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Bucknell University
Bucknell Digital Commons
Faculty Journal Articles
Faculty Scholarship
2014
Significance of the Deformation History within the
Hinge Zone of the Pennsylvania Salient,
Appalachian Mountains
Mary Beth Gray
Bucknell University, mbgray@bucknell.edu
Peter B. Sak
sakp@dickinson.edu
Zeshan Ismat
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Part of the Tectonics and Structure Commons
Recommended Citation
Sak, P.B., Gray, M.B., and Ismat, Z., 2014, Significance of the Juniata culmination in the deformation history of the Pennsylvania
salient, Appalachian Mountains. Journal of Geology 122, 367-380
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Significance of the Deformation History within the Hinge Zone of the
Pennsylvania Salient, Appalachian Mountains
Peter B. Sak,1,* Mary Beth Gray,2 and Zeshan Ismat3
1. Department of Earth Sciences, Dickinson College, Carlisle, Pennsylvania 17013, USA; 2. Department of
Geology, Bucknell University, Lewisburg, Pennsylvania 17837, USA; 3. Department of Earth and
Environment, Franklin and Marshall College, Lancaster, Pennsylvania 17603, USA
ABSTRACT
Two competing models exist for the formation of the Pennsylvania salient, a widely studied area of pronounced
curvature in the Appalachian mountain belt. The viability of these models can be tested by compiling and analyzing
the patterns of structures within the general hinge zone of the Pennsylvania salient. One end-member model suggests
a NW-directed maximum shortening direction and no rotation through time in the culmination. An alternative model
requires a two-phase development of the culmination involving NNW-directed maximum shortening overprinted by
WNW-directed maximum shortening. Structural analysis at 22 locations throughout the Valley and Ridge and southern
Appalachian Plateau Provinces of Pennsylvania are used to constrain orientations of the maximum shortening direction and establish whether these orientations have rotated during progressive deformation in the Pennsylvania
salient’s hinge. Outcrops of Paleozoic sedimentary rocks contain several orders of folds, conjugate faults, steeply
dipping strike-slip faults, joints, conjugate en echelon gash vein arrays, spaced cleavage, and grain-scale finite strain
indicators. This suite of structures records a complex deformation history similar to the Bear Valley sequence of
progressive deformation. The available structural data from the Juniata culmination do not show a consistent temporal
rotation of shortening directions and generally indicate uniform, parallel shortening directions consistent with the
single-phase model for development of the Pennsylvania salient.
Introduction
SW segment plunge to the SW and record shortening directions that underwent a counterclockwise rotation through time (Nickelsen 1988, 2009).
The folds in the NE segment plunge to the NE, and
the shortening directions within these rocks preserve evidence of a temporal clockwise rotation
(Nickelsen 1979; Geiser and Engelder 1983; Gray
and Mitra 1993; Markley and Wojtal 1996; Zhao
and Jacobi 1997; Younes and Engelder 1999). The
segments of this salient meet in the hinge zone that
contains a NW-trending structural high, the Juniata
culmination (fig. 1b). The folds within the Juniata
culmination are doubly plunging to the NE and SW.
The hinge zone of the Pennsylvania salient encompasses the Juniata culmination and the area adjacent to it—it is defined as the area of maximum
curvature, in plan view, of the Pennsylvania salient
(fig. 1b).
Currently, two competing end-member kinematic models attempt to explain (1) the formation
The Pennsylvania salient is one of the most prominent features in the Appalachian mountain system
and has been extensively studied for more than 150
years (e.g., Dana 1866; Rogers 1958; Nickelsen
1963; Gwinn 1967; Thomas 1977; Ong et al. 2007).
The structures in the Valley and Ridge Province are
the result of tectonic shortening and thickening associated with the closure of the Iapetus Ocean, culminating in the Permian continent-continent collision of Gondwana with Laurentia during the
Alleghanian orogeny (Rodgers 1949; Hatcher et al.
