ML19296B822
| ML19296B822 | |
| Person / Time | |
|---|---|
| Issue date: | 10/03/1979 |
| From: | Wastler S Office of Nuclear Reactor Regulation |
| To: | NRC OFFICE OF STANDARDS DEVELOPMENT |
| References | |
| CON-NRC-01-78-004, CON-NRC-1-78-4, TASK-OS, TASK-SS-802-9 NUDOCS 8002220112 | |
| Download: ML19296B822 (31) | |
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i y ac A CHARACTERIZATION AND CLASSIFICATION OF GEOLOGIC FAULTS I!! THE APPLACHIA14 FOLDBELT.
b M
nb P
Prepared by:
D.E. Dunn, J.T.
- Engelder, y
P.A. Geiser, R.D. Hatcher, S.A. Kish, A.L.
Odom.
S. Schamel, and D.U. Wise Prepared for:
The U.S. Iluclear Regulatory Commission Office of Standards Development Under contract:
NRC-01-78-004 to The Florida State University Tallahassee, Florida A.L. Odom project director C'
Sua
5 C.
Feult Classes I.
Bedding plane thrusts - Decollements.
2.
Faults penecontemporaneous with wildflysch.
$ Thrusts rooted in low grade to unmetamorphosed crystaliIne basement.
$ Block faults.
h Faults associated with local centers (diapirs, cauldron subsidence, intru-sions, and crypto-volcanic structures).
6.
Pre-to synmetamorphic f aults in medium to high grade terrane.
7.
Postmetamorphic thrusts in medium to high grade terrane.
Strike slip faults.
Q Complex faults with long, repeated movement history.
10.
Enigmatic faults, inferred f aults such as the 38th parallel lineament.
- 11. Geomorphic faults with Tertiary history overprinting older Appalachian structures.
Faults with demonstrable Cenozoic movement.
Table I below summarizes the movement history of each of the twelve fault classes. The vertical extent of each lIno represents the time during which faults of this class were active, while the width of each line represents the degree of activity during various time Intervals.
Fault Classes Most Subject to Reactivation Faultclasses3,4,5,8,gand12arebrittlefaults,eitherunhealedor filled (this 1rcmir-ology is defined in Section 11. below); consequently, these faults are planes or zones of mechanical discontinuity with respect to thecountryrockenclosingthem.hAllmustbeconsideredaspossiblecandi-dates for reactivation, depending on the magnitude of the resolved shear stress they support, because they ha.se lower shearing strength than the coun.-
e try rock (see Appendix XIX.A. for a discussion of critical fault nrien-tations with respect to h situ stress orientation).
Particular attention must be given to Class 5 faulA.
The largest historic earthquakes located in the Appalachian Orogen were at Cape Ann, Massachusetts (1755) and Charleston, South Carolina (1886). Both were on 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, Classh faults have long histories of reactivation and at least some portions of their total movement histories were brittle. More-cver, there it. some modern sei;micity on both the Ramapo and Brevard faults; consequently these faults must be considered as possible candidates for f
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furth~er reactivation.
QOT E.
Fault Classes i. east Subject To Reactivation Fault Classes 6,7, and 9 are either healed brittle faults or ductile faults (terminology defined in Section II. below); consequently, they are
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not planes or zones of mechjnical 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 thay 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,
9 II.
DEFINITION OF KEY TERMS
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The usual distinction between a fault and a joint or fracture is scale-i p~p' dependent. k feature mapped as a fault at a scale of 1 inch equals 10 feet f
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)
d 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, feature.= variously
'8 described as " shear zones" or " zones of displacement" or " dislocation zones" 5
may or may not be treated as faults, even though they clearly involve shear M
mo tion. We prefer less ambiguous terminology and will adhere to the follow-A
., p
/
ing definitions throughout this report.
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A.
Faul t A fault is a surface or tabular zone along which true or apparent dis-placement parallel to the surface or zone is at least ten times greater than displacement normal to the surface.or zone.
l.
Ductile fault - A ductile fault involves no loss of cohesion normal to the fault at the time of last motion. Cohesion as used here refers to the tensile strength of the fault surface or zone in contrast to the unfaulted rocks on either side.
2.
