ML19296B825

Forwards Comments on Draft Rept, Characterization & Classification of Geologic Faults in Appalachian Foldbelt. No Taxonomic Basis for Classifications of Faults.Annotated Pages of Rept Encl

ML19296B825
Person / Time
Issue date:	11/28/1979
From:	Alterman I Office of Nuclear Reactor Regulation
To:	Odonnell E NRC OFFICE OF STANDARDS DEVELOPMENT
References
CON-NRC-01-78-004, CON-NRC-1-78-4, TASK-OS, TASK-SS-802-9 NUDOCS 8002220115
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{{#Wiki_filter:* f h, UNITED STATES 3 -r, g NUCLEAR REGULATORY COMMISSION y ,,.l, ' j W ASHINGTON, D. C. 20555 %'% j[ NOV 2 8 lug MEMORANDUM FOR: led O'Donnell Site Safety Standards Branch, OSD THRU: Leon Reiter, Leader Geology and Seismology Section Geosciences Branch, DSS FROM: Ina Alterman, Geologist Geology and Seismology Section Geosciences Branch, DSS

SUBJECT:

FAULT CLASSIFICATION REPORT I am enclosing herewith the draft manuscript of "A Characterization and Classification of Geologic Faults in the Appalachian Foldbelt" which contains some of my detailed coments. Also enclosed are my general review coments of the report and those of Richard McMullen. Last week, Sandra Wastler of our branch sent you her review coments of the report..The present enclosures should be included with hers to complete the reviews by Geosciences Branch geologists. We hope our comments and observations will be of value to the authors in the successful completion of their task of meeting NRC goals for their project. Ina B. Alterman, Geologist Geology and Seismology Section Geosciences Branch Division of Systems Safety cc: R. Jackson L. Reiter I. Alterman 220 8002 a22 29

sq g sie : c. a. in (arc) General Cc:.ments on "A Characterization and Classification of Geologic Faults in the Appalachian Foldbelt" I. Basis of Classification: There is no taxonomic basis for the classification. The lack of a consistent or uniform basis for class-ification has led to contradictions and repetitions. The most serious consequenceofthisnon-taxcnoaicclassificationisthatseheralfaults haYebeenputintomorethanoneclass(e.g.WesternBerkshiremassif thrusts are listed as type example of Class 3, Subclass 3 and also as an example of Class 7; Goat Rock is listed as Class 6 and Class 7; Ramapo is both a Class 7 and a Class 9 example). Anadhantagetoataxenomic classification is that it pays more attention to the similarities and groups entitiesonthatbasis,subdihidingbydistinctionsinthedetailsinsome consistent way. Theaforementionedfaultsobhiouslyhahemoresimilarities than differences and would probably fare better classed together and subdihidedbytheirminordifferences(whichseemstobetheirtimeofemplace-mentrelatihetoregionalmetamorphism,accordingtotheauthors' scheme). II. Addressing NRC purposes and goals of this project: The stated objectihe of the project is to establish some comon terminology in order to improhe comunication between applicants and the NRC. As faults and faulting, in theirNmanifestations, are a major concern in site selection and design, some standardized methods of identifying and classifying the wide range of structures was sought that could organize or systematize ~ types of faults which may be associated with potential or actual sites. The classificationaspresentedinthisreportdoesnotseemtomeetthatobjectihe. Astheauthorsthemselhesdonotappeartobeinagreementastotheclasstowhich

a particular fault may be assigned, it seems unlikely that geologists not associated with this report would be able to classify a new fault found in the field according to their scheme. III.SuggestionsforImprohement: Some suggestions are offered that might bring the present report and classificationclosertoachiehingsomeoftheNRCobjectihes. A. In order to clarify some of the details of the classification where distinctionsbetweenclassesarehagueornotpronouncedenough,itissuggested thatachartlistingthetweiheclassesandsomeoftheirmajorfeatures orcharacteristicsmighthelptomakeaquick,hisualdistinctionbetween them. Such a chart might include for each class, for example SurfaceShape(Planar,curhiplanar, folded) Surface Character (smooth, brecciated, mylonitized, healed, etc.) Surface orientation (parallels regional trend, cross-cuts, etc.) (high angle, low angle, horiz. etc.) NatureofMotion(Normal, thrust, strike-slip,hariableetc.) Interpretedmechanism(Brittle, ductile,grahityslide, compression, extension) RocksInholhed(Sed.coher,coherandbasement,seafloorcrust,basementover coher,etc.) Time of Motion (Syndep., syntect., synmet, pre-met., etc.) Ace of earliest motion Age of Latest Motion Assoc.w/RecentSeismicity(withinactiheseismicbelt, assoc.withrecent surfacemohement,noknownassoc.,etc.) PotentialforReactihation (none,' slight, moderate,high,etc.) These or any other pertinent types of information that might serhe to distinguish between the classes would be helpful. Much of the information could beputonthechartincodedform,eitherabbrehiationsorsymbolstosahespace.

, 8. Since the distinctions between some of the classes require microscopic detialsoffabricandtexturalelements(metamorphicoherprinting, granulation, etc.) some photomicrographs that characterize the types would be helpful as guides to recognition of those classes. Photographs showing the distinctions between healed, mineralized, etc. surfaces should help. C. There is considerable difficulty with the maps, cross-sections, and diagrams. A table at the beginning is missing, yet seems important (p.5-Table 1summarizingmohementhistoryofthetweihefaultclasses) Seheralmapsareillegible,manydiagramsandmapsarenotreferredtoin the text, some diagrams are mislabelled, and some do not show what they are purported to show. It is suggested that someone be designated to deal with diagrams to see that they are in their proper order, are labelled correctly so they can be identified, that all diagrams present have proper references in the text to direct the reader to them, that those that are not clear be redone, and all diagrams show what they are purported to show. D. The reasons for the juxtaposition of classes, why for example, Class 4 is Class 4 rather than some other number eludes me. I see no fundamental reason for the order of Classes as they appear, but it seems that Classes 11 and 12 shouldbereYersed,asClass11dealswithdisturbancesthatdonotpenetrate bedrock or that are not tectonically induced, which set them apart from all cthers. Perhapsgroupingatleastbymechanism(thruststogether,etc.)so that, for example, Classes 1 through 5 are all Yariations of thrust or thrust-related(graYity-slide) faults.Wouldtherenotbesomelogictothatkindof . grouping? See manuscript for a suggestion on renumbering of the classes. Perhaps some discussion in the introduction of what the basis is for the classification would be helpful.

~ lCf'/79 fl]- JL /, g-ly(..u-k Qff k .' i l / ~ r / v 4) r,- (m.-: A don.$ d2ffa m n, L w f.y,4-h/tf ff / 2x A CHARACTERIZATION AND CLASSIFICATI'ON OF GEOLOGIC FAULTS IN THE APPLACHIAN FOLDBELT. Prepared by: D.E. Dunn, J.T.

Engelder, P.A. Geiser, R.D.

Hatcher, S.A. Kish, A.L.

Odom. S. Schamel, and D.U. Wise Prepared for: The U.S. Nuclear Regulatory Commission Office of Standards Development \\ Under contract: NRC-01-78-004 to The Florida State University Tallahassee, Florida A.L. Odom project director 80022 20

5 ,a , p. FaulI Classes / l. Bedding plane thrusts - Decollements. 1 2. Faults penecontemporaneous with wildflysch. ,'7 m 3 3. Thrusts rooted in low grade to b amorphosed cr'ystallige basement. 6 4. Block faults. ~ 7 5. Faults associated with !o'.si centers (diapirs, cauldron subsidence, Intru-sions, and crypto-volcanic structures). V 6. Pre-to synmetamorphic faults in medium to high grade terrane. 5 7. Postr5etamorphic thrusts in medium to high grade terrane. d' 8. Strike slip faults. 9. Complex f aults with long, repeated movement history. 10. Enigmatic faults; inferred f aults such as the 38th parallel lineament. ) p 11. Geomorphic faults with Tertiary history overprinting c:fer Appalachian structures. )(12. Faults with demonstrable Cenozoic movement. m D elow summarizes the movement history of each of the twelve Table Ij / fault classes. The vertical extent of each line represents the time during I;.u.% e; which faults of this class were active, while the width cf each line represents the degree of activity during various time intervals. Fault Classes Most Subject to Reactivation Fault classes 3,4,5,8, and 12 are brittle f aults, either unhealed or filled (this terminology is defined in Section 11. below); consequently, these f aults cre planes or zones of mechanical discontinuity with respect to the country rock enclosing them. All must be considered as possible candi-dates for reactivation, depending on the magnitude of the resolved shear stress they support, because they have lower shearing strength than the coun-w

f try rock (see Appendix XIX.A. for a discussion of critical fault orien-tations with respect to f_n, situ stress orientation). s n 6 ff# rticular attention must be given to Class 5 faults. The largest h storic earthquakes located in the Appalachian Orogen were at Cape Ann,

1. M >'

Massachusetts (1755) and Charler, ton, South Carolina (1886). ~ Both were on L the flanks of local intrusive centers where stress intensification exists because of a mismatch of mechanical properties between the intrusion and the country rock (Kane,1977; Simmons,1978). Finally, Class 9 faults have long histories of reactivation and at least some portions of their total movement histories were brittle. More-cver, there is some modern sei micity on both the Ramapo and Brevard faults; consequently these faults must be considered as possible candidates for further reactivation. E. Fault Classes Least Subject To Reactivation j Fault Classes 6,7, and 9 are either healed brittle faults cr ductile faults (terminology defined in Section II. below); consequently, they are not planes or zones of mechanical discontinuity with respect to the coun-try rocks enclosing them. A word of caution is in order with respect to the foregoing generalization. Mylonitic rocks frequently have a higher degree of preferred orientation and more penetrative foliation than the rocks from which they were derived. In this case the penetrative mylonitic foliation does have lower shearing strength than the country rocks. Jackson (1973) has demonstrated as much as 62% reduction in shearing strength for mylonites compared to their source rocks, when the mylonitic foliation was oriented near the plane of maximum resolved shear stress. The extent of this effect is demonstrated by the common occurrence of brittle faults superimposed on faults of Classes 6,7, and 9 Typically the brittle features constitute the more easily eroded and less well exposed portions of these fault zones,

7 4 A so some care must be exercised in determining the likel hood of reacti-vation. F. Successful Techniques For Dating Last Motion 1. Oikes, veins, or other rocks which cross-cut a fault may be dated isotopi-cally or placed in the relative geologic time scale. 2. Rocks which cover a fault may be dated isotopically or placed in the relative geologic time scale. 3. Faults may be partially or totally filled by minerals.which can be dated isotopically, or whose stability fields suggest growth at elevated T-P condi tions. The T-P condition implies a certain depth of burial at the time of mineral c.owth. If reasonable estimates can be made of the rate of erosion unloading, the time required to expose the minerals in question can be calculated.

• 4.

Fault movement generates microcracks in mineral grains adjacent to the movement surface. New mineral growth begins to heal or fill these micro-cracks as soon as they form. If mineral growth kinetics are known for the appropriate T-P condition, the volume of crack healing or filling is a direct measure of the age of the crack. If mineral growth kinetics are unknown it may still be possible to estimate age, by comparing the degree of healing or filling in the subject cracks to the degree of healing or filling in cracks associated with faults of known age and similar thermal history. G. Relation of Seismicity to Surface Breaks There are two curious aspects to modern seismicity in the Appalachian 0rogen. First, the relation of intensity effects to epicentral distance suggests that there is less attenuation of seismic energy in the crust of the eastern and central U.S., than in the western U.S. (Bollinger,1977).

8 rE Presumably this reflects the dernes4 sty ductile $ nature of the most intense deformations to affeq he reg [ ?. and water fountains, c Although ground breakage in the form of fissures, craterlets, and sand ten is well documented for the Cape Ann and Charleston events (BoIIInger, 1977; Simmons, 1978); surface breakage is conspicuously absent for most modern seismic events. Thus, surfIclal geology may be a relatively poor guide to the potential hazards of a specific site. e

~ 9 _II. DEFINITION OF KEY TERMS The usual distinction between a fault and a joint or fracture is scale- [w dependent. A feature mapped as a fault at a scale of 1 inch equals 10 feet might be treated as a joint at a map scale of 1 inch equals 1 mile. We pre-fer a definition which is independent of map scale. Also, features variously described as " shear zones" or " zones of displacement" or " dislocation zones" may or may not be treated as faults, even though they clearly involve shear mo tion. We prefer less ambiguous terminology and will adhere to the follow-ing definitions throughout this report. A. Faul t at;qv L p t dis A u r 4 A fault is a surface or tabular zone along whichtruev ppar placement parallel to the surface or zone is a st ten times greater than displacement normal to the surface or zone. 1. Ductile fault - A ductile fault involves no loss of cohesion normal to d the fault at the time of last motion. Cohesion as used here refers MP%y to the tensile strength of the fault surface or zone in contrast to l uf, +o rw 4 g(, i.D " M'the unfaul ted rocks on either side.

• y 2.

Brittle fault - A brittle fault is characterized by loss of cohesion ( M normal to the fault at the time of last motion. Brittle faults hp p(1 't may be subdivided into three categcries. M a. Unhealed - An unhealed brittle fault has remained essentially unchanged M e/ V y r k O k u M since its last motion. y,7y b. Filled - A filled brittle fault has been modified by new mineralization ' 7g 6 T which partially or totally fills and cements open space along the g country rocks enclosing the fault.