1989; Stamatakos et al. 1996; Faill 1998). In map
view, the Pennsylvania salient forms a smooth arc
with an interlimb angle of ∼135⬚ (fig. 1). This arc
links two relatively linear segments, the NNESSW-trending SW segment and the ENE-WSWtrending NE segment (fig. 1). The folds within the
Manuscript received July 29, 2013; accepted January 14, 2014;
electronically published June 11, 2014.
* Author for correspondence; e-mail: sakp@dickinson.edu.
[The Journal of Geology, 2014, volume 122, p. 367–380] 䉷 2014 by The University of Chicago.
All rights reserved. 0022-1376/2014/12204-0002$15.00. DOI: 10.1086/675907
367
368
P. B . S A K E T A L .
Figure 1. a, Location of the study area; b, the Central Appalachian Salient, highlighting major lithologic contacts
in the Valley and Ridge Province, the hinge zone, the Juniata culmination, internal crystalline thrust sheets comprising
the Blue Ridge and Reading Prong, and location of study sites (black dots).
of the salient’s arcuate pattern, (2) the temporal
rotation of shortening directions within the salient,
and (3) the formation of the Juniata culmination
and general hinge zone (Gray and Stamatakos 1997;
Wise 2004; Wise and Werner 2004). The first model
is referred to here as the “single-phase” model and
the second as the “two-phase” model (fig. 2). Both
models assume that the shape and position of the
Pennsylvania salient has been inherited from a preexisting continental reentrant in the Iapetan rifted
margin of eastern Laurentia (North America). This
guided and shaped the present-day geometry of the
Pennsylvania salient during Alleghanian deformation (Thomas 1977, 2006; Beardsley and Cable
1983; Ong et al. 2007), such that major crystalline
thrust sheets within the internides (e.g., Reading
Prong and Blue Ridge) are also parallel to the segments of the reentrant (fig. 1b). In both of the models, the Juniata culmination forms at the corner of
this Eocambrian reentrant (fig. 1b). Despite these
similarities, the two models predict distinctively
different progressive deformation paths, particu-
Journal of Geology
D E F O R M AT I O N W I T H I N T H E P E N N S Y LVA N I A S A L I E N T
369
Figure 2. Schematic diagrams of the single-phase (a) and two-phase (b) models for Alleghanian tectonic shortening.
Juniata culmination enclosed in rectangle. Black arrows p early-stage shortening directions, gray arrows p late-stage
shortening directions. Stereonets show predicted maximum shortening directions in the Juniata culmination under
each of the models. A color version of this figure is available online. MSD p maximum shortening direction.
larly within the hinge zone of the Pennsylvania
salient.
While deformation paths on the salient segments
differ, the single-phase model proposes a unidirectional transport toward the corner of the Eocambrian reentrant (fig. 2a; Stamatakos et al. 1996;
Gray and Stamatakos 1997; Wise 2004). As a result,
a single shortening direction, approximately parallel to the NW trend of the culmination, should
be preserved within the salient’s hinge (Gray and
Stamatakos 1997). The two-phase model suggests
that the salient formed by two separate shortening
events, each directed approximately perpendicular
to the salient’s segments (fig. 2b; Geiser and Engelder 1983; Wise 2004; Wise and Werner 2004). If
this model is valid, both phases should be represented by overprinting shortening directions
within the salient’s hinge.
In this article, we have compiled structural observations preserved at 22 locations within the
hinge zone of the Pennsylvania salient—15 of these
sites are within the Juniata culmination (fig. 1b).
These structures are then compared to those predicted by the single- and two-phase models to determine whether the data are consistent with either
of the models.