Brittle fault - A brittle fault is characterized by loss of conesion normal to the fault at the time of last motion.
Brittle faults may be subdivided into three categories.
a.
Unhealed - An unhealed brittle fault has remained essentially unchanged since its last motion.
b.
Filled - A filled brittle fault has been modified by new mineralization ~
which partially or totally fills and cements open space along the country rocks enclosing the fault.
12 III.
OUTLINE FOR DISCUSSION OF EACH FAULT CLASS A.
Name and Generalized Description 1.
Possible subclasses 2.
List of typical examples, i.e., specific faults B.
Description of Fault _ Class 1.
Basic geometry a.
strike length b.
width perpendicular to strike c.
spatial orientation d.
displacement 4 crnImq lc <6CL Mb b
/w N e.
continuity f.
curvature
~
g.
termination along strike 2.
Tectonic setting 3.
Characteristics of surface or zone a.
type of fault, i.e., brittle vs. ductile b.
surface texture c.
material present, i.e., gouge, mylonite, etc.
d.
metamorphism and/or mineralization e.
datable materials
]3 4.
Relation to country rock a.
parallel or across regional grain (scale) b.
salient or re-entrant c.
thick-skinned or thin-skinned d.
relation to stratigraphic thickness changes (isopachs) e.
stratigraphic interval affected f.
relation to folds g.
relation to S-surfaces h.
change in fault character with changing lithology i.
P-T conditions V
- j. relation to isograds 5
k.
relation to intrusions g
o 1.
tectonic injections f3. / 9 ),'*(
p.
5.
History pp p
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a.
age of inception b.
recognition of syndepositional effects c.
radiometric ages d.
relation to erosional unloading e.
indications of last motion 6.
Stress field
,s r
a.
orientation of principal stresses at inception A
k
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b.
magnitude of principal stresses and strains F
c.
variation through time of stress and strain d.
present h situ stress e.
seismic first motion studies f.
rates of motion g.
fluid pressure changes and effects
15 CHAPTER IV CLASS I BECDING PLANE THRUST - 0$COLLEMENTS A.
Generalized Description eedding piane (Fig. 4) and decollement thrusts (Fig. 5) are the character-Istic fault phencmena of " thin skinned" deformation. The faults are one of the p_rincipal structural features in foreland deformation. Their spacing generally controls the locations of the major anticlines which they initiate by splaying or climbino strattarachic section (Fig. ' ).
6 Although bedding plane and dicollement thrusts are best known from the foreland, recent work in"the Moine of Scotland and the Grandfather Mountain Window, North Carolina, has demonstrated that thrusting in these more internal metamorphic terranes retains the gecmetry of the thrusting seen in the foreland. Apparently any volume of rock which centains any tyoe of large-scale planar mechenical anisotropy, is capable of falling in similar ways.
Typical bedding plane and de'collement thrusts (as shown in Figs. 4 Aand
- 5) have the following properties:
- 1) Thrusts cut up section in the direction of tectonic transpcrt.
- 2) The faults tend to parallel the bedding in " units" behaving as the weaker layers and cut up section in the buttressing layers.
In general weak layers are units such as evaporites, shales or coals; however, localities are known in the Cordillera (Burchfield, personal ccemunication, 1979) where the thrusts parallel bedding within the apparently ccepetent units (lime-stones) and appear to ignore weaker shale Interbeds.
- 3) Thrusts need not change the overall thickness, but, if they do, they thicken the section by repetition of strata. These thrust do not cause bed omission
. ]7 I
except in ano=alous cases (see Fig. 7).
- 4) Thrusts place older beds on younger except in anomalous cases (Fig. 7 ).
The terminology which has been applied to thrust faults is shown in Figs. '8a,b.
Complications which add complexity to thrust fault geometry typically occur in the region of ramps and at the trailing and leading edges of thrusts. These effects are shown in Figs. 9 and 10.
Syndepositional effects of foreland thrusting tend' to be associated with colassic sedimentation (e.g., Price and Montjoy,1970).
Syndepositional thrusting in more internal areas is associated with flysch and wilde flysch sedimentation, where the location of the emergent thrust may be marked by precursory olistostromes (Elter and Treyison,1973).
