoloav 47: 537-550,1984.

I Gupta, V.K., F.S. Grant, Mineral-exploration aspects of gravity and aeromagnetic surveys in the Sudbury-Cobalt area, Ontario, The Utility of Reaional Gravity and Maanetic Anomaly Maps. W.J.

Hinze, ed. Tulsa, OK: Society of Exploration Geophysicists: 392-412,1984.

Hardyman, R.F., J.S. Oldow, Tertiary tectonic framework and Cenozoic history of the central I

WalkerLane, Nevada, Geoloav and Ore Deposits of the Great Basin. Symposium Proceedinas.

G.L. Raines, R.E. Uste, R.W. Schafer, and W.H. Wilkinson, eds. Reno, NV: Geological Society of Nevada,1991.

Hasenaka, T., l.S.E. Carmichael, The cindercones of Michoacan-Guanajuato, centralMexico:

Theirage, volume and distribution, and magma discharge rate, Joumal of Volcanoloav and Geothermal Research 25: 105-124,1985.

Hawkesworth, C., S. Tumer, K. Gallagher, A. Hunter, T. Bradshaw, N. Rogers, Calc-alkaline magmatism, lithospheric thinning and extension in the Basin and Range, Joumal of Geoohvsical Research 100(B7): 10,271-10,286,1995.

Heming, R.F., Pattems of Quatemary basaltic volcanism in the northem North Island, New i

Zealand, New Zealand Journal of Geoloav and Geophysics 23: 335-344,1980.

Hill, B.E., Cunent Concems Regarding DOE /LANL Volcanism GeochemicalData, Al 5702-441-547, San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1995.

Hill, B.E., Constraints on the Potential Subsurface Area of Disruption Associated with Yucca Mountain Region Basaltic Volcanoes, IM 5708-461-701. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1996.

Hill, B.E., CNWRA dose calculations for volcanic disruption of the proposed repository, Presentation to Advisory Committee on Nuclear Waste, Rockville, MD, April 22,1997.

Hill, B.E., and C.B. Connor, Volcanic Systems of the Basin and Range, NRC High-Level I

Radioactive Waste Research at CNWRA, July-December 1995, CNWRA 95-02S. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses: 5-1 to 5-21,1996.

Hill, B.E., C.B. Connor, and J.S. Trapp, Igneous activity, NRC High-Level Radioactive I

123

Waste Program Annual Progress Repor1, Fiscal Year 1996, CNWRA 96-01 A. San Antonio, TX:

I Center for Nuclear Waste Regulatory Analyses: 2-1 to 2-32,1996.

Hill, B.E., B.W. Leslie, and C.B. Connor, A Review and Analysis of Dating Techniques for Neogene and Quatemary Volcanic Rocks, CNWRA 93-018. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1993.

l Hill, B.E., C.B. Connor, Field Volcanism, NRC High-Level Radioactive Waste Research at CNWRA, July-December 1994, CNWRA 94-02S. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses. 141-154,1995.

Hill, B.E., C.B. Connor, M.S. Jarzemba, P.C. La Femina, M. Navarro, W. Strauch,1995 eruptions of Cerro Negro volcano, Nicaragua, and risk assessment forfuture eruptions, Geoloaical Society of America Bulletin. In press,1998.

Hill, B.E., F.M. Conway, C.B. Connor, P. La Femina, Field Volcanism. NRC Hiah-Level g

Radioactive Waste Research at CNWRA. JanuarvWune 1995. CNWRA 95-01S. San Antonio, g,

TX: Center for Nuclear Waste Regulatory Analyses: 7-1-7-22, 1995.

Ho, C.H., Time trend analysis of basaltic volcanism at the Yucca Mountain site, Joumat of Volcanoloav and Geothermal Research 46: 61-72,1991.

Ho, C.H., Risk assessment for the Yucca Mountain high-level nuclear waste repository site: Estimation of volcanic disruption, Mathematical Geoloav 24: 347-364,1992.

Ho, C.H., E.l. Smith, D.L. Feurbach, T.R. Naumann, Eruptive probability calculation forthe Yucca Mountain site, USA: Statistical estimation of recurrence rates, Bulletin of Volcanoloav 54:

50-56,1991.

Hogg, R.V., E.A. Tanis, Probability and Statistical /nference, New York, NY: Macmillan Co.,

1988.

Howard, N.W., Variation in Properties of Nuclear Test Areas and Media at the Nevada Test Site, UCRL-53721. Livermore, CA: University of Califomia, Lawrence Livermore National Laboratory,1985.

Hudson, M.R., D.A. Sawyer, R.G. Warren, Paleomagnetism and rotation constraints for the Miocene southwestem Nevada volcanic field, Tectonics 13: 256-277,1994.

Inbar, M., J.L. Hubp, L.V. Ruiz, The geomorphological evolution of the Paricutin cone andlava flows, Mexico, 1943-1990, Geomorpholoav 9: 57-76.,1994.

Jarzemba, M.S., P.A. LaPlante, Preliminary Calculations of Expected Dose from Extrusive Volcanic Events at Yucca Mountain, IM 5078-771-610. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1996.

Jarzemba, M.S., P.A. LaPlante, K.J. Poor, ASHPLUME Version 1.0- A Code for Contaminated Ash Dispersal and Deposition, Technical Description and User's Guide, CNWRA 97-004. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1997.

124

'I

I Jarzemba, M.S., Stochastic radionuclide distributions aftera basaltic eruption forperformance assessments of Yucca Mountain, Nuclear Technoloov 118(2): 132-141,1997.

{

i Jolly, R.J.H., D.L. Sanderson, A Mohr circle construction for the opening of a pre-existing fracture, Joumal of Structural Geoloov 19: 887-892,1997.

Kear, D., Volcanic alignments north and west of New Zealand's central volcanic region, j

New Zealand Joumal of Geoloov and Geophysics 7: 24-44,1964.

j Knutson, J., T.H. Green, Experimental duplication of a high-pressure megacryst/ cumulate assemblage in a near-saturated Hawalite, Contributions to Minerafoov and Petrofoov 52:

121-132.,1975.

Kuntz, M.A., D.E. Champion, E.C. Spiker, R.H. Lefrebre, Contrasting magma types and steady-state, volume-predictable basaltic volcanism along the Great Rift, Idaho, Geoloaical Society of America Bulletin'97: 579-594,1986.

Lachenbruch, A.H., P. Morgan, Continental extension, magmatism and elevation: formal relations and rules of thumb, Tectonophysics 174: 39-62,1990.

Langenheim, V.E., D.A. Ponce, Depth to Pre-Cenozoic basement in southwest Nevada, Proceedinas of the Sixth Annual Intemational Conference on Hiah-level Radioactive Waste Manaoement. La Grange Park, IL: American Nuclear Society: 129-131,1995.

Langenheim, V.E., KS. Kirchoff-Stein, H.W. Oliver, Geophysicalinvestigations of buried volcanic centers near Yucca Mountain, southwestem Nevada, Proceedinas of Fourth Annual

.i Intemational Conference on Hiah-Level Radioactive Waste Manaaement. La Grange Park, IL:

1 American Nuclear Society: 1,840-1,846,1993.

Leeman, W.P., The isotopic composition of strontium in Late Cenozoic basalts from the Basin-Range Province, westem United States, Geochimica et Cosmochimica Acta 34: 857-872, 1970.

Leeman, W.P., J.G. Fitton, Magmatism associated with lithospheric extension: Introduction, Joumal of Geophysical Research 94(B6): 7,682-7,684, 1989.

Link, R.L., S.E. Logan, H.S. Ng, F.A. Rokenbach, K.J. Hong, Parametric Studies of Radiological Consequences of Basaltic Volcanism, Sandia National Laboratory Report SAND 81-2375, Albuquerque, NM: Sandia National Laboratory,1982.

Lister, J.R., R.C. Kerr, Fluid-mechanical models of crack propagation and their application to magma transport in dykes, Jour e hnvo;G.; Nsearch 96: 10,049-10,077,1991.

Lorence, A., A Review of the United States NuclearRamjet Program. UCRL-51342. Livermore, CA: Lawrence Livermore National Laboratory,1973.

Luedke, R.G. R.L. Smith, Map Showing Distribution, Composition, and Age of Late Cenozoic Volcanic Centers in Califomia and Nevada, U.S. Geoloaical Survey Miscellaneous investiaations Series Map _l-1091-C. Reston, VA: U.S. Geological Survey,1981.

125

7 Luhr, J.F., T. Simkin, Paricutin, the Volcano Bom in a Mexican Comfield, Phoenix, AZ:

Geoscience Press.1993.

Lutz, T.M. An analysis of the orientations oflarge scale crustal structures: a statistical approach based on areal distribution of point-like features, Joumal of Geophysical Research 91:

421--434,1986.

Lutz, T.M., J.T. Gutmann, An improved method of determining the alignment of point-like features and implications for the Panicked volcanic field, Mexico, Joumal of Geophysical Research 100: 17,659-17,670,1995.

Macedonio, G., F. Dobran, A. Neri, Erosion processes in volcanic conduits and application to the AD 79 eruption of Vesuvius, Earth and Planetary Science Letters 121: 137-152,1994.

j Magsino, S.L., C.B. Connor, B.E. Hill. J.A. Stamatakos, P.C. LaFemina, D.A. Sims, R.H. Martin, 1998, CNWRA Gropund Magnetic Surveys in the Yucca Mountain Region, Nevada (1996-1997),

Center for Nuclear Waste Regulatory Analysis, San Antonio, Texas,1998.

Magus' kin, M.A., V.B. Enman, V.S. Tselishchev, Changes in height, volume and shape of the New Tolbachik volcanoes of the Northem Breakout, The Great Tolbachik Fissure Eruption.

Geolooical and Geophysical Data 1975-1976. S.A. Fedotov, and Ye, K. Markhinin, eds.

Cambridge, MA: Cambridge University Press: 307-315,1983.

Malin, M.C., D. Dzurisin, R.P. Sharp, Stripping of Keanakakoitephra on Kilauea Volcano, Hawaii, Geoloaical Society of America Bulletin 94: 1,148-1,158,1983.

Manteufel, R.D., R.G. Baca, S. Mohanty, M.S. Jarzemba, S.A. Stothoff, C.B. Connor, G.A. Cragnolino, A. Chowdhury, T.J. McCartin, T.M. Ahn, Total-System Performance Assessment (TPA) Version 3.0 Code: Module Descriptions and User's Guide,1M 20-5708-762-730. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1997.