Background
Fold-thrust belt culminations commonly form in
salients and are topographic structural highs away
from which the folds plunge (Boyer 1978; Elliott
and Johnson 1980; Boyer and Elliott 1982). Cul-
minations often are a result of duplexing and/or
basement uplift, possibly due to excess sediment
in the original miogeoclinal package (Lageson 1984;
DeCelles and Mitra 1995). Culminations may also
reflect concentrated shortening resulting from preexisting reentrants along convergent plate boundaries (Thomas 1977, 2006; Lageson 1980; Mitra
1997; Faill and Nickelsen 1999). These topographic
highs are often the source for sediment shed into
the adjacent foreland basin. Culminations may also
help to maintain critical taper during fold-thrust
belt evolution (DeCelles and Mitra 1995; Mitra
1997). In addition, culminations are key targets for
oil and gas exploration (Lageson 1984).
The Juniata culmination, is a classic example of
a fold-thrust belt culmination (fig. 1). We have conducted numerous small-scale structural studies of
outcrops along roads, in quarries and stream cuts,
within and around the culmination, with an eye to
discerning kinematics. We have complied the results of these studies and use the findings to test
the viability of the single-phase and two-phase
models. Each model is discussed in greater detail
below.
Single-Phase Model. The single-phase model proposes that thrust sheets were transported toward
320⬚–340⬚, approximately parallel to the bisector of
the Eocambrian cratonic corner (Gray and Stamatakos 1997; fig. 2a). The clastic wedge is stratigraphically thickest at the corner of this reentrant
(Thomas 1977). As the excess sediment was shortened during Alleghanian deformation, a duplexcored wedge-shaped culmination formed, which
370
P. B . S A K E T A L .
Table 1. Structures Used to Determine Maximum Shortening Directions (MSDs)
Structural
stage
Structural feature
Interpretation of MSD
a
Joints (mode I fractures)
Cleavagea
Wedge faults
Strike-slip faultsb
Folds (and parasitic folds)
Flexural-slip slickenlines
Reverse faults
In plane of fracture and parallel to the strike of the fracture
Normal to cleavage plane
Parallel to the trend of slickenlines and fault poles
Parallel to the strike of the acute planar bisector of conjugate faults
Perpendicular to fold axes
Perpendicular to fold hinge
Parallel to the trend of slickenlines and fault poles
Early Late
X
X
X
X
X
X
X
X
X
X
a
Joints and cleavage are associated with both the early and late stages of deformation. A fold test is used to
determine the temporal relationship between joint sets or cleavage and folding.
b
Strike-slip faults are associated with both the early and late stages of deformation. Early strike-slip faults
are characterized by slickenlines oriented parallel to bedding planes. Slickenlines associated with late-stage
deformation are not folded by outcrop-scale folds.
was tapered toward the foreland and laterally away
from the Juniata culmination. This model, compared to the two-phase model, suggests greater
shortening within the corner culmination zone.
This increased shortening and thickening eventually led to late-stage, radial gravitational spreading
(Gray and Stamatakos 1997; fig. 2a). Because gravitational spreading was concurrent with shortening, this model does not require significant tangential extension around the arc of the salient. In other
words, as the clastic wedge continued to thicken
during transport and shortening, it provided a continual source of sediment that spread to the foreland, along the axis of the culmination, and laterally away from the culmination’s axis. If the
curvature postdated sediment deposition, then a
significant amount of tangential extension would
be required to form the salient’s hinge.
Gravitational spreading combined with a unidirectional transport direction produced a shortening
history with a clockwise sense of rotation in the
northeast segment and a counterclockwise rotation
in the southwest segment of the Pennsylvania salient (Gray and Stamatakos 1997; Wise 2004; fig.
2a). This rotation is preserved as secondary shortening directions toward 010⬚ in the northeastern
limb and 280⬚–290⬚ in the southwestern segment
(Gray and Stamatakos 1997). The proposed threedimensional tapered wedge also helped to form the
NE- and SW-plunging folds on either side of the
culmination. Moreover, orientation patterns of
characteristic and secondary paleomagnetic components in the strata exposed in the culmination
are uniquely explained by this model alone (Gray
and Stamatokos 1997).
