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Basement control of structure in the Gettysburg rift basin, Pennsylvania and Maryland
281
Tectonophysics, 166 (1989) 281-292
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Basement control of structure in the Gettysburg rift basin, Pennsylvania and Maryland SAMUEL Department
of Geology,The
College
I. ROOT of Wooster, Wooster, OH 44691 (U.S.A.)
(Received July 28,1988; revised version accepted January 10,1989)
Abstract Root, S.I., 1989. Basement control of structure in the Gettysburg rift basin Pennsylvania and Maryland. Tecronophysics, 166: 281-292.
Jurassic faulting formed the 93 km long Gettysburg basin as an extensional half graben paralleling the basement structural grain. Preserved in the basin are rift-related Carnian to Rhaetian strata that were tilted 20-30“ NW into a SE dipping, listric normal fault at the northwest border of the basin. Vertical displacement on the border fault approaches 10 km. The border fault developed parallel to the trend of the terminal Paleozoic Alleghenian South Mountain cleavage of the Blue Ridge basement along 80% of its extent. However, it is only roughly parallel to discordant to dip of the cleavage. Relations~p of cleavage and later border faulting may be the result af persistent reactivation of the original App~a~~an ~ntinental margin. LocaI complex structures in the half graben are related to reactivation of two subvertical, pre-Mesozoic faults that transect basement structural grain (cteavage) at a large angle. The northern S~p~nsburg fauh was reactivated during basin normal faulting, offsetting the border fault in a right-lateral sense by 3.5 km and forming within the basin a fold and a fault sliver of basement. The southern Carbaugh-Marsh Creek fault was not reactivated, but is the locus of a 20 ’ -30 o change of trend of both the basement cleavage and later border fault. However, two large, NW trending, left-lateral wrench faults, antithetic to the Carbaugh-March Creek fault, developed here offsetting the border fault and forming en echelon folds and horst blocks of basement rock within the basin.
1. Introduction Rift-associated Mesozoic basins are exposed along the eastern margin of North America in a 2000 km long belt from South Carolina to Nova Scotia. Occupying a central position in this belt is a major depositional basin of Late Triassic to Early Jurassic age, extending nearly 500 km from Virginia to New York (Fig. 1). Rotation and faulting of beds in this basin produced four halfgrabens, including the Gettysburg basin, dipping west and northwest into a major bounding fault at the basin border (Fig. 1). Subsequent erosion of the Mesozoic strata separated the Gettysburg basin from the Culpeper basin on the south. Northeast 004O-1951/89/$03.50
d 1989 Eisevier Science Publishers B.V.
of the Gettysburg basin, outcrops of Mesozoic strata decrease in width in an area termed the Narrow Neck basin, but they widen farther northeast and become the well known Newark basin. The striking congruence of the configuration of the Mesozoic basins and arcuation of the older Appalachians is evidence for a genetic relationship between the two (see discussion by Swanson, 1986) (Fig. 1). Lindholm (1978) and Swanson (1986) attributed all border faulting of these exposed Mesozoic basins to reactivation of pre-Triassic structures. The best d~umentation of reactivation of older basement structures during Mesozoic extensional faulting is found in the Newark basin (Ratchffe et al., 1986).
S.I. ROOT
282
Fig. 1. Relation and Newark
of the Mesozoic
basins.
Precambrian-Paleozoic
rift basins
Note congruence age. Dot-dash
(stippled
pattern)
of the basins
line in Piedmont
Wavy lines are Atlantic
to basement
to the Appalachian
coastal
rocks. Narrow structural
is the Martic
Line. Coastal
waters. Washington,
Neck basin (not labelled)
grain represented plain
is structurally
unrelated
provinces
Cenozoic
of
cover.
D.C. and New York City are shown.
To date there have been no substantive studies of Mesozoic reactivation of basement structure in
and development may be detailed.
the Gettysburg basin. Although this study is based only on outcrop information (Stose, 1932; Stose
Basement rocks
and Jonas, 1939; Stose and Stose, 1946; Fauth, 1978; Root, 1977 and unpublished data), supplemented by regional gravity and magnetic studies
links Gettysburg
by several geologic
of Mesozoic
basin
structures
Structure of the basement adjacent to the western margin of the Mesozoic
basins is important
(Daniels, 1985; Phillips, 1985; Sumner, 1977), significant relationships between basement structure
because the major fault that borders these half-
Fig. 2. Geologic
of basin border
change
across
map of the Gettysburg Carbaugh-Marsh
grabens
basin. Note trend parallelism
Creek (C.-M.C.)
fault. Based on mapping F-Fairfield;
is on the west (Figs.
of many
and E-Emmitsburg.
fault to basement workers
cleavage
1 and 2).
Im-
and 20 O-30 o trend
cited in the text. Y.S-York
Springs;
BASEMENT
CONTROL
OF STRUCTURE
IN GEnYSBURG
RIFT
283
BASIN
mediately adjacent to the Gettysburg basin along most of its extent are Precambrian Catoctin Group metavolcanics and Lower Cambrian Chilhowee Group quart&es and phyllites that form the mount~nous Blue Ridge (Fig. 2). The quartz&es are overlain by a thick sequence of CambroOrdovician carbonate rocks and shales that underlie the Great Valley flanking the Blue Ridge on the west. Together these rocks were deformed during the Alleghenian orogeny into the cleavage dominated South Mountain anticlinorium described by Cloos (1947). This fold is a large westward overturned anticliiorium that is part of, the Blue Ridge thrust plate whose sole thrust is about -4500 m adjacent to the Culpeper basin (Kulander and Dean, 1986). The cleavage dips steeper in the upper limb (> 3O”SE) than the inverted lower limb ( < 30 “SE) forming a fan open to the northwest. Cleavage strike can vary as much as rt15* about a domain mean strike, but it typically varies by about +7” (see Table 1). Strike variation is generally related to position of cleavage on the plunging fold. Orientation of this principal cleavage, termed the South Mountain cleavage (Mitra and Elliot, 1980), is important to development of the border faults at the edge of the Mesozoic half-graben. Other minor cleavages are present but are not of regional significance (Fauth, 1978). The Valley and Ridge fold belt is developed in a thick Cambrian-Pennsylvanian sedimentary sequence that has been folded together with the South Mountain fold but lacks a pervasive cleavage. At the north end of the Gettysburg basin, the basement becomes more complex as the Blue Ridge is overridden by a sequence of Cambro-Ordovician nappes composed of shales and carbonate rock that are emplaced on a late Alleghenian, subhorizontal thrust (Root, 1970, 1977). Multiple cleavages are developed in this thrust sheet. The general structural grain of the nappe system is parallel to the grain of the Valley and Ridge. This area is the locus of a profound change in the trend of the structural grain of the basement and Mesozoic basins from northeast to nearly east-west. The southeast margin of the basin is an overlap of Triassic red beds upon Piedmont rocks. Northwest of the Martic Line (Fig. l), Piedmont rocks
are principally Cambro-Ordovician carbonate rocks and siliciclastics. Southeast of the Martic Line is a metamorphosed sequence of Lower Paleozoic schists and chloritic quartzites. Even though these strata have been deformed at least three times and multiple cleavages occur (Freedman et al., 1964), the principal structural grain southeast of the Mesozoic basins generally conforms to the Appalachian structural grain to the northwest. Rocks in the Gettysburg basin Sedimentary strata preserved in the Gettysburg basin are a rift-related red bed sequence ranging in age from mid-Camian to uppermost Rhaetian (Traverse, 1987). Various measured sections at surface aggregate to a composite thickness of nearly 9000 m. Because the adjacent Culpeper and Newark basins contain strata as young as Pliensbachian (Traverse, 1987), equivalent strata probably were deposited in the Gettysburg basin but subsequently eroded. These eroded strata could have aggregated to an additional 500-1000 m of section. However, it may be that strata thin depositional~y toward the basin center so that a maximum of 7000 m basin fill is a more conservative thickness. Mesozoic strata in the basin are extensively intruded by the Gettysburg diabase sheet which is estimated to be 500 m thick on its concordant, southern up-dip edge. On the north and south margins, it is highly discordant forming irregular cross-cutting bodies and isolated intrusives. The southern edge of the sheet intruded along a fault to be discussed later in this paper. Numerous vertical to nearly vertical dikes, up to a few tens of meters thick, tens of kilometers long and trending N to N20 “E, cut across the basin. They are not shown on the figures. Strata1 dip is the result of post-Pliensbachian monoclinal rotation and concomitant major border faulting with accompanying igneous and volcanic activity. This deformation may have occurred during regional Toarcian taphrogenesis. It appears that only minor faulting is associated with deposition of the Carnian to Rhaetian strata in the Gettysburg basin.