Margulies, T., L. Lancaster, N. Eisenberg, L. Abramson, Probabilistic analysis of magma scenarios forassessing geologic waste repositoryperformance, American Society of Mechanical Enaineers. Winter Annual Meetina. New York, NY: American Society of Mechanical Engineers, 1992.

McBimey, A.R., Volcanology, Techniques for Determining Probabilities of Geologic Events and Processes, intemational Association for Mathematical Geology, Studies in Mathematical Geolooy No. 4. R.L. Hunter and C.J. Mann, eds. New York, NY: Oxford University Press:

167-184,1992.

McDuffie, S.M., C.B. Connor, K.D. Mahrer, A simple 2-D stress model of dike-fracture interaction, EOS, Transactions of the American Geophysical Union 75(16): 345,1994.

McGregor, J.K., C. Abston, Photographs from the U.S. Geological Survey Photographic Library (Earthquakes, Volcanoes, Geolodc Hazards, and OtherPhenomena)...S. Geoloaical Survev Diaital Data Series DDS-8. Reston, VA: U.S. Geological Survey,1992.

McKague, H.L., Summary of Measured Medium Properties of Paleozoic Rocks at the DOE Nevada Test Site, UCRL-52884. Livermore, CA: University of Califomia, Lawrence Livermore 126 I

I I

National Laboratory,1980.

McKenzie, D., M.J. Bickle, The volume and composition of melt generated by extension of the I

lithosphere, Joumal of Petroloov 29: 625-679,1988.

)

i Morris, A., D.A. Ferrill, D.B. Henderson, Slip-tendency analysis and fault reactivation, Geolooy 24(3): 275-278,1996.

Nakamura, K, Volcanoes as possible indicators of tectonic stress orientation-principles andproposal, Joumat of Volcanoloov and Geothermal Research 2: 1-16,1977.

Ofoegbu, G.L., D.A. Ferrill, Finite Element Modeling of Listric Normal Faulting, CNWRA 95-008.

San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1995.

O' Leary, D.W., Synthesis of tectonic models for the Yucca Mountain area, Seismotectonic

\\

Framework and Characterization of Faultino at Yucca Mountain. Nevada. J.W. Whitney, ed. U.S.

I I

Geological Survey Report to the U.S. Department of Energy, Milestone 3GSH100M, Reston, VA:

U.S. Geological Survey: 8-1 to 8-153,1996.

1 Parsons, T., G.A. Thompson, The role of magma overpressure in suppressing earthquakes and

\\

topography: Worldwide examples. Science 253: 1,399-1,402,1991.

Pedersen, T., H.E. Ro, Finite duration extension and decompression melting, Earth

)

and Planetary Science Letters 113: 15-22,1992.

I l

.g Performance Assessment Working Group, Branch TechnicalPosition on a Performance l

Assessment Methodology for Low-Level Radioactive Waste Disposal Facilities, NUREG-1573.

Washington, DC: U.S. Nuclear Regulatory Commission,1997.

l Ratcliff, C.D., J.W. Geissman, F.V. Perry, B.M. Crowe, P.K. Zeitler, Paleomagnetic record of a geomagnetic Reld reversal from late Miocene masc intmsions, southem Nevada, Science 266:

412-416,1994.

I 1

I Reches, Z., J. Fink, The mechanism of intrusion of the Inyo Dike, LongValley Caldera, Califomia, Joumal of Geoohvsical Research 93(B5): 4,321-4,334,1988.

l i

Rogers, N.W., C.J. Hawkesworth, D.S. Ormerod, Late Cenozoic basaltic magmatism in the Westem Great Basin, Califomia and Nevada Joumal of Geophysical Research 100(B7)-

10,287-10,301,1995.

Sagar, B., NRC High-Level Radioactive Waste Program Annual Progress Report: Fiscal Year 1996, NUREG/CR-6513. Washington, DC: U.S. Nuclear Regulatory Commission,1997.

Segerstrom, K, Emsion Studies at Paricutin, State of Michoac6n, Mexico, U.S. Geolooical Survey Bulletin 965-A. Reston, VA: U.S. Geological Survey,1950.

I Segerstrom, K, Erosion and Related Phenomena at Paricutin in 1957, U.S. Geoloolcal Survey Bulletin 1104-A. Reston, VA: U.S. Geological Survey,1960.

Sehmel, G.A., Radionuclide Particle Resuspension Research Experinwnts on the Hanford 127 i

Reservation, BNWL-2081. Richland, WA: Pacific Northwest Laboratory,1977.

Self, S., R.S.J. Sparks, B. Booth, G.P.L. Walker, The 1973 Heimaey strombollan scoria deposit, 3'

iceland, Geoloaical Macazine 111: 539-548,1974.

E-Schweikert, R.A., M.M. Lahren, Strike-slip fault system in the Amargosa Valley and Yucca Mountain, Nevada, Tectonophysics 272: 25-41,1997.

Scott, R.B., Tectonic setting of Yucca Mountain, southwest Nevada, Basin and Ranae Extensional Tectonics Near the Latitude of Las Veaas. Nevada. B.P. Wemicke, ed. Geological Society of America Memoir 176: 251-282,1990.

Sheridan, M.F., A Monte Carlo technique to estimate the probability of volcanic dikes, Proceedinos of the Third Intemational Conference on Hiah-Level Radioactive Waste Manaaement. La Grange Park, IL: American Nuclear Society: 2,033-2,038,1992.

Silverman, B.W., Density Estimation for Statistics and Data Analysis, New York, NY:

Chapman and Hall,1986.

Smith, E.l., D.L. Feuerbach, T.R. Naumann, J.E. Faulds, Annua / Report of the Center for Volcanic and Tectonic Studies forthe Period 10-1-89 to 9-30-90, Reoort 41. Carson City, Nevada: The Nuclear Waste Project Office,1990.

Sparks, R.S.J., The dimensions and dynamics of volcanic eruption columns, Bulletin of Volcanolooy 48: 3-15,1986.

Sparks, R.S.J. Bursik, M.I., G. Ablay, R.M.E. Thomas, S.N. Carey, Sedimentation of tephra by volcanic plumes, Part 2: Controls on thickness and grainsize variations in tephra fall deposits, Bulletin of Volcanolooy 54: 685-695,1992.

Sparks, R.S.J., M.I. Bursik, S.N. Carey, J.S. Gilbert, L.S. Glaze, H. Sigurdsson, A.W. Woods, Volcanic Plumes, John Wiley and Sons: New York,1997.

Spence, D.A., D.L. Turcotte, Magma-driven propagation of cracks, Joumal of Geophysical Research 90(B1): 575-580,1985.

Stablein, N.K., Letter (August 1) to S.J. Brocoum, U.S. Department of Energy, Washington, DC:

- U.S. Nuclear Regulatory Commission,1997.

Stamatakos, J.A., D.A. Ferrill, Tectonic processes in the central Basin and Range region, NRC HiDh-Level Radioactive Waste Research at CNWRA, July-December 1995, B. Sagar, ed.

CNWRA 95-02S. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses: 6-1 to 6-25, 1996.

Stamatakos, J.A., C.B. Connor, R.H. Martin, Quatemary basin evolution and basaltic volcanism of Crater Flat, Nevada, from detailed ground magnetic surveys of the Little Cones, Joumal of Geoloav 105: 319-330,1997a.

Stamatakos, J.A., D.A. Ferrill, H.L. McKague, Technical /nput on Review and Acceptance Criteria forIssue Resolution Status Report on Tectonic Models, IM 5708-471-700. San Antonio, 128 I;

TX: Center for Nuclear Waste Regulatory Analyses,1997b.

Stamatakos, J.A., D.A. Ferrill, KH. Spivey, Paleomagnetic Constraints on the Tectonic Evolution of Bare Mountain, Nevada, IM 20-5708-471-730. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1997c.

Stevens, B., The laws ofintmsion, Transactions of the American Institute of Minina Enaineers 41: 650-672,1911.

Stewart, J.H., Basin and range simcture: A system of horsts and grabens produced by deep-seated extension, Geoloaical Society of America Bulletin 82(4): 1019-1044,1971.

Suzuki, T., A TheoreticalModel for the Dispersion of Tephra, D. Shimozuru and I. Yokiyama, eds.

Arc Volcanism: Physics and Tectonics. Tokyo Japan: Terra Scientific Publishing Company:

95-113,1983.

Swadley, W.C, W.J. Carr, Geologic Map of the Quaternary and Tertiary Deposits of the Big Dune Quadrangle, Nye County, Nevada, andInyo County, Califomia, U.S. Geoloaical Survey Miscellaneous investiaations Series Mao 1-1767. Reston, VA: U.S. Geological Survey,1987.

Takada, A., The influence of regional stress and magmatic input on styles of monogenetic cnd polygenetic volcanism, Joumal of Geoohysical Research 99: 13,563-13,574, 1994.

Takahashi, E., I Kushiro, Melting of a dry peridotite at high pressures and basalt magma genesis, American Mineralocist 68: 859-879,1983.

Tanaka, KL., E.M. Shoemaker, G.E. Ulrich, E.W. Wolfe, Migration of volcanism in the San Francisco volcanic field, Arizona, Geoloaical Society of America Bulletin 97: 129-141,1986.

Tegen, I., and I. Fung, Modeling of mineral dust in the atmosphere: Sources, transport and cpticalthickness, Joumal of Geoohvsical Research 99(11): 22,897-22,914,1994.

Thomas, R.M.E., R.S.J. Sparks, Cooling of tephra during fallout from emption columns, Bulletin of Volcanoloov 54: 542-553.,1992.

Thordarson, T., S. Self, The Laki(Skaft6rFires) and Grimsvotn emptions in 1783-1785, Bulletin of Volcanoloav 55: 233-263,1993.

1 Tokarev, P.I., Calculation of the magma discharge, growth in the height of the cone and dimensions of the feeder channel of Crater 1 in the Great Tolbachik Fissure Eruption, July 1975, The Great Tolbachik Fissure Eruption. S.A. Fedotov and Y.K Markhinin, eds. New York, NY:

Cambridge University Press: 27-35,1983.

TRW Environmental Safety System inc., Total System Perfortnance Assessment - Viability Assessment (TSPA-VA) Method and Assumptions, Prepared for U.S. Department of Energy, Office of Civilian Radioactive Waste Management,1000 Independence Avenue, S.W.,

Washington, D.C. 20565,1997.

Turrin, B.D., D. Champion, R.J. Fleck, *ArPArage of the Lathrop Wells volcanic center, Yucca Mountain, Nevada, Science 253: 654-657,1991.