A variation to this single-phase model suggests
that there was oblique convergence with the Laurentian craton (Ong et al. 2007). This model suggests that there was a single convergence direction
parallel to the Blue Ridge. The Reading Prong acted
as a buttress, so most of the rotation took place in
the salient’s NE limb. With this variation, we
would still expect to see only one shortening direction within the hinge of the salient, supporting
a single-phase model (Ong et al. 2007). Although
this model is not critical to our article, it is interesting to note that this approach would likely result
in a similar suite of structures preserved throughout the hinge zone of the Pennsylvania salient as
is suggested by our single-phase model.
Two-Phase Model. In the two-phase model, two
successive stages of noncoaxial transport are used
to explain the structures within, and the curvature
of, the Pennsylvania salient (fig. 2b). An initial
shortening event directed toward 325⬚ is followed
by a second shortening event directed toward 292⬚
(Wise 2004; Wise and Werner 2004; fig. 2b). Each
event is directed approximately perpendicular to
the edges of a preexisting, corner-shaped continental reentrant (Thomas 1977; Geiser and Engelder
1983; Wise 2004; fig. 1b).
The linear Reading Prong and Blue Ridge basement uplifts trend perpendicular to the first and
second transport directions, respectively (figs. 1b,
2b). The Reading Prong and the Blue Ridge delineate the NE and SW segments, respectively, of the
Pennsylvania salient. These basement uplifts are
proposed to be a result of each shortening event
(Wise and Werner 2004). During both events, thrust
sheet motion was impeded by the reentrant’s corner, resulting in rotational drag (Wise 2004). This
produced the observed clockwise rotation in shortening direction within the NE limb of the salient
and counterclockwise rotation in the SW limb of
the salient.
The change in direction of tectonic transport resulted in overprinting and duplexes piling up at the
intersection of the two shortening directions, that
Journal of Geology
D E F O R M AT I O N W I T H I N T H E P E N N S Y LVA N I A S A L I E N T
371
Figure 3. Geologic map draped over a shaded relief digital elevation model of the study area showing the locations
of the 22 study sites and orientations of the early and late maximum shortening directions.
is, along the reentrant’s corner, producing the Juniata culmination (Wise 2004). This type of salient
is referred to as an intersection orocline because it
is produced in a corner of a cratonic margin by more
than one overlapping directions of tectonic transport (Marshak 2004; Weil and Sussman 2004). Based
on this model, we should expect to see evidence of
both the 325⬚ and 292⬚ shortening directions within
the hinge zone of the Pennsylvania salient. And,
similar to the one-phase model, this model does
not require significant longitudinal extension
around the arc of the salient.
Methods
In general, the structural stages preserved throughout the Pennsylvania salient’s hinge zone mimic
those found in other parts of the Pennsylvania
Valley and Ridge fold-thrust belt. Here, the deformation is due to the Alleghany orogeny. Nickelsen
(1979) first unraveled a structural sequence of the
Valley and Ridge Province from work done in the
Bear Valley Strip Mine in Shamokin, Pennsylvania.
The so-called Bear Valley sequence consists of five
Alleghanian deformation stages (II–VI; Nickelsen
1979). These stages have since been recognized at
other locations in the Valley and Ridge (e.g., Gray
and Mitra 1993; Gray and Stamatakos 1997). Spiker
and Gray (1997) documented the Bear Valley sequence on the Appalachian Plateau along the Alleghany front in the vicinity of Williamsport, Pennsylvania. A similar sequence is also recognized by
Gray and Mitra (1993) throughout the middle and
southern Anthracite regions of Pennsylvania.
Structures formed during the five-stage deformation sequence reported by Gray and Mitra (1993)
are found in table 1 along with a brief explanation
of how the orientations of these structures were
used to establish mean shortening directions.