284
Basin structure
The basin is a half-graben in which strata dip 20”-30” NW toward the border fault at the northwest margin of the basin and exhibit relatively minor disruption by intrusion of the diabase sheet. The border fault is mapped as continuous (Berg, 1980; Cleaves et al., 1968) and with a large displacement. This is opposed to the model of Fail1 (1973) which considers the fault discontinuous. East of the Lisburn fault, the border-fault strike is N70”-80 o E, but this area should be considered part of the Narrow Neck basin because en echelon folding dominates here. The Gettysburg basin extends for 93 km from Frederick to the Lisburn fault (Fig. 2). Overall deformation in the Gettysburg basin is of extensional character with the vertical diabase dikes in the basin thought to represent the principal plane of stress thus suggesting a direction of principal extension oriented about 105 O/285 O; normal to the basement cleavage trend south of the Carbaugh-Marsh Creek fault (C.-M.C. fault) and subnormal to cleavage trend north of the fault (Fig. 2, Table 1). Both minor and major structures in the basin indicate dominantly normal fault, dip-slip movement. A few wrench structures occur but are considered unique and the product of special basement structures to be described later. It is difficult to calculate the total amount of vertical displacement across the border fault. Based on the thickness of preserved Mesozoic strata and the diabase sheet, at least 7000 m of displacement is required in the structurally deepest segment of the half graben sited near the border fault southwest of York Springs (Fig. 2). Vertical displacement along the border fault decreases progressively southwest to the termination of the basin near Frederick where the Blue Ridge basement is faulted against the Piedmont basement (Fig. 2). An indication of the total ma~mum amount of vertical displacement may be inferred from basement rocks exposed at three localities within the half graben adjacent to the border fault (Fig. 2). The basement here is composed of Cambro-Ordovician carbonate rocks. However, across the border fault, exposed basement rocks are Blue Ridge metavolcanics and quartzites. At the initiation of
S.1. ROOT
Mesozoic sedimentation the carbonate rock floor of the basin was probably 1000-2000 m stratigraphically above the underlying Blue Ridge rocks. Add to this the estimated 7000 m of preserved Triassic strata, 500 m of diabase sheet, plus 500-1000 m of Jurassic rocks that have probably been eroded from the half graben and total vertical border fault displacement approaches 10 km. ReIation of the border fault to the basement structure There is little to suggest that the Gettysburg basin border fault may be a reactivated Paleozoic fault trending subparallel to cleavage. Geologic mapping in the basin and Blue Ridge does not reveal a fault in the basement complex that can be mapped into the border fault as with the Ramapo fault in the Newark basin (Ratcliffe et al., 1986). Indeed there is not much Alle~enian thrusting in the Blue Ridge in this area. As shown by Cloos (1947), folding is the dominant deformational mode here. A number of faults in the Blue Ridge are actually normal faults related to Mesozoic border faulting (Fig. 2). This conclusion derives from the presence of younger beds on the southeast side of the presumably SE dipping (Root, 1970) faults, because in the case of thrusting, older beds are to be expected on the southeast. The border fault in the southern Gettysburg and northern Culpeper basins is virtually rectilinear for 110 km (Fig. 1); a condition not to be expected from a reactivated thrust in a terrain of moderate relief. The pervasive South Mountain cleavage, developed during the late Paleozoic Alleghenian deformation, is the principal anisotropy in the basement and therefore a potential plane for later normal faulting. It has been demonstrated (Root, 1970) that this cleavage can be divided into northem and southern domains separated by the C.-M.C. fault (Fig. 2) which previously behaved as a zone of disjunction during Alleghenian deformation. South of this fault cleavage trends NIOO-20° E; north of the fault cleavage trends N40”-SO0 E (Fig. 2). The southern domain (NIOO-20’ E) extends at least 160 km south of the C.-M.C. fault where it is adjacent to the
BASEMENT
TABLE
CONTROL
OF STRUCTURE
IN GETTYSBURG
RIFT
285
BASIN
1
Relation of border fault to basement cleavage Domain
Cleavage
Border fault length
strike
(km)
(“)
Mean difference strike
n
dip (SE)
(“)
(“)
014*
4
43*
fault-cleavage strike (“)
A
5
019 f 1
11
I
5
B
5
010 + 1
9
019+
7
29 + 11
9
C
5
009 f 1
4
010*
4
53*
6
1
D
4
010 * 1
2
005 *
3
30 + 10
5
E
5
026 I!Z3
4
0185
6
58*
F
3
039 + 1
5
G
3
035 f 2
10
H
2
013 * 1
3
I
4
036 + 4
21
039 + 15
39*
J
11
064 Ifr4
35
019 + 42
28511
2
8
053 + 6
44 + 12
14
032 + 12
61 + 11
3
015+
51+ 14
2
9
9
3 15
Border fault strike determined from means of 1 km fault segments of mapped fault traces within domains. n = number of cleavage measurements. Values for strike are azimuth and standard deviation. Domains A-F domains G and L from Fauth (1978), domain I from Freedman (1967)
are based on data from Whitaker (1955).
domain J from Root (1977). See Fig. 3 for location
of
domains.
Culpeper
basin.
The border
fault is parallel
to the
cleavage along most of its extent here (Lindholm, 1978; Mitra and Elliot, 1980; Whitaker, 1955). Only near Emmitsburg, Maryland (Fig. 2) does it deviate from this trend; the reasons for this deviation will be discussed later. The structural grain in the northern domain extends 45 km from the C.-M.C. fault to near the Susquehanna River where the Blue Ridge disappears beneath the thrust sheet emplacing the nappe system (Root, 1970; 1977). The Mesozoic
border
fault
basin
‘shows a marked
trend
of the South Mountain
of the Gettysburg
parallelism cleavage
to strike
or
in the Blue
Ridge along 80% of its extent (Fig. 2). Results of detailed mapping by various workers are presented in Table 1. Their results are divided into ten domains, extending from the border fault about 2 km into the Blue Ridge and of varied length along the border fault (Fig. 3). Except for domains F and J the difference between mean strike of the border
fault and cleavage
in the Blue Ridge in the
various domains ranges from lo-9” with a mean difference of only 4’ (Table 1). The large deviation of 14O between strikes in domain F south of Emmitsburg (Fig. 2) is attributed to complex processes involving wrenching on nearby NW trending faults concomitant with development of
Fig. 3. Index map showing location of domains A-J
that were
used to prepare Table 1. Heavy line is basin border fault. Note domain J is located astride nappe-bearing thrust sheet.
S.I. ROOT
286
the border
fault. Deviation
observed
where
Cambro-Ordovician Ridge
nappe
basement.
the border
of 15” in domain
the basement
consists
sequence
It will be shown
fault parallels
cleavage
J is
of a thin above
Blue
north
2) a distance
of Fairfield of about
to Emmitsburg
20 km, the border
N20 o -30 o W and truncate “E
cleavage.
(Table
(Fig. fault is
1, domains
(Fig.
2) on the main
basin.
minor
F, G,
exposed
border
dip
depth
cleavage
between
faults
associated canics,
Two faults,
faulting
These
synthetic faults are gently arced in map view and follow the trend of cleavage in the Piedmont
but
their
faulting
Magnetite
mining
to
of low-angle
basins.
In the Nar-
half of the Newark
is well established operations
an
metavol-
persistence
Evidence
exists in the adjoining
as there The few
including
in Precambrian
60”-70”SE
low-angle
and fault plane.
in the basin,
may be questioned.
of
fault
to assess the degree of
cleavage
fault
row Neck and western
and H), are wrench faults rather than the normal fault typical of the basin border. At the southeast margin of the Gettysburg basin normal faulting produced three, second-order half-grabens
exists (Fig. 2), it is difficult
The dip of the border fault is uncertain have been no drilling or seismic studies.
which
N15”-50
of trend border
in the overrid-
to Blue Ridge
trending
parallelism
and the Mesozoic
later that here
not parallel trend
unambiguous cleavage
dip parallelism
den Blue Ridge. From
Although basement
basins,
at the border.
and unvailable
turn-
of-the century exposures show that the border fault dips 25 “-45OSE. Ratcliffe et al. (1986) have conclusively
demonstrated
that the Ramapo
fault
segment forming part of the border of the Newark basin dips 25”-35OSE and represents Mesozoic normal fault reactivation of Paleozoic thrust slices.
mapped by Mitra and Elliot (1980). The Martic Line, a complex and much debated Paleozoic boundary between terrains of metamorphosed rocks on the southeast and non- or little metamorphosed rocks on the northwest, locally may have been reactivated as a SE dipping normal
Unlike the Gettysburg basin, this area shows a significant component of left-lateral wrenching as large-scale en echelon folds, and abundant horizontal slickenlines on related fault surfaces occur
fault bounding and 4).
adjacent to the border fault (Manspeizer, The border fault trends N70”-80°E
one of the half-grabens
(Figs.
1
Within the Blue Ridge, four normal faults occur near the border fault (Fig. 2). North of the C.-M.C. fault these faults parallel the border fault. South of the C.-M.C. fault they are oblique to the border fault in this area but still maintain parallelism to the border fault north of the C.-M.C. fault.