]9 that the hanging wall seems to have moved up or down, giving a false impression of the real sense of slip such that the use of the terms normal and thrust faults are misleading and, therefore, not appropriate. An additional component of hori-zontal displacement due to layer-parallel shortening must be. added to that due to slip on the fault surface. This component has only been partially documented in the Appalachians, primar-ily in the New York and Pennsylvania Plateaus (Nickelsen,1966; Engelder and Geiser,1979) and the Central Appalachlan Valley and Ridge (Faill, 1977, 1979). In this region, Engelder (1979) has shown that this component may almost double che total lateral shortening. e. Continuity - 1. Parallel tc strike; faults of this class are continuous surfaces at the time of inception; may follow a single bedding surface (individual horizon) for hundreds of km; subsequent folding and erosion may isolate segments of the sheet. ii. Normal to strike (profile section) - Bedding plane thrusts characteristically climb section (ramp) in the direction.of tec-tonic transport, forming anticlines in the ramp area (Fig. 8a). Numerous complications develop in the ramp areas, among these are: q P^3 a. Imbrication: back-breaking (Fig. b) and front-breaking n.Jee 5 p $7 (Fig!11) s 2-I k ,w 4 C e, f p 1,

20 b. Duplexing (Fig.101 T c. Thrusts may terminate in ra=p areas either as splays (Fig. 9) or as imbricate stacks (Fig.10 ). f. Curvature - ~ 1. Map view - Two types of outcrop pattern. are found: Bowe shaped (Fig.12al and rectangular (Fig. lib'l. Fault terminations

h g a g in these two patterns are distinctly different. Bow-shaped ter-p t.op minate in anticlines while rectangular terminate in tear faults.

The bow-shaped types dominate in the Appalachians. Elliot (1976) presents evidence that displacement on bow shaped faults is 'directly proportional to their length, with maximum displacenent expected near their nid-point. It has been suggested that the bow shape may represent either the shape of the -sedimentary basin, geometry of the loading mass,or geometry of the detachment hori-Zon. ii. Local curvature - Ramps occur where thrusts cut' sharply up section from one ddcollement plane to another, with local A7 antic 11nes created above the ramps. Splaying cor=1on at lead-ing edge, ramps, and trailing edge (truncated rear end). CF Slices coc= ton in the vicinity of ramps and splays. g. Termination along strike - Bedding plane thrusts may terminate abruptly in strike-slip faults (tear faults) (Fig. 13). They may gradually lose displacement along their length and terminate in folds (Fig. 14), or in a series of en echelon thrusts or folds called transfer zones (Fig.14) which conserve the dis-placement by carrying it to the next major thrust.

21 Tear fault terminations may be a single large fault or multiple faults. Tear faults are sometimes manifested at the surface by en echelon folds or a region of fold terminations (Fig. 15). 2. Tectonic Setting This style of faulting occurs in the thick miogeoclinal wedge and'cratonic foreland; displacement decreases t' ward craton; o some early stages of Piedmont thrusting could conceivably involve a bedding plane threst (e.g. Safe Harbor fault, PA ' Wise, 1970)], -] but evidence of these thrusts HQ bably been obscured. ends gg[ M % flatten eastward with dcy m ma Men disappTal""Md'erMrontal h b zone of the Blue Ridge and related thrusts: master decollements N Master Q H are best developed in ductile horizons such as Rome shale. ^fhk thrustsrisetothewestornorthwest%eitherjemergeorgointo G.,g-J blind folds and die normal to strike. Many do not extend through the mid-Appalachian salient. In most mountain belts, the timing of thrusting is generally associated with Molasse and Flysch sedimentation. However, in the Appalachians this has been a matter of considerable controversy, as this would extend the development of the foreland back to the Taconic and implies the presence of syndepositional thrusting. Although scattered evidence for such activity has been reported from the Appalachian foreland, the main thrusting and fold events in the foreland are still regarded as late Carboniferous.

22 It should be noted that until recently the basic tectonic framework of the Appalachians has been a catter of some debate, often referred to.as the " thick skin - thin skinned" controversy. Proponents of the thick-skinned school held that defor=ation in the Appalachians was largely due to vertical motions of portions of the basement beneath major folds. These concepts were primar-ily supported by sedimentalogic and stratigraphic evidence that the folds were growing during deposition (Cooper,1967; Rodgers, 1970). Thus, " thick-skin" ideas held that the deformation of the Appalachian orogen (particularly the Valley and Ridge) had been very long-lived. " Thin-skinned" proponents (see Rodgers,1970), pointing to the example of the French Jura and geophysical evidence that the ~ basement beneath the Valley and Ridge and Appalachian Plateau was almost completely flat, held that the deformation of these regions was one of bedding-plane thrusting above detach =ent surfaces. In general those supporting these ideas have held that the deforma-tion of the Valley and Ridge and Plateau occurred as a single late Paleozoic, rather than as an ongoing process through much of the Paleozoic. Although the structural and geophysical evidence have over-whelmingly supported thin-skinned ideas and even extended them into the Blue Ridge and Piedmont, now also known to be allocthonous, evidence for syndepositional deformation is still extant. Geiser (1977) has noted that thin-skinned tectonics and syndepositional

. 23 ^ deformation are not incompatible phenomena. Nunerous cases of this relationship are well-known from the European literature. Thus, the possibility still remains open that the detornation of the Appalachien foreland may ' extend back considerably further - than the late Paleozoic. The, structural behavior of the Southern and Central Appalachian foreland shows a marked contrast in style, changing fecm dominantly thrusting in the Southern Appalachains to dcminantly folding in the Central Appalachians. However, considerable evidence exists that demonstrates that thrusting is still the dominant mechanism in the Central Appalachians ina.

h as the major anticlines are apparent-ly cored by thrusts (Gwinn,1970) and decollement surfaces can be traced far out onto the Plateau (Prucha,1968; Engelder and Geiser.

1979). The northern termination of the Valley and Ridge Province is characterized by an abrupt narrowing of the fold thrust belt into a belt of apparenti clinal ipping beds on the east side of fPw o ~~~' the Poconos and Catskills. However, there is strong evidence (Geiser, A' unpublished data) that much of this region represents a narrow zone hM of imbricated thrusts, although the total displacement is ale.ost \\\\\\ certainly less than that of the Central Appalachians. This zone has been mapped along the Helderburg Escarp =ent north of Rosendale, New York. The presence of the zone of imbrication north of the Delaware Water Cap implies that either: 1) the Hudson River Valley - Great Valley sequence contains unidentified thrusts, or 2) that the western v

24 margin of this region from the Delaware Water Cap to Albany marks l'"ikJh the site of an imbricated footvall at the leading edge of a major Q thrust sheet emerging from the Normanskill Formation (see Fig. 99). M 3. Characteristics of Fault Surface or Zone Type of fault - Althvugh little is known from direct observation a. of growing bedding-plane thrusts, both textural features (see below) and theoretical studies (Elliott, 1973; Chapple, 1978) N ~ - -. ) - si2ggest that the faults are an aseismic creep phenomenon. ~3 3 J b. Surface Texture - 1. Mineralization generally sparse; quartz and calcite may deposit on surface. ii. Fault surface usually identified in Ramp areas where fault - J zone crosses (jumps) sections. iii. Typically motion is concentrated on a few stratigraphic surfaces in a major sedimentary pile but minor motions can be distributed on hundreds of fault surfaces. Flexural slip d'aring folding may be responsible for some slip; Cloos was able to separate flexural slip from ddcollenent by calculating the total possible displace =ent from flexural' slip or the wrong sense of displacement on a fold. Within major zones, slice on slice on slice may be created to producejyup to several hundreds of meters ,N thicksection;consistinglargelyofchaotic,lensoidalslicken-sided or polished pieces. exanple - McConnellsburg Thrust, ?a.

27 Stratigraphic interval - Bedding thrusts are directly controlled e. by the stratigraphic interval of the detabh=ent surface. The faults follow given intervals presumably abandoning them as ' stresses 5 1 T1in'theoverlyingstiff5ayer ~ 3 ,p g g (LC-k N b rupture and ramping of the fault. [Ra= ping 3 11 be controlled A.W b by lateral facies changes. Other control mechanisms might.in-clude changes in strain rate during fault growth. The princi-pal ddcollement horizons for the Appalachians are given in w_. le 1. ' ' Tab i f. Relation to folds - In general faulting precedes major folding l in the Appalachian fold belt, the precise relation in plateau is still a sub' ject of debate. The migrating toe reflects motion ph 7, in zones of thrusting o of the fold belt. Folding locks ~ the throughgoing bedding plane thrust; subsequent faulting may then cut folds, g. Relation to s-surfaces - The development of local cleavage sur-faces along bedding-plane thrusts has been docu=ented by Alvarez i et al. (1978) in the Apennines and discussed by Elliott (1976). Alvarez et al. (1978) have suggested that thrust tips may migrate by utilizing " damage" zones created by cleavage development. h. Relation of fault character to rock type - There are two controls on the fault character by the rock type:

i. Ramping generally occurs in more competent units, e.g.. sand-stones and limestones.

ii. The deformation textures along the fault surface generally change with rock type; in thick weak units (shales, salts,' etc.) s e

28 slip is distributed in a wide zot.e, in some cases hundreds of meters wide; in strong units, slip is restricted to narrow zones. i. P-T conditions - Bedding-plane thrusting characteristically occurs under the conditions of the low temperature (<100*C) and low (21 kb) pressures of the foreland. Since active thrusts are known to emerge at the surface it is apparent that the pro-cess can occur at surface conditions. The independence of thrusting and temperature in the southern and central Valley and Ridge has been demonstrated by Harris et al. (1978) through use of Coned Attenuation Index (CAI) isograd data. A similar independence is suggested for the New York Plateau where cleavage related to a i decollement surface in the Salina Group is independent of the CAI isograds (see Fig.17). i

j. Relation to isograds - The root zones of these thrusts may cut

./* and displace isograds and paleo-isotherms. This behavior is classically demonstrated by the Blue Ridge thrust in Virginia and Tennessee (Bryant and Reed, Jr.,197C' while a few, such as the Greenbrier have isograds superposed on them (Bryant and Reed, Jr., 1970). k. Relation to intrusions - none. 1. Tectonic injections or forced intrusions - Tectonic injections M kX> ofgougeandhighlg its such as shales, occur locally /\\F Ck along the thrust surface. W. N"'

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29 ^ 5. History a. Age of inception - The age c^. thrusting in the Appalachians is a subject of some dispute, since the age of inception is not indicated by the age of the youngest rocks cut. Most of the foreland thrusting in the Appalachians is probably Carboniferous or younger in age. However, there is good reason and some evidence in the form of age dates and cleavages to suggest that some of the thrusting is Devonian. Evidence for iai2 thrusting has been found in 9 t, the eastern Valley and Ridge of V (Lowry,1971), while j m b much of imbrication along the Martic zone is c

Wisa, b

' W" 1970), yo.Y,5< b Recognition of syndepositional effects - Syndepositional thrust - r g 7 ing has been documented in Vitginia in the form of the Fincastle conglomerate (Lowry and others,1971) and in the Coosa Basin by Gilbert (1977). The evidence is primarily in the form of auto-conglomerates occupying synclinal basins. It is probable that other evidence of this type can be found elsewhere in the Appal-achians if the appropriate sedimentologic and stratigraphic studies are made. Radiometric ages - Pierce and Armstrong (1966) give the only c. known age determination; a whole rock K/Ar date of 390 + 50/-15 my. from the Tuscarora fault of central Pennsylvania. d. Relationship to unloading - As far as is known, these faults are unaffected by unloading.

30 e. Indications of last motion - No bedding plane thrust is known to have moved since the end of the Paleozoic. However, there are no good indicators which demonstrate a d} finite time of last motion, except locally where overlain by Quarternary alluvium. 6. Stress Field The regional stresses approximate Anderson's (1951) theoretical a. model where at is'approximately normal to the structural grain, a2 is approximately parallel and c3 is vertical. b. Magnitude of stress and strain 1. Stress-Deviatoric stress values arranged across a section through the trailing edge are almost certainly less than 200 bars (Elliott, 1976), and according to Groshang (1975), in some cases may be less than the yield strength of calcite twinning (probably less than 65 bars). ii.. Strain - Body strain within the thrust plate varies from 1 - 2% shortening at the leading edg3 to greater than 100% at the trailing edge. This strain is partitioned among, finite a=pli-tude folding, solution loss, intra-and intergranular strain, jointing, vedging and recoverable elastic strain. c. Variation in stress and strain - 1. Stress - The highest stress levels are developed along the fault surface; possible local stress concentrations may reach levels on the order of > 1 kb (Handin and Hager (1957). Maximum mean values within the sheet are less than 200 bars and are be-(,)(,_ lieved to be achieved only in the trailing edge ghedeviatoric stress levels attenuate towards the c:'p where current evidence indicates they are on the order of a few 10's of bars. i

31 ii. Strain - The distribution of strain magnitudes associated with thrusting may follow the distribution of stress magnitudes however, only the bst preliminary quantitative data are available on this (Engelder and Geiser,1979; Alvarez et al.,1978). An added problem is that virtually nothing is known about the distribution of strain partitioning within thrust sheets. d. g situ stres. - Not known. e.. Seismic first motion studies - No seismicity has been connected with the Appalachian bedding-plane thrusts. f. Rates of motion - When active the thrusts are believed to move at -12 -14 rates of 10 - 10 k=/ cec. (Elliott,1976a).b' Bow 1ver,neotec-toIseismic data from present-day r$untain belts suggests that some cpl l* 'y ,f \\seismicactivitymayoccuralongthesefaultsg(nfortunatelythedata J u ( are not of sufficient quality to provide. unambiguous answers. g. Fluid pressure changes and effects - Not known, however changes in fluid pressure should effect behavior of fault (see Hubbert and Rubey, 1959). 7. Geophysical and Subsurface Characteristics a. Seismic activity levels - Nil, although induced seismicity is possible under loading. b. Subsurface offsets - Commonly offsets both stratigraphy and struc-ture above the detachment zone. Not known to produce any effects in the crystalline basement, although basement irregularities may control the location of offsets. c. Relations to anomalies - May produce local gravity anomalies where imbricate thrusting develops " stacking" of section.