Typical Examoles - The classic example of a bedding plane thrust is the Pine Mountain fault of Tennessee, Virginia and Kentucky (Rich,1934; Harris and Milici, 1977).
Other major faults of this type in the Appalachian Valley and Ridge are the Pulaski thrust of Virgfnia and Tennessee and the o
Little North Mountain fault of Virginia and Maryland. Thrust faults are not restricted to the Valley and Ridge Province but also extend well out into the Plateau; the Burning Springs Anticline is located 180 km from the Allegheny front. On the New York Plateau, Prucha (1968) has recognized the presence of a ddcollement surface beneath the northernmost anticline some 2
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150 km north of the Allegheny front, while Engelder and Geiser (1979) show that this decolle=ent extends to the Helderburg escarpment some 80 km beyond the outermost fold of the Plateau.
29 5.
History a.
Age of inception -
The age of 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 Ordovician thrusting has been found in the eastern Valley and Ridge of Virginia (Lowry,1971), while much of imbrication along the Martic zone i< Ta conic (tN
- e,
1_970).
b.
Recognition of syndepositional effects - Syndepositional thrust -
ing has been documented in Virginia in the form of the Fincastle conglo=erate (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 unde.
Radiometric ages - Pierce and Armstrong (1966) give the only c.
known age determination; a whole rock K/Ar date of 390 + 50/-15 cv.
from the Tuscarora fault of central Pennsylvania, d.
Relationship to unloading - As far as is known, these faults are unaffected by unloading.
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31 11.
Strain - The distribution of strain magnitudes associated with thrusting may follow the distribution of stress magnitudes; howeve:, only the most 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 distributien of strain partitioning within thrust sheets.
d.
g situ stress - Not known.
bd f$4 h!.
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Seismic first motion studies - no seismicity has been connected with the Appalachian bedding-plane thrusts, b DL Nd f.
Rates of motion - When active the thrusts are believed to move at
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rates of 10
- 10 k=/sec. (Elliott, 1976a). How2ver, neotec-tonic seismic data from present-day rountain belts suggests that some seismic activity cay occur along these faults; unfortunately the data 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 - Cc==only 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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Class 3 THRUSTS ROOTED Ifl LCW GRADE TO UNMETAMOPPHCSED CRYSTALLINE BASEMENT 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 have associated with these a regional penetration cleavage.
Faults of this class characteristically occur along the foreland / metamorphic core boundary as transported exrernal massifs. Such struc.tures may be found in similar positions in any of the world's major thrust mountain chains.
At the base of these thrust sheets, the f ault zone is ductile marked by greenschist mylonites and/or brittle cataclasites*. Thicknesses of these zones range f rem I-2 m.,
rarely to more than 10 m.
Subclasses 1.
Thrusted fold nappes with basement cores -(Fig. 32)- Reading Prong type Wusconetcong nappe system; Drake, 1978). Thrust nappes of this sub-class consist of detached fold nappes deformed initially by a shear mechanism with ductilly deformed cover.
2.
Blue 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 folding and some later. faults. They occur alor.g the western edge of the Blue Ridge (Fig. 33).
- 3. hSerkshire Highlands type - Thrusts of this class are relatively high angle but may aiso be Icw angIe ductile faults which deform the core
dA 0 4 A / G l dtf M(Le YAlat. #6 Uy kW ##/ 3 y
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Figure 30. I ap showing region of foldbelt where Class 3 faults are known to occur. Solid line marks western boundary of the region and broken line marks the eastern boundary.
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63 for mylonitic rocks along a thrust in north Georgia, d.
Relationships to Unloading - Not appilcable (see 5b above).
e.
Indications of Last Motion - Cataclastic veins in the Berkshires are overprinted by Acadian metamorphism end f aulting (Ratclif fe and Harwood, 1975). The Cross Mountain transcurrent fault (post-Alleghanian?) cuts the Blue Ridge subclass f aults in northeast Tennessee f. King and Ferguson, 1950; Hardecan, 1966). Triassic-Jurassic diabase dikes cut all subclasses.
6.
Stress Field a.
Orientation of Principal Stresses at inception - The orientation of a; during the initial stages of movement was probably oriented toward the northwest to west.
b.
Magnitude of Principal Stresses and Strains - Since the masses of material moved are of considerab.!e size the principal stresses must have been of considerable magnitude.