Lisburn
fault where there is a 30 o shift of trend to
the N40 “-50 ’ E trend of the northern Gettysburg basin. To the south, in the Culpeper basin, an extension of the Gettysburg basin, seismic profiles across the basin western margin may be interpreted to indicate Div., 1982).
a low-angle
-NW
border
fault (Virginia
SE-
A BLUE
1981). to the
GETTYSBURG RIDGE
Pennsylvania
P;
BASIN !Maryland
PIEDMONT
Fig. 4. Cross-section across the Gettysburg basin showing relatively uniform dip of Triassic New Oxford and Gettysburg formations and listric normal faults in the basin. No vertical exaggeration. See Fig. 2, A-A’
for section location.
BASEMENT
CONTROL
OF STRUCTURE
IN GETTYSBURG
RIFT
287
BASIN
There is also, however, evidence of steep faulting at the border of these basins. The border fault on the eastern half of the Newark basin, at five localities, dips 50 o -70 OSE (Ratcliffe and Burton, 1985). It is interesting to note that in this portion of the Newark basin there is little evidence of wrenching. Cleavage in the Blue Ridge thrust plate dips 30 “-60 o SE (mean 46 O, Table 1) in the upright fold limb which is adjacent to the border fault along much of its extent. Dip of Blue Ridge cleavage and minor faults in the Mesozoic basin are generally similar from which is inferred that major faults are roughly subparallel to cleavage at surface except for Domain J (Table 1). The effect of cleavage on the angle of shear fracture has been studied experimentally by Donath (1961) who showed that shear fractures tend to develop subparallel to pervasive planar anisotropy for inclinations of up to 45 “-60 o to the direction of maximum pressure, comparable to relations between inferred vertical principal stress and cleavage in this region. Cleavage in the Blue Ridge cannot be considered as simple uniform planar surfaces extending to depths of several kilometers or more. Internally, the Blue Ridge thrust plate may be of the order of 4500 m thick (Kulander and Dean, 1986), composed of lithologies that show appreciable cleavage refraction. Cleavage at the thrust plate sole will be subhorizontal (Mitra and Elliot, 1980). Beneath the Blue Ridge thrust plate are younger Cambro-Ordovician carbonate rocks and shales, probably dominated by a series of complex duplex structures in which cleavage dip will vary according to position of the fold limb (Fig. 4). Where cleavage is uniform in o~entation to depths of several kilometers, faults will develop subparallel to these planes of anisotropy. However, in the Blue Ridge where dip of cleavage in vertical succession varies non-systematically, the border fault will probably transect some domains of cleavage dip. Geometrically, the border fault and cleavage have the same strike but not the same dip. This problem of cleavage dip variation in basement becomes particularly acute at the north end of the Blue Ridge, where it is overridden by a nappe sheet about 200 m thick in this area. Major
foliation in the nappe is folded and dip varies from subhorizontal to 30”SE, with the trend of the border fault deviating about 15” from the trend of nappe foliation (Domain J, Table 1). Trend of the border fault, however, is nearly parallel to cleavage in the Blue Ridge (067O) where it is last observed passing beneath the nappe sequence (Table 1). At the Susquehanna River, just east of the Gettysburg basin, an exposure no longer available shows the contact between the Mesozoic basin and basement to dip 45 “SE (Wherry, 1913). Although Wherry (1913) considered this contact to be an unconformity, I suggest it is actually a fault contact and that such a fault dip, if maintained, transects in the footwall the nappe sheet, the Blue Ridge thrust plate and subBlue Ridge thrust plate, all within internal variations in cleavage dip. From relations in this area an important conclusion is derived. It appears that the border fault was initiated at depth, parallel to trend of the Blue Ridge cleavage and was propagated upward, transecting nappe foliation of the overlying thin sheet. Border fault processes
It is appealing to infer a genetic relationship between the border fault and basement cleavage because of their trend parallelism. Indeed, Lindholm (1978) concluded that in the Culpeper basin faulting is controlled by foliation. A clear example of basement foliation controlling subsequent normal faulting occurs in the Red Sea rifts where Miocene faults are reactivated on Pan-African schistosity planes (Jarridge et al., 1986). However, such a genetic relations~p requires parallelism of both strike and dip between the cleavage and the later normal faults. Divergence of dip between these two surfaces at the border of the Gettysburg basin is sufficiently large, especially where the nappe sheet forms outcropping basement, that some other process is required. The border fault apparently was initiated at depth, possibly at the bottom of the brittle crust, and propagated upwards. Under such conditions there is no requirement of parallelism between border fault and cleavage. Where some degree of parallelism occurs, a common process must be
S.I. ROOI
288
sought. Relationship of cleavage and the later normal faulting may be ascribed to persistent reactivation of the original Appalac~an continental margin. Thomas (1977), showed that the shape of this margin is the result of transform faulting along a Late Precambrian rift and that distribution of Paleozoic sedimentary units and outlines of subsequent compressional structures conform to this. Separation along this margin during Mesozoic rifting will account for parallelism between the rift basins and Appalachian structure (Fig. 2) as well as the relationship of normal faulting to cleavage. Such reactivation is a complex process because basement is allochthonous with overprints of both Taconic and Alleghenian collisions preceeding Mesozoic rifting. Basin cross-section Behavior of the border fault at depth is important in construction of the basin cross-section. The 60”SE dip of minor faults in the basin is considered to indicate the dip of the border fault at surface. In comparable basins such as Bahia Bay, China, the normal faults are listric, with a 60 “-70 o dip at surface flattening to 20*-30 o at depth, and basin extension is 30-40% (Hefu, 1986). Distinction between listric and planar faults is important in terms of the amount of extension required to produce strata1 rotation. Wernicke and Burchfiel (1982) show that for a given maximum strata1 rotation and fault dip, listric geometry requires far less basin extension than planar geometry. This is demonstrated in experiments with analogue models involving extension above a uniformly expanding basement (M&lay and Ellis, 1987). When the basement fault is modelled as a planar fault, extension of 25% produces strata1 rotation of 8”, but when modelled as a listric fault, the same amount of extension produces dips of 25”. The values derived from these listric fault experiments are fairly close to Gettysburg basin values of actual strata1 rotation and the amount of basin extension as inferred from the Bohia Bay analogue. The cross-section (Fig. 4) of the basin is therefore const~cted with a listric shape to all major faults. They approximate a set of related planar or “domino” faults that involves some ductile deformation at their base (Barr, 1987).
Wrench related structures in the basin Even though extension is the dominant deformational mode, two types of wrench structures are recognized in the Gettysburg basin. One type is related to reactivation of the Shippensburg fault (Figs. 2 and 5). The other type is developed at the C.-M.C. fault where there is an abrupt change in trend of the basement cleavage and concomitant change in trend of the border fault (Figs. 2 and 5). Both the C.-M.C. and Shippensburg faults are segments of the Transylvania fault zone, which transects the Appalac~~ structural grain at a large angle, and extends more than 300 km west from the Blue Ridge onto the Appalachian Plateau in Pennsylvania (Root and Hoskins, 1977) and into Ohio (Gray, 1982). This extensive fault zone is considered to represent a vertical fracture zone in the continental plate that has been active in the Paleozoic and is probably a Precamb~an structure (Root and Hoskins, 1977). The Transylvania fault zone must have extended some distance farther east into what is now the Gettysburg basin, Shippensburg fault The border fault is sharply offset 3.5 km in a right-lateral sense where it intersects the Shippensburg fault (Fig. 5). Within the basin, at this offset, is a narrow basement inlier of Paleozoic limestones with locally preserved Triassic red beds. The inlier extends finger-like eastward from the border fault for 4 km, past York Springs. Stose (1949) attempted to account for this occurrer&e of basement outcrop by suggesting an older, second border fault buried east of the Paleozoic inlier. The idea is untenable in the light of present mapping which requires that faults bound the inlier, making it a fault sliver. Both north and south of the fault bounded basement fault sliver, Triassic strata dip 30”40 o NW. The southern fault is probably an extension or splay of the S~ppensburg fault in the basin. Within the inlier, Triassic strata, which are unconformable upon the Paleozoic limestones, also dip 35 o NW {McLaug~in, 1961). The relative uniformity of Triassic bedding attitudes both adjacent to and within the fault sliver demonstrate the dominance here of extensional tectonics with nor-
BASEMENT
CONTROL
OF STRUCTURE
IN
GETTYSBURG
RIFT
289
BASIN
Me,sozpic
faults
Fig. 5. Various types of faults associated with Jurassic development of the Gettysburg structures. Stippled pattern within the basin indicates Cambro-Ordovician
associated
with
regional
basin. All are related to various basement
limestone basement. Bedding attitudes shown in areas of
complex basin structures related to Jurassic reactivation of C.-M.C. and Shippensburg faults. See Fig. 2 for diabase sheets omitted in this figure.
ma1 faulting, not major wrenching, producing a flower structure fault sliver. However, in extensional tectonics it would be peculiar if the basement inlier was an isolated narrow, upfaulted sliver bounded
by faults
thousand Paleozoic
meters. Therefore, it is proposed that the limestones are a really small horst-like,
large displacement block. Questioned
with displacements
of several
sliver on a much larger fault limits of this proposed larger
structure produced by relatively sional wrenching. Consideration of the structure fault sliver and folded Triassic with
the
Shippensburg
pre-existing reactivated
fault
minor
transpres-
of the basement strata on strike
indicated
that
this
regional basement fault was locally as a right-lateral wrench during border
faulting to transtensional
produce both transpressional and structures in the basin. The 3.5 km
of offset of the border
fault across
the Shippens-
block are suggested in Figs. 2 and 5. En echelon with the basement fault sliver is a fold with a sense of right-lateral motion. The
burg fault should not be construed as the amount of wrench displacement. If wrenching with 3.5 km
folding
of displacement
occurs
as regional
nent strike ridges of Triassic belt.
