ML20003F918
| ML20003F918 | |
| Person / Time | |
|---|---|
| Issue date: | 09/19/1980 |
| From: | Murley T NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Harold Denton, Monogue R Office of Nuclear Reactor Regulation, NRC OFFICE OF STANDARDS DEVELOPMENT |
| References | |
| RIL-105, NUDOCS 8104230885 | |
| Download: ML20003F918 (4) | |
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4-UNITED STATES
[p ng NUCLEAR REGULATORY COMMisstON o
WASHINGTON, D. C. 20555 g ) ),,
g SEP 191980 MEMORANDUM FOR: Harold R. Denton, Director Office of Nuclear Reactor Regulation Robert B. Minogue, Director Office of Standards Development FROM:
Thomas E. Murley, Acting Director Office of Nuclear Regulatory Research
SUBJECT:
RESEARCH INFORMATION LETTER N0. 105 GEOPHYSICAL CORRELATIONS AND MODELING OF A SELECTED AREA ACROSS THE CLINTON-NEWBURY/ BLOODY BLUFF FAULT ZONES IN NORTHEASTERN MASSACHUSETTS INTRODUCTION AND
SUMMARY
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This memorandum transmits the results of a gravity survey of the Clinton-Newbury and Bloody Bluff fault zones in northeastern Massachusetts.
It is a study correlating gravity anomalies, geomagnetic contours, and the surface bedrock. The work was doo,e by the Massachusetts Institute of Technology under contract to the Office of Nuclear Regulatory Research (contract number NRC-04-76-209). The principal investigator was Dr. Steven R. Taylor.
Previous geophysical work in the survey area consists of a Bouguer gravity map of the Boston Basin region, Ginsburg (1959), and a simple Bouguer gravity map of Massachusetts, Bromery (1967). Total intensity aeromagnetic maps published by the U.S. Geological Survey (USGS) were available for the l
area of this survey. The data were collected along a nearly east-west traverse with a flight spacing of 0.8 km and elevation of 150 m abo've i
ground level.
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A geological map was compiled from preliminary geologic maps of the Shirley (Russel and Allmendinger, 1975), Ayer (Gore, 1975), Westford (Albord, 1975),
and Maynard (Hansen,1956) quadrangles and was supplemented with information on the Boston 2 sheet (Barosh,1977). The fault zone consists of a series of northeast striking middle to late Paleozoic imbricate thrusts which dip to the northwest. The stratigraphy is characterized by northwest dipping thick eugeosynclinal metasediments and metavolcanics ranging in age from late Precambrian to middle Paleozoic. Numerous Acadian intrusives are present in the stratified sequences.
The area surveyed crosses a highly deformed northeast-trending Paleozoic
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thrust belt extending from southern Connecticut through eastern Massachusetts.
Magnetic anomalies associated with the formations in the thrust belt suggest that the faults continue offshore in an east-northeast direction into the Gulf of Maine (Weston Geophysical,1976), and possibly into New Brunswick (Nelson,1976).
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Harold R. Denton Robert B. Minogue 2
The Clinton-Newbury fault is recognized to be a major structural feature in the region.
It eccupies a prominent strike valley along which a mylonite zone of variable thickness can be traced. The fault forms a conspicuous magnetic lineament between an area of strong magnetic relief to the south-east, and moderate relief to the northwest (Castle,1976). No stratigraphic units can be positively correlated across the fault which suggests offsets on the order of tens of kilometers.
Southeast of the Clinton-Newbury fault zone exists a belt of northwest dipping imbricate thrust faults (Skehan,1969). The stratigraphic units dip steeply to the northwest and are cut by numerous acidic to intermediate intrusions. Between the Clinton-Newbury and the Bloody Bluff fault zones, stratigraphic units include the Tadmuch Brook schist, the members of the Nashoba formation, and the Marlboro formation.
Intrusive rocks include the Assabet quartz diorite, Andover granite, and Dedham granodiorite.
The Bloody Bluff fault zone marks the southeast border of the thrust belt.
Southeast of the Bloody Bluff fault zone the metamorphic isograds decrease to chlorite grade and the structural trends shift from the northeast to the east. The Bloody Bluff fault zone is also associated with a magnetic linea-ment and can be traced on the gravity map of this survey.