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~ ? /, 9 t m ,4' r-2 0 l T / / / ry 4,/ t ~ n / ~ ) f.3 a)' R x a "S Y A / 4(=C U %E S / r CI f n / y?n j a.y, / ,. / O 3.o

46 Figure 18. Map showing known locations of Class 2 faultr. Stippled pattern indicates lower allochthons, resting on wild-flysch: allochthon 1 of Bird and Dewey (1970). Horizontally lined pattern indicates higher allochthon - ductile thrust slices: allochthon 2 of Bird and Dewey (1970). Numbers are (1) Stanbridge nappe, (2) Gliddings Brook slice, (3) Peapeck allochthon, (4)' Jutland allochthon, (5) Hamburg Klippe. W u <-{ h ( v%

48 Hamburg Kliepe of central and east-central Pennsylvania (Stose,19k6; Platt and others,1972; Root and MacLachlin,1978), an extensive terrain ^ \\\\ n which the cutochthonous exogeosynclinal, Martinsburg $]3 alp of Middle r Qc? .i p ( and Late Ordovician age is supplanted along strike by an allochthonous O omplex of blocks and slabs of vn dimen ut Taconide lithologic wm affinities. Many of the blocks have a Taconic-age structural fabric, but the entire allochthonous complex and underlying autochthonous shelf sequette (Cumberland Valley sequence) were strongly deformed and foreshortened during 12 IJ the Alleghanian orogeny. The regional slaty cleavnge, at least at the g p[M( vestern end of the allochthon, is of Allechanian age (Root,1977). See N, figures 23,'24 and 25. L B. Description of fault class 1. Basic geometry a. strike length - Two of the lan$est allochthons, the Giddings -) Brook slice and the Hamburg Klippe, have lengths of 198 and 125 kilometers, respectively. Their original lengths may have been somewhat greater. However, the Hamburg Klippe is not a single, ccherent sheet of rock, but rather is a complex of innu=erable allochthon slabs of uncertain dimensions (Root and ' Maclachlan, 1978; Alterman, 1971). The smallest of the princi-pal Taconic allochthons, the Sunset Lake slice, is approximately 10 kilometers long. b. length perpendicular to strike - The present vidth of the Giddings Brook slice'is 26 kilometers, althou6h the original vidth probably exceeded 35 kilometers (voight and Cady, 1978). The minimum vidth of the He;: burg Klippe is 22 kilometers, but

49 the northern edge of the allochthon is buried beneath the upper Martinsburg Formation or younger strata and post-Taconic thrusting obscures its southern edge. All other allochthens are considerably narrower. c. orientation - The allochthons are nor= ally parallel to the regional structural Frain, except where they h.syc been ntrongly redeformed by superimposed structures. Motion of the allochthonc was not.necessarily perpendicular to their present strikes. d. displacement - The Taconic allochthon appears to have moved approximately 100 kilometers vestward from a site east of the Cheshire-Dalton shelf facies boundary, the earl / Paleozoic shelf edge (Ratcliffe, 1975). This site lay east of the palinspastic position of the Green Mountain and Berkshire massifs. The original site of the Hamburg Klippe rocks probably was southeast of the Baltimore gneiss domes (Platt and others, 1972); they have been transported a mini =um distance of 70 kilometers to the north and nortt} vest. e. continuity - along the base of the allochthons are ontinuous through the entire length of the allochthon, however, ! %'T"h ~ -) the thrusts are not known to be rooted. Present exposures are l -vkef.,MbA [b erosional re nants of originally more extensive sheets. Late folding and erosion may give rise to separated portions of the same allochthon slab (Zen, 1972). f. curvature - The fault surfaces may be strongly curved, especially at the ends of the ' slices and along the trailing margins where the allochthons have been folded and cut by later thrusts. g. termination along strike - Erosion appears to have removed the original terminations of the allochthon slabs.

52 = b. surface texture - Execpt for thructs at the base of the higher Taconic slicoc, which are probably not of thic class, the faults lack slickensides and mineralized surfaces. c. character of the zone - The actual fault surfaces may be cryptic, uneven boundaries between different colored shnles or chales and greyvackes of different ages. Co=monly the boundaries are welded or are marked by highly cheared shale that breaks into small, polished, lozenge-shaped fragments (scaly shale or "argille scagliose"). Within the Taconic allochthon, wildflysch-like zones are generally not recognized associated with slices above the Giddings Brook slice; the thrusts associated with the higher slices are more brittle in character. Potter (1972a) describes " crushing, e shearing, and mineralization" along the Rensselaer Plateau Thrust (figure 29). Carbonate slivers derived from the.under- ~ lying autochonous shelf sequence are distributed along this thrust. d. metamorphism and mineralization - E= placement of the Hamburg Klippe and the lower slices of the Taconic allochthon pre-dates ~ ~ ~. the regional slaty cleavage and greenschist facies metamorphism, w s.- / which 1.s_ Tags.nf r(ZeM972.) in New E:;g).and and Alleghan@N, . \\,}g OW J ( , c, g.,. a,.,v ' n Pennsylvania (Root and Maclachlan, 1978).,2Eorite or prehnite- % %' 'Y k G* g gs Q,,b \\ W ', pumpellyite facies metano ism may have accompanied emplacement e of the allochthon. ~ b) M, 6 0g b, e g w p e. datable material - None. d \\ f A / x %.- Q 4 f M H b W h y G., g le $ w d 4 & a l - a. w s.s L A 4. l' y 09 RdlQ.f % kg

54 ' L-1 xCF+m /-s W-burg Klippe complex, however, com=only conta.n hkh - a slaty cleavage that had formed prior to their emplacement y M GJh to the Martinsburg basin 7 The orientation of the S cleavage J y bb# varies from slab to slab and is strongly overprinted by the regional S cleavage f Alleghanian age (Root, 1977). 2 h. relation of fault character to rock type - none described. 1. P-T conditions - The thrust slices were emplaced onto or near the seafloor at relatively lov temperatures and pressures.

j. isograds - Regional metamorphic isogrades (up to biotite and garnet-isograd in the vestern Teconic allochthon) are super-imposed across the allochthens and post-datz.their emplacemer.t.

k. relation to intrusions - Large blocks of basaltic and andesitic o I /ko \\ f'pillov lavh(Jonestown volcanics in Pennsylvania and Sterkes w 1 MM9 / Knob in New York) are incorporated in the vildflysch complex M "Y (Platt and othe.s,1972; Bird and Dewey,1970) and pre-date ~ N emplacement of the allochthons. Near West Rutland at the northern end of the Tacor.ic alloch-then, late, post-tectonic lamprophyre dikes cut rocks of the allochthon. Hornblende from one such dike was dated by the K-Ar method as 105 h m.y. or Late Cretaceous (Zen, 1972). 1. tectonic injections or forced intrusions - Although clastic dikes are known to exist within the Taconic sequence, they have not been linked with specific movement horizons (Voight and Cady, 1978). 5 History age of inception - The Giddings Brook slice contains rocks a. as young as Middle Ordovician (graptolite cone 12) and is emplaced into and across Normanskill Formation of late Middle Ordovician

7 phd 58 CHAPTER VI h &!>f.hM "b Class 3 THRUSTS ROOTED IN LCW GRADE TO UNMETAMONSED CR ALLINE BASEMEfff A. Generalized Description Thrusts of this class are generally large low angle thrusts which carry base-ment rocks (Fig. 31). Yet they behave as thin-skinned thrusts where they involve and carry cover rocks. They are associated with either low grade or no metamorphism. Where Paleozoic metamorphic rocks have been involved the grade is generally no higher than greenschist f acies. They may or may not M- ~ { have associated with these a regional penetration cleavage. Faults of this class characteristically occur along the foreland / metamorphic core boundary as transported external massifs. Such struc.tures may be found in similar positions in any of ints world's major thrust mountain chains. At the base of these thrust sheets, the fault zone is ductile marked by greenschist mylonites and/or brIMie cataclasites. Thicknesses of these zones rance from I-2 m., rarely to more than IG m. Subclasses I. Thrusted fold nappos with basement cores -(Fig. 32)- Reading Prong type (f. sconetcong nappe stem; Drake, 1976). Thrust nappes of this sub-p 4 04 [ class consist of detached fold nappes deformed initially by a shear 3 g,T (

• f t

g mec.hanism with ductilly deformed cover. 2. Side Ridge type - Thrusts of this subclass consist of large thrust sheets which transport basement rocks (King and Ferguson, 1960). A fold mechanism is not involved. Most have been subsequently deformed by foiding and some later faults. They occur alor.g the western edge of the Blue Ridge (Fig. 33). 3. Berkshire Highlands type - Thrusts of this class are relatively high p> ud 6% d

61 4. Relationships to Ccuntry Rocks a. Class 3 faults are generally parallel to the structural grain or a regional basis. However, they exhibit cross-cutting relation-ships where examined in detall. b. Prcrnentories or Embayments - These f aults generally occur at major promontories within the mountain chain and' die into embayments, except those in the Reading Prong. Thick-skinned or Thin-skinned - Faults of Class 3 are obviously c. thick-skinned but exhibit many of the behavioral characteristics of thin-skinned thrusts. d. Relationships to isopachs - These thrusts may relate to original basement highs where late Precambrian and Eocambrian sedimentation was absent. A similar relationship involving the same kinds of thrusts has been noted by Burchfiel and Davis (1975) in the U. S. Cordillera. ( ~ e. Stratigraphic Interval Affected - Basement rocks and the immediate cover are involved in these thrusts. f. Relationships to Folds - Most faults of this class appear to die into folds. The most obvious relationships is the Blue Ridge type (Subclass 2) where northeastward termination of thrusts into the Blue Ridge anticlinorium occurs (Fig.35). These thrusts are characteristically folded. 4,.Le ;g,q ) g. Relationships to S-surf aces - Mylonitic follations generally s parallel the fault zones. S-surfaces related to low grade regional tamorphism may be coeval with thrusting. s ( h. Changes in Fault Character with Changing Lithology - Where deforTned \\ \\ S \\ zones are thick within or adjacent to basement rocks there is a f\\ } notable thinning (sometimes to a knife edge) of the deformed zone w %. Lev k S U Mo- .m wm w+wH 'Qb u% (% Ng t%7)

63 for mylonitic rocks along a thrust in north Georgia. ~ d. Relationships to Unloading - Not applicable (see 5b above). I i e. Indleations of Last Motion - Cataclastic veins in the Berkshires ~ are overprinted by Acadian metemorphism and f aulting (Ratclif fe and Harwood, 1975). The Cross Mountain transcurrent fault (post-Alleghanlan?) cuts the Blue Ridge subclass faults in northeast Tennessee f. King and Ferguson, 1950; Hardeman, 1966). Triassic-Jurassic diabase dikes cut all subclasses. 6. Stress Fleid Orientation of Principal Stresses at inception - The orientation a. of o during the initial stages of movment was probably oriented g toward the northwest to west. b. Magnitude of Principal Stresses and Strains - Since the masses of matetial moved are of considerable size the principal stresses b f' g f

• g [ must 4 ave been of-onsideFable magn

~ ~

d r

r ~- I estimate strains because of the nature and complexities of deforma-tional processes affecting these rocks. Because of the large { amounts of transport of slabs of considerable size, it can be~ con-l cluded that stresses were immense. Strain within a particular f ault zone varied with position in the zone and the nature of the 0 ^p g processes (Mn at a particular time. c. Variation in Time of Stress and Strain - There is aburdant evidence of reactivation of Class 3 thrusts in the Berkshires (Ratcliffe and Harwood, 1975) and the vestern Blue Ridge. Most thrust sheets of this size probably experienced an episodic movement history rather than a single emplacement event. d. Present _I_n_ situ stress - Unknown. n t

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.d South Mount.wi Catocian Mounta c-===-%q,7.--.~ >k m. um, r,&n,,,, Q- }y (m{kY$N;g7 .-l , J _ a ~ af o/ c /Y4

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g- _ ~~ fed c g, . 58%~'1 [' d;.s4. -7 s... A n g > ;w.u: 6 6 u.ss.er t ,,a, %d%'i,!bb..., Y,'M f.?!O!'5:ft.'jBl*.?,?:$f.$, i.~: ; Q ,,5 hI bks li@, s s yy' ""'*** xa E**"' i O o p' otu. nes. sn.,, m C.toctut Mount.in A ,,fp5b I(**'jf ,. cc y } ? r=' 5.:.'.':.3..'.?.5@h. '*-;*iid-p ' S** ' eve S '),$.hh '."Sh ': << (9'$S'.'$'YE,S'I >$.*iE~'l$M,//.fdN'i-r a e, n=.: u'?P'f;j', ; m w :. my nn.g..d2,O. i9 ' .S. a d'f as e a .u.. @h a io...... Y ~ - - -. ~ - N Figure 3S Structuro s<crion and index map of Blue Ridge anticlinorium (from Espenshade, 1970). Rs, Triassic sandstono a:.d shale; OGI, Cambrian and Ordovician limestone and delomite; GI, Cambrian limestone, doloralte, and shale; Gc, Cambrian and Cambrian (?) quartzite end phyllite (Chilhowee Group); Pze, Paleozoic (?) phyllite and schist (Evington Group); pGv, upper Precambrian metabasalt, phyllite, and quartzite (Catoctin Formation and Swif t Run Formation); pGl, upper Precambrian mica gneiss and schist (Lynchburg Formation, including Rockfish Conglomerate Member, and metasedimentary rocks of Mechum River); pCg, older Precambrian basement complex. 9 N

f* CHAPTER VII }. l i CLASS 4: BLOCK FAULTS A. General description Faults of this class are steeply dipping faults that extend to and disrupt basement (antithetic faults in headwall are truncated by master fault). Though they might have exp'rienced strike-siip or even reverse motion, e these faults are primarily normal faults. Faults of this class have pro-bably developed at several times in the history of the Appalachians: In the late Precambries' associated with crustal extension and rifting (Bird and Dewey, 1970) within the Grenville crust, during latest Precembrian - Carrbrian time associated with the Avolonian activity in the northern Appalachians (McCartney, 1969) and the Carolina Slate belt in the southern Appalachians (Long, 1979),during Pennsylvanian-Permian activity in the Narragansett-Boston-Norfolk Basin area [though faults here are primarily thrusts (Skehan and others,1979)], and during the Mesozoic Era associated - with tite continental separation. l Faults of this class which might have formed - and probably did - prior to Pennsylvanian time have not been documented, and because of later a

w. q oF deformational activity probably no longer can be identifled as belonging h

ktothisclass. nW%..am / '@p c .@ d e ($ y g The Mesozoic subclass contains the overwhelming majority of faults in ~ --..._ d 3 Class 4. /Also the Mesozoic f aults are the only members which can be m y)$ _. / f' r y g 3 /said tojGQ cmc' hcharacterize n area of the Appalachians.fThus y <the example and detailed description given in this chapter, refers to Y ~ g the Mesozoic subclass. Class 4 faults are common throughout the Piedmont Province and in the coastal plain subsurf ace (Fig. 36). All high angle f aults within the