A stereonet
plot
deflections
of promi-
strata in a 8 km long
defines
a fold
plunging
N25 o W/32O. North of the fold axis beds trend northeast, but approaching the fold, the axis changes towards a more E-W orientation. South of the fold, the beds change back to a NE trend (Fig. 5). The range of dip values across the area of deflection indicates this is not sedimentary drape over a basement block. This fold is the surface expression of a right-lateral fault in basement, probably a splay of the Shippensburg fault, and may be considered as an extremely simple flower
trending
occurred,
en echelon
folds
it would rather
produce than
NE
a single,
nearly E-W zone of strata1 deflection oriented along the continuation of the Shippensburg fault in the basin. For example, a modest amount of wrenching produces an array of en echelon folds near Emmitsburg (Fig. 5). Border fault offset across the Shippensburg fault is attributed to a combination of shift of position during border fault propagation and right-lateral displacement of the Shippensburg fault. The reason for rightlateral motion of the Shippensburg fault may be related to the regional eastward arcuation of the
S.I. ROOT
290
Appalachian
structural
grain north
of the Gettys-
that, during tion
burg basin (Fig. 1).
occurred
consistent Carbaugh-Marsh Though as a wrench
active
(cleavage)
the border
discontinuity. changes
65”
angle between fault
fault, it was
Basement
trend here abruptly
about 25 o (Root, 1970) forming border
ment
fault was not reactivated
fault offsetting
a significant sotropy
Creek fault
the C.-M.C.
within fault.
by
the basin a
the two segments
at the C.-M.C.
ani-
of the later Under
such
conditions there can be an overlap of fault trends related to different trends of the border. Overlap of fault trends occurs in the Blue Ridge basement south of the C.-M.C. fault. Here, two large normal faults follow the trend of the border fault north
of the
C.-M.C.
fault.
Indeed,
one
fault that follow the structural trend of the south. Within the basin, two large, uniquely NW trending high-angle faults occur south of the C.-M.C. fault and for a distance of 20 km offset the NE trending border fault (Fig. 5). Because of associated normal faulting, the displacement is considerably less than the amount of horizontal offset. The fault north of Emmitsburg is obscured by diabase intrusion, but discordance of bedding across the diabase (NW dipping beds strike into the NW trending syncline) demonstrates its preswrenching
with
on
the
right-lateral pensburg
faults.
C.-M.C.
these
of observable fault
displacement
transferred
inhibited
trends
across
faults
comparable
because
to the Shipfault
of convergence
this fault faults.
from other
is
displace-
on the C.-M.C.
to the antithetic
these wrench
mo-
This
of the associated
fault. Displacement
of cleavage
and
motion
To distinguish
faults
of differing
they are shown on Fig. 5 as faults associ-
ated with change
Effects ture
left-lateral
along
the lack
may have been
process,
reactivation,
of trend
of basement
of basin development
grain.
on basement
struc-
fault
mapped by Fauth (1978), may well be an extension of the border fault north of the C.-M.C. fault (Fig. 5). There are no faults north of the C.-M.C.
ence (Fig. 2). Transpressional
Jurassic
again
occurs on
these two faults producing several en echelon folds, the largest of which is a 12 km long syncline passing through Emmitsburg (Fig. 5). En echelon geometry of the folds and displacement of the border fault suggest left lateral wrenching of the faults. Complex interaction of normal faulting
Regional Gettysburg
Jurassic extension that formed the half graben affected basement rocks
substantially. Involved is 20 “-30 o rotation about a horizontal axis in the area of the half graben as well as development of normal faults over a larger area. Basement, northwest of the border fault, has behaved
as a passive rift shoulder
and been rotated
not more than a few degrees. Regionally, geometry of Blue Ridge and Great Valley structures lack impress of substantial Jurassic rotation. For example, fold axial surfaces change progressively from inclined in the Blue Ridge to upright in the Valley and Ridge without interruption. Fail1 (1973) also arrives at this conclusion even though his basin model differs. In detail, a narrow band of basal Triassic strata is preserved fault near the Susquehanna
north of the border River. Strata1 attitude
ranges from subhorizontal to 10 “-15 “SE where they are dragged into the border fault (Root, 1977). Immediately south of the border fault, Triassic strata
and
underlying
basement
have been
concomitant with the wrenching form two small horst-like fault blocks of Paleozoic limestone base-
rotated 20”-30” 40 km southeast
ment adjacent to the border fault near Fairfield and 14 km northwest of Frederick (Figs. 2 and 5). Origin of the two left-lateral wrench faults is unclear. Their orientation and sense of movement relative to the C.-M.C. fault suggests that they are antithetic to the main C.-M.C. fault. It is suggested that these two faults were pre-Mesozoic left-lateral antithetic to the C.-M.C. fault and
that this regional rotation extends southeast beyond the erosional limits of the basin is problematic. In the Piedmont, about 60 km southeast of the border fault, late stage arching of Paleozoic cleavage has been mapped as the Tucquan anticline (Freedman et al., 1964). Possibly this arch could mark the horizontal rotational hinge of the Gettysburg half graben. Indeed, Freedman et al.
NW in a belt extending at least of the border fault. The distance
BASEMENT
CONTROL
OF STRUCTURE
(1964) recognized
a possible
for this deformational Normal
faults
occur
previously,
of younger
the border
ing is a brittle ing involves thetic
the basin these
deformed
process, mapped
extend
faults
rocks of the Pied-
whereas,
extends
the earlier
margin
at the
50 km
where it faults
northeast
fault-
edge
syn-
margin
the Piedmont.
at least
the Piedmont
fault-
The second-order
well into
of
One of southwest
the overlap
of the
Culpeper
with
ment horst-like
I greatly R.T.
of simple
and
complex
half-graben.
structure
of base-
ment have structural analogues of corresponding complexity in the half-graben. Within the basin, it is possible
to recognize
three types of contempora-
faults that formed regime (Fig. 5).
Faculty
in a dominantly
exten-
This study was supported
by a
Grant
of structures
orientated
cleavage.
normal
Ultimately,
in the basement
or subnorparallelism
and half-graben
probably related to reactivations Appalachian continental margin.
are
of the original Reactivation of
a basement fracture zone at a large angle to the continental margin, produced complex structures within the Gettysburg basin. The Shippensburg basement fault, within this basement fracture zone, was reactivated to produce in the basin a fold, a basement fault block, and offset of the border fault. The related C.-M.C. fault does not offset the border fault directly, but associated antithetic wrench faults offset the border fault substantially,
from the College
1980. Geologic
E., Edward,
of
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Donath,
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Faill, R.T., 1973. Tectonic ark-Gettysburg
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Barr, D., 1987. Structural/stratigraphic
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The border fault and second-order synthetic faults are extensional normal faults that rigorously parallel the trend but not the dip of the pervasive
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Wooster.
Cloos,
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T. Engelder,
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timore,
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Interaction faults
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Faill,
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Structural grain of Precambrian/ Paleozoic basement affects, in unremitting fashion, the structure
folds.
normal
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echelon
250,000. Pennsylvania
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through
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mylonitization.
faults
base-
is inferred
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Tectonophysics, 166 (1989) 281-292
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Basement control of structure in the Gettysburg rift basin, Pennsylvania and Maryland SAMUEL Department
of Geology,The
College
I. ROOT of Wooster, Wooster, OH 44691 (U.S.A.)
(Received July 28,1988; revised version accepted January 10,1989)
Abstract Root, S.I., 1989. Basement control of structure in the Gettysburg rift basin Pennsylvania and Maryland. Tecronophysics, 166: 281-292.