It is difficult to 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 si::e, it can be con-
]
cluded that stresses were immense. Strain within a particular r
g fault zone varied with position in the zone and the nature of the 6
processes operation at a particular time.
c.
Variation in Tine of Stress and Strain - There is aburdant evidence 49 of reactivation of Class 3 thrusts in the Berkshires (Ratcliffe r
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and Harwood, 1975) and the western Blue Ridge. Most thrust sheets of this size probably experienced an egvement history N
Ok rather than a single emplacement event, d.
Present h situ stress - Unknown.
68 Exa=ple:
Jonesboro fault - The Jonesboro forms the eastern border fault of the Durham and Sanford Triassic basins (collectively known as the Deep River Triassic basin). The fault was named by Campbell a:.J Kimball (1923) for exposures of the fault near the village of Jonesboro; which is now part of the city of Sanford.
The fault extends nearly 160 kilometers along striks- (figure '37).
It is bordered on the east by a narrow belt of low-grade metamorphic rocks, further east is a large antiformal region (Raleigh belt) con-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 g,
by cross faults which had significant strike-slip component and these faults were subsequently intruded by diabase dikes (figure 39).
tk.
More recent studies (Bain and Harvey, 1977) reinterpret the structures
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%g and suggest the Jonesboro is not cut by younger faults but does in fact cut diabase dikes (figure 40).
In this case latest movement on the fault post-dates the approximately 180 Ma-old dikes, however 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 interbedded with granite boulders.
The fault surface is described as
69 being sharp and fairly straight. Shearing is also present in both the granite and claystone adjacent to the fault.
In most exposures the fault zone is only a few meters wide and movement appears to have been localized along a single surface (Reinemund, 1955).
Carpenter (1970) has described siliceous breccia zones extending north of the Jonesboro fault into low grade metamorphic rocks.
The Jonesboro fault has been considered to have been the major
,[
fault of the Durham-Sanford basins (figure 41).
Recent 9
studies, utilizing geophysical methods (figure 42), suggest the
!\\ 09 border fault actually has relatively minor displacement, with the major faulting occurring within the basin (Bain and Harvey,1977].
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70 B.
Description of Fault Class 1.
Basic geonetry a) Strike Length - Meters to hundreds of kilometers.
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, locaIly departing due to basement anisotropy.
d) Displacement - Dominantly dip-siip motion end. most ccemonly nornal d
g M movenent. The total apparent thickness of stratigraphic section in
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basin might greatly exceed the total relief on the basement and
&lf displacement along any one f ault. Displacement is f rem meters (or
[h even centimeters to a f ew ki tcmeters, maximum). Cross-faults tend to have smaller displacements.
g j
r Present border f aults might not be the major fault, but only an
" accident" of erosion. The major fault may be cutside the basement.
e) Centinuity - Individual faults tend to link through a series of roughly contemporaneous normal faults and cross faults, especially along the down dip margin of the basins.
f) Curvature -(0 along dip - The faults may be listric. Many faults may flatten into subhorizontal basement faults with depth. At shallow depths the dips are 60 to verticle.
(ii)along strike - Tends to parallel regional strike of basement.
Most lack significant curvature, but may locally show curvature due to local anisotrophy.
Ccmposites or ceupling of contemporaneous straight segments can give a regic- indication of curvature.
73 regard).
J) Relation to Isograds - Regional isograds are displaced by the faults.
Displacement of isograds can be used to establish limits to magnituc3 of displacement. However, caution must be exercised in areas of Inverted isograds.
k) Relation to intrusions - Usually dolerite dikes predate faulting or are penecontemporaneous.
Faults may wrap around older, unfoilated granites (such as the Jonesboro fault) giving a false irrpression of the granite intruding the fault.
5.
History a) Age of inception - For the Mesozoic subclass trajor trotion is not known with certainly, some may represent reactivation of older 1
0 (Paleozoic faults). Major motions are Triassic and Jurassic.
4 ff. a6
[D L
Motion frequently occurred during sedimentation.