129

r..

Turrin, B.D., D. Champion, R.J. Fleck, Measuring the age of the Lathrop Wells volcanic center at Yucca Mountain: Response, Science 257: 556-558,1992.

Turrin, B.D., J.C. Dohrenwend, R.E. Drake, G.H. Curtis, K-Ar ages from the Cima volcanic field, eastem Mojave Desert, Califomia, Isochron / West 44: 9-16,1985.

United States Department of Energy, Repository Safety Strategy: U.S. Department of Energy's Strategy to Protect Public Health end Safety After Closure of a Yucca Mountain Repository, Office of Civilian Radioactive Waste Management, Washington, D.C. 20585,1998.

U.S. Department of Energy, Regionaland Local Wind Pattems Near Yucca Mountain, B00000000-01717-5705-00081. Washington, DC: U.S. Department of Energy, Office of Civilian Radioactive Waste Management,1997.

United States Nuclear Regulatory Commission, Branch TechnicalPosition on the Use of & pert Elicitation in the High-Level Radioactive Waste Program, NUREG-1563. Washington, DC,1996.

United States Nuclear Regulatory Commission, Disposal o/High-leve/ Radioactive Wastes in Geologic Repositories, Code of Federal Reaulations. Title 10 - Enerav. Chapter 1-U.S.

g Nuclear Regulatory Commission Part 60.152, Washington, DC: U.S. Govemment Printing Office, g

1997.

Valentine, G.A., K.R. Groves, Entrainment of country rock during basaltic eruptions of the Lucero Volcanic Field, New Mexico, Joumal of Geoloov 104: 71-90,1996.

Vaniman, D.T., B.M. Crowe, E.S. Gladney, Petrology and geochemistry of Hawaiite lavas from CraterFlat, Nevada, Contributions to Mineraloav and Petroloav 80: 341-357,1982.

Viswanathan, R., Damaae Mechanism and Life Assessment of Hiah-Temperature Components.

E]

Materials Park, OH: ASM Intemational, Inc,1989.

E Wadge, G., A. Cross, Quantitative methods fordetecting alignedpoints: An application to the vents of the Michoac6n-Guanajuato volcanic field, Mexico, Geoloav 16: 815-818,1988.

Wadge, G., P.A.V. Young, \\.J. McKendrick, Mapping lava flow hazards using computer simulation, Joumal of Geophysical Research 99: 489-504,1994.

Walker, G.P.L., Basaltic-volcano systems, Magmatic Processes and Plate Tectonics, Geoloaical Society Special Publications 76. H.M. Prichard, T. Alabaster, N.B.W. Harris, and C.R.

Neary, eds. London: Geological Society: 3-38,1993.

Walker, G.P.L., R. Croasdale, Chanateristics of some basaltic pyroclastics. Bulletin of Volcanoloav 35: 303-317,1972.

Walker, G.P.L., S. Self, L. Wilson., Tarawera 1886, New Zealand - A basaltic plinian fissure eruption, Joumal of Volcanoloav and Geothermal Researgt1 21: 61-78,1984.

Watson, A., Windflow characteristics and aeolian entrainment, Arid Zone Geomorpholoav.

D.S.G. Thomas, ed. New York, NY: John Wiley and Sons,1989.

130 t

I

Wells, S.G., L.D. McFadden, C.E. Renault, B.M. Crowe, Geomorphic assessment of late Quatemarv volcanism in the Yucca Mountain area. southem Nevada: Implications for the proposed hiah-level radioactive waste repository. Geoloav 18: 549-553,1990.

Wells, S.G., B.M. Crowe, L.D. McFadden, Measuring the age of the Lathrop Wells volcanic centerat Yucca Mountain: Comment. Science 257: 555-556,1992.

I E

'Wemicke, B., J.L. Davis, R.A. Bennet, P. Elosegui, M.J. Abolins, R.J. Brady, M.A. House, N.A.

Niemi, J.K. Snow, Anomolous Strain Accumulation in the Yucca Mountain Area, Nevada, Science, Vol 279, pp 2100,1988.

Whitney, J.W., R.R. Shroba, Comment on Geomorphic assessment oflate Quatemary volcanism in the Yucca Mountain area, southem Nevada: Implications forthe proposed high-I levelradioactive waste repository, Geoloav 18: 661,1991.

Wilson, L., R.S.J. Sparks, T.C. Huang, N.D. Watkins, The control of volcanic column heights by eruption energetics and dynamics. Joumal of Geophysical Research 83(B4): 1,829-1,836, 1978.

I Wilson, M.L., J.H. Gauthier, R.W. Bamard, G.E. Barr, H.A. Dockery, E. Dunn, R.R. Eaton, D.C. Guerin, N. Lu, M.J. Martinez, R. Nilson, C.A. Rautman, T.H. Robey, B. Ross, E.E. Ryder, A.R. Schenker, S.A. Shannon, L.H. Skinner, W.G. Halsey, J.D. Gansemer, L.C. Lewis, A.D.

I Lamont, l.R. Trisy, A. Meijer, D.E. Morris, Total-System Performance Assessment for Yucca Mountain - SNL Second Iteration (TSPA-1993), Sandia National Laboratories Report SkND 93-2675. Albuquerque, NM: Sandia National Laboratories,1994.

Woods, A.W., The dynamics and thermodynamics of eruption columns. Bulletin of Volcanoloav 50: 169-191,1988.

Woods, A.W., Moist convection and the injection of volcanic ash into the atmosphere,.Joumal of Geophysical Research 98: 17,627-17,636,1993.

I Woods, A.W., The dynamics of explosive volcanic eruptions, Reviews in Geoohvsics 33:

495-530,1995.

I Woods, A.W., M.l. Bursik, Particle fallout, thermal disequilibrium and volcanic plumes, Bulletin of Volcanoloav 53: 559-570.,1991.

Yogodzinski, G.M., E.I. Smith, Isotopic domains and the area ofinterest for volcanic hazard assessmentin the Yucca Mountain area, EOS, Transactions of the American Geophysical Union 76(46): F669,1995.

Young, S.R., G.L. Stirewalt, A.P. Morris, Geometric Models of Faulting at Yucca Mountain, CNWRA 92-008. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses,1992.

Zoback, M.L., State of stress in the northem Basin and Range Province, Joumal of Geophysical Research 94: 7,105-7,128,1989.

l I

131

I I

I I

I I

I g

APPENDIX A I

COMPILATION OF DATES FOR BASALTIC ROCKS g

OF THE YUCCA MOUNTAIN REGION

'I I

I I

I I

I I

I Compilation of K-Ar and Ar/Ar dates for post-il Ma basaltic rocks of the YMR. Units and sample numbers as reported by authors. Wgtd mean corresponds to weighted mean on n samples and 1-sigma best estimated errors (Taylor,1990), multiplied by the square root of the mean square of the weighted deviates I

(MSWD) if MSWD > 1 (e.g., Fleck, et al.,1996). Arcrage is average i l standard deviation of reported dates. Urin. is unknown, unpub. res. is unpublished research, and pers. comm. is personal communication.

Coordinates for vent locations in Universal Transverse Mercator meters, Zone 11 North American Datum 1927. Vent locations determined by cited authors and field observations (B. liill, unpub. res., 1993-1996).

Lathrop Wells: 543780E,4060380N. Normal magnetic polarity (Champion,1991).

ilo Date error Unit / Sample #

hiethod (Ala)

(hfa)

Notes Reference Unkn.

K/Ar 0.06 0.03 Smith, et al.,1990 Unkn.

K/Ar 0.23 0.02 Vaniman, et al.,

1982 Qlla Ar/Ar 0.14 0.05 Wgtd mean, Turrin, et al.,1991 n=8 Qlla Ar/Ar 0.125 0.005 Turrin,1995 Q12a Ar/Ar 0.18 0.02 Wgtd mean, Turrin, et al.,1991 n = 16 Ql2a/31472 Ar/Ar 0.142 0.019 Turrin, et al.,1992 I

Q13 Ar/Ar 0.22 0.05 Wgtd mean, Turrin, et al.,1991 n=4 Qs2?/Qsu Ar/Ar 0.15 0.05 Wgtd mean, Turrin, et al.,1991 n=8 Q12a/TSV-1 K/Ar 0.29 0.2 Vaniman and Crowe, 1981 Qs3/TSV-129-78 K/Ar 0.30 0.10 Vaniman, et al.,

1982 Average: 0.1810.08 Afa,51SWD = 4.92, Wgtd mean = 0.13 i 0.01 hia Not included in Lathrop Wells data set due to questionable apparent accuracy I

Unkn./71 K/Ar 2.0 0.6 Anomalously Marvin, et al.,1973 old Lava K/Ar 0.30 0.02 Wgtd mean, Sinnock and n = 25 Easterling,1983 I

I A-1

I Little Black Peak: 522120E, 4110340N. Normal magnetic polarity (Champion,1991).

t io Date envr Unit / Sample #

Method (Ma)

(Ma)

Notes Reference NNTS89-111 K/Ar 0.19 0.10 Fleck, et al.,19%

Unkn. lava K/Ar 0.21 0.13 Crowe & Perry,1991 Unkn. lava K/Ar 0.22 0.10 Crowe & Perry,1991 Unkn. lava K/Ar 0.24 0.22 Crowe, et al.,1982 Unkn. lava K/Ar 0.29 0.11 Crowe, et al.,1982 Unkn. lava K/Ar 0.32 0.15 Crowe, et al.,1982 TSV-5-77 K/Ar 0.33 0.03 Fleck, et al.,1996 TSV-6-77 K/Ar 0.39 0.07 Fleck, et al.,1996 Avemge: 0.26 t 0.07 Ma, MSWD = 0.69, Wgtd mean = 0.31 t 0.02 Ma Hidden Cone: 523400E, 4112600N. Normal magnetic polarity (Champion,1991).

g i lo E

Date ermr Unit / Sample #

Method (Ma)

(Ma)

Notes Reference Unkn. lava K/Ar 0.32 0.20 Crowe & Perry,1991 RDSBal4 K/Ar 0.34 0.07 Fleck, et al.,1996 NNTS89-109 K/Ar 0.37 0.07 Fleck, et al.,1996 Unkn. lava Ar/Ar 0.38 0.02 Turrin,1995 TSV-64-78 K/Ar 0.40 0.09 Fleck, et al.,1996 913-8B2 K/Ar 0.43 0.11 Fleck, et al.,1996 Avemge: 0.37 + 0.04 Ma, MSWD = 0.14, Wgtd mean = 0.38 + 0.02 Ma I

Nonhem Cone: 540350E, 4079360N. Reversed magnetic polarity (Champion,1991).

t io Date ermr Unit / Sample #

Method (Ma)

(Ma)

Notes Reference Unkn. lava K/Ar 1.04 0.03

" unpublished Crowe, et al.,1995 USGS date" A-2 I

1 Nonhem Cone: 540350E, 4079360N. Reversed magnetic polarity (Champion,1991).

t io Date enor Unit / Sample #

Method (Ma)

(Ma)

Notes Reference Unkn. lava K/Ar 1.05 0.07 Feldspar Smith, et al.,1990 separate I

NNTS 7-86 K/Ar 1.06 0.05 Fleck, et al.,1996 TSV-128-78 K/Ar 1.07 0.04 Vaniman, et al.,1982 TSV-128-78 K/Ar 1.09 0.03 Fleck, et al.,1996 Unkn. lava K/Ar 1.09 0.07 Feldspar Faulds, et al.,1994 separate TSV-128 K/Ar 1.14 0.3 Vaniman and Crowe, i

1981 Unkn. lava K/Ar 1.66 0.5

" unpublished Crowe, et al.,1995 USGS date" Avemge:1.15 t 0.21 Ma, MSWD = 0.44, Wgtd mean = 1.07 t 0.02 Ma E

Age must be <0.98 Ma or > 1.05 Ma to correspond with Cir.In subchron boundaries in I

Cande and Kent (1992).