Jurassic faulting formed the 93 km long Gettysburg basin as an extensional half graben paralleling the basement structural grain. Preserved in the basin are rift-related Carnian to Rhaetian strata that were tilted 20-30“ NW into a SE dipping, listric normal fault at the northwest border of the basin. Vertical displacement on the border fault approaches 10 km. The border fault developed parallel to the trend of the terminal Paleozoic Alleghenian South Mountain cleavage of the Blue Ridge basement along 80% of its extent. However, it is only roughly parallel to discordant to dip of the cleavage. Relations~p of cleavage and later border faulting may be the result af persistent reactivation of the original App~a~~an ~ntinental margin. LocaI complex structures in the half graben are related to reactivation of two subvertical, pre-Mesozoic faults that transect basement structural grain (cteavage) at a large angle. The northern S~p~nsburg fauh was reactivated during basin normal faulting, offsetting the border fault in a right-lateral sense by 3.5 km and forming within the basin a fold and a fault sliver of basement. The southern Carbaugh-Marsh Creek fault was not reactivated, but is the locus of a 20 ’ -30 o change of trend of both the basement cleavage and later border fault. However, two large, NW trending, left-lateral wrench faults, antithetic to the Carbaugh-March Creek fault, developed here offsetting the border fault and forming en echelon folds and horst blocks of basement rock within the basin.
1. Introduction Rift-associated Mesozoic basins are exposed along the eastern margin of North America in a 2000 km long belt from South Carolina to Nova Scotia. Occupying a central position in this belt is a major depositional basin of Late Triassic to Early Jurassic age, extending nearly 500 km from Virginia to New York (Fig. 1). Rotation and faulting of beds in this basin produced four halfgrabens, including the Gettysburg basin, dipping west and northwest into a major bounding fault at the basin border (Fig. 1). Subsequent erosion of the Mesozoic strata separated the Gettysburg basin from the Culpeper basin on the south. Northeast 004O-1951/89/$03.50
d 1989 Eisevier Science Publishers B.V.
of the Gettysburg basin, outcrops of Mesozoic strata decrease in width in an area termed the Narrow Neck basin, but they widen farther northeast and become the well known Newark basin. The striking congruence of the configuration of the Mesozoic basins and arcuation of the older Appalachians is evidence for a genetic relationship between the two (see discussion by Swanson, 1986) (Fig. 1). Lindholm (1978) and Swanson (1986) attributed all border faulting of these exposed Mesozoic basins to reactivation of pre-Triassic structures. The best d~umentation of reactivation of older basement structures during Mesozoic extensional faulting is found in the Newark basin (Ratchffe et al., 1986).
S.I. ROOT
282
Fig. 1. Relation and Newark
of the Mesozoic
basins.
Precambrian-Paleozoic
rift basins
Note congruence age. Dot-dash
(stippled
pattern)
of the basins
line in Piedmont
Wavy lines are Atlantic
to basement
to the Appalachian
coastal
rocks. Narrow structural
is the Martic
Line. Coastal
waters. Washington,
Neck basin (not labelled)
grain represented plain
is structurally
unrelated
provinces
Cenozoic
of
cover.
D.C. and New York City are shown.
To date there have been no substantive studies of Mesozoic reactivation of basement structure in
and development may be detailed.
the Gettysburg basin. Although this study is based only on outcrop information (Stose, 1932; Stose
Basement rocks
and Jonas, 1939; Stose and Stose, 1946; Fauth, 1978; Root, 1977 and unpublished data), supplemented by regional gravity and magnetic studies
links Gettysburg
by several geologic
of Mesozoic
basin
structures
Structure of the basement adjacent to the western margin of the Mesozoic
basins is important
(Daniels, 1985; Phillips, 1985; Sumner, 1977), significant relationships between basement structure
because the major fault that borders these half-
Fig. 2. Geologic
of basin border
change
across
map of the Gettysburg Carbaugh-Marsh
grabens
basin. Note trend parallelism
Creek (C.-M.C.)
fault. Based on mapping F-Fairfield;
is on the west (Figs.
of many
and E-Emmitsburg.
fault to basement workers
cleavage
1 and 2).
Im-
and 20 O-30 o trend
cited in the text. Y.S-York
Springs;
BASEMENT
CONTROL
OF STRUCTURE
IN GEnYSBURG
RIFT
283
BASIN
mediately adjacent to the Gettysburg basin along most of its extent are Precambrian Catoctin Group metavolcanics and Lower Cambrian Chilhowee Group quart&es and phyllites that form the mount~nous Blue Ridge (Fig. 2). The quartz&es are overlain by a thick sequence of CambroOrdovician carbonate rocks and shales that underlie the Great Valley flanking the Blue Ridge on the west. Together these rocks were deformed during the Alleghenian orogeny into the cleavage dominated South Mountain anticlinorium described by Cloos (1947). This fold is a large westward overturned anticliiorium that is part of, the Blue Ridge thrust plate whose sole thrust is about -4500 m adjacent to the Culpeper basin (Kulander and Dean, 1986). The cleavage dips steeper in the upper limb (> 3O”SE) than the inverted lower limb ( < 30 “SE) forming a fan open to the northwest. Cleavage strike can vary as much as rt15* about a domain mean strike, but it typically varies by about +7” (see Table 1). Strike variation is generally related to position of cleavage on the plunging fold. Orientation of this principal cleavage, termed the South Mountain cleavage (Mitra and Elliot, 1980), is important to development of the border faults at the edge of the Mesozoic half-graben. Other minor cleavages are present but are not of regional significance (Fauth, 1978). The Valley and Ridge fold belt is developed in a thick Cambrian-Pennsylvanian sedimentary sequence that has been folded together with the South Mountain fold but lacks a pervasive cleavage. At the north end of the Gettysburg basin, the basement becomes more complex as the Blue Ridge is overridden by a sequence of Cambro-Ordovician nappes composed of shales and carbonate rock that are emplaced on a late Alleghenian, subhorizontal thrust (Root, 1970, 1977). Multiple cleavages are developed in this thrust sheet. The general structural grain of the nappe system is parallel to the grain of the Valley and Ridge. This area is the locus of a profound change in the trend of the structural grain of the basement and Mesozoic basins from northeast to nearly east-west. The southeast margin of the basin is an overlap of Triassic red beds upon Piedmont rocks. Northwest of the Martic Line (Fig. l), Piedmont rocks
are principally Cambro-Ordovician carbonate rocks and siliciclastics. Southeast of the Martic Line is a metamorphosed sequence of Lower Paleozoic schists and chloritic quartzites. Even though these strata have been deformed at least three times and multiple cleavages occur (Freedman et al., 1964), the principal structural grain southeast of the Mesozoic basins generally conforms to the Appalachian structural grain to the northwest. Rocks in the Gettysburg basin Sedimentary strata preserved in the Gettysburg basin are a rift-related red bed sequence ranging in age from mid-Camian to uppermost Rhaetian (Traverse, 1987). Various measured sections at surface aggregate to a composite thickness of nearly 9000 m. Because the adjacent Culpeper and Newark basins contain strata as young as Pliensbachian (Traverse, 1987), equivalent strata probably were deposited in the Gettysburg basin but subsequently eroded. These eroded strata could have aggregated to an additional 500-1000 m of section. However, it may be that strata thin depositional~y toward the basin center so that a maximum of 7000 m basin fill is a more conservative thickness. Mesozoic strata in the basin are extensively intruded by the Gettysburg diabase sheet which is estimated to be 500 m thick on its concordant, southern up-dip edge. On the north and south margins, it is highly discordant forming irregular cross-cutting bodies and isolated intrusives. The southern edge of the sheet intruded along a fault to be discussed later in this paper. Numerous vertical to nearly vertical dikes, up to a few tens of meters thick, tens of kilometers long and trending N to N20 “E, cut across the basin. They are not shown on the figures. Strata1 dip is the result of post-Pliensbachian monoclinal rotation and concomitant major border faulting with accompanying igneous and volcanic activity. This deformation may have occurred during regional Toarcian taphrogenesis. It appears that only minor faulting is associated with deposition of the Carnian to Rhaetian strata in the Gettysburg basin.