Seismic focal
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_ mechanisms indicate that in central Appalachians some mesozoic d\\
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boarder faults are still active - but elsewhere rost are apparently p
- Inactive, b) Recognition of. Syndepositional Ef fects - easin f r iis and fangiomer-ngp p
()
ates, thickness variations of volcanic flows in northern basins, g
rotation of initial dips with associated gravity slides.
There is some question as to the extent to which basin configuration was D#
controlled by the faults.
Local ponding affects can be found.
c) Radiometric ages - K-Ar and fission track ages have been measured on fault filling material.
Depending on mineralegy, condition and history of samples, the analytical ages either approach or are grcatly less than the true ages.
d)
Relationship to Unloading - Not applicable.
e)
Indications of Last f'otion - Cross cuttirg dikes (though rare),
74 growth of minerals in fault zones and the age of those minerals which can be shown to post date movement; coaftal plain overlap; relation to fluvial deposits; estimates from P - T conditions of movement or fault fill and possible range of uplift and erosional rates; seismic Information where applicable.
6.
Stress Field a) Orientation of Principle Stresses - ci is vertical and a3 is g
normal to strike at time of faulting. Slic.kensides and seismic p
Nt.
fIrst motion studies indicate that stress orientation can and in
-rhtdd #g ph gp some cases have changed (e.g. seismic activity associated with h pp cth-Re1apo fault zone), c2 gener fly parallel to basin border fault, gg but locally considerable re-orientation cf stress axis occurred during Mesozoic.
b) Magnitude of Stress and Strain - Unknown.
c)
Variation of Str=ess and Strain - Unknown.
d) in situ Stress - Unknown.
e)
Seismic First Motion Studies - Unknown.
f) Rates of Motion - Unknown.
g)
Fluid Pressure Changes and Effects - Unknown.
7.
Geophysical and Subsurface Characteristics a)
Seismic Activity Levels 'Where networks are suf ficiently dense,
[UMCA.
microseismicity has been found associated with the faults, b)
Subsurface Offset - Displacements have been revealed by gcephysical methods indicating subsidiary faults of this class beneath basin fill - and air beneath Coastal Plain sediments (even displacing Coastal Plain sedirents; e.g. Charleston, South Carolina area)-
c) Relations to Anomalies - Cut magnetic ancmal!es and generally parallel or nearly parallel gravity anomalies.
31 the strongest shocks ever recorded in the region.
In New England, part of the region of the White Mountain plutons is included in a high seismicity area (Sykes, 1978).
Kane (1977) suggests a cor- _. /
% door relation of mafic and ultramafic bodies with centers of major seismic activity g g based on a correlation of these areas with local gravity highs. Tha center of g h the Charleston activity of 1886 includes several small gravity highs sug-M I fOobTcra.-
gestive of shallow mafic intrusives. Basalt was recovered from a drill core taken frem one of the highs (Rankin, 1978).
h@
The reasons for this assoc!ation of present day seismic activity with Lt KanfAr Precambrian, Paleozoic or Mesozoic intrusive bodies are f ar f rom clear.
n (1977) suggests tha t creep of rocks surrounding a more rigid mafic plug can cause the storage and sudden release of elastic energy in those rccks. The presence of serpentinization in the border zone of the mafic body would aid this process.
In suppcrt of this, Kane notes that the major seismic activity in these regions is peripheral to the gravity highs rather than centered on them.
Long (1976) proposed a variation of this medel based on the same kind of data in which the mafic body was considered weaker than the surrounding rocks. An additional ccmplication for several of the largest quake areas is the proximity of a continental margin and buried Mesozoic basin. Mesozoic basins are interpreted as being just east of the Charleston and the Coastal Massachusetts areas (Rankin 1977).
In addition to these complexities several of the largest quake areas lie on the possible projections of ancient transform faults.
Sykes (1978) points out the association of the Charleston Area with the Blake Spur Fracture zone and the New England seismic region with the possible projection of the Kelvin Fracture zone. He suggests a possible causal relationship with the activity and the concentration of present day stresses along much older zones of deep crusta l weakness.