I 1

[

[

[

E A-3

'W W

I, Black Cone: 538840E, 4074120N. Reversed magnetic polarity (Champion,1991).

t io Date ermr Unit / Sample #

Afethod (Afa)

(Afa)

Notes Reference Northern flow K/Ar 0.71 0.06 Feldspar Smith, et al.,1990 E

separate E

NNTS 9-86 K/Ar 0.79 0.09 Fleck, et al.,1996 l

Lava S of cone K/Ar 0.8 0.M

" unpublished Crowe, et al.,1995 USGS date" NNTS 8-86 K/Ar 0.82 0.07 Fleck, et al.,1996 Summit lava lake K/Ar 0.83 0.09

" unpublished Crowe, et al.,1995 USGS date" NNTS-106-89 K/Ar 0.91 0.07 Fleck, et al.,1996 S lava /BC6FVP Ar/Ar 0.94 0.05 Crowe, et al.,1995 Duplicate of BC3FVP Ar/Ar 0.%

0.15 Crowe, et al.,1995 l

NNTS 105-89 K/Ar 1.03 0.06 Fleck, et al.,1996 N lava /BCl2FVP Ar/Ar 1.05 0.08 Crowe, et al.,1995 Summit lava lake /BCIFVP Ar/Ar 1.05 0.14 Crowe, et al.,1995 TSV-2A-77 K/Ar 1.06 0.05 Fleck, et al.,1996 TSV-2-77 K/Ar 1.07 0.05 Fleck. et al.,1996 TSV-2A K/Ar 1.07 0.4 Vaniman and Crowe, g

1981 3

Lava lake S K/Ar 1.09 0.12 Feldspar Smith, et al.,1990 separate TSV-2 K/Ar 1.09 0.3 Vaniman and Crowe, Avemge: 0.95 t 0.12 Afa, AfS%'D: 3.12, %'gtd mean = 0.94 t 0.03 Ma Age must be <0.98 Ma or > 1.05 Ma to correspond with Cir.in subchron boundaries in l

Cande and Kent (1992).

Red Cone: 537580E, 4071880N. Reversed magnetic polarity (Champion,1991).

t io g

Date ermr g

Unit / Sample #

Afethod (Afa)

(Afa)

Notes Reference NNTS 10-86 K/Ar 0.84 0.12 Fleck, et al.,1996 A-4 l

l

Red Cone: 537580E, 4071880N. Reversed magnetic polarity (Champion,1991).

t io Date error Unit / Sample #

Method (Ma)

(Ma)

Notes Referenee Flow E of cone K/Ar 0.84 0.15

" unpublished Crowe, et al.,1995 USGS date" Sandia 6 K/Ar 0.93 0.1 Fleck, et al.,1996 Top of main cone K/Ar 0.95 0.08 Feldspar Smith, et al.,1990 separate Sandia 7 K/Ar 0.97 0.04 Fleck, et al.,1996 Sandia 5 K/Ar 0.97 0.06 Fleck, et al.,1996 Scoria-mound dike K/Ar 0.98 0.1 Feldspar Smith, et al.,1990 separate Main cone K/Ar 1.01 0.06 Feldspar Smith, et al.,1990 separate Flow E of cone K/Ar 1.07 0.34

" unpublished Crowe, et al.,1905 USGS date" NNTS 11-86 K/Ar 1.09 0.2 Fleck, et al.,1996 Sandia 213 K/Ar 1.09 0.16 Fleck, et al. 1996 Sandia 214 K/Ar 1.11 0.09 Fleck, et al.,1996 TSV-378-81 K/Ar 1.5 0.1 Vaniman, et al.,1982 l

Avemge:1.03 t 0.17Ma, MSWD = 2.64, Wgtdmean = 1,01 t 0.04 Mal Age must be <0.98 Ma or > 1.05 Ma to correspond with Cir.in subchron boundaries in Cande and Kent (1992).

Not included in Red Cone data set due to questionable apparent accuracy Lab B K/Ar 1.12 0.27 Average Sinnock and n=6 Easterling,1983 I2b C K/Ar 1.55 0.15 Average Sinnock and n=6 Easterling,1983 Lab A K/Ar 1.55 0.31 Average Sinnock and n=6 Easterling,1983 I

ll h

A-5

,,ve+

_____n

little Cones: 535200E,4069360N; bocca at 535480E,4069560N. Reversed magnetic polarity

'3 (Champion,1991).

t 10 g

E, ikte error Unk/ Sample #

Afethod (Afa)

(Ma)

Notes Reference SE lava K/Ar 0.76 0.2 Crowe, et al.,1995 NE cone summit K/Ar 0.77 0.04 Feldspar Smith, et al.,1990 separate Unkn.

Ar/Ar 0.904 0.011 sanidine Heizier, et al.,1994 xenocryst g

SE lava /CF15FVP Ar/Ar 1.02 0.1 Crowe, et al.,1995 E

Unkn./TSV-3-77 K/Ar 1.04 0.05 Fleck, et al.,1996 SW lava /TSV-3 K/Ar 1.11 0.3 Vaniman and Crowe, 1981 Avemge: 0.93 t 0.15 Ma, MSW = 4.18 Ma, Wgt Mean = 0.90 t 0.02 Ma Age must be <0.98 Ma or > 1.05 Ma to correspond with Clr.In subchron boundaries in Cande and Kent (1992).

1 I,

I I

I A-6 I

Buckboard Mesa: 555180E, 4109200N; second vent possble at about 555500E, 4108500N (Lutton, 1%9). Normal magnetic polarity (Minor, et al.,1993).

t io Date ermr Unit / Sample #

Method (Ma)

(Ma)

Notes Reference TSV-413-82 K/Ar 2.7 0.2 Marvin, et al.,1989 WDH-12 K/Ar 2.77 0.08 Fleck, et al.,1996 Unkn.

K/Ar 2.79 0.1 Crowe, et al.,1982 Unkn.

K/Ar 2.82 0.N Crowe, et al.,1982 WDH-Il K/Ar 2.82 0.N Fleck, et al.,1996 NNTS-15 Ar/Ar 2.91 0.01 Turrin,1995 Unkn.

K/Ar 2.93 0.03 Crowe, et al.,1995 NNTS 16-86 K/Ar 2.95 0.06 Fleck, et al.,1996 Unkn.

K/Ar 3.07 0.29 Crowe, et al.,1995 I

NNTS 15-86 K/Ar 3.21 0.12 Fleck, et al.,1996 Avemge: 2.90 t 0.15 Ma, MSWD = 2.50 Ma, Wgtd Mean = 2.90 t 0.01 Ma l

Age must be 2.60-3.05 Ma to correspond with C2An.In subchron boundaries in Cande and Kent (1992).

I I

I I

I I

[

[

A-7 A

Cmter Flat: 540330E, 4070050N: 540420E, 4068780N: 540360E, 4%8440N: 540680E, 4M8820N:

540700E,4068260N: possible vent area at 540300E, 4071600N. All reversed magnetic polarity (Champion, 1991).

Date envr Unit / Sample #

Afethod (Afa)

(Afa)

Notes Reference Sandia 32 K/Ar 3.59 0.06 Fleck, et al.,1996 Sandia 17 K/Ar 3.64 0.03 Fleck, et al.,1996 VH-1-126 K/Ar 3.64 0.10 Fleck, et al.,1996 CF79-26-1 K/Ar 3.64 0.13 Vaniman and Crowe, E

1981 E

S vent /CF10FVP Ar/Ar 3.65 0.06 Crowe, et al.,1995 Central vent /CF12FVP Ar/Ar 3.69 0.05 Crowe, et al.,1995 Sandia 212 K/Ar 3.69 0.06 Fleck, et al.,1996 Sandia 217 K/Ar 3.70 0.11 Fleck, et al.,1996 Sandia 211 K/Ar 3.71 0.11 Fleck, et al.,1996 Sandia 20 K/Ar 3.73 0.02 Fleck, et al.,1996 Sandia 215 K/Ar 3.73 0.04 Fleck, et al.,1996 Sandia 31 K/Ar 3.73 0.17 Fleck, et al.,1996 N lava /CF14FVP Ar/Ar 3.75 0.04 Crowe, et al.,1995 Unkn.

K/Ar 3.75 0.12 Crowe, et al.,1995 NNTS-14-86 K/Ar 3.75 0.07 Fleck, et al.,1996 Sandia 19 K/Ar 3.77 0.05 Fleck, et al.,1996 Sandia 29 K/Ar 3.78 0.06 Fleck, et al.,1996 Sandia 30 K/Ar 3.79 0.03 Fleck, et al.',1996 Sandia 18 K/Ar 3.81 0.08 Fleck, et al.,1996 CF79-72-24-8 K/Ar 3.84 0.15 Marvin, et al.,1989 CF79-24-8 K/Ar 3.85 0.13 Fleck, et al.,1996 NNTS-14 Ar/Ar 3.86 0.02 Turrin,1995 Unkn.