284
Basin structure
The basin is a half-graben in which strata dip 20”-30” NW toward the border fault at the northwest margin of the basin and exhibit relatively minor disruption by intrusion of the diabase sheet. The border fault is mapped as continuous (Berg, 1980; Cleaves et al., 1968) and with a large displacement. This is opposed to the model of Fail1 (1973) which considers the fault discontinuous. East of the Lisburn fault, the border-fault strike is N70”-80 o E, but this area should be considered part of the Narrow Neck basin because en echelon folding dominates here. The Gettysburg basin extends for 93 km from Frederick to the Lisburn fault (Fig. 2). Overall deformation in the Gettysburg basin is of extensional character with the vertical diabase dikes in the basin thought to represent the principal plane of stress thus suggesting a direction of principal extension oriented about 105 O/285 O; normal to the basement cleavage trend south of the Carbaugh-Marsh Creek fault (C.-M.C. fault) and subnormal to cleavage trend north of the fault (Fig. 2, Table 1). Both minor and major structures in the basin indicate dominantly normal fault, dip-slip movement. A few wrench structures occur but are considered unique and the product of special basement structures to be described later. It is difficult to calculate the total amount of vertical displacement across the border fault. Based on the thickness of preserved Mesozoic strata and the diabase sheet, at least 7000 m of displacement is required in the structurally deepest segment of the half graben sited near the border fault southwest of York Springs (Fig. 2). Vertical displacement along the border fault decreases progressively southwest to the termination of the basin near Frederick where the Blue Ridge basement is faulted against the Piedmont basement (Fig. 2). An indication of the total ma~mum amount of vertical displacement may be inferred from basement rocks exposed at three localities within the half graben adjacent to the border fault (Fig. 2). The basement here is composed of Cambro-Ordovician carbonate rocks. However, across the border fault, exposed basement rocks are Blue Ridge metavolcanics and quartzites. At the initiation of
S.1. ROOT
Mesozoic sedimentation the carbonate rock floor of the basin was probably 1000-2000 m stratigraphically above the underlying Blue Ridge rocks. Add to this the estimated 7000 m of preserved Triassic strata, 500 m of diabase sheet, plus 500-1000 m of Jurassic rocks that have probably been eroded from the half graben and total vertical border fault displacement approaches 10 km. ReIation of the border fault to the basement structure There is little to suggest that the Gettysburg basin border fault may be a reactivated Paleozoic fault trending subparallel to cleavage. Geologic mapping in the basin and Blue Ridge does not reveal a fault in the basement complex that can be mapped into the border fault as with the Ramapo fault in the Newark basin (Ratcliffe et al., 1986). Indeed there is not much Alle~enian thrusting in the Blue Ridge in this area. As shown by Cloos (1947), folding is the dominant deformational mode here. A number of faults in the Blue Ridge are actually normal faults related to Mesozoic border faulting (Fig. 2). This conclusion derives from the presence of younger beds on the southeast side of the presumably SE dipping (Root, 1970) faults, because in the case of thrusting, older beds are to be expected on the southeast. The border fault in the southern Gettysburg and northern Culpeper basins is virtually rectilinear for 110 km (Fig. 1); a condition not to be expected from a reactivated thrust in a terrain of moderate relief. The pervasive South Mountain cleavage, developed during the late Paleozoic Alleghenian deformation, is the principal anisotropy in the basement and therefore a potential plane for later normal faulting. It has been demonstrated (Root, 1970) that this cleavage can be divided into northem and southern domains separated by the C.-M.C. fault (Fig. 2) which previously behaved as a zone of disjunction during Alleghenian deformation. South of this fault cleavage trends NIOO-20° E; north of the fault cleavage trends N40”-SO0 E (Fig. 2). The southern domain (NIOO-20’ E) extends at least 160 km south of the C.-M.C. fault where it is adjacent to the
BASEMENT
TABLE
CONTROL
OF STRUCTURE
IN GETTYSBURG
RIFT
285
BASIN
1
Relation of border fault to basement cleavage Domain
Cleavage
Border fault length
strike
(km)
(“)
Mean difference strike
n
dip (SE)
(“)
(“)
014*
4
43*
fault-cleavage strike (“)
A
5
019 f 1
11
I
5
B
5
010 + 1
9
019+
7
29 + 11
9
C
5
009 f 1
4
010*
4
53*
6
1
D
4
010 * 1
2
005 *
3
30 + 10
5
E
5
026 I!Z3
4
0185
6
58*
F
3
039 + 1
5
G
3
035 f 2
10
H
2
013 * 1
3
I
4
036 + 4
21
039 + 15
39*
J
11
064 Ifr4
35
019 + 42
28511
2
8
053 + 6
44 + 12
14
032 + 12
61 + 11
3
015+
51+ 14
2
9
9
3 15
Border fault strike determined from means of 1 km fault segments of mapped fault traces within domains. n = number of cleavage measurements. Values for strike are azimuth and standard deviation. Domains A-F domains G and L from Fauth (1978), domain I from Freedman (1967)
are based on data from Whitaker (1955).
domain J from Root (1977). See Fig. 3 for location
of
domains.
Culpeper
basin.
The border
fault is parallel
to the
cleavage along most of its extent here (Lindholm, 1978; Mitra and Elliot, 1980; Whitaker, 1955). Only near Emmitsburg, Maryland (Fig. 2) does it deviate from this trend; the reasons for this deviation will be discussed later. The structural grain in the northern domain extends 45 km from the C.-M.C. fault to near the Susquehanna River where the Blue Ridge disappears beneath the thrust sheet emplacing the nappe system (Root, 1970; 1977). The Mesozoic
border
fault
basin
‘shows a marked
trend
of the South Mountain
of the Gettysburg
parallelism cleavage
to strike
or
in the Blue
Ridge along 80% of its extent (Fig. 2). Results of detailed mapping by various workers are presented in Table 1. Their results are divided into ten domains, extending from the border fault about 2 km into the Blue Ridge and of varied length along the border fault (Fig. 3). Except for domains F and J the difference between mean strike of the border
fault and cleavage
in the Blue Ridge in the
various domains ranges from lo-9” with a mean difference of only 4’ (Table 1). The large deviation of 14O between strikes in domain F south of Emmitsburg (Fig. 2) is attributed to complex processes involving wrenching on nearby NW trending faults concomitant with development of
Fig. 3. Index map showing location of domains A-J
that were
used to prepare Table 1. Heavy line is basin border fault. Note domain J is located astride nappe-bearing thrust sheet.
S.I. ROOT
286
the border
fault. Deviation
observed
where
Cambro-Ordovician Ridge
nappe
basement.
the border
of 15” in domain
the basement
consists
sequence
It will be shown
fault parallels
cleavage
J is
of a thin above
Blue
north
2) a distance
of Fairfield of about
to Emmitsburg
20 km, the border
N20 o -30 o W and truncate “E
cleavage.
(Table
(Fig. fault is
1, domains
(Fig.
2) on the main
basin.
minor
F, G,
exposed
border
dip
depth
cleavage
between
faults
associated canics,
Two faults,
faulting
These
synthetic faults are gently arced in map view and follow the trend of cleavage in the Piedmont
but
their
faulting
Magnetite
mining
to
of low-angle
basins.
In the Nar-
half of the Newark
is well established operations
an
metavol-
persistence
Evidence
exists in the adjoining
as there The few
including
in Precambrian
60”-70”SE
low-angle
and fault plane.
in the basin,
may be questioned.
of
fault
to assess the degree of
cleavage
fault
row Neck and western
and H), are wrench faults rather than the normal fault typical of the basin border. At the southeast margin of the Gettysburg basin normal faulting produced three, second-order half-grabens
exists (Fig. 2), it is difficult
The dip of the border fault is uncertain have been no drilling or seismic studies.
which
N15”-50
of trend border
in the overrid-
to Blue Ridge
trending
parallelism
and the Mesozoic
later that here
not parallel trend
unambiguous cleavage
dip parallelism
den Blue Ridge. From
Although basement
basins,
at the border.
and unvailable
turn-
of-the century exposures show that the border fault dips 25 “-45OSE. Ratcliffe et al. (1986) have conclusively
demonstrated
that the Ramapo
fault
segment forming part of the border of the Newark basin dips 25”-35OSE and represents Mesozoic normal fault reactivation of Paleozoic thrust slices.
mapped by Mitra and Elliot (1980). The Martic Line, a complex and much debated Paleozoic boundary between terrains of metamorphosed rocks on the southeast and non- or little metamorphosed rocks on the northwest, locally may have been reactivated as a SE dipping normal
Unlike the Gettysburg basin, this area shows a significant component of left-lateral wrenching as large-scale en echelon folds, and abundant horizontal slickenlines on related fault surfaces occur
fault bounding and 4).
adjacent to the border fault (Manspeizer, The border fault trends N70”-80°E
one of the half-grabens
(Figs.
1
Within the Blue Ridge, four normal faults occur near the border fault (Fig. 2). North of the C.-M.C. fault these faults parallel the border fault. South of the C.-M.C. fault they are oblique to the border fault in this area but still maintain parallelism to the border fault north of the C.-M.C. fault.
Lisburn
fault where there is a 30 o shift of trend to
the N40 “-50 ’ E trend of the northern Gettysburg basin. To the south, in the Culpeper basin, an extension of the Gettysburg basin, seismic profiles across the basin western margin may be interpreted to indicate Div., 1982).
a low-angle
-NW
border
fault (Virginia
SE-
A BLUE
1981). to the
GETTYSBURG RIDGE
Pennsylvania
P;
BASIN !Maryland
PIEDMONT
Fig. 4. Cross-section across the Gettysburg basin showing relatively uniform dip of Triassic New Oxford and Gettysburg formations and listric normal faults in the basin. No vertical exaggeration. See Fig. 2, A-A’
for section location.