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(
_ CLASS 6: PRE-TO SYNf4ETNEPHIC THRUSTS IN HIGH GRADE TERPANES MI A.
eneralized Description Thrusts of Class 6 consist of brittle Cincluding Imbricate) thrusts, which possibly exhibited bedding plane type behavior, that were emplaced and Y
subsequently were overprinted by metamorphism. All the characteristics of Yx bedding thrusts (Class I) may apply to these. Most were completely annealed h(
by the themal event; so e were reactivated later during or af ter the O
thermal peak producing mylonites along faults. Probably most Class 6
}g faults formed as early compressive features as a result of the initial
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stages of the themal/ metamorphic event with which they are associated.
However, there were probably a varlety of faults, thrusts, strike-slip, and nomal faults, which fomed in the early Paleozoic orogen.
Some of these may have been reactivated as synmetamorphic thrusts.
Several faults of this class are transitional into recumbent basement-cored nappes as well as into late to post-metamorphic thrusts in high grade terranes (Class 6).
- Those, such as the Honey Hill, include metamorphic reactions as part of their strain mechanism.
Seme, like Cameron's Line in Connecticut (Rodgers, 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 dif ferent 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 examples of Class 6 faults may be grouped into two subclasses:
Cl) brittle premetamcrphic bedding thrusts which were for the most part h %
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Faults of the Pine fbuntain belt of Georgia and Alabama N
s' U
(including the Towaliga and Goat Rock faults) contain extensive mylonites.
They Juxtapose high (sillimanite?) grade terranes of the Inner Piedmont and pp 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 Ammencosuc f ault of New Hamp--
shire and Maine is another candidate for Class 7 f ault.
8.
Description of Fault Class I.
Basic Geometry (a) Strike Length - 10's to 100's of kilometers (Hollins Line f ault),
may have klippes (Alto allochthon).
(b) Width - Less than a meter to tens of reters.
(c) Orientations - Parallel to subparallel to the dcminant structural grain. Traces bounding the ends of klIppes and folded f aults 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 few kilcmeters to tens of kilometers have been generally accepted.
Larger displacements have been suggested (Hatcher, 1978a). Geophysical data supports the latter (Hatcher and Zietz, 1978).
(e) Continuity - May be demonstrated along their lengths. These faults rarely sp!ay.
(f) Curvature - Cctrnonly folded, so this is reflected in outcrop patterns.
Subhorizontal segments exist in some.
Have out-crop traces strongly influenced by the topcgraphic contours.
112 3)
Faults Reactivated with Strike-Slio Motion W
g Faults of this subclass are described elsewhere in this report as
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omplex faults.
Some complex faults have de=onstrable strike-slip dis-y
- placements 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 mylonitization 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... carks the extension of the fault zone to the northeast rather than the open work breccia of the youngest fault episode. Cataclastic defor=ation took place at various tices in the Canopus area, judging from the cross-cutting relationships of cylonite zones. However, the details are i= perfectly known at present. The field relationships as presently understood are presented in Figurc [9]. The area was capped by the writer at a scale of 1 in.: 1000 ft during inves-tigation which spanned a threc-vcek period.
The mylonite zones shown on Figure [9] are all marked by strong development of minor 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 map (Fig. [9], Loc. 2), suggests a right-lateral displacement of 4 km (2.48 mi).
The age rela-tionships of this fracturing vill be discussed in the section dealing with the intrusive rocks."
- 4) Tear Faults Associated with Decollement Tectonics Co==only the ddcollecent sheets of the Central Appalachians coved as units separated by tear faults.
Figure 67 shows major tear faults
118 ep CHAPTER XII gg N.
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[/f['V C
CLASS 9 COMPLEX FAULTS g
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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 and cataclastic rocks extending f rom central Alabama to northern North Ca olina (Fig. 74).
Mylonites are both prograde and retregrade with major segments having undergone a later brittle movement history. The dominant mylonitic foliation in the Brevard zone is parallel or subparallel to the dcminant regional (S ) follation in the southern Appalachians.
Bat the dcoinant 2
foliation in the Brevard zone transpcses the regional S.
It is in large 2
part stratigraphy controlled:
distinctive lithologies (graphitic phyllonite, marble and quartzite) are associated with the structurally defined zone.
Slices of platform carbonates have been brought into the f ault zone frem the footwalI 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 4 km.
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4 Fig. 74 Location and known extent of the Brevard Zone and other major Late Paleczoic thrusts of the Southern Appalachians (frcm Hatcher, 1971),
125 Y
a
)v# a aR e RAMAPO FAULT SYSm!