K/Ar 3.91 0.2 Crowe, et al.,1995 NNTS-13-86 K/Ar 4.01 0.13 Fleck, et al.,1996 Avemge: 3.75 t 0.10 Afa, AfSWD = 2.94, Wgtd mean = 3.76 t 0.02 Afa Age must be 3.55-4.03 Ma to correspond with C2An.3n-C3n.In subchron boundaries in Cande and Kent (1992).

Not included in Crater Flat data set due to questionable apparent accuracy A-8

[

Cmter Flat: 540330E, 4070050N; 540120E, 4068780N 540360E, 4%8440N: 540680E, 4068820N; 540700E,4068260N; possible vent area at 540300E,4071600N. All reversed magnetic polarity (Champion, 1991).

1 10 Date errt?

Unit / Sample #

Method (Ma)

(Ma)

Notes Reference

[

Lava 4, Lab B K/Ar 3.69 0.09 Average Sinnock and n=6 Easterling,1983 Lava 3, lab B K/Ar 3.73 0.06 Average Sinnock and

[

n=6 Easterling,1983 Lava 3 Lab C K/Ar 3.89 0.17 Average Sinnock and n=6 Easterling,1983 Lava 4, Lab C K/Ar 4.00 0.13 Average Sinnock and n=6 Easterling,1983

[

Lava 4, Lab A K/Ar 4.22 0.08 Average Sinnock and na6 Easterling,1983 Lava 3,I2b A K/Ar 4.27 0.46 Average Sinnock and n=6 Easterling,1983

(

{

{

l A-9 e

i I

Amargosa Desert Aeminagnetic Anomaly "B": Estimated vent location 553700E,4052900N. Reversed magnetic polarity (Langenheim, et al.,1993).

t io Ikte error Unit / Sample #

Afethod (Ala)

(Afa)

Notes Reference Well FF-251 Ar/Ar 3.85 0.05 Crowe, et al.,1995 i

Well FF-25-1 Ar/Ar 4.11 0.07 B. Turrin, unpub.

res.,1995 Well FF-25-1 Ar/Ar 4.19 0.05 B. Turrin, unpub.

res.,1995 E

Well FF 25-1 Ar/Ar 4.19 0.06 B. Turrin, unpub.

E res.,1995 Well FF-25-1 Ar/Ar 4.19 0.07 B. Turrin, unpub.

res.,1995 Well FF-25-1 Ar/Ar 4.15 0.07 B. Turrin, unpub.

g res.,1995 gi Well FF-25-1 Ar/Ar 4.16 0.06 B. Turrin, unpub.

res.,1995 Well FF-25-1 Ar/Ar 4.16 0.05 B. Turrin, unpub.

J res.,1995 Average: 4.13 t 0.11 Ma, AfSWD = 4.95, Wgtd Afean = 4.11 t 0.05 Ma.

Age must be 3.55-4.03 Ma or 4.12-4.27 Ma to correspond with C2An.3n-C3n.In subchron boundaries in Cande and Kent (1992).

I I

A-10

[

Amargosa Desen Aeromagnetic Anomalies "A ": Estimated vent locations for A1: 546100E, 4055100N;

(

A2: 546100E,4053100N: A3: 544500E, 4051400N (Connor, et al.,1997). All reversed magnetic polarity. Age of 4.1 0.1 Ma estimated by analogy with Anomaly B.

f Amargosa Desen Aemmagnetic Anomaly "C": Estimated vent location 547000E, 4042900N. Reversed magnetic polarity (L.angenheim, et al.,1993). Age of 4.1 i 0.1 Ma estimated by analogy with Anomaly B.

[

Amargosa Desen Aemmagnetic Anomaly "D": Estimated vent location 549400E, 4040000N. Normal l

magnetic polarity (Langenheim, et al.,1993). Age of 4.1 i 0.1 Ma estimated by analogy with Anomaly B and correspondence with 4.03-4.13 Ma C3n.In normal subchron in Cande and Kent (1992).

Amargosa Desen Aemmagnetic Anomaly "E" Estimated vent location 538300E,4047200N. Normal magnetic polarity (Langenheim, et al.,1993). Age of 4.1 i 0.1 Ma estimated by analogy with Anomaly B and correspondence with 4.03-4.13 Ma C3n.In normal subchron in Cande and Kent (1992).

Magnetic Anomaly, SW Crater Mat Estimated vent location 535000E, 4067800N, normal magnetic polarity (Kane and Bracken,1983; Connor, et al.,1997). Age of 6 i 1 Ma estimated by applying 0-1 Ma sedunenration rate of 0.03 mm yr~' at Little Cones (Stamatakos, et al.,1997) to modeled 150-200 m depth of burial to causative body (Ceaar, et al.,1997).

l Thirsty Mountain: Main vent location 529520E, 4112150N. Small boccas at 529480E, 4112040N and 529540E, 4111680N. Reversed magnetic polarity (Fleck, et al.,1996).

(

t io Date erwr Unit /Sannple #

Method (Ma)

(Ma)

Notes Reference f

913-6A K/Ar 4.60 0.04 Fleck, et al.,1996 9134B K/Ar 4.66 0.03 Fleck, et al.,1996

[

913-6C K/Ar 4.61 0.10 Fleck, et al.,1996 NE-10-1-91-1 Ar/Ar' 4.68 0.03 Crowe, et al.,1995

[

NE-10-1-91-2 Ar/Ar 4.88 0.04 Crowe, et al.,1995 Avernge: 4.69 t 0.11 Ma, MSWD = 7.34, WgtdMean = 4.69 t 0.05 Ma.

[-

Age must be 4.61-4.69 Ma to correspond with C3n.3n normal subchron boundaries in Cande i

and Kent (1992).

Nye Canyon: Main vent locations are Nonh unit: 604680E, 4094260N; Middle unit: 602170E, 4088960N; South unit 600950E,4085920N and 600550E,4085450N; Ring dike main vent: 599160E, 4085820N; Scarp Canyon vent: 597930E,4082470N (Hinrichs and McKay,1%5; Tschanz and Pampeyan 1970; Crowe, et al.,1986). Normal magnetic polarity (D. Champion. unpub. res.,1994).

A-11

I t io Date ermr Unit / Sample #

Afethod (Afa)

(Afa)

Notes Reference North /TSV-293-80 K/Ar 6.3 0.2 Crowe, et al.,1983a Middle /TSV-63D K/Ar 6.8 0.2 Crowe, et al.,1983a South /TSV-%

K/Ar 7.2 0.2 Crowe, et al.,1983a South /I 7420 Ar/Ar 7.27 0.03 B Turrin, unpub.

res.,1995 Ring dike /L-7405 Ar/Ar 7.34 0.03 B. Turrin, unpub.

j res.,1995 Middle /L-7400 Ar/Ar 7.36 0.05 B. Turrin, unpub.

Avemge: 7.05 t 0.07Ma, MSWD = 7.14, Wgtd Afean = 7.30 t 0.05 Afa.

Age must be 7.25-7.38 Ma to correspond with C4n.In subchron boundaries in Cande and Kent (1992).

Ml g

I I

I I

Ii I

A-12 I

Frenclunan Flat: Basaltic lava in drill holes Ue51 (286-293 m), 595260E, 4080980N; and Ue5K (289-299 m), 593520E,4081480N (Carr, et al.,1975). Drill hole locations used as vent proxies. Carr.

I et al. (1975) show 2 discrete lavas based on interpretations.of magnetic and gravity data. Magnetic polarity unknown.

i lo Date error Unit / Sample #

Method (Ma)

(Ma)

Notes Reference UES-1/940' Ar/Ar 8.57 0.21 Turrin,1995 UE5-l/950' Ar/Ar 8.4 0.16 Turrin,1995 UES-1/960' Ar/Ar 8.67 0.08 Turrin,1995 I

UE5-K/960' Ar/Ar 8.37 0.21 Turrin,1995 UES-K/970' Ar/Ar 8.43 0.19 Turrin,1995 UES-K/980' Ar/Ar 8.59 0.29 Turrin,1995 Average: 8.51 i 0.12 Ma, MSWD = 0.83, Wgtd Mean = 8.57 0.06 Ma.

Yucca Flat: Basalt [c lava in drill holes UElh (226-308 m),582980E,4095280N; UElj (415-429 m 582440E,4096580N; and UE6d (about 1000 m depth; Carr, 1984), 583740E, 4093400N (Fernald, et al.,1975). Drill hole locations used as vent proxies. Magnetic polarity unknown.

i lo Date error Unit / Sample #

Method (Ma)

(Ma)

Notes Reference UEth/784' K/Ar 8.1 0.3 Carr,1984 Rocket Wash: Main vent location estimated at 536100E,4109100N (Lipman, et al.,1966; O' Conner, et al.,1966; Crowe, et al.,1995). Unknown magnetic polarity.

I i lo Date error i

Unit / Sample #

Method (Ma)

(Ma)

Notes Reference Lava K/Ar 8

0.2 Crowe, et al.,1983a I

I A-13

~

Palute Ridge: Main vent locations estimated at 592400E, 4106800N; 592800E, 4105900N (Beyers and Barnes,1%7; Crowe, et al.,1983b); 593400E,4105500N. Cogenetic vents in northern Scarp Canyon at $94800E, 4107900N and 595800E, 4106300N. Other small, dike-fed vents are possible in the complex. Transient normal-to-reversed magnetic polarity (Ratcliff, et al.,1994).

t io Date error Unit / Sample #

Method (Ma)

(Ma)

Notes Reference Scarp Canyon dike K/Ar 8.7 0.3 Crowe, et al.,1983b TSV-309-80 K/Ar 8.5 0.3 Crowe, et al.,1983b Sanidine xenocrysts Ar/Ar 8.65 0.1 Ratcliff, et al.,1994 Sanidine xenocrysts Ar/Ar 8.66 0.18 Ratcliff, et al.,1994 Sanidine phenocryst Ar/Ar 8.59 0.07 Ratcliff, et al.,1994 Average: 8.62 t 0.08 Ma, MSWD = 0.13, Wgtd Mean = 8.61 t 0.0S Ma.

Pbhute Mesa: Main vent locations estimated at 548900E, 4133300N; 554100E, 4134500N: and 562400E, 4132700N (Ekren, et al.,1966; Noble, et al.,1%7; Crowe, et al.,1995). Unknown magnetic polarity.

t io Date enor Unit / Sample #

Method (Ma)

(Ma)

Notes Reference NE Basalt Ridge / TSV-55 K/Ar 8.8 0.1 Crowe, et al.,1983a Basalt Ridge / TSV-17 K/Ar 10.4 0.4 Crowe, et al.,1983a Basalt Ridge dike /TSV 16 K/Ar 9.1 0.7 Crowe, et al.,1983a Average: 9.4 t 0.9 Ma, MSWD = 7.57, Wgtd Mean = 8.9 t 0.1 Ma.