G7 Piedmont Province, and certainly those which can be shown to be normal, post-metamorphic f aults, are prime candidates for this class. Regionally 9 and temporally associated with these f aults are sedimentary basin fills and dolerite dikes which fill NW to NE trending fracture systems. bb b The example of a Class 4 fault described is the Jonesboro f aultp 0 addition to the description given, the reader is referred to " Fault Investigations, Shearon Harris Nuclear Power Plant Units I, 2, 3, 4" (Carolina Power and Light Company) report prepared by Ebasco Services, incorporated. '( ~

G8 Example: Jonesboro fault - The onesboro forms the eastern border fault I ]' of the Durham and Sanford Triassic basins (collectively known as the L Deep River Triassic basin) The fault was named by Campbell and u)hy Kimball (1923) for exporures of the fault near the village of Jonesboro; which is now part of the city of Sanford. The fault extends nearly 160 kilometers along strike- (figure."37). It is bordered on the east by a narrow belt of low-grade metamorphic rocks further east is a large antifor.ul region (Raleigh belt) con-g taining high grade schist, gneiss, and plutonic bodies. The trace of the Jonesboro fault is not linear over long dis-tances (figure 38). Lindholm (1978) suggested the local orientation of foliation may have affected the trend of the fault, producing a curvilinear trace. From a detailed study of the fault trace, Reinemund (1955) suggested that the Jonesboro fault is cut and offset by cross faults which hid significant strike-slip component and these faults were subsequently intruded by diabase dikes (figure 39). More recent studies (Bain and Harvey,1977) reinterpret the structures and suggest the Jonesboro is not cut by younger faults but does in fact cut diabase dikes (figure 40). In this case latest movement GC on the fault post-dates the approximately 180 Ma-old dikes p uever it has not disturbed early(?) Cretaceous sediments which cover the fault south of Sanford, N. C. Reinemund (1955) has described several exposures of the fault contact. At one location the contact is between a light-gray late Paleozoic granite and brown and gray banded Triassic claystone interbcdded with granite boulders. The fault surface is described as

70 B. Description of Fault Class 1. Basic geometry a) Strike Length - Meters to hundreds of kilcrneters. b) Width - (Dimension perpendicular to strike) - Downdip extensions uncertain, but probably disappear into ductile zone at great depth. Antithetic f aults may truncate against synthetic f aul.ts. c) Orientation - Strike generally, parallel to regional grain of the Appalachians, locally departing due to basement anisotropy. d) Displacunent - Dominantly dip-slip motion and most commonly nomal movement. The total apparent thickness of stratigraphic section in basin noght greatly exceed the total relief on the basement and displacement along any one fault. Displacement is f rem meters (or even centimeters to a few kilcrneters, maximum). Cross-faults tend to have smaller displacunents. ~j Present border faults might not be the major fault, but only an h " accident" of erosion. The major fault may be outside t e) Continuity - Individual faults tend to link thrcugh a series of roughly contenporaneous normal faults and cross faults, especially along the down dip margin of the basins. f) Curvature -(I) along dip - The faults may be' Many faults / \\ may flatten into subhorizontal basement fa h depth. At } shallow depths the dips are 600 to verticle. (Ii)along strike - Tends to parallel reglonal strike of basement. Most lack significant curvature, but may locally show curvature due to local anisotrophy. Composites or coupling of contemporaneous straight segments can give a regional indication of curvature. J

71 g) Termination along Strike - May terminate in rotational f aults, splays, monoclines or other folds which decrease in magnitude, or may extend into crystalline basement where f ault is dif ficult to trace. 2. Tectonic Setting Mesozoic block faults are located east of the billion year old Pre-cambrian massifs of the Long Blue-Green axis both in the Piedmont and Coastal Plain provinces. These faults were formed in response to the rifting and separation of continental crust which produced the Atlantic ocean. 3. Characteristic of Fault Surface or Zone a) Type of Fault - Typically in brittle demain and non healed, par-tially filled to completely filled - occessionally silicified; breccia and scuge common, with zones from tens of centimeters to several tens of meters. l b) Surface Texture - Slickensides - wear groves are common on sur-faces; fiber veins much less common. Slickensides can have diverse orientations, including horizontal, but the predominant direction is parallel to dip slip direction. c) Metamorphism and Mineralization - Post barrovian metamorphism. COh p + f.,y There is a range of hydrothermal mineralization up to greenschist e w % w^ f-4. i f g facies. Zeolites are ccmmon; carbonates are calcite, ankerite, NY}'% and siderite; sulfide mineralization; wide zones of silicification py V\\ y4' Including box-work and multiple development of veins. Open space p N crystal growth Indicating pressures lower than hydrostatic are conmon. Oxide minerals may be present. 4. Relation to Country Rock a) Parallel or Cross Regional Grain - Short fault segments corrmonly

~ 72 cross regional grain, but long fault segments usually parallel regional grain, b) Promontory or re-entrant - No obvious association. ~ c) Thick or thin-skinned - Faults cut and displace basement. d) Relation to isopachs - Border faults truncate sedimentary basin fills. Faults within basins commonly are surfaces across which thicknesses change abruptly. For faults outside of b3 sins, this does not apply. See Fgure 41. e) Stratigraphic Interval Transected - Maximum P E,thru Mesozoic. Most include rocks as young as Jurassic and as old as lower Paleozoic. In Coastal Plain, faults have been found to cut Cretaceous strata. f) Relation to Folds - Broad warping of strata in basins and local folding marginal to faults; dragfolds, reverse drags and flexures associated with terminations of f aults. i g) Relation to S-surfaces - No genetically associated s-surfaces on / a regional scale, but locally kink bands might develope adjacent to fault. Also numerous, closed spaced, parallel fractures adjacent to faults are observed in some areas. h) Relation of Fault Character to Rcck Type - Faults are not af fected by rock type except change in strike which might result from rock anisotrophy. I) P - T Conditions - Crystalline basement involved in f aulting is below the brittle-ductile transition and not above the chlorite zone of greenschist facies; generally low temperature and confining pressure. Locally precise P - T conditions can be detennined by fluid inclusion studies or fecm stabilities of vein or other associated minerals (zeolite assemblages have been used in this J

73 regard). J) Relation to Isograds - Regional isograds are displaced by the f aults. /' Displacement of isograds can be used to establish ilmits to magnitude of displacement. However, caution must be exercised in areas of inverted Isograds. k) Relation to intrusions - Usually dolerite dikes predate f aulting or are penecontemporaneous. Faults may wrap around older, unfollated granites (such as the Jonesboro f ault) giving a false impression of the granite intruding the fault. 5. History a) Age of inception - For the Mesozoic subclass cajor motion is not known with certainly, some may represent reactivation of older rA 7 (Paleozoic faults). Major motions are Triassic and Jurassic. W-b cW = A Motion frequently occurred during sedimentation. Seismic focal G (, mechanisms indicate that in central Appalachians someysozoic , I b rder faults are still active - but elsewhere most are apparently

inactive, b) Recognition of Syndepositional Effects - Basin fills and fanglomer-ates, thickness variations of volcanic flows in northern basins, rotation of initial dips with associated gravity slides. There is some question as to the extent to which basin configuration was controlled by the faults.

Local ponding af fects can be found. c) Radiometric ages - K-Ar and fission track ages have been measured on fault filling material. Depending on mineralegy, condition ar3 history of samples, the analytical ages either approech or are greatly less than the true ages. d) Relationship to Unloading - Not applicable. e) Indications of Last P'otion - Cross cuttirg dikes (though rare), u

75 d) Geophysical Lineaments Not applicable. 8. Geomorphic Relationships Fault line scarps are frequently developed where basin fill, adjoins the 7 fault. Springs are frequently aligned; colluvium and colluvial sliding may be present. Strong topographic changes of ten occur across border faults of this class. Faults of this class might also exhibit topographic expression due to the nature of fill. 9. Methods of identification and Detection a) Best detected by evidence of basin' fill and offset of basin fill. Fault h eDnd buried scarps detected by geophysics and drill b) 4' holes. g c) Zones of silicification, zeolite mineralization, micro breccia can be clue - but must be considered with other features; open box-work h{(i DhM sIIIca is often characteristic. d) Ccmmonly associated with increase fracture density. ( e) Abrupt changes in metamorphic isograds not associated with Coastal Plain or thrust faults. f) Faults offsetting dolerite are good candidates. 6' g) localized zones of metamcephic retregression in high grac, < terranes %.S might be a candidate for this class. h) A'onoclinal flexures and draps in basin fill may indicate buried faults at depth or exposed along their projection. I) Brittle reactivation of earlier steeply dipping faults might be a clue of this class of faults. 10. Pitfalls in Recognition a) Mere brittle behavior of fault with strike parallel to regional grain is not adequate evidence, b) The present border fault may be a younger feature than the basin u

78 CHAPTER VIII CLASS 5: FAULTS ASSOCI ATED WlTH LOCAL CENTERS Several classes of structures tc ming small local disturbances have f aulting associated with their origins. In addition there is some circumstantial evidence linking some members of these groups to modern seismic activity. These associations include two of the most destructive earthquakes of eastern U. S. (Charleston and Cape Ann) localized near small late plutonic bodies. A second class of possible quake-related structures includes meteorite impact or crypto-explosion structures. In terms of overall Appalachian tectonics, these two classes of structures are of minor Ir.1portance but in terms of seismic risk ar.alyses, they descrve proportionally much more te.-Icus consideration. FAULTS ASSOCI ATED WITH LOCAL INTRUSIVE CENTERS Small intrusive bodies of late or post-metamorphic age ccmmonly have faulting associated with their emplacement. They are also included in the list - of prime suspects for the localization of present day major seismic events (Sykes 1978). Best known of the late to post-metamorphic intrusive bcdles is the White Mountain line of Jurassic and Cretaceous age ring dikes and Intrusives trending NNW across New England. A second series of analogous bodies is the group of Paleozoic coastal plutons of Maine. Both sets of intrusions may have been localized by regional fractures and have had subsidence faulting associated with their emplacement. Chapman (1968) notes reticulate arrangement of the coastal plutons of Maine w!_th..dcrainant4Ltections NNE angg He notes that the country rock near yjh'thegraniticcontactsappearstobedraggeddownwardandi 4 and brecciated. Some of the shattered zones are up to a half mile in width

79

• with fracturing increasing toward the contact to culn!nate in a thoroughly jumbled breccia mass.

Blocks of breccia range up to 100 m in diameter, with hornfels and small granite dikes. Chapman (1968) also notes reticulated patterns oriented WNW and BI associated with the Mesozoic White Mountain intrusives of New England and 'j N suggests that both the Maine and White Mountain sets of Intrusions were localized along the intersections of regional deep seated fracture systems. Similar to the Maine examples, dips of the White Mountain country rocks are inward tcward the complex. The central regions of the _cceplex are commonly downf aulted blocks of Most Volcanics surrounded by ring dikes. Thicknesses of at least a mile of volcanics in these cauldron subsidences (Billings,1945) Indicate major vertical motions dropping and preserving small portions of a much more extensive volcanic pile which former!y covered the intrusive belt. The fault contacts of the White Mountain Magma series are not well exposed in general and for the most part are obscured by later stages of intrusion. Billings (1945) gives a general discussion of the emplacement g mechanisms of these plutons including some of the faulting aspect more specific discussions of the faulting exist in the literature. Kings!cy (1931) notes existence of two radial faults in the Ossipee Mountains of New Hampshire within the ring dike bounding the infaulted Moat ^ 7 volcanics against older granite)k 0 She describes one outcrop of the fault as Urb / granite grading into a " crush rock" with the fault itself marked by breccia intruded by quartz porphyry and basalt. Apparently the fault was used as a volcanic condult. She suggests that the magnitude of settling of this central ( cauldron was at least 5000 feet. Billings, et al (1946) in the Mt. Washington quadrangle of New Hampshire map the very conspicuous Pine Peak fault separating and dropping the Mt. Washington block in the SE fran the Pliny Range ring dikes and intruzions of u o

83 and by a slight negative gravity ancmaly. It is interpreted by Isachsen as a possible ancient impact buried beneath the Devonian clastics and manifesting itself by upward propagation of joints above the deeper circular rim and walls. None of the impact sites within the U. S. have marked concentrations of modern seismic activity associated with them. Only the Charlesvoix structures of all the Canadian impact features has cancentration of such activity. Sykes (1978) suggests this may be a local phenomenon caused by the offects of a higher stress zone of the Saint Lawrence Valley being concentrated in the impact weakened rocks. Within the eastern U. S., these sites seem reasonably safe from seismic activity but probably should be monitored with a micro-seismic network before any critical siting decisions are reached. Summary data and additional references can be found in Fiench and Short (1968) and E. A. King (1976). [25 N g p5gu 5 c - (oe d ' ".. _.w,e. ^4 ~ j i 3M n ( ,\\ 4 F '"\\ ' ~