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- 1) Basic Geometry
[S )1nB[V L
i p
i St a) Strike length - The Ramapo fault proper extends approximately 80' q
V km from Stoney Point on the Hudson River to Peapack, !!ew Jersey (Fig. 79).
The fault system may extend 18 km northeast into the Hudson Highlands where the Canopus Hollow Fault system, Dennytown fault and.Peekskill Hollow fault may.be traced (Fig. 80).
Individual faults in the Ramapo fault system 1,nclude the Thiells fault, Cedar Flats fault, Ambreys Pond fault, Timp Pass fault, Blanchard Road fault, Willow Grove fault, Bald Mountain fault, Buckberg Mountain fault, Letchworth fault, and Mott Farm Road fault.
b) Width perpendicular to strike - The Ramapo fault occupies a zone less than 100 m wide; but parallel faults near the northeastern end of the system spread over a zone up to 4 km wide.
~
l 140 lack of sufficient understanding as to the precise nature of the structures.
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10.
Pitfalls in identification: The principal pitfalls lie in an over-enthusiastic use of aerial photographs and LANDSAT imagery, where a tendency develops to generate enormous families of spurious align-ments (see Appendix B). An additional problem is a failure to consider that the zones are highly complex features, consequently a field study of only a few of the many possible manifestations of the phenomena may produce false negative results..
11.
Possibility of Reactivation:
FaultswithinaCSDshouldbetreated,{3)#
t c
with suspicion as a number of CSD's are known to be seismically active.
CSD's also have long histories of activity, consequently reactivation of structures within the CSD should be considered a possibility and studies should be made to judge its capability.
O
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 undei-Section B (Description of fau' t class) other fault types will be covered I
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 natorial 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.
4 h-g[
Fault Subclasses a
,9ci$UV Thrust faults
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Schsfer (1979) and (Block et al. (1979h 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-iMrRLJ1 oles used for blasting operations
p
-l' 153 h
Block faul and growth faults Late blesozoic and Cenozoic faults of this type have not been documented within the Appalachians proper, but they 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).
t liigh-angle reverse faults This type of fault is the only type in the Appalachians which has been shown to have well-documented post->!esozoic displacement. The apparent 3
localization of these faults along the Fall Line between the Piedmont and l
Coastal Plain (figure 91) may be real (perhaps due to a "hing-line effect")
or more likely.it is due to presence of a thin, relatively continuous veneer j
of Cretaceous and Tertia 7 sediments which allow ready identification of 5
faults of relatively small displacement.
Likewise, none of these faults have been identified north of the Coastal Plain of Maryland, probably due i
to the absence of suitable sedimentary cover over older crystalline and sedimentary rocks. T ummarizes pertinent features for all recog-N
/
nized faults of this typ.
Examples:
Stafford fault system - The Stafford fault system is 1 coated along the Fall Line and Potomac estuary of northeastern Virginia, approximately 40 kilo-
&y)-
meters southwest of Nashington, D. C. (figure 93). The fault syster:t will f['%
be used as an example for the characteristic of fault of this class.
D While the possibility of youthful faulting has been suspected for k
sone time (McGee, 1888); the true extent of faulting has been only recently documented (Mixon and Newell,1977 and 197S).
Rocks in this region consist O
TABLE 3 cone.
Hap Fault name Strike Strike Age of movement Vertical Coment s Reference I
reference or location orientation length displacement 10 Cliftone Forge.
Va.
N30 W 41 km
- m. Tertiary (f) 5m Upthrown block on Su side Mitte (1962) 11 Raleigh N.C.
East-West 41 km
- a. Pleistocene or Pliocene 3m Upthrown on south side, relatively Parker (1979) low angle (40 ) fault.
12 Cheraw S.C.
H80*W 41 km
- a. Post-Cretaceous 1.2 m Upthrown on NE side, relatively Howell and 13 Stanleytown.
N12E 1 km a, llolocene 6m low sogle (420) fault.
Zupan (1974)
Va.
dip is 62SE upthrown block on Nif sider soveral faults present Conley and Toewe M968) 1
- a. age of major movement b age of latest movement G
uah so Abox abon y