TSV-17 overlays 9.40 0.03 Ma Rocket Wash Tuff of the Thirsty Canyon Group (Ekren, et al.,1966; Sawyer, et al.,1994). Correlative lavas 20 km W of Basalt Ridge overlay 9.15 0.02 Ma Gold Flat Tuff of the Thirsty Canyon Group (Rogers, et al.,1%8; Sawyer, et al.,

1994).

I A-14

Basalt of Sleeping Butte: Vent locations at 525700E, 4112100N; 524300E, 4113600N; others likely.

Unknown magnetic polarity.

t io Date ermr Unit / Sample #

Method (Ma)

(Ma)

Notes Reference 913-8C Ar/Ar 9.70 0.13 Fleck, et al.,1996 913-61 Ar/Ar 9.70 0.21 Fleck, et al.,1996 913-8D Ar/Ar 9.29 0.06 R. Fleck, unpub.

res.,1995 I

RDSBc23 Ar/Ar 9.81-0.07 R. Fleck, unpub.

l res.,1995 I

RDSBb1 Ar/Ar 9.84 0.07 Fleck, et al.,1996 NNTS89-107 Ar/Ar 9.85 0.19 Fleck, et al.,1996

]

Average: 9.7 t 0.2 Ma, MSWD = 9.89, WgtdMean = 9.6 t 0.1 Ma.

Overlain by 9.40 i 0.04 Ma Rocket Wash Tuff (Fleck, et al.,1996 Sawyer, et al.,1994).

Solitario Canyon: Vent location estimated at 546800E, 4082400N, after Scott and Bonk (1984).

Unknown magnetic polarity.

t io Date er w r Unit / Sample #

Method (Ma)

(Ma)

Notes Reference TSV-168-79 K/Ar 10.0 0.4 Crowe, et al.,1983a dike K/Ar 11.7 0.3 Groundmass Smith, et al.,1997 feldspar separate large disparity in ages precludes averaging. Dike intrudes 12.70 i 0.03 Ma Tiva Canyon Tuff (Sawyer, et al.,1994). Compositionally distinct from other Miocene Crater Flat basalt (i.e.,

Crowe, et al.,1986).

A-15

Miocene Basalt of SW Cmter Flat: Vent locations estimated at 536400E,4064000N: and 534700E, 4066500N after Swadley and Carr (1987) and Connor, et al. (1997). Reversed magnetic polarity.

t io Date ermr Unit / Sample #

Method (Ma)

(Ma)

Notes Reference 1

Main vent area K/Ar 10.5 0.1 Crowe, et al.,1983a W Lava /DFCF-1 Ar/Ar 11.19 0.13 This report W Lava /DFCF-2 Ar/Ar 11.29 0.12 This report j

Avemge: 11,0 t 0.4 Ma, MSWD = 15.73, Wgtd mean = 10.9 t 0.3 Ma.

l Overlies 11.45 i 0.03 Ma Ammonia Tanks Tuff of the Timber Mountain Group (Sawyer, et al.,

1994). Compositionally similar to basalt in VH 2 drill core (i.e., Crowe, et al.,1986).

Basalt of VH-2: Vent location (s) unknown, estimated at VH-2 location 537900E, 4072950N. May extend S and W of VII-2 (Crowe, et al.,1995). Reversed magnetic polarity (Carr and Parrish,1985).

t io

\\

Date ermr Unit / Sample #

Method (Ma)-

(Ma)

Notes Reference VH-2-1200 K/Ar 11.3 0.4 Carr and Parrish, 1985 Overlies 11.45 i 0.03 Ma Ammonia Tanks Tuff of the Timber Mountain Group (Sawyer, et al.,1994).

Compositionally similar to Miocene basalt in SW Crater Flat (i.e., Crowe, et al.,1986). Possibly correlative dike in VH 1 drill core intruding Topopah Spring Tuff at 353 m (Carr,1982).

I I

I li Il 3 !

A-16 EI I

Basalt of Skull Mountain: Unknown vent locations and magnetic polarity.

t io Date enor Unit / Sample #

Method (Ma)

(Ma)

Notes Reference Unkn.

K/Ar 10.2 0.5 Crowe, et al.,1983a Overlies 11.45 i 0.03 Ma Ammonia Tanks Tuff of the Timber Mountain Group (Sargent and Stewart, 1971). May correlate with basalt in eastern Amargosa Desert (i.e., Swadley,1983).

Basalt of Kiwi Mess: Vent locations at 568940E, 4078740N and 568820E, 4079000N; others possible.

Unknown magnetic polarity.

t la Date error i

Unit / Sample #

Method (Ma)

(Ma)

Notes Reference TSV-382-81 K/Ar 11.2 0.5 Kiwi Mesa '

Marvin, et al.,1989 basalt at Little Skull Mountain TSV-382-81 K/Ar 11.4 0.5 Kiwi Mesa Marvin, et al.,1989 basalt at Little Skull Mountain Unkn.

K/Ar 9.7 0.3 Crowe, et al.,1983a TSV-370-81 K/Ar 9.9 0.4 Marvin, et al.,1989 TSV-370-81 K/Ar 10.0 0.4 Marvin, et al.,1989 Average: 10.4 t 0.9 Ma, MSWD = 3..M, Wgtd Mean = 10.2 & 0.3 Ma.

Overlies 11.45 i 0.03 Ma Ammonia Tanks Tuff of the Timber Mountain Group (Ekren and Sargent,1%5).

l A-17

Basalt of Little Skull Mountain: Unknown vent locations and magnetic polarity. Multiple flow units, compositionally diverse (Crowe, et al.,1986),

i lo Date error Unit / Sample #

Method (Ma)

(Ma)

Notes Reference Upper lava K/Ar 8.4 0.4 Crowe, et al.,1995 l

Overlies 11.45 i 0.03 Ma Ammonia Tanks Tuff of the Timber Mountain Group (Sargent and Stewart,1971). Date thought too young (Crowe, et al.,1995) based on correlation with basalt of Jackass Flat.

I Basalt of Jackass Flat: Unknown vent locations and magnetic polarity.

la Date error Unit / Sample #

Method (Ma)

(Ma)

Notes Reference j

TSV-372-81 K/Ar 11.0 0.5

" Poor date" Marvin, et al.,1989 TSV-372-81 K/Ar 9.6 0.4

" Preferred Marvin, et al.,1989 date" Overlies 12.80 0.03 Ma Topopah Spring Tuff of the Timber Mountain Group (McKay and Williams, 1964). Marvin, et al. (1989) identify 11.0 Ma date as " poor" and the 9.6 Ma date as the

" preferred date" based on analytical uncertainties.

Basalt of Northeastern Amargosa Desert: Near-vent location at 563300E,4046500N (i.e., Swadley, 1983); other locations likely. Compositionally distinct from Basalt of Southeastern Amargosa Desert; contains quartz phenocrysts (?) and may correlate with Basalt of Little Skull Mountain.

Likely correlates with shallow, highly reflective unit in western part of AV-1 seismic reflection line (Brocher, et al.,1993). Unusual shallow inclination to paleomagnetic direction may correlate with Basalt of Southeastern Amargosa (D. Champion, pers. comm.,1995).

Il I

I A-18

Basalt of Southeastern Amargosa Desen: Exposed only in drill cores and cuttings at drill holes MSH-C (565300E,4039700N; 149-157 m) and water wells around $69200E, 4043600N (Johnston,1%8). Likely correlates with shallow, highly reflective unit in eastern pan of AV-1 seismic reflection line (Brocher, et al.,1993).

t io Date enor Unit / Sample #

Method (Ma)

(Ma)

Notes Reference MSH-C-501 K/Ar 9.6 0.1 R. Fleck, unpub.

res.1996

)

Shallow inclination to MSH-C n=l=naPR direction (30 ) similar to direction of Northeastern Amargosa Desen basalt outcrops (D. Champion, pers. comm.1995). Fine-grained holocrystalline olivine-pyroxene basalt compositionally distinct from Northeastern Amargosa Desert basalt (Hill and Luhr, unpub, res.1997).

Basalt of the Specter Range: Small outcrops of subvolcanic basalt in the Specter Range at 571500E, 4057600N and 571900E, 4055350N (Sargent and Stewart,1971). Compositionally primitive and distinct from other basalts within 20 km (Hill and Luhr, unpub. res.). Intrudes carbonate rocks of the Cambrian Bonanza King Formation (Sargent and Stewart,1971).

l i

A-19

i I

Basalt of Dome Mountcin: Unknown vent locations, normal magnetic polarity (Minor, et al.,1993).

t io Date enor Unit / Sample #

Method (Ma)

(Ma)

Notes Reference Unkn.

K/Ar 10.8 0.5 Kistler,1%8 Upper /RR30al6R Ar/Ar 10.4 0.4 R. Fleck, unpub res.,1995 Main /RR30a4 Ar/Ar 10.7 0.2 R. Fleck, unpub res.,1995 Lower /RR30al4 Ar/Ar 10.7 0.07 R. Fleck, unpub res.,1995 Basal /RR30al5 Ar/Ar 10.5 0.05 R. Fleck, unpub res.,1995 j

Avemge: 10.6 t 0.2 Ma, MSWD = 1,98, Wgtd mean = 10.58 t 0.06 Afa.

Overlies 11.2-11.4 Ma Beatty Wash Formation and is overlain by 10.3 Ma Rhyolite of Shoshone Mountain (Christiansen and Lipman,1%5; Minor, et al.,1993). Date concordant with 9.78-10.83 normal polarity chron C5n.2n (Candie and Kent,1992).

Basalt of the Amargosa Range: Diffuse outcrops of highly eroded basaltic rock between 505000E 4077000N and 521000E,4062000N, on the eastern flank of the Amargosa Range, Nevada.

t la Reference (Ma)

Unit / Sample #

Method Date ermr h%tes (Ma)

Basalt BF-383 K/Ar 9.0 0.3 Marvin, et al.,

1989 Basalt BF-380 K/Ar 10.3 0.4 Marvin, et al.,

1989 Basalt BF-379 K/Ar 7.5 0.3 Marvin, et al.,

1989 Basalts BF-383 and -380 overlay 11.45 i 0.03 Ma Ammonia Tanks Tuff of the Timber Mountain Group (Marvin et al.,1989: Sawyer, et al.,1994).