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CHAPTER IX CLASS 6: PRE-TO SYNMETAMORPHIC THRUSTS IN HIGH GRACE TERRANES A. Generalized Description Thrusts of Class 6 consist of brittle Cincluding Imbricate) thrusts, which possibly exhibited bedding plane type behavior, that were emplaced and sub S ently were overprinted by metamcephism. AlI the characteristics of 'JLw bedding thrusts (Class 1) may apply to these. Most were completely annealed M v^*gg gs p by the thermal event; some were reactivated later during or after the 7 b thermal peak producing mylonites along faults. hcM Probably most Class 6 g faults formed as early compressive features as a result of the initial stages of the thermal / metamorphic event with which they are associated. However, there were probably a variety of faults, thrusts, strike-slip, and nonnal faults, which formed in the early Paleozoic orogen. Some of these may have been reactivated as synmetamorphic thrusts.) Several faults of this l -clas's are transitional into recumb*ent basement-cored nappes as well as into -m, o post-metamorphic thrusts in h!gh grade terranes (Clas,sp Those, j such as the Honey Hill, include metamorphic reactions as part of their strain mechanism. Some, like Cameron?s Line in Connecticut (Redgers,1970), may be sutures with ultramafic associations. These are typically the syn-metamorphic thrusts and exhibit a ductile, rather than a brittle, strain history. Faults of Class 6 are recognized by telescoping of stratigraphic successions, metamorphic overprinting with no disruption of isograds, aligned ultramafic bodies and juxtaposition of markedly different strati-graphic and/or petrologic sultes. Most of these faults in the Appalachians are Ordovician and as such present no hazard today. Typical Examples Typical examnles of Class 6 f aults may be grouped into two subclasses: (1) brittle premetamcrphic bedding thrusts which were for the most part

85 healed and not reactivated, but were severely deformed later; and (2) syrmetamorphic thrusts exhibiting a ductile history, were probably, reactivated, but were also severely deformed later. Subclass (1) faults e include the Martic (Fig. 50) (Wise, 1970), Hayesville (Hatcher, 1978; Hatcher and others, 1979) Greenbrier (Hadley and Goldsmith, 1963; King, 1964; Hadl_ey and Nelson, 1971) and Peach Bottcm f ault ' Baltimore h bbro ophiolite t ust sheets and Bay of I nds phiolites probably fall W }2 g Into this subclass as wel1. Subclass (2) faults include the Rosemont 5 (Armstrong, 1941), Coatesville - Doe Run (Bailey and Mackin, 1937) and 0 { ' (- .S"( 9 g Vy Honey Hill (Fig. 51) (Dixon and Lundgren, 1968; Wintsch, 1979) faults. Basi'c Gecmetry e A " 7 $0 Subclass (1) - Premetamorphic %p h./

1. jf p If these faults are still recognizable as such after metamorphism and have

,#'Y G*-s involved predominantly sedimentary sequences, the geometric characte.-Istics [yt Y of bedding thrusts (Class I) are applicable here. There is an association V 9 of some faults of this class (e.g., Cameron's Line) with ultramafic rocks, y-bringing forth the possibility that several of these faults may be sutures and former plate boundaries. Generally, metamorphic isograds extend across these faults and they have been sufficiently % annealed that subsequent motion Y is unlikely but may occur in the brittle realm on properly oriented seg-ments. They are recognized by telescoping of stratigraphic successions, locally aligned ultramafic bodies and/or juxtaposition of two or more markedly dif ferent stratigraphic /petrolog,1c suites. Subclass (2) - Syneetamorphic a. Strike Length - Tens to hundreds of kilcmeters. b. Width - Generally recognizable width of fault zone is on the order of cm to meters, may be " knife-sharp" contact.

88 3. Relationships to Country Rocks a. Dominant metamorphic foliation commonly defines the regional grain. Premetamorphic faults may exist ~at any angle to the dominant follation. Synmetam- . hic f aults generally relate to and may be aligned parallel to one or more major S-surf aces. b. There are no obvious relationships of these f aults to salients and re-entrants. The premetamorphic thrusts may exhibit thin-skinned behavior in part c. but generally basement of other crystalline rocks are involved. ~Synm orphic t b bs nvolve crystalline rocks and thick ~ O,

1. gd 4

3 h(p sIabs. FattLtlas-in-the-latteF6c5fs at7.oFsIderabf e-depths,-therefore great thicknesses of rock materials must be moved. V d. Relationships to isopachs - There are only possible Indirect relation-ships of these faults to isopact.3 in the Appalachians. They could effect W >gX,pghedimentation which was taking place more or less cosvally 4 to the west L of the metamorphic core: the stacking of pre-to synmetamorphic thrust W )k (/) D j sheets could have effected uplift of the core which supplied clastics p to the foreland shelf beginning in middle Ordovician time. d e. Stratigraphic Interval Af fected - The interval af fected in both pre-and synmetamorphic thrusts includes rocks as deep as the crystalIine base-ment (either continental or oceanic) and whatever cover rocks may i. ave been present. Some premetamorphic thrusts may in places involve only cover rocks, such as along portions of the Greenbrier f ault (Hadley and \\ \\ Goldsmith, 1963). \\ \\ f. ~R2 atienships to Folds - Faults of this class may develop synchronously w'th folds. Faults developed in this relationship would be subparallel \\ Q to the axes of these folds. In many instances the faults represent the excised cores of folds which became closed during formatfor and movement (.

89 continued on the resultant thrust. They are commonly overprinted by later folds. g. Relation to S-surfaces - S-surfaces overprint premetamorphic thrusts. Synmetamorphic thrusts have a mylonitic foliation along them which is the dominant s-surface Inside and outside the fault zone, h. Change in Fault Character with changing Lithology - Premetamorphic thrusts would exhibit the detachment - ramp properties of thin-skinned thrusts, if confined to cover sequences. The fault character of syn-metamorphic thrusts would change according to lithology as well, particularly with regard to the type of mylonitic material present along the fault with varying protolIth. I. P-T Conditions - Premetamorphic thrusts af f ect unmetamorphosed rocks to rocks metamorphosed during a previous thermal / deformation cycle during their movement histories. Synmetamorphic thrusts form under conditions $g 7 ranging from greenschist to granullte facies conditions (300 - 700 C) .s gJ' and at depths corresponding to pressures of I-Skb (5 - 20 km). They are generally associated with Barrovian metamorphic conditions. J. Relationships to isograds - Premetamorphic thrusts are overprinted by isograds (Fig. 3). Synmetamorphic thrusts are generally subparallel to ( ) isograd surfaces. Late stage movement may produce truncation of iso-( : e-.% ) ( grads, but this is not widespread in faults of this class. k. Relationships to intrusion Generally there is no known or clearcut relationship to intrusions. I. Relationships to Tectonic Injections - It is likely that some faults of this class brought with them masses of ultramafic rocks and/or pieces of oceanic crust. ~ Th[ophilolite sheets of the Bay of Islands in f- ~ J (,,~, h C! M '( {):% i. (,c C'm . r y

91 are reactivated by brittle deformation, these were cut by quartz-i feldspar veins which yielded age dates (minimum ages) of 210 Ma. (Chorokee PSAR). 5. Stress Fii 1 - Crientation - Stress field orientations in premetamorphic thrusts in-a. volving cover rocks would be similar to that of thin-skinned thrust with ~ c, oriented toward the northwest. A similar condition prevailed during formation of synmetamorphic thrusts with og. oriented toward the northwest or west. b. Magnitude of Principal Stresses and Strains - Stresses and strains for premotamorphic thrusts are considerable. At elevated temperatures the constitutlw relationships between stress and strain are a function of strain rate. Therefore no legitimate assessment of principal stress magnitude of synmetamorphic thrusts is possible. Strains associated with synmetamorphic thrusts are huge. For discussion of the problems and methods of determinations of' finite strain in highly strained rocks, see Ramsay (1967) and Mitra (1978). Variations of Stress and Strain Through Time - Stress and strain c. magnitudes probably varied considerably throughout the movement histories of these faults. They probably move incrementally; synmetamorphic thrusts move with formavion of mylonites, d. Present in situ stress - since these are pre-to synmetamorphic thrusts, N there should be little relationships to present stress fields. Seismic First Motion Studies - Not app!! cable. e. f. Rates of Motion - Essentially unknown. See discussion under c. above. g. Fluid Pressure Changes and Effects - Class! cal fluid pressure relat'lonships, as deduced by Hubbert and Rubey (1959), could be applied M to premetamorphic thrusts in cover rocks. In synmetamorphic thrusts f/ A (tk $ 0 3i ACS k

92 fluid pressure changes would be a function of metamorphic additions-and the availability of water in the system. Cehydration reactions pro-babi play a part in'the movement of syr. metamorphic thrusts. For example, ser ntine is altered easily until it begins to dehydrate, r 6. Geophysical and Subsurf ace Characteristics Seismic Activity - No clearcut associa?!on of seismic activity and these a. faults has been documanted. b. Subsurface Displacanent - Seismic reflection studies in the southerr} Appalachains '(Cook and others,1979) have not revealed clearcut examples of known faults of this class. The Hayesville fault may be discernable in the seismic section. c. Relationships to Gravity and Magnetic Anomalles - Geophysical signature, particularly inadequate, is dependent upon the rock types brought into 44f Juxtaposition by faulting. Thefaultfzenesofthis(exceptperhapsthe wA , g (,, Goat Rock) do r.ot have a characteristic magnetic signature.,antf*gther magnetit ; or gravity relationships are actually obscure, unless two distinctly dif-ferent terranes are brought together. This is true with portions of the Hayesville fault. d. Relationships to Geophysical Lineanents - No obvious genetic relationships. 7. Geomorphic Relationships - Faults of this class are generally not well expressed in the topography. Expression depends upon the nature and weathering characteristics of rocks on opposite sides of these faults. Very subtle. topographic expression may exist, as small notches in ridges, slightly aligned tributary streams and subtle dif ferences in erosional character *of rocks on either side of faults. 8. Methods of Identification - Faults of this class are best identified by differences in rock type across these faults. MylvJtes and other

93 mylonitic rocks in scme synmetamorphic faults may help, along with cata-clasites where'they occur. Overprinting by metamorphism and no of fset of isograds along the boundary may be used carefully but one must be able to discern otherwise that a contact in question is a fault. 9. Pitfalls in identification a. Failure to identify subtle differences in stratigraphy on either side of faults, b. Failure to recognize mylonites along syr. metamorphic faults. c. In a high-grade highly, deformed area, the most homogenous, planar, l l 2 gQ L fine p rock is most ilkely to be the most highly deformed, d. Age dating o ather than motion. pu hd/A / \\ e. Mistaking the age determined for a superposed brittlo event for the age of mylonite or f aulting in premetamorphic faults. f. Tendency to link vaguely defined epicenters with f aults of this class. g. Brittle deformation zones are likely to be missed (except in drill cores) because they are generally.very thin, on the order of 100 times less than the thickness of mylonite zones. h. Much money and time can be wasted trying to prove isotopically that the metamorphism and movanent in question is Paleozoic, whereas the basic problem may be a premetamorphic event or, in the case of cata-clastically reactivated faults, younger than the metamorphic event. I. Change in character of protolith with metamorphic overprinting.

10. Possibility of Reactivation,

a. Premetam;rphic thrusts are unlikely candidates for reactivation. Synmetamorphic are not a major cause for concern, but given a choice 7 -they'~woulif E good to a D 6 h b~ Mylonitic foliation in synmetamorphic thrusts is the most preminent planar feature and commonly has a lower shear strength. Hence this a gav a Ac o-yeJ--u 1% obw daq& {Ah

94 Is the most likely material for reactivation. Younger cataciasis does occur on these faults indicating reactivation c. has occurred in the past. However, the cataclasts is a very localized phenomenon Indicating local offsets. Doming, cooling, differential stresses, uplift are all possible causes. There is no concentration of seismic activity along these zones at present.. h d. Some[C gaults occur along province boundaries, as possible crustal boundarles in sone instances (e.g., the Hayesville thrust), therefore stress concentrations may occur here. I I, Selected References List Armstrong (1941) Dixon and Lundgren (196oi Hadley and Nelson (1963) Hatcher (1978) Hatcher and others (1978) King (1964) - ) Lundgren (1972) Roper and Dunn (1973) Wise (1970) .,.e

95 o CHAPTER X Class 7 LATE-TO POST-METAMORPHIC THRUSTS iti MEDIUM TO HIGH GRACE TERRANES (PALE 0 ZOIC CYCLE) f A. General Description (gy Thrusts of this class are generally thrus sN n which there has been enough motien (minimally) to Juxtapose metamorphic isograds -(Fig. 53). However, to accomplish this a considerable amount (at least a.few kilometers) of either horizontal or vertical transport, or a combination, must have occurred. These faults may or may not involve basement rocks. Contacts may be knife sharp, like those of many decollement thrusts, or have relatively thin mylonite zones along them Indicating time of formation was probably late stage synmetamorphic. Cataciustic and/or retregrade zones along '5ese f aults or portions thereof may indicate recurrent movement at a later time when the rock mass had cooled suf ficiently to exhibit brittle behavior. Some of these therefore could be classed as complex faults. ( Some Class 7 faults reside beneath extensive continuous sheets with defineable roots, while others may occur beneath erosional remnants as dis-membered allochthons and kilppes. Splays are seldom observed along faults of this class. All are f aults of the metamorphic core of the Appalachians. Subclasses of Class 7 include: (!) those faults with abundant mylonites which Juxtapose Paleozoic isograds; (2) faults without mylonites; and (3) erosional remnants of allochthonous sheets. Examples of Class 7 f aults include the Linville Falls f ault in the Grand-g father Mountain ~ window in North Carolins (Bryant and Reed, 1970a, 1970b), the p f h(s Hollins Line f ault of Alabama (Tu ll,1978), the Alto allechthon of Georgia (j and South Carolina (Hatcher, 1978b), the Bowen Creek-Ridgeway faults of Virginia and North Carolina (Conley and Henika, 1973), inner Piedmont nappes (Griffin, 1971, 1974) and the west side of the Beri< shires (Ratcliffe and s A 3 ~ U 'Nb % :GV9 (JQ M z CJ.cf cg