BASEMENT
CONTROL
OF STRUCTURE
IN GETTYSBURG
RIFT
287
BASIN
There is also, however, evidence of steep faulting at the border of these basins. The border fault on the eastern half of the Newark basin, at five localities, dips 50 o -70 OSE (Ratcliffe and Burton, 1985). It is interesting to note that in this portion of the Newark basin there is little evidence of wrenching. Cleavage in the Blue Ridge thrust plate dips 30 “-60 o SE (mean 46 O, Table 1) in the upright fold limb which is adjacent to the border fault along much of its extent. Dip of Blue Ridge cleavage and minor faults in the Mesozoic basin are generally similar from which is inferred that major faults are roughly subparallel to cleavage at surface except for Domain J (Table 1). The effect of cleavage on the angle of shear fracture has been studied experimentally by Donath (1961) who showed that shear fractures tend to develop subparallel to pervasive planar anisotropy for inclinations of up to 45 “-60 o to the direction of maximum pressure, comparable to relations between inferred vertical principal stress and cleavage in this region. Cleavage in the Blue Ridge cannot be considered as simple uniform planar surfaces extending to depths of several kilometers or more. Internally, the Blue Ridge thrust plate may be of the order of 4500 m thick (Kulander and Dean, 1986), composed of lithologies that show appreciable cleavage refraction. Cleavage at the thrust plate sole will be subhorizontal (Mitra and Elliot, 1980). Beneath the Blue Ridge thrust plate are younger Cambro-Ordovician carbonate rocks and shales, probably dominated by a series of complex duplex structures in which cleavage dip will vary according to position of the fold limb (Fig. 4). Where cleavage is uniform in o~entation to depths of several kilometers, faults will develop subparallel to these planes of anisotropy. However, in the Blue Ridge where dip of cleavage in vertical succession varies non-systematically, the border fault will probably transect some domains of cleavage dip. Geometrically, the border fault and cleavage have the same strike but not the same dip. This problem of cleavage dip variation in basement becomes particularly acute at the north end of the Blue Ridge, where it is overridden by a nappe sheet about 200 m thick in this area. Major
foliation in the nappe is folded and dip varies from subhorizontal to 30”SE, with the trend of the border fault deviating about 15” from the trend of nappe foliation (Domain J, Table 1). Trend of the border fault, however, is nearly parallel to cleavage in the Blue Ridge (067O) where it is last observed passing beneath the nappe sequence (Table 1). At the Susquehanna River, just east of the Gettysburg basin, an exposure no longer available shows the contact between the Mesozoic basin and basement to dip 45 “SE (Wherry, 1913). Although Wherry (1913) considered this contact to be an unconformity, I suggest it is actually a fault contact and that such a fault dip, if maintained, transects in the footwall the nappe sheet, the Blue Ridge thrust plate and subBlue Ridge thrust plate, all within internal variations in cleavage dip. From relations in this area an important conclusion is derived. It appears that the border fault was initiated at depth, parallel to trend of the Blue Ridge cleavage and was propagated upward, transecting nappe foliation of the overlying thin sheet. Border fault processes
It is appealing to infer a genetic relationship between the border fault and basement cleavage because of their trend parallelism. Indeed, Lindholm (1978) concluded that in the Culpeper basin faulting is controlled by foliation. A clear example of basement foliation controlling subsequent normal faulting occurs in the Red Sea rifts where Miocene faults are reactivated on Pan-African schistosity planes (Jarridge et al., 1986). However, such a genetic relations~p requires parallelism of both strike and dip between the cleavage and the later normal faults. Divergence of dip between these two surfaces at the border of the Gettysburg basin is sufficiently large, especially where the nappe sheet forms outcropping basement, that some other process is required. The border fault apparently was initiated at depth, possibly at the bottom of the brittle crust, and propagated upwards. Under such conditions there is no requirement of parallelism between border fault and cleavage. Where some degree of parallelism occurs, a common process must be
S.I. ROOI
288
sought. Relationship of cleavage and the later normal faulting may be ascribed to persistent reactivation of the original Appalac~an continental margin. Thomas (1977), showed that the shape of this margin is the result of transform faulting along a Late Precambrian rift and that distribution of Paleozoic sedimentary units and outlines of subsequent compressional structures conform to this. Separation along this margin during Mesozoic rifting will account for parallelism between the rift basins and Appalachian structure (Fig. 2) as well as the relationship of normal faulting to cleavage. Such reactivation is a complex process because basement is allochthonous with overprints of both Taconic and Alleghenian collisions preceeding Mesozoic rifting. Basin cross-section Behavior of the border fault at depth is important in construction of the basin cross-section. The 60”SE dip of minor faults in the basin is considered to indicate the dip of the border fault at surface. In comparable basins such as Bahia Bay, China, the normal faults are listric, with a 60 “-70 o dip at surface flattening to 20*-30 o at depth, and basin extension is 30-40% (Hefu, 1986). Distinction between listric and planar faults is important in terms of the amount of extension required to produce strata1 rotation. Wernicke and Burchfiel (1982) show that for a given maximum strata1 rotation and fault dip, listric geometry requires far less basin extension than planar geometry. This is demonstrated in experiments with analogue models involving extension above a uniformly expanding basement (M&lay and Ellis, 1987). When the basement fault is modelled as a planar fault, extension of 25% produces strata1 rotation of 8”, but when modelled as a listric fault, the same amount of extension produces dips of 25”. The values derived from these listric fault experiments are fairly close to Gettysburg basin values of actual strata1 rotation and the amount of basin extension as inferred from the Bohia Bay analogue. The cross-section (Fig. 4) of the basin is therefore const~cted with a listric shape to all major faults. They approximate a set of related planar or “domino” faults that involves some ductile deformation at their base (Barr, 1987).
Wrench related structures in the basin Even though extension is the dominant deformational mode, two types of wrench structures are recognized in the Gettysburg basin. One type is related to reactivation of the Shippensburg fault (Figs. 2 and 5). The other type is developed at the C.-M.C. fault where there is an abrupt change in trend of the basement cleavage and concomitant change in trend of the border fault (Figs. 2 and 5). Both the C.-M.C. and Shippensburg faults are segments of the Transylvania fault zone, which transects the Appalac~~ structural grain at a large angle, and extends more than 300 km west from the Blue Ridge onto the Appalachian Plateau in Pennsylvania (Root and Hoskins, 1977) and into Ohio (Gray, 1982). This extensive fault zone is considered to represent a vertical fracture zone in the continental plate that has been active in the Paleozoic and is probably a Precamb~an structure (Root and Hoskins, 1977). The Transylvania fault zone must have extended some distance farther east into what is now the Gettysburg basin, Shippensburg fault The border fault is sharply offset 3.5 km in a right-lateral sense where it intersects the Shippensburg fault (Fig. 5). Within the basin, at this offset, is a narrow basement inlier of Paleozoic limestones with locally preserved Triassic red beds. The inlier extends finger-like eastward from the border fault for 4 km, past York Springs. Stose (1949) attempted to account for this occurrer&e of basement outcrop by suggesting an older, second border fault buried east of the Paleozoic inlier. The idea is untenable in the light of present mapping which requires that faults bound the inlier, making it a fault sliver. Both north and south of the fault bounded basement fault sliver, Triassic strata dip 30”40 o NW. The southern fault is probably an extension or splay of the S~ppensburg fault in the basin. Within the inlier, Triassic strata, which are unconformable upon the Paleozoic limestones, also dip 35 o NW {McLaug~in, 1961). The relative uniformity of Triassic bedding attitudes both adjacent to and within the fault sliver demonstrate the dominance here of extensional tectonics with nor-
BASEMENT
CONTROL
OF STRUCTURE
IN
GETTYSBURG
RIFT
289
BASIN
Me,sozpic
faults
Fig. 5. Various types of faults associated with Jurassic development of the Gettysburg structures. Stippled pattern within the basin indicates Cambro-Ordovician
associated
with
regional
basin. All are related to various basement
limestone basement. Bedding attitudes shown in areas of
complex basin structures related to Jurassic reactivation of C.-M.C. and Shippensburg faults. See Fig. 2 for diabase sheets omitted in this figure.
ma1 faulting, not major wrenching, producing a flower structure fault sliver. However, in extensional tectonics it would be peculiar if the basement inlier was an isolated narrow, upfaulted sliver bounded
by faults
thousand Paleozoic
meters. Therefore, it is proposed that the limestones are a really small horst-like,
large displacement block. Questioned
with displacements
of several
sliver on a much larger fault limits of this proposed larger
structure produced by relatively sional wrenching. Consideration of the structure fault sliver and folded Triassic with
the
Shippensburg
pre-existing reactivated
fault
minor
transpres-
of the basement strata on strike
indicated
that
this
regional basement fault was locally as a right-lateral wrench during border
faulting to transtensional
produce both transpressional and structures in the basin. The 3.5 km
of offset of the border
fault across
the Shippens-
block are suggested in Figs. 2 and 5. En echelon with the basement fault sliver is a fold with a sense of right-lateral motion. The
burg fault should not be construed as the amount of wrench displacement. If wrenching with 3.5 km
folding
of displacement
occurs
as regional
nent strike ridges of Triassic belt.