I Beatty Basalt: Vents preserved at $25300E, 4085600N; 527400E, 4085200N; and 514800E, 4090800N (Maldonado and Hausback,1990); other vents likely. Unknown magnetic polarity. Diffuse outcrops of highly eroded basaltic rock between 502000E,4081000N and 529000E,4092000N.

t la Reference (Afa) g Unit / Sample #

Method Date enor Notes (Afa)

E Lava Tb2 K/Ar 10.3 0.4 Maldonado and Hausback (1990)

A-20 I

r l

Beasty Basalt: Vents preserved at 525300E, 4085600N; 527400E, 4085200N; and 514800E, 4090800N (Maldonado and Hausback,1990); other vents likely. Unknown magnetic polarity. Diffuse outcrops of highly eroded basaltic rock between 502000E, 4081000N and 529000E, 4092000N.

t la Reference (Ma)

Unit / Sample #

Method Me ermr Notes (Ma)

Basalt BF-380A K/Ar 8.1 0.4 Marvin, et al.,

1989 Tbl/102887-3 K/Ar 10.7 0.2 Monsen, et al.,

1992 Lava TB2 overlain by 10.0 i 0.4 Ma locally erupted latite lavas (unit TI), Maldonado and Hausback (1990). Basalt BF-380A overlies 9.36 i 0.02 Ma Pahute Mesa Tuff of the Thirsty Canyon Group (Marvin, et al.,1989: Sawyer et al.,1994).

Basalt of the Grapevine Mountains: Eroded vents at 476400E, 4101800N, 476800E, 4102700N, and 477900E, 4106600N; more vents likely to west. Poor age constraints, likely late Pliocene or Pleistocene (Albers and Stewart,1972). Similar isotopic character as YMR basalts (Yogodzinski and Smith,1995: Hill and Connor,1996).

Basd of the Fhneral Fomesson: Over 25 vents exposed between about 515000E, 4010000N and 542000E, 4030000N (McAllister, 1970,1971,1973; Conway, et al.,1997). Normal and reversed magnetic polarities.

Similar isotopic character as YMR basalts (Yogodzinski and Smith,1995 Hill and Connor,1996).

i t la Reference (Ma)

{

Unit / Sample #

Method Me erm r Notes (Ma)

TSV-383-81 K/Ar 4.0 0.1 Marvin, et al.,

1989 N Ryan area K/Ar 4.03 0.12 McAllister,1973 E Black Mtns.

K/Ar 4.90 Unkn.

Asmerom, et al.,

1994 Funeral Fm., upper?

K/Ar 3.20 Unkn.

Asmerom, et al.,

1994 A-21

I Basalt of Southern Death Valley: Cinder liill vent at 523900E, 3977100N. Multiple vents possible for Shoreline Butte, summit 526200E, 3973700N. Unknown magnetic polarities. Similar isotopic character as YMR basalts (Yogodzinski and Smith,1995; Hill and Connor,1996).

t la Reference (Mal g

Unit / Sample #

Method Date ermr Notes (Ma) g:

Shoreline Butte K/Ar 1.5 Unkn.

Wright and Troxel, 1984 Cinder liill K/Ar 0.69 Unkn.

Wright and Troxel.

{

1984 1

I' I

I I

I A-22 I

APPENDIX REFERENCES l

Albers, J.P., J.H. Stewart, Geology and Mineral Deposits of Esmeralda County, Nevada, Nevada Bureau of Mines and Geoloav Bulletin 78. Reno, NV: University of Nevada,1972.

Asmerom, Y., S.B. Jacobsen, B.P. Wernicke, Variations in magma source regions during large-scale continental extension, Death Valley region, westem United States, Earth and Planetary Science Letters 125: 235-254,1994.

Beyers, F.M., H. Barnes, Geologic Map of the Paiute Ridge Quadrangle, Nye County, Nevada, U.S. Geoloaical Survev Geoloaical Quadranale Mao GO-577. Reston, VA:

U.S. Geological Survey,1967.

I Brocher, T.M., M.D. Carr, K.F. Fox, P.E. Hart, Seismic reflection profiling across Tertiary extensional structures in the eastem Amargosa Desert, southem Nevada, Basin and Range province. Geoloaical Society of America Bulletin 105: 30-46,1993.

Cande, S.C., D.V. Kent, A new geomagnetic polarity time scale for the late Cretaceous and Cenozoic, Joumal of Geoohvsical Research 97(B10): 13,917-13,951,1992.

Carr, W.J., Volcano-TectonicHistory of CraterFlat, Southwestem Nevada, as Suggested by New Evidence from DrillHole USU-VH-1 and Vicinity, U.S. Geoloaical Survey Ooen-File Reoort 82-457.

Reston, VA: U.S. Geological Survey,1982.

Carr, W.J., Regional and Structural Setting of Yucca Mountain, Southwestem Nevada, and Late Cenozoic Rates of Tectonic Activityin Part cf the Southwestem Great Basin, Nevada and Califomia, U.S. GeoloaicalSurvey Ooen-File Renort 84-854. Reston, VA: U.S. GeologicalSurvey, 1984.

Carr, W.J., L.D. Parrish, Geology of Drill Hole USW VH-2, and Structure of Crater Flat, Southwestem Alevada, U.S. Geolooice.' Survey Ooen-File Reoort 85-475. Reston, VA:

U.S.

Geological Su'rvey,1985.

Carr, W.J., G.D. Bath, D.L. Healey, R.M. Hazelwood, Geology of Northem Frenchman Flat, 4

Nevada Test Site. U S. GeolooicalSurvey Ooen-File Reoort 474-216. Reston, VA: U.S. Geological Survey,1975.

Champion, D.E., Volcanic episodes near Yucca Mountain as determined by paleomagnetic studies at Lathrop Wells, Crater Flat, and Sleeping Butte, Nevada, Proceedinas of the Second AnnuallntemationalConference on Hiah-Level RadioactiveWaste Manaaement La Grange.cark, IL: American Nuclear Society: 61-67,1991.

Christiansen, R.L., P. Lipman, Geologic Map of the Topopah Spring NW Quadrangle, Nye County, Nevada. U.S. Geoloaical Survey Geolooical Quadranule Mao GO-444 Reston, VA:

U.S.

Geological Survey,1965.

A-23

Connor, C.B., S. Lane-Magsino,J.A. Stamatakos, R.H. Martin, P.C. La Femina, B.E. Hill, S. Lieber, Magnetic surveys help reassess volcanic hazards at Yucca Mountain, Nevada, EOS, Transactions of the American Geopbysical Union 78(7): 73-78,1997.

E E

Conway, F.M., C. Connor, B. Hill, D. Ferrill, Landsat TM, SPOTand SLAR interpretationof volcanic and structural features of the Greenwater and Saline Ranges, Inyo County, Califomia, USA, g

Volcanic Activity and the Environmert. lAVCEI Abstracts. Unidad Editorial: Guadalajara, Mexico, l

68,1997.

Crowe, B., F. Perry, Preliminarygeologicmap of the SleepingButte Volcanic Centers, Los Alamos National Laboratorv Reoort LA-12101-MS. Los Alamos, NM: Los Alamos National Laboratory, 1991.

Crowe, B.M., M.E. Johnson, R.J. Beckman, Calculation of the probability of volcanic disruption of a high-level nuclear waste repository within southern Nevada, USA, Radioactive Waste Manaoement and the Nuclear Fuel Cvele 3: 167-190,1982.

Crowe, B.M., D.T. Vaniman, W.J. Carr.1983a, Status of Volcanic Hazard Studies for the Nevada Nuclear Waste Storage Investigations, Los Alamos National Laboratorv Reoort LA-9325-MS.

Los Alamos, NM: Los Alamos National Laboratory,1983a.

Crowe, B., S. Self, D. Vaniman, R. Amos, F. Perry, Aspects ofpotentia/ magmatic disruption of a high-level radioactive waste repository in southem Nevada, Joumal of Geoloov 91: 259-276, 1983b.

Crowe, B.M., K.H. Wohletz, D.T. Vaniman, E. Gladney, N. Bower, Status of Volcanic Hazard Studies for the Nevada Nuclear Waste Storage Investigations, Los Alamos National Laboratory Reoort LA-9325-MS. Vol. II. Los Alamos, NM: Los Alamos National Laboratory,1986.

Crowe, B.M., F.V. Perry, J. Geissman, L. McFadden, S. Wells, M. Murrell, J. Poths, G.A. Valentine, L. Bowker, K. Finnegan, Status of Volcanic Hazard Studies for the Yucca Mountain Site g

CharacterizationProject, Los Alamos National Laboratorv Reoort LA-12908-MS, Los Alamos, NM:

E Los Alamos National Laboratory,1995.

Ekren, E.B., and K.A. Sargent, Geologic Map of the SkullMountain Quadrangle, Nye County, Nevada, U.S. Geolooical Survev Geological Quadranole Mao GO-387. Reston, VA:

U.S.

Geological Survey,1965.

Ekren, E.B., R.E. Anderson, P.P. Orkild, E.N. Hinrichs, Geologic Map of the Silent Butte Quadrangle, Nye County, Nevada, U.S. Geolooical Survev Geological Quadranole Mao GO-493, g

Reston, VA: U.S. Geological Survey,1966.

E Faulds, J.E., J.W. Bell, D.L. Feuerbach, A.R. Ramelli, Geologic Map of the Crater Flat Area, Nevada. Nevada Bureau of Mines and Geoloov Mao 101. Reno, NV: Nevada Bureau of Mines and Geology,1994.

I A-24

n Femald, A.T., F.M. Beyers, Jr., J.P Oh!., LithologicLogs and Stratigraphic Units of Drill Holes and Mined Shaltsin Areas 1 and 6, Nevada Test Site, U.S. GeoloaicalSurvev Reoort 474-206. Reston, VA: U.S. Geological Survey,1975.

Fleck, R.J., B.D. Turrin, D.A. Sawyer, R.G. Warren, D.E. Champion, M.R. Hudson, S.A. Minor, Age and character of basaltic rocks of the Yucca Mountain region, southem Nevada, Joumal of Geoohvsical Research 100(B4): 8,205-8,227,1996.

Heizier, M.T., W.C. McIntosh, F V. Perry, B.M. Crowe, "ArMArResults ofIncompletely Degassed Sanidine Age of Lathrop We/Is Volcanism, U.S. Geolooical Survev Circular 1107. Reston, VA:

U.S. Geological Survey: 113,1994.

Hill, B.E., and C.B. Connor, Volcanic Systems of the Basin and Range. NRC High-Level Radioactive Weste Research at CNWRA, July-December 1995, CNWRA 95-02S. San Antonio, TX: Center for Nuclear Waste Regulatory Analyses: 5-1 to 5-21,1996.