Y h gg & ch $ O 96 Harwood, 1975). Faults of the P.Ine Moun belt of Georgia and Alabama r m (including the Towaliga anQat Rock faults)/ contain extensive mylonites. They Juxtapose high (sillimanite?) grade terranes of the Inner Piedmont and Uchee belt against the slightly lower (kyanite) grade rocks of the Pine Moun-tain belt (Clarke, 1952; Bentley and Neathery, 1970). Until the metamorphic grades of these respective terranes have been accurately determined, the proper class of these faults will remain unknown. The Ammonoosuc fault of New Hamp-shire and Maine is another candidate for Class 7 fault. B. Description of Fault Class 1. Basic Geometry (a) Strike Length - 10's to 100's of kilometers (Hollins Line fault), may have k!!ppes (Alto allochthon). ^ ~ (b) Width - Less than a meter to tens of meters. (c) Orientations - Parallel to subparallel to the dominant structural grain. Traces bounding the ends of klippes and folded faults of ( this class may cut across the regional strike (e.g., Linville Falls fault). They generally have a low angle of dip. (d) Displacement - A f ew kilometers to tens of kilcmeters have been generally accepted. Larger displacements Fave been suggested T (Hatcher, 1978a). Geophysical data supports the latter (Hatcher and Zietz, 1978). (e) Continuity - May be demonstrated along their lengths. These faults rarely splay. (f) Curvature - Ccmmonly folded, so this is reflected in outcrop patterns. Subhorizontal segments exist in some. Have out-crop traces strongly influenced by the topcgraphic contours. 9

103 Identification very difficult, even though the chronology of deformation and mechanics may be identical to that of other faults of this class. (d) Planes or zones of prograde or retrograde low grade metamorphism within a high grade terrane should be treated with suspicion. (e) Variations in the properties of a given contact might cause misclassification or interpretation as a simple stratigraphic contact. (f) Dating retrogressive mineral assemblages may be possible but determination of exactly what is being dated requires very careful study of mineral paragenesis. II. Possibilities for Reactivation Reactivation of Class 7 faults is unlikely, except along high angle segments. y' ]7 12. Best Ref erences on Class 8 Faults Bryant and Reed (19706) Conley and Henika. (1973) Griffin (1974) Hatcher (1978a, 1978b) Ratcliffe and Harwood (1975)

104 C:IAPTER XI CLASS 8: STRIKE-SLIP FAULTS A) GENERALIZED DESCRIPTION 1) Introduction Strike-slip faults include faults on all scales whose major component of slip is parallel to the fault strike. At least seven subclasses of strike-slip faults can be distinguished based either on scale, tectonic history or geologic setting: 1) Major strike-slip faults; 2) Cross-J go structure faults with a horizontal component of slip; ' Faults reactii -D vated with strike-slip motion; 4) Tear faults associated with decollement b4Y$$n tectonics; 5) Small displa' cement strike-slip faults on the limbs of folds; 7.,,I L'"cs

6) Small displacement strike-slip faults in flat-lying sediments; and

(

7) Strike-slip faults in Mesozoic basins.

The United States Appalachian Mountains have few examples of well exposed major strike-slip faults. In contrast, several major strike-slip faults are known in the Maritime Appalachians of Canada and may extend into Maine. Strike-slip faults with narrow fault zones, less than 100 m of slip and mappable less than a few kilometers parallel to strike are the most likely faults of this class to be encountered in the U.S. Appalachians. The seismicity of the Appalachian Mountains includes few examples of strike-slip faulting as determined by focal mechanis=s. This trend in seismicity is another indicator of the small role that strike-slip

hM

xs AS 3)

Faults Reactivated with Strike-Slio Motion Faults of this subclass are described elsewhere in this report as complex faults. Sone complex faults have demonstrable strike-slip dis-l P acements based on offset of local rock units. One example is the Ramapo fault system of New York that has as much as 4 km of right-later-al slip in the Paleozoic (Ratcliffe, 1971). The Canopus fault of the Ra=apo fault system is characterized by extensive cylonitization along a complex fault system (Fig. 66). Ratcliffe (1971) describes the Canopus fault: "It is significant that cataclastic defor=ation similar to that ascribed to the older faulting... marks the extension of the fault zone to the northeast rather than the opea work breccia of the youngest fault episode. Cataclastic deformation took place at various times in clie'Canopus area, judging from the cross-cutting relationships of cylonite zones. However, the details ) are imperfectly known at present. The field relationships as presently understood are presented in Figurc [9]. ~he area was mapped by the writer at a scale of 1 in.: 1000 ft during inves-tigatien which spanned a threc-vcek period.... The mylonite zones shown on Figure [9] are all c:arked by strong development. of miaor folds showing right-lateral shear sense and near vertical fold axes. Evdience for right-lateral transcurrent faulting is best' displayed along the western side of the Canopus Valley marble belt in the vicinity of the Canopus pluton. Here, offset of a distinctive 1.5-to 3-ft-thick magnetite deposit, shown by a special symbol on the c:ap (Fig. [9], Loc. 2), suggests a right-lateral displacement of 4 km (2.48 mi). The age rela-tionships of this fracturing will be discussed in the section dealing with the intrusive rocks."

4) Tear Faults Associated with Decollement Tectonics Cor:=only the decollement sheets of the Central Appalachians moved as units separated by tear faults.

Figure 67 shows major tear faults

117 7) Strike-Slip Faults Associated with Mesozoic Basins Slickensides with horizontal stria are coc=on within the Meso-zoic basins of the App =1=rhfans. Some are associated with faults that can be traced several km within the Mesozoic. basins (Dames and Moore Report, 1977), whereas others are found within the border fault zones (Ratcliffe,1979, personal coccunication). Simfinr strike-slip move =ent domains can be traced westward into the cry-stalline terranes of the Berkshires fres the Connecticut basins. In general the sense of slip abag the border faults is mixed (i.e., both right-and left-lateral). Within the basins slip along indivi-dual faults is consistent. For exa=ple, the northern end of the Newark basin shows left-lateral slip along several faults subpar-allel to Ramapo (Lomando and Engelder, unpublished manuscript). Strike-slip faults within the Newark basin have been traced into g adjacent crystalig (Fig. 72). Ae'(c9' Displace =ent on cost slickenside surfaces is on the order of t=t to cm. Some of the intrabasin faults have a few meters of slip. The abundance of these faults is illustrated by the Limerick P.S.A.R. Zeolite minerals are found in some of these Mesozoic strike-slip faults. List of Tvee Examoles a) Rockland Lake fault, New York (Dames and Moore Report,1977) b) Ramapo fault, New York (Ratcliffe, personal coc=unication)

8 CHAPTER XII CLASS 9 CCMPLEX FAULTS Complex faults are defined herein as those with a repeated movement history. While most f aults in the Appalachians, particularly the larger f aults in the crystallines, have experienced multiple movement histories, they have had a single major episode of movement, usually associated with a particular orogenic event. However, those faults classed as complex faults have experienced several major episodes of movement. Discussion of the class will be through two well-known, but still Incompletely understood, examples: the Brevard zone and the Ramapo fault. Recent investigations in Virginia have revealed the complex history of the Hylas zone (Bobyarchic and Glover,1979). BREVARD ZONE A. Generalized Description The Brevard zone consists of a linear belt of mylonitic cnd cataclastic rocks extending from central Alabama to northern North Carolina (Fig. 74). ( Mylonites are both prograde and retrogrcde with major segments having undergone a later brittle movement history. The dominant nylonitic foliation in the Brevard zone is parallel or subparallel to the dominant regional (S ) foliation in the southern Appalachians. But the dominin? > _2 s I~ation in the Brevard zone transposes the regional S. It is in large 2 'k p part stratigraphy contro!!ed: distinctive lithologies (graphitic 1 M Q M phyllonite, marble and quartzite) are associated with the structurally t# t, " defined zone. Slices of platform carbonates have been brought into the Cr >i f ault zone from the footwall of the Blue Ridge thrust sheet (Fig. 75). B. Description of Fault Class I. Basic Geometry a. Strike Length - 400 km. b. Width Perpendicular to Strike - I - 4 km. s e

121 d. Relation to Isopachs - None known. e. Stratigraphic Interval Af fected - Rocks involved in the Brevard c zone range frem late Precambrian to early Paleozoic with slices of Cambro-Ordovician and possibly Grenville basement rocks. Associated Igneous units indirectly involved may be as young a.s Silurian (Lmmon, 1973). f. Relationship to Folds - Mylonitic foliation is axial planar to Taconic and Acadian folds but is deformed by younger more open-folds. Fault terminations in some cases are in the vicinity of bhad folds but may not be related in time. g. Relationships to S-surfaces - Early ductile faulting along the 'CW Brevard zone developed subparallel to regional _S but transposed e 2 ~_e- ,II,g-bb h all earlier S-surfaces.;Later movement served to transpose b (4.t.c C.7 9binitial mylonitic S-surfaces (Roper and Dunn,1973). ~~ ~ ~~ ' \\Y ( *> v Op()" A h. Change in Fault Character with Changing Lithology - Protoliths c M A* M]p. p ~ ,N

• control the character of mylonites and mylonitic follation but

( -t 9NO.,(3ch j not the orientation (Fig. 75). p 1 + \\ M-1. P-T Conditions - Earliest movement is probably pre-Taconic thermal . ry 1 g peak; major movment occurred at staurelite-kyanice conditions, r S' b f,I f V then later during Acadian greenschist conditions, then again under i p bGfY g., lower to subgreenschist conditions when the brittle event occurred. Tv' isograds Indicate a decrease in grade approaching the Brevard zone N-I% both from the northwest and southeast, reflecting the synclinal p.

VtOb yko 3

g f,./ foldi_Dg of the isograds in the Chauge bolt. <f k J. Relation to intru'slons - Cambrian Henderson Gnesis (535 f/a Rb/Sr anu zircon Odem and Fullagar, 1973; 600 Ma zircon Sinha and Glover,1978) transformed intomylonites. Stirewalt and Dunn (1973) presented evidence that 3revard zone faults cut the Mount Airy Granite in North Carolina. e e-e m ee e m ee me eve-omeeeme we e-e

133 r CHAPTER XIII .. ~ CLASS 10: ENGitAATIC STRUCTURES (Co0SS STRIKE DISCONTINUITIES?) A. Generalized Description These structures represent perhaps the most controversial class of large-scale structural discontinuities. They have been variously described as lineaments, fracture zones,linears and most recently cross-strike structural discontinuities (CSD) (Wheeler et al., 1978; appendix b). The fea-tures consist of a complex of. structures arranged in linear belts of vary-ing sizes. The structures run t.he ga=ut from igneous intrusions, fault-ing, fracturing, fold plunge outs, geomorphic patterns to photo linears; they may have geophysical and stratigraphic expression as well. Where studied the features are frequently found to have very long \\ histories in some cases extending from the Eo-Cambrian to the present. Some contain regions of present-day seismicity and thus should be treated with caution. Motions on these features are generally complex and are us-ually described as a region of " flexing" or " jostling". Some of the structures show evidence of basement involvement in either an active or passive mode (i.e., simply reflecting basement " roughness" vs. basement motion). Other CSD's are thin-skinned, and appear to be associated with regions where detachments cut up section at a large angle to strike. 1. Possible Subclasses a. Basement controlled CSD's ig. 85a) CSD's formed by any type of motion which Ib,h3M) is seated in the crust.

134 ii. Passive: (Fig. 85b) CSD's formed by interference or control by inactive irregularities in the basement, e.g., a fault con-Y" h ($ 'd trolling facies distribution which in turn controls thrust ,. ;.y' boundaries. M d.,1, l b. Supra-crustal CSD's (Fig. 86) - CSD's controlled by discontinui-tv ties in thin-skinned structures, e.g., tear fault at depth, C. y boundary of " keystone" structure (Engelder,1979, Fig. 5) 2. Typical examples a. 38th parallel lineament b. Petersburg and Parsons CSD c. Transylvania fault (Root and Hoskins, 1977) B. Description of Fault Class 1. Basic Geometry - ~ Strike length - Map scale, varying from a few kilometers to a. over 1000, b. Length perpendicular to strike - The discontinuities are thin relative to length. Widths are generally proportional to length and vary from a few 100 meters to up to 80 km. Orientation - The structures occur at a large acute angle to c. the regional strike. Present knowledge indicates that the active CSD's have strikes close to 090 (except see Clarendon-Linden fault, Hutchins et al.,1979), while the supra-crustal CSD's seem to occur within 20' - 30' of the norcal to the regional strike (e.g., Wheeler et al., 1974). d. Displacement - Supracrustal displacements occur only on individual faults within the CSD. Dsiplacements are small, varying from a 9

few meters to several hundred (rare). Displacements are char-acteristically mixed, i.e., both nor=al as well as strike co-tinne, and lack any consistent pattern. Continuity - The CSD's do not have a well developed continuity e. as they consist of an assemblage of structures of which any one or more may define the zone at any given locality. Since there is variation in both the intensity of development as well as the particular set of structures which forms the lineament, the surface continuity is generally poor. f. Termination alcng strike - Almcst nothing is known about the 9 mode of termination, other than the zones of disturbance simp ]ly __/ ^ ' ~ ~ ~ gn no longer be found. 2. Tectonic Setting - Basement controlled CSD may extend across the entire fold belt and deep into the craton (e.g., 38th parallel linea-~ \\ ment [CSD7]). The supra-crustal CSD's are recognized most readily in the Plateau and rarely cross into the Valley and Ridge. 3. Characteristics of Zone (see Appendix B) - The CSD's are character-ized by one or more of the following: a) An increase in the inten-sity of development of a structure or set of structures, generally jointing or faulting, in a zone at a high angle to the tectonic ? grain. Examples: 40th Latitude Lineament (Root and Hoskins, 19 ): _77, rv Parsons Lineament (Holland and Wheeler,1977); b) Termination or change in trend of some structure or structural elements, e.g., fold plunge outs, strike disruptions. Example: Petersburg Linea-ment (Sites et al.,1976); c) Belts of igneous activity, facies and thickness changes across the zone. Exa=ple: 38th Parallel Linea-ment (Heyl, 1972). t e