Tectonic Implications a
The northern Appalachians are the result of one semi-continuous orogenic f
episode extending from late Precambrian to Cretaceous-time. Most of the 1
protoliths of the metasediments and metavolcanics found in the area repre-I sent eugeosynclinal deposits laid down in early Paleozoic time. The l
predominant structural features, metamorphic zoning, and intrusions are the result of middle to late Paleozoic Acadian and Alleghanian orogenies.
Nelson (1976) suggests that the Clinton-Newbury and Bloody Bluff fault zones represent the suture zone in an area of continental convergence.
METHODOLOGY Measurements J
A total of 232 stations were occupied in the area under study. The survey area was confined to a strip approximately 40 km in length and 5 km in width oriented in a northwest-southeast direction perpendicular to the j
strike of the Paleozoic thrust belt in ecstern Massachusetts.
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i Harold R. Denton 3
Robert B. Minogue
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Gravity measurements were made at most of the available USGS benchma In areas without adequate at Department erf Public Works survey markers. elevation control, r a
accurately on a topographic map, such as road intersections, All of the readings were tied to a base station located in the courtyard at Draper Labs in Cambridge, Massachusetts, where the absolute a gravity transfer from the Air Force Cambridge Research Laboratories is known to be 980.3874 gal.
A LaCoste and Romberg gravity site in the Haskell Seismic-Gravity Facility.
model G gravimeter was used in the transfer.
Crustal Models_
The strip covered in this study is oriented perpendicular to the strike of the regional structural fabric and, therefore, is analyzed using the two-The vertical dimensional modeling technique of Talwani et al. (1959)
At specified points above model two-dimensional bodies of arbitrary shape.is found by summing all of the contribu the structure being modeled, gz The calculations of the anomalies require from each side of the polygons.
This fact only the polygon shapes and the relative density contrasts.Without any emphasizes the nonuniqueness inhereht in the gravity models.
geologic or density control, the infinite infonnation regarding densities, structures, and mapped distributions of geologic formations will decrease Densities were the number of degrees of freedom involved in the modeling.
measured for all of the formations included in the structural mod Based on the constraints of structure and stratigraphy, the initial modelIto n had all of the units dipping at an angle of 60The value of 2 km was selected average wavelength of tne residual gravity field is approximately 15 km.
to a depth of 2 km.
Comparison of this value with the wavelength of the gravitational field associated with a buried horizontal cylinder suggests that the contribution of the surrounding material to the observed anomalies become negligible depths greater than 2 km.
The final model was derived by adjusting dips of various formations, varyi the shape of intrusive bodies, and truncating units ag For example, geological evidence indicates an unconformable contact betw faults.
the Devens-Long Pond Gneiss and the overlying Eliot Formation (Gore Adjustments made in constructing the final structural model suggest contact is an angular unconfonnity.
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Harold R. Denton Robert B. Minogue 4
CONCLUSIONS A gravity strip map crossing a major thrust belt in eastern Massachusetts has been prepared.
Good correlation exists between the gravity field and the mapped geology. The two locations of highest gravity gradient occur over the Bloody Bluff and Clinton-Newbury faults which support previous geological and geophysi-cal interpretations that these are the major faults in a broad thrust zone.
Using measured densities, a two-dimensional gravity profile has been computed which is consistent with a series of northwest dipping formations and thrusts.
Data from total intensity aeromagnetic maps were used to test the gravity model.
l The unreasonably high susceptibilities required to fit the observed magnetic anomalies suggest that a substantial component of natural remanent magnetism exists in many of the formations between the Clinton-Newbury and Bloody Bluff fault zones.
RECOMMENDATIONS This is a significant study providing further corroborative detail to the previously mapped Clinton-Newbury and Bloody Bluff fault zones.
It is recom-mended that these results be incorpgrated into the growing body of knowledge concerning the geomorphology, tectoni'cs, and seismicity of the region, and they be taken into consideration in future estimates of the seismic hazard associated with these fault systems.
Note: This study is published in its entirety in NUREG/CR-1186.
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Thomas E. Murle
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