Hinrichs, E.N., E.J. McKay, Geologic Map of the Plutonium Valley Quadrangle, Nye and Lincoln Counties, Nevada, U.S. Geoloaical Survev Geoloaical Quadranale Mao GO-384. Reston, VA: U.S.

Geological Survey,1965.

Johnston, R.H., U.S. Geological Survey Tracer Study, Amargosa Desert, Nye County, Nevada, Part 1: ExploratoryDrilling, Tracer Well Construction and Testing, and Preliminary Findings.U.S.

Geological Survey Open-File Report 68-152. Reston, VA: U.S. Geological Survey,1968.

Kane, M.F., R.E. Bracken, Aeromagnetic Map of Yucca Mountain and Surrounding Regions, SouthwestNevada, U.S. GeoloaicalSurvey Open-File Reoort 83-616. Reston, VA: U.S. Geological Survey,1983.

1 Kistler, R.W., Potassium-argon ages of volcanic rocks in Nye and Esmeralda Counties, Nevada.

Geoloaical Society of America Memoir 110. Rau! der. CO: Geological Society of America:

251-262,1968.

Langenheim, V.E., K.S. Kirchoft-Stein, H.W. Oliver, Geophysicalinvestigations of buried volcanic

\\

centers near Yucca Mountain, southwestem Nevada, Proceedinas of Fourth Annual IntemallQual Conference on Hiah-Level Radiondive Weste Manaaement. La Grange Park, IL: American Nuclear Society: 1,840-1,846, 1993.

Lipman, P.W., W.D. Quinlivan, W.J. Carr, R.E. Anderson, Geologic Map of the Thirsty Canyon SE Quadrangle, Nye County, Nevada. U.S. Geoloaical Survev Geoloaical Quadranale Mao GO-489.

Reston, VA: U.S. Geological Survey,1966.

Lutton, R.J., intemal structure of the Buckboard Mesa basalt, Bulletin of Volcano'oav 33:

579-593,1969.

A-25 l

I Maldonado, F., and B.P. Hausback.1990. Geologic Map of the Northeast Quarter of the Bullfrog

(

15-Minute Quadrangle, Nye County, Nevada. U.S. Geological Survey Miscellaneous investigatiors Map l-2049. Reston, VA: U.S. Geological Survey,1990.

I Marvin, R F., H.H. Mehnert, E.H. McKee, A summary of radiometricages of Tertiary volcanic rocks in Nevada and eastem California, Part Ill: southeastern Nevada. IsochronlWest 6: 1-30,1993.

l Marvin, R.F., H.H. Mehnert, C.W. Naeser.1989, U.S. Geological Survey radiometric ages:

Compilation C, part three: Califomia and Nevada. Isochron / West 52: 3-11,1989.

]

McAllister, J.F., Geology of the FuneralCreek Borate Area, Death Valley, Inyo County, Califomia, California Division of Mines and Geoloov Mao Sheet 14. Sacramento, CA: Califomia Division of Mines and Geology,1970.

McAllister, J.F., Preliminary Geologic Map of the Funeral Mountains in the Ryan Quadrangle, Death ValleyRegion, Inyo County, Califomia, U S. Geological Survey Mao 71-187. Reston, VA:

U.S. Geological Survey,1971.

McAllister, J.F., Geologic Map Sections of the Amargosa Valley Borate Area - Southeast Continuation of the Fumace Creek Area - Inyo County, Califomia, U.S. Geoloaical Survev Miscellaneous Geolocical Investiaations Mao I-782. Reston, VA: U.S. Geological Survey,1973.

McKay, E.J., W.P. williams, Geologic Map of the Jackass Flats Quadrangle, Nye County, Nevada, U S. GeologicalSurvev GeologicalOuadrangle Mao GO-368. Reston, VA: U.S. GeologicalSurvey, 1964.

Minor, S.A, D.A. Sawyer, R.R. Wahl, V.A. Frizzell, Jr., S.P Schilling, R.G. Warren, P.P. Orkild, J.A.

Coe, M.R. Hudson, R.J. Fleck, M.A. Lanphere, W.C. Swadley, J.C. Cole, Preliminary Geologic Map g

of the Pahute Mesa 30'x60' Quadrangle, Nevada, U.S. Geolooical Survey Ooen-File Reoort 93-3 299, Reston, VA: U.S. Geological Survey,1993.

Monsen, S.A., M.D. Carr, M.C. Reheis, P.P. Orkild, Geologic Map of Bare Mountain, Nye County, Nevada, U.S. Geological Survev Mao 1-2201. Reston, VA: U.S. Geological Survey,1992.

Noble, D.C., R.D. Krushensky, E.J. McKay, J.R. Ege, Geologic Map of the Dead Horse Flat Quadrangle, Nye County, Nevada, U.S. Geoloaical Survev Geoloaical Quadranole Mao GO-614.

Reston, VA: U.S. Geological Survey,1967.

O' Conner, J.T., R.E. Anderson, P.W. Lipman, Geologic Map of the Thirsty Canyon Quadrangle, Nye County, Nevada, U.S. Geological Survev Geological Quadranale Mao GO-524. Reston, VA:

U.S. Geological Survey,1966.

Ratcliff, C.D., J.W. Geissman, F.V. Perry, B.M. Crowe, P.K. Zeitler, Paleomagnetic record of a geomagnetic field reversal from late Miocene mafic intrusions, southem Nevada, Science 266:

412-416,1994.

I A-26

[

Rogers, C.L., E.B. Ekren, D.C. Noble, J.E. Weir, Geologic Map of the Northem Half of the Black

. Mountain Quadrangle, Nye County, Nevada,- U S. Geoloaical Survey Miscellaneous Geoloaic Investiaations Mao I-545. Reston, VA: U.S. Geological Survey,1968.

. Sargent, K.A., J.H. Stewart, Geologic Map of the Specter Range NW Quadrangle, Nye County, Nevada, U S. Geoloaical Survev Geolooical Quadranale Mao GO-884. Reston, VA:

U.S.

Geological Survey,1971, Sawyer, D.R., R.J. Fleck, M.A. Lanphere, R.G. Warren, D.E. Broxton, and M.R. Hudson, Episodic caldera volcanism in the Miocene southwestem Nevada volcanic field: Revised stratigraphic framework, *Ar/"Ar geochronology, and implications for magmatism and extension, Geologscal

~

Society of America Bulletin 106:1,304-1,318,1994.

Scott, R.B., J. Bonk, Preliminary Geologic Map of Yucca Mountain, Nye County, Nevada, with GeologicSections, U.S.GecloaicalSurveyOnen-FileRanart84-494. Reston,VA: U.S. Geological Survey,1984.

Sinnock, S., R.G. Easterling, EmpiricallyDetermined Uncertaintyin Potassium-AtgonAges for Plio.

Pleistocene Basalts from Crater Flat, Nye County, Nevada, Sandia National t ahnratorv Rooort SAND 82-2441. Albuquerque, NM: Sandia National Laboratory,1983.

Smith, E.l...D.L. Feuerbach, T.R. Naumann, J.E. Faulds, AnnualReport of the Center for Volcanic and Tectonic Studies for the Period 10-1-89 to 9-30-90, Report 41. Carson City, Nevada: The Nuclear Waste Project Office,1990.

Smith, E.1., S. Morikawa, A. Sanchez, Summary of the Activities of the Center for Volcanic and Tectonic Studies, University of Nevada, Las Vegas, for the Period 1986-1996, Carson City, Nevada: The Nuclear Waste Project Office,1997.

Stamatakos, J.A., C.B. Connor, R.H. Martin, Quatemarybasin evolution and basaltic volcanism of

\\

CraterFlat, Nevada, from detailedgroundmagneticsurveys of the Little Cones, Jaumalof Geoloav 105: 319-330,1997.

Swadley, W.C., Map Showing Surficial Geology of the Lathrop Wells Quadrangle, Nye County, Nevada, U.S. Geoloaical Survey Miscellaneous investiaations Series Maa 1-1361. Reston, VA:

U.S. Geological Survey,1983.

Swadley, W.C, W.J. Carr, Geologic Map of the Quatemary and Tertiary Deposits of the Big Dune Quadrangle, Nye County, Nevada, and Inyo County, California, U.S. Geoloaical Survey Miscellaneous investiaations Series Mao I-1767. Reston, VA: U.S. Geological Survey,1987.

Taylor, J.K., Statistical Techniques for Data Analysis. Boca Raton, FL: CRC Press,1990.

1 l

A-27

i Tschanz, C.M, E.H. Pampeyan, Geology and Mineral Deposits of Lincoin County, Nevada, Nevada Bureau of Mines and Geoloav Bulletin 73. Reno, NV: Nevada Bureau of Mines and Geology,1970.

Turrin, B., Geochronology data for the Yucca Mountain Region, Presentation to the DOE Probabilistic Volcanic Hazard Assessment Elicitation Panel, February 22, 1995, Meeting in Phoenix, Arizona, San Francisco, CA: Geomatrix Consultants,1995.

Turrin, B.D., D. Champion, R.J. Fleck, *ArPAr age of the Lathrop Wells volcanic center, Yucca Mountain, Nevada. Science 253: 654-657,1991.

Turrin, B.D., D. Champion, R.J. Fleck, Measuring the age of the Lathrop Wells volcanic center at Yucca Mountain: Response. Science 257: 556-558,1992.

I Vaniman, D., B. Crowe, Geology and Petrology of the Basalts of Crater Flat: Applications to Volcanic Risk Assessment for the Nevada Nuclear Waste Storage Investigations, Los Alamos NationalLaboratorv Recort LA-8845-MS, Los Alamos, NM: Los Alamos NationalLaboratory,1981.

Vaniman, D.T., B.M. Crowe, E.S. Gladney, Petrology and geochemistry of Hawaiite lavas from Crater Flat, Nevada, Contributions to Mineraloav and Petroloav 80: 341-357,1982.

j Wright, L.A., B.W. Troxel, Geology of the North 1/2 Confidence Hills 15 Minute Quadrangle,

/nyo County, Califomia, Califomia Division of Mines and Geoloav Mao Sheet 34, Sacramerto, CA:

California Division of Mines and Geology,1984.

Yogodzinski, G.M., E.l. Smith, Isotopic domains and the area ofinterest for volcanic hazard lj assessmentin the Yucca Mountain area, EOS, Transactions of the American Geoohvsical Union E

76(46): F669,1995.

i a

I 4

A-28