.I. 136. 4. Relation to Country Rock a) As indicated by their name, CSD's almost exclusively occur at a large angle to the regional grain. However, this relationship may be an artifact of the difficulty in recognizing a linea-ment parallel to the regional grain, since much of the basis of its recognition is founded on the transverse disruption of a the regional trends. The possibility that there are lineaments approximately parallel to the regional trend is suggested by a magnetic lineament described by King and Zeitz (1977) which parallels the Appalachian orogen along its western border. To date, no structural expression of th'is feature has' been re-cognized, although Dennison (19' ) has presented evidence for f-N 1 C a " Keel zone" for the Appalachian Basin, which is quite close ,1 f in location and orientation to Zeitz's magnetic lineament. b. There is no direct relation to echayments or promontories. However, the two most prominent CSD's, the 38th and 40th Parallel Lineaments are located at the changes of regional trend which mark the Virginia Promontory and the New York Embayment. Only a tentative statement can be made regarding the extent of c. crustal involvement; however, the evidence thus far suggests an ultimate thick-skinned origin, although the i==ediate expression is thin-skinned as in the Transverse step ups (Fig. 86.) suggested by Kulander and Dean (1978). e

CHAPTER XIV /))as W Q w& n%Oi"~)> "'Ns.At U h We CLASS 'll: FAULTING RELATED TO GEOMCRPHIC PHENCMENA A. Generalized Description Faults of small to moderate scale can be produced by a number of processes which are fundamentally geomorphic in nature. These structures are technically faults even though their relationships to true tectonic processes are distant at best. Nevertheless, their presence constitutes additional " noise" in the complicated small scale patterns of tectonic faulting present in most regions of the Appalachians. Most of the relevant geomorphi~c processes have been active in very recent time, and the possibilities of misinterpreting their effects are numerous. Their interpretation as true tectonic disturbances can cause unwarranted concern about the level of neotectonic activity in a region. The possibility of continued motion on geomorphically produced faults obviously needs to be taken into account in the engineering design at any site. However, these problems fall more in the realm of soil mechanics and - 's civil engineering than in the analysis of seismotectonics. If a fault dis-placement can be demonstrated to be soleiv the result of some surficial process, it should be excluded from the usual seismotectonic restrictions required for site certification. However, many of these geomorphic effects can be con-centrated along older fault zones as a result of bedrock contrasts, deeper weathering along fracture zone, more water in the fault zone, weaker gouge in the zone, etc. Consequently, the mere identification or label of some structure as a geomorphically produced fault should not automatically exclude it from all tectonic considerations. However, once complete independence of these geomorphic structures from true tectonic structures has been established by deeper excavation, determination of map movement patterns independent of bedrock fault patterns, drilling, etc., then this class of faults and related structures should be excluded from the usual seismo-tectonic safety consider- \\_ ations for that site.

147

• Pemafrost structures Extensive permaf rost during the Pleistocene produced a variety of sur-ficial structures throughout the Appalachians, both within and beyond the glacial border. The southward limit of Pleistocene permafrost is not well established. One model for the origin of the Carolina Bays of the Coastal Plain is as permafrost thaw lakes similar to those of the Arctic coastal 5

plain of Alaska. Fels (German for seas of rocks) may occur at higher elevations in alI areas in.:luding the upland of the Southern Appalachians as a result of extensive frost shattering and heaving in both Pleistocene and recent time. Locally, these rock fields dev' eloped flow characteristics during o<3C.5, the Pleistocene, somewhat analogeous to rock glaciers as at Blue @ Pen-nsylvania (Potter and Moss, 1969). A summary of permafrost features is given in books by Washburn (1973), Pewe (1969), and King (1976). In particular, the ef fects of former Ice wedges, patterned ground and sollfluction can minic true tectonic, structures. Ice wedging and lifting of bedrock sheets can pro-duce effects along Joints which are easily mistaken for faults. Deeper excavation and careful attention to map patterns or internal structures of fer the best methods of distinguishing these structures. Washburn, A. L., 1973, Periglacial processes and environments, 320 p. St. Martin's Press, N.Y. Pewe, T., 1969, The Periglacial processes and environment, 487 p. McGlil-Queens Univ. Press,tentreal. King, C.A.M. (eg.), 1976, Periglacial Processes, 459 p. Benchmark pcpors in geology, v. 27, Doden, Hutchinson, and Ross, Inc. Stroudsburg Pa.

Potter, N., and Poss, J.,

l968, Origins of the Blue Rocks block field and adja-cent deposits, Berks Co., Penns., Geol. Soc. Am. Bull. v. 79, p. 255-262.

150 does not mean that all offset drill holes or similar features should be regarded as neo-tectoni.c effects. Most offset drill holes are gravity slumping of joint bloc):s or frost heaves. Most others represent release of in situ stress by ramoval of the surrounding rock mass. Among the better known examples of the type of modern deformation produced by exca-vation is the Niagra power canal as described by Lee and Low ( There j the Ieckport Dolomite, sandwiched between two shale layers, has consider-m ably more residual / stress than the adjacent shales. Turbines put into a -__w' U b p; h I deep cut early in this century have required repeated readjustments because of progressive movement of the dolomito. The deformation consisted of two parts; an instantaneous elastic release followed by continuing viscous deformation. Near surface stress release features are a normal aspect of deep ( excavation and of many areas of the Appalachians. Their presence suggests. at least moderate levels of residual stress in an area and calls for more careful and more complete in situ stress measurements than might ordinarily be made. Their presence should not automatically be considered as evidence of modern tectonic activity. However, the presence of microearth activity, as in the Moodus area, in association with strong development of these release phenomena should be cause for concern. G

151 CHAPTER XV Class 12-Faults with Demonstrable Late Mesozoic or Cenozoic Movement A. Generalized description Included in this class are all faults with displacement which can be demonstrated to post-date major Triassic-Jurassic block faulting; as such', these faults do not necessarily share a common genetic history.

However, a very large majority of documented faults of this class appear to be high-angle and reverse in nature. These faults will be reviewed in detail'under

()g, Section B (Description of fault class ther fault types wi'l be covered under a discussion by subclass. Recognition of faults of this class is usually dependent upon their (/(, association with faulted Cretaceous or Cenozoic sediments,in the absence of ~ ( such material documentation of late Mesozoic or Cenozoic movement is very difficult. For this reason the distribution of these faults is uncertain, although they appear to be present in all the major geologic provinces of the southern and central Appalachians (figure 91). Knowledge of the extent of faults of this class in the northern Appalachians is limited due to the absence of Cretaceous and Tertiary sediments. Fault Subclasses Thrust faults SchEfer (1979) and Block et al. (1979) have recently described evidence for modern thrusting in the Valley and Ridge province and crystalline portions of the Appalachians Both studies justified the presence of modern thrusting by the presence of offsets in drill holes used for blasting operations R

152 during the construction of roadcuts. Similar structures have been observed in numerous quarry excavations in the Piedmont of the southern Appalachians 1 (J. R. Butler, pers. comm., 1979). Schafer (1979) described offsets in both sedimentary layers and in modern (10 year-old), drill holes present in folded and thrusted Pennsylvanian shales and sandstones (figure 92). He attributed the offset of drill holes to reactivation along previously existing yW fault planes. Block et al. (1979) described offsets in drill holes present in an interbedded sequence of quartz-biotite-plagioclase schist and cale-silicate gneiss. Offsets are along pre-existing foliation surfaces and fault planes i ( which also display well-developed slickensides. Repeated measurements which were taken over an 8-year period indicated a relatively continuous rate of e g> offset of 2.8 mm/ year. The authors suggest that this motion was at least in part responsible for local microseismic activity and associated "Moodus noises." ~ Unfortunately these examples are completely based on sites where large ~ k scale rock excavation has taken place. While the structures described are \\,\\ ? technically faults, their origin may be due to the local release of L \\ stress caused by unloading rather than regional tectonic stresses capable of -g producing macroseismic activity. Until additional studies can substantiate s \\M [ a megascopic (map-scale) character for these structures they should be con-d.O C yD '/ sidered to be features analogous to " pop-ups" described under geomorphic

1. r q

\\ 'ty faulting. 7 [M c Conley and Drummond (1965) have described possible thrust-faulted Pleistocene or Pliocene alluvium, colluvium, and underlying gneiss located near the base of the Blue Ridge topographic escarpment in southwestern North Carolina. The presence of the structure near the base of a slope and the weathered nature of the bedrock suggest this may actually be a geomorphic feature produced by large-scale slumping.

153 Block faults and growth faults Late Mesozoic and Cenozoic faults of this type have not been documented within the Appalachians proper, but the~y may be present in ths subsurface of the adjacent Atlantic Coastal Plain of Virginia (Cederstrom,1945), North Carolina (Baum and Prowell,1979), and Georgia (Cramer,1969). Cretaceous and Tertiary faults of this type are'well-documented in the Gulf Coastal Plain of Alabama (Copeland et al., 1977). High-angle reverse faults This type of fault is the only type in the Appalachians which has been f shown to have well-documented post-Mesozoic displacement. The apparent localization of these faults along the Fall Line between the Piedmont and j Coastal Plain (figure 91) may be real (perhaps due to a "hing-line effect") l or more likely.it is due to presence of a thin, relatively continuous veneer i of Cretaceous and Tertiary sediments which allow ready identification of faults of relatively small displacement. Likewise, none of these faults have been identified north of the Coastal Plain of Maryland, probably due to the absence of suitable sedimentary cover over older crystalline and sedirantary rocks. Table summarizes pertinent features for all recog-nized faults of this type. F.xamples: LJ M Stafford fault system - The Stafford fault system is 1 coated along the Fall gf Line and Potomac estuary of northeastern Virginia, approximately 40 kilo-meters southwest of Washington, D. C. (figure 93). The fault system will [gd be used as an example for the characteristic of fault of this class. Mt While the possibility of youthful faulting has been suspected for some time (McGee, 1888); the true extent of faulting has been only recently documented (Mixon and Newell, 1977 and 1978). Rocks in this region consist

154., of a basement of crystalline schist and gneiss nonconforcably overlain by lower and upper Cretaceous coarse fluvial sediments of the Potomac Group which are in turn overlain by thinner units of Paleocene, Eocene, and Miocene marine sediments. The sequence is capped by thin units of r.ppermost Tertiary (?) or Pliocene-Pleistocene gravels. Faulting is en echelon in nature, with faults being offset in a sinistral manner (figure 94). Structural-contour maps of Cretaceous and Paleocene , f gp&& f 11thostratigraphic units show that displacement on faults increases downward, u, ith the Cretaceous-bedrock contact having as much as 60 meters of displace-t 4" w

1. h meat while displacement within Tertiary units is less than 20 meters (Mixon N

QJM l Newell 1967 6 1968), indicating probable recurreat novement. Units of the lower to middle Miocene Calvert Formation are not extensively affected f/(, by faulting. however at one location the. Fall Hill fault has displaced }Y Pliocene-Pleistocene Rappahannock river terrace alluvium by approximately 0.5 meter. The apparent thickening of sediments across faults (figure 95) is probably the effect of recurrent faulting and erosional truncation rather than original tectonic control of sediaentation. Nixon and Newell (1977) suggest the alignment of the Stafford and adjacent Brandywine fault systems with the adjacent faults in the Farmville and Richmond Triassic basins may indicate reactivation of old, unhealed fault under a new stress regime. B. Description of fault class 1. Basic geometry a. strike length - Major fault systems have demonstrable lengths of 5 to 30 kilometers; in several cases these systems are en echelon in nature, individual faults may be less than one

155 I kilometer in length. Sunor faults usually cannot be traced from a single exposure. I b. length perpendicular to strike - Uncertain. The high-angle 'of the faults, and their extensive length suggest they are deep seated in nature. orientation A majority of faults trend northeasterly, parallel ~ c. to regional structural grain. Faults in the Pine ?!ountain belt near Warm Springs, Georgia (Reunhardt et al., 1979) trend north-west. Faults present in the Carolina slate belt (Parker, 1979; Howell and Zupan, 1979) trend east-west. d. displacement - Major faults have measured vertical di' placements s of 10 to 75 meters; usually displacement has been found to increase with depth in stratigraphic section, suggesting recurrent movement. continuity - Many fault systems.are en echelon in nautre, individual e. faults may range from 0.5 to 15 kilometers. In some cases minor ~ k splays are present. f. curvature - Faults exhibit very linear surface traces, curvature perpendicular la strike at depth has not been determined. Fault , surfaces usually refract and decrease in dip when in unconsolidated sediments. g. termination along strike - Faults terminate as splays with de- , g c_. creasing displacement or monoclinal foi p many cases they can be traced into crystalline rocks where the actual termination cannot be mapped due to the lack of marker units. 2. Tectonic setting Faults of this class occur in all major geologic provinces of the Appalachians (figure 91). As previously stated, the locali-zation of faults near the Fall Line is probably an artifact of ideal

180 DECO:.LEMENT: Detachment along stratigraphic surfaces as a result of g V'1,}/ _ *. a O L 9 deforma tion' (Dennis,1967). j.pe f-

gdb4, DEFORMATION: 'The' net change of position, with respect to a fixed coordi-

') b 7, k nate sys f every point with)n/ a body. deformation af h body translation]and/or t gid bodyytalion) 'f caar t: i Ncr./ orc. Informall ' ML, AAha)Ns @R(b eformation refers to the process L 1. ,and d g by which the above changes occur, for example, a brittle de- [%-4$g .e n < - TL*W 0* '"f AQ< formation (Hobbs, Means, and Williams,1976). y45 % 4e DIAPIR: A fold or plug-like flow structure whose mobile core pierces overlying less mobile rock (Dennis,1967; Billings,1972). kiridMM ' DISPLACEMENT: g ach material point in an undeformed body may be connected to E 4 g r.k,, Go J E j L( - vector. The total array of displacement vectors constitutes Q& .hW p'& g Mbl- '\\ the displacement field (Hobbs, Means, and Williams,1976). .g g o.d, [. Informally, displacement can refer to the relative movement A~ between two bodies, but is a non-precise term including both slips and separations (Gary, McAfee, and Wol f,1972). The proces% ksd C Q M k', 2, % b'4QAu W Mww b* Vded g../ DRAG: s resulting in drag folds. W a tnr.kbokc b l 3 up u .-y DRAG FOLD: Minor folds produced in certain rock layers by differential move-ment of adjacent layers (Dennis,1967). EN ECHELON: An overlapping or staggered arrangment, in a zone, of geologic features which are oriented obliquely to the orientation of the zone as a whole. The individual features are short relative to the length of the zone (Dennis,1967).