A stereonet
plot
deflections
of promi-
strata in a 8 km long
defines
a fold
plunging
N25 o W/32O. North of the fold axis beds trend northeast, but approaching the fold, the axis changes towards a more E-W orientation. South of the fold, the beds change back to a NE trend (Fig. 5). The range of dip values across the area of deflection indicates this is not sedimentary drape over a basement block. This fold is the surface expression of a right-lateral fault in basement, probably a splay of the Shippensburg fault, and may be considered as an extremely simple flower
trending
occurred,
en echelon
folds
it would rather
produce than
NE
a single,
nearly E-W zone of strata1 deflection oriented along the continuation of the Shippensburg fault in the basin. For example, a modest amount of wrenching produces an array of en echelon folds near Emmitsburg (Fig. 5). Border fault offset across the Shippensburg fault is attributed to a combination of shift of position during border fault propagation and right-lateral displacement of the Shippensburg fault. The reason for rightlateral motion of the Shippensburg fault may be related to the regional eastward arcuation of the
S.I. ROOT
290
Appalachian
structural
grain north
of the Gettys-
that, during tion
burg basin (Fig. 1).
occurred
consistent Carbaugh-Marsh Though as a wrench
active
(cleavage)
the border
discontinuity. changes
65”
angle between fault
fault, it was
Basement
trend here abruptly
about 25 o (Root, 1970) forming border
ment
fault was not reactivated
fault offsetting
a significant sotropy
Creek fault
the C.-M.C.
within fault.
by
the basin a
the two segments
at the C.-M.C.
ani-
of the later Under
such
conditions there can be an overlap of fault trends related to different trends of the border. Overlap of fault trends occurs in the Blue Ridge basement south of the C.-M.C. fault. Here, two large normal faults follow the trend of the border fault north
of the
C.-M.C.
fault.
Indeed,
one
fault that follow the structural trend of the south. Within the basin, two large, uniquely NW trending high-angle faults occur south of the C.-M.C. fault and for a distance of 20 km offset the NE trending border fault (Fig. 5). Because of associated normal faulting, the displacement is considerably less than the amount of horizontal offset. The fault north of Emmitsburg is obscured by diabase intrusion, but discordance of bedding across the diabase (NW dipping beds strike into the NW trending syncline) demonstrates its preswrenching
with
on
the
right-lateral pensburg
faults.
C.-M.C.
these
of observable fault
displacement
transferred
inhibited
trends
across
faults
comparable
because
to the Shipfault
of convergence
this fault faults.
from other
is
displace-
on the C.-M.C.
to the antithetic
these wrench
mo-
This
of the associated
fault. Displacement
of cleavage
and
motion
To distinguish
faults
of differing
they are shown on Fig. 5 as faults associ-
ated with change
Effects ture
left-lateral
along
the lack
may have been
process,
reactivation,
of trend
of basement
of basin development
grain.
on basement
struc-
fault
mapped by Fauth (1978), may well be an extension of the border fault north of the C.-M.C. fault (Fig. 5). There are no faults north of the C.-M.C.
ence (Fig. 2). Transpressional
Jurassic
again
occurs on
these two faults producing several en echelon folds, the largest of which is a 12 km long syncline passing through Emmitsburg (Fig. 5). En echelon geometry of the folds and displacement of the border fault suggest left lateral wrenching of the faults. Complex interaction of normal faulting
Regional Gettysburg
Jurassic extension that formed the half graben affected basement rocks
substantially. Involved is 20 “-30 o rotation about a horizontal axis in the area of the half graben as well as development of normal faults over a larger area. Basement, northwest of the border fault, has behaved
as a passive rift shoulder
and been rotated
not more than a few degrees. Regionally, geometry of Blue Ridge and Great Valley structures lack impress of substantial Jurassic rotation. For example, fold axial surfaces change progressively from inclined in the Blue Ridge to upright in the Valley and Ridge without interruption. Fail1 (1973) also arrives at this conclusion even though his basin model differs. In detail, a narrow band of basal Triassic strata is preserved fault near the Susquehanna
north of the border River. Strata1 attitude
ranges from subhorizontal to 10 “-15 “SE where they are dragged into the border fault (Root, 1977). Immediately south of the border fault, Triassic strata
and
underlying
basement
have been
concomitant with the wrenching form two small horst-like fault blocks of Paleozoic limestone base-
rotated 20”-30” 40 km southeast
ment adjacent to the border fault near Fairfield and 14 km northwest of Frederick (Figs. 2 and 5). Origin of the two left-lateral wrench faults is unclear. Their orientation and sense of movement relative to the C.-M.C. fault suggests that they are antithetic to the main C.-M.C. fault. It is suggested that these two faults were pre-Mesozoic left-lateral antithetic to the C.-M.C. fault and
that this regional rotation extends southeast beyond the erosional limits of the basin is problematic. In the Piedmont, about 60 km southeast of the border fault, late stage arching of Paleozoic cleavage has been mapped as the Tucquan anticline (Freedman et al., 1964). Possibly this arch could mark the horizontal rotational hinge of the Gettysburg half graben. Indeed, Freedman et al.
NW in a belt extending at least of the border fault. The distance
BASEMENT
CONTROL
OF STRUCTURE
(1964) recognized
a possible
for this deformational Normal
faults
occur
previously,
of younger
the border
ing is a brittle ing involves thetic
the basin these
deformed
process, mapped
extend
faults
rocks of the Pied-
whereas,
extends
the earlier
margin
at the
50 km
where it faults
northeast
fault-
edge
syn-
margin
the Piedmont.
at least
the Piedmont
fault-
The second-order
well into
of
One of southwest
the overlap
of the
Culpeper
with
ment horst-like
I greatly R.T.
of simple
and
complex
half-graben.
structure
of base-
ment have structural analogues of corresponding complexity in the half-graben. Within the basin, it is possible
to recognize
three types of contempora-
faults that formed regime (Fig. 5).
Faculty
in a dominantly
exten-
This study was supported
by a
Grant
of structures
orientated
cleavage.
normal
Ultimately,
in the basement
or subnorparallelism
and half-graben
probably related to reactivations Appalachian continental margin.
are
of the original Reactivation of
a basement fracture zone at a large angle to the continental margin, produced complex structures within the Gettysburg basin. The Shippensburg basement fault, within this basement fracture zone, was reactivated to produce in the basin a fold, a basement fault block, and offset of the border fault. The related C.-M.C. fault does not offset the border fault directly, but associated antithetic wrench faults offset the border fault substantially,
from the College
1980. Geologic
E., Edward,
of
J., Galser,
Harrisburg,
J.D., 1968. Geologic
Maryland
Geological
1: Pa.
Map of
Survey,
Bal-
D.L.,
praisal.
deformation
in the South
Mountain
Bull. Geol. Sot. Am., 58: 843-913.
1958. Gravimetric
Gettysburg
Donath,
Survey,
Md.
E., 1947. Oolite
the
for extensional
Geol., 9: 491-500.
Map of Pennsylvania,
Geological
1: 250,000.
fold, Maryland. Daniels,
models
type. J. Struct.
Berg, T.M. (compiler),
basin,
character
and anomalies
Pennsylvania-a
in
preliminary
ap-
U.S. Geol. Surv., Circ., 946: 128-132. F.A.,
1961. Experimental
anisotropic
study
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failure
in
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Faill, R.T., 1973. Tectonic ark-Gettysburg
development
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Geol.
New-
Sot.
Am.
Furnace
and
Maryland
Ge-
Bull., 84: 725-740. Fauth,
J.L., 1977. Geologic
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of they
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basement cleavage. These structures basin and their development was
the by
comments
M.A.
Barr, D., 1987. Structural/stratigraphic
Blue Ridge Summit
dominate enhanced
and
References
Fauth,
mal to regional
in the basin.
Wilson;
Development
The border fault and second-order synthetic faults are extensional normal faults that rigorously parallel the trend but not the dip of the pervasive
extension
base-
Wooster.
Cloos,
Gettysburg
of the
produce
the critical
T. Engelder,
were most helpful.
timore,
of the Mesozoic
Interaction faults
fault blocks
appreciate
Faill,
Maryland,
Structural grain of Precambrian/ Paleozoic basement affects, in unremitting fashion, the structure
folds.
normal
Acknowledgements
Cleaves,
Conclusions
neous sional
echelon
250,000. Pennsylvania
basin.
Areas
en fault
side
to recog-
at the southeast
through
from
the later normal
mylonitization.
faults
base-
is inferred
are difficult
However,
form
fault. As
beds on the southeast
of the fault. Such geometries nize in the multiply
and
C.-M.C.
their geometry
basement.
age
in the Blue Ridge
noted
291
BASIN
event.
at least 10 km beyond
mont
RIFT
post-Alleghenian
ment
the presence
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ological
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