EPI-ANTICLINAL FAULT: A longitudinal or transverse fault associated with a doubly plunging minor anticline and formed con-currently with the folding (Gary, McAfee, and Wolf, W K(Gary, McAfee, and Wolf,1972). EXTENSIONAL FAUST: Qension A tension y4( fault must, of necessity, form in the ai-az plane at ~ right angles to a3 This precludes any shear motion , _ [. fM" '. V^ s because the fault lies in a plane of zero resolved shear ch SNG ' {3 b i.S{ M "**,

• stress.( The term should be abandoned, because any shiear 1 d' 99"

/ M motion must be superimposed on a pre-existing fracture. FAULT: A fault is a surface or tabular zone along which true or apparent displacement parallel to the surface or zone is at least ten times greater than displacement normal to the zone. This definition is not scale-dependent. Cobpare with Billings (1972), Dennis (1967), Gary, McAfee, and Wolf (1972). FIBERS:. Parallel arrays of elongate crystal growths marking the direction of extension along a fracture or fault. FLINTY CRUSHED ROCK: Silicified microbreccia or gouge; a cataclastic rock. FLUXION STRUCTURE: A mylonitic foliation. FOLIATION: A general term for a planar arrangement of textural or structural features in any ',ype of rock, e.g., cleavage in slate or schistosity r. 0#-()# -1 in schist (Gary, McAfee, and Wol f,1972). ,) N- ) b uxW.

182 FRACTURE: A general. term for any brittle break in a rock, whether or not it causes displacement. Fracture includes cracks, joints, and brittle faults (Modified from Gary, McAfee, and Wolf,1972). A surface along which loss of cohesion has taken place (Dennis, 1967). FRICTION: The force resisting slip on a surface. For coefficients of friction, internal friction, dynamic friction, sliding friction, and static friction, see Jaeger and Cook (1976). GASH FRACTURE: Small scale tension fractures, having highly eccentric e: k elliptical crnss sections, occuring at in angle to a fault, which remain open or are filled by secondary mineralization. GLACIAL OVER-RIDING: The process by which moving ice sheets exert shearing stress on the rocks beneath the ice. Glacial over- ) riding structures include minor faults and folds. G0UGE: Clay-like rock material formed by crushing and grinding along a faul t. Most individual fragments are too small to be visible to the unaided eye (Higgins,1971). A ultra fine-grained cataclastic rock. GROWTH FAULT: A fault in sedimentary rock that forms contemporaneous 1y and continuously with deposition, so that the throw increases with depth and the strata of the downthrown side are thicker than the correlative strata on the upthrown side (Gary, McAfee, and Wol.f,1972)

183 IMBRICATE: The geometric array of a sucession of nearly parallel overlapping thrust or reverse faults which are approximately equidistant and f have approximately the same displacement (Modified from Dennis, 1957). IN-SITU: In place, existing at the present time; e.g., in,-site stress is that stress state currently existing in rocks as opposed to paleo-stress. JOINT: A joint is a fracture along which true or apparent displacement par.allel to the surface is less than ten times greater than dis-placement normal to the surface. KINK BAND: A sharply defined tabular zone, on any scale, within which planar fabric elements are abruptly rotated with respect to their orientation outside the zone. ( LAG: Listric normsl fault. LIMB ATTENUATION: The tectonic thinning of the limb of a fold, either by flattening or by the development of a ductile fault oblique to the limb. Limb attenuation is d a kh .#rumu ty process. dwg

184 LINEAMENT: Straight or gently curved, lengthy features of the Earth's surface, frequently expressed topographically as depressions or lines of depressions; these are prominent on relief models, high altitude air photograohs, and radar imagery. Their meaning has been much debated; some certainly express valid structural features such as faults and zones of intense jointing, but the meaning of others is obscure (Gary, McAfee, and Wolf,1972). For an extended discussion see Appendix B. LISTRIC: A fault plane with decreasing dip at depth; a shovel-like Y,. [ h geometry; af ter the Greek for shovel -- listron. r U& V L C' D MASS WASTING: The gravitationally driven transport of surficial material p down a topographic slope. Mass Wasting has no tectonic signi-ficance except that some tectonic events can trigger' mass wasting; it is included here only because some mass wasting (slump blocks) can be confused with faulting. Mass wasting differs from faulting because mass wasting does not extend to appreciable depth. MEGACRYSTS: A nongenetic term for any crystal or grain, in an igneous or metamorphic rock, that is significantly larger than the surrounding groundmass or matrix. Megacrysts include pheno-crysts, porphyroblasts and porphyroclasts (Gary, McAfee, and Wol f, 1972 ).

185 MILLING: The process of granulation and comminution of rocks in a brittle fault zone producing fault gouge. Lapworth thought milling was an b important process in the production of the Moine mylonites; but t is is incorrect because those mylonites were produced by plasti- -U? 44ty processes. MULLION: b.b MYLONITE: A fined-grained, highly foliated c. rock, re-7 sulting from intense' ductile strD,where strain rate exceeds recovery and/or recrystallization rate. The strain is accom-plished by the nucleation, glide, and climb of dislocations. The mylonitic foliation is invariably an axial plane foliation. Mylonites are finer-grained than their protoliths. Quartz-rich protoliths yield mylonites containing quartz ribbons flattened and extended in the foliation, and these quartz ribbons usually display recovery textures such as strain-free subgrains. (Hatcher,- 1978; Hobbs, Means, and Williams,1976) MYLONITIZATION: A p12 " ' 77processinvolvinghighductilestrainandik (c6diplete recoverk Dimunition of grain siz[is characteristic 'r' #eh of this process (Hatcher,1978). Mylonitic rocks include: )w I.s &e,Wb Jckb phyllonite, blastomylonite, mylonite, and ultramylonite. g-M ,3 <h e ~ SW (e# NAPPE: A large allocthonous, sheet-like tectonic unit that has moved along a predominately subhorizontal floor (Dennis,1967). An allochthon. gas s u,.se ap

186 NORMAL: A line taken perpendicular to a plane, and whose orientation specifies the orientation of the plane. NORMAL FAULT: A fault whose hanging wall has moved down relative to its footwall. Typically normal faults are brittle faults. OBLIQUE: Across, not parallel to. Features may be parallel or oblique or ' perpendicular. When the net slip of a fault has both dip slip and strike slip OBLIQUE SLIP: components it is oblique slip. Oblique slip has rake angles greater than zero degrees, but less than ninety degrees (Billings, 1972). PALE 0 CRY 0TURBATION: The past disruption of surficial materials caused by the expansion of water upon freezing, or by glacial over-riding. PENETRATIVE: A structure is penetrative if it is repeated statistically at imperceptible distances on the scale of the domain under con-sideration, so that it effectively pervades the body and is in the same average orientation in every sample. (Dennis,1967). The concept is scale-dependent and a structure may be penetrative at one scale but not at another. PHYLLONITE: A rock of phyllitic appearance formed by mylonitization of a schistose protolith (Modified from Higgins,1971). PLASTICITY: That property of rocks and minerals whereby permanent (non-elastic, non recoverable) strain is achieved in a non-hydro-static stress field. Plastic strain is permanent strain under constant nonhydrostatic stress (Jaeger and Cook, 1976). Plastic

187 debrmation is a constant volume deformation, that is, it p arises only from the deviat@c part of the applied stress. The mechanisms of plastic deformation are dislocation glide, dislocation climb, and grain boundary sliding (Nicolas and Poirier, 1976). Plastic flow is a solid state process in contrast to fhk 9 viscous flow; nevertheless an effective coefficient of viscosity can be calculated for plastic flow even though 'he flow is non-(Newtonian. d POP-UP: An outcrop scale anticline formed by natural or artificial erosional release causing the upbowing of a surficial bedrock slab. PROT 0 LITH: The parent rocks from which cataclastic or mylonitic rocks are derived. PROTOMYLONITE: A mylonitic rock containing greater than 10% megacrysts. In contrast to blastomylonites, the megacrysts in protomylonites do not have rims of neomineralization and/or recrystallization. PSEUD 0TACHYLITE: A dark fine-grained, often glassy looking cataclastic rock which frequently occurs in discordant veins in fault zones. Although pseudotachylites usually are interpreted as the product of friction-induced melting, they have the microtex-tures of intense brittle deformation at high strain rates and low temperatures (Wenk,1978). RAMP: That portion of a bedding-plane thrust which cuts up-section producing a fold in the rocks of the thrust sheet.

{, 1.33 RECESS: An arcuate portion of an orogenic belt which is convex toward the craton. Synonymous with reentrant. REC 0VERY: The process which lowers the total strain ene~rgy of a_c..rystal by p ~ G.T dislocation climb, dilocation annihilation, polygonization) and 'A C annealing recrystallization. Recovery is a M rature phenomenon, and the rate competition between strain hardening and recovery processes determines the texture of plastically deformed rocks (Nicolas and Poirier,1976). RESIDUAL STRAIN: Elastic (recoverable) strain stored in a rock because of the constraints imposed by surround rock, matrix, or cement. Residual strain is relieved when the rock or mineral grain is freed from its surroundings. For example, dissolving the cement from a sandstone allows individual sand grains to elastically recover their undeformed shape. RESIDUAL STRESS: The stress equivalent of residual strain (Jaeger and Cook, 1976). r REVERSE FAULT: A steeply dipping fault (more than 45 degrees) in which the hanging wall block moves up relative to the footwall block. Many thrusts faults emerge from the ground as reverse faults. (Billings,1972). Reverse faults may be brittle or ductile. ROTATION: Rigid body rotation describes a change in the spatial orientation of the bounding surfaces of a body measured with respect to a fixed coordinate system. Internal rotation involves a change in the angular relation between material lines within a body, and is a manifestation of shearing strain (Hobbs, Means, and Williams, 1976). 6

191 STICK SLIP: A type of frictional sliding characterized by abrupt accelerations of sliding velccity and abrupt decreases in shearing stress. On a force displacement curve, stick slip is the region of sharp peaked oscillations, as the sliding surface accelerates then " locks up" (Jaeger and Cook,1976). b. The change in size and/or shape of a body in response to str !s. STRAIN: Of) -pf5&Fa1 straigare changes in the lengths of material line within p se a body, either extensional or contractional; and shearJn [ g stra (h are charige5~liMiangular relations between materJal lines within e ~ 0 the body. kerM three mutually perpendicular directions with TM s, o H kp an unstrained Lody which re, main perpendicula fn the strained state. \\~ . E These are the principal strain axes,and are normal strains, either vW e / . qi h extensional or contractional representing extreme values in change t j y p of the length of material' lines. Because the principal strain axes.q, p \\o - j remain perpendicular there is no shearing strain in those directions. g y' h*V p# In all other directions there is a combination of shearing and normal strains. There is no necessary correspondance between principal j \\J Q_ l stress axes and principal strain axesgoreover, the total defor-l 7 i, mation may involve rigid body rotation; if so the final orienti. tion \\g k of the principal strain axes will not correspond to the initial \\ I position. (Jaeger and Cook,1976; Hobbs, Means and Williams,1976)./ STRESS: The intensity of force per unit area, acting at every point within a body, due to the existence of body forces and the application of surface forces on the boundaries of the body. At every point within a body there are three mutually perpendicular directions ~n which normal stress (o) attains extreme (maximum or minimum) values. These directions are the principal stress directions and are designated: 01 (maximum principal stress) > oz (intermediate principal stress)

192 > a3 (least principal stress), where the relations are algebraic and compression is positive. In the principal stress directions, no shear stresses exist. In all other directions, normal stresses have values less than et but greater than o3, and shear stresses (t) exist. Any plane not perpendicular to one of the principal stresses, 'has both a normal stress acting perpendicular to it and shearing the stress state is stresses acting within it. When ot=o2"33 hydrostatic and no shearing stress exists in any direction within the body. (Jaeger and Cook,1976). TEAR FAULT: Strike-slip or oblique-slip faults which terminate a thrust fault or exist within a thrust sheet. (Modifie_d from Denr.is,1967). TECTONIC INJECTION: \\ J TENSILE STRENGTH: The magnitude of the least principal stress (o3) at the instant of tensile failure. The term usually refers to the uniaxial tensile strength when ot=o2=o and a3 = tension. Tensile strength is a material property. (Jaeger and Cook, 1976). THICK. SKINNED: An informal term describing deformation, either folding or faulting, which extends into and involves crystalline basement. THIN SKINNED: An informal term describing deformation, either folding or faulting, which does not extend into crystalline basement.

194 VIRGATION: Sheaf-like diverging of fold axes in an orogenic belt (Dennis,1967). WEAR GROOVES: Grooves on a frictional sliding surface produced by abrasion of opposite sides of the surface. Wear grooves are indiv-idually recognizable on a sliding surface but grade con-tinuously into slikensides. WEDGES: C. WRENCH FAULTS: A synonym for strike-slip faults, especially transcurrent faul ts. Unfortunately a fallaciour concept, known as wrench fault tectonics, carries erroneous genetic implications in violation of Newtonian mechanics. The term should be abandoned. /}}