Following the methodology employed by Nickelsen (1979) and Gray and Mitra (1993), the stages
of progressive deformation were established at each
of the 22 sites incorporated into this study (fig. 3;
table 2). Stages of deformation can be precisely defined by observing cross-cutting relationships
among structures. In some cases it is possible to
distinguish up to six stages of Alleghanian deformation. For the purposes of the regional compilation of multiple studies by different authors, it is
most convenient to condense all progressive deformation into two general Alleghanian stages, early
and late (Nickelsen 1963; fig. 4).
Early, or prefolding, structures are those that are
clearly demonstrated to have been rotated on the
limbs of folds (fig. 4). In the Valley and Ridge, these
structures typically include features that accom-
372
P. B . S A K E T A L .
Table 2. Locations of Field Sites and Orientation of the Maximum Shortening Directions
MSDa
Site
Latitude Longitude
(⬚N)
(⬚W)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
41.2816
41.227
41.2158
41.1829
41.1304
41.0899
41.0445
40.9663
40.8836
40.8759
40.8674
40.8738
40.7665
40.7256
40.6748
40.6241
40.6066
40.6
40.4893
40.4715
40.4674
40.459
77.0649
77.325
76.9833
77.3369
77.4047
76.8807
76.8506
76.6462
76.8903
76.6685
76.5858
77.2367
77.0697
77.0239
76.8343
77.2483
77.2377
77.4333
76.9543
76.9494
77.1186
77.0252
Formation
Lock Haven
Lock Haven
Bald Eagle
Lock Haven
Mifflintown
Tuscarora
Bloomsburg
Bloomsburg/Mifflintown
Keyser/Tonoloway
Mahantango
Mahantango
Bloomsburg
Keyser/Tonoloway
Keyser/Tonoloway
Keyser/Tonoloway
Keyser/Tonoloway
Mahantango
Tuscarora
Pocono
Irish Valley Mbr, Catskill Fm
Irish Valley Mbr, Catskill Fm
Trimmers Rock
Early stage Late stage
338⬚
326⬚
332⬚
338⬚
329⬚
358⬚
348⬚
020⬚
353⬚
343⬚
341⬚
329⬚
342⬚
322⬚
340⬚
325⬚
327⬚
340⬚
298⬚
335⬚
340⬚
326⬚
355⬚
323⬚
338⬚
354⬚
002⬚
348⬚
340⬚
348⬚
348⬚
342⬚
351⬚
348⬚
348⬚
340⬚
325⬚
343⬚
345⬚
338⬚
Source
Spiker and Gray 1997
Lunde 2012
Miller 1984
This study
Miller 1984
Nickelsen and Cotter 1983
This study
Nickelsen and Cotter 1983
Miller 1984
Miller 1984
Nickelsen and Cotter 1983
This study
Johnson 2000
Miller 1984
Bajak 1981
Miller 1984
Miller 1984
Herbert 2009
Miller 1984
Wills and Sak 2010
Miller 1984
This study
a
The maximum shortening direction (MSD) for the early and late phases of the Alleghanian orogeny were determined
from field relationships. Absence of an MSD indicates insufficient data to support an interpretation. Mbr p Member;
Fm p Formation.
plished layer-parallel shortening during the earliest
stages of the orogeny such as joint sets, cleavage,
wedge faults, and conjugate strike slip fault sets.
Late-stage structures include folds and structures
associated with folding, such as flexural-slip slickenlines, hinge extension fractures, fold-transecting
cleavage, conjugate faults, and parasitic folds (fig.
4).
Structural Data
The rocks exposed in the Valley and Ridge Province
range in age from the Ordovician to the Pennsylvanian and commonly form alternating layers of
competent (e.g., quartz sandstone and quartz pebble
conglomerate) and incompetent (e.g., shale and micritic limestone) beds. In this investigation, we focus on the folds and the structures found within
the folds, throughout the 22 field sites (fig. 3). These
structures are classified as either early or late stage
and are described as either being primarily found
in the competent or incompetent units (fig. 4; table
1). The sequence of the structures is determined
from cross-cutting relationships.
Early Stage. The initial, prefolding, stages of deformation were accommodated by layer-parallel
shortening structures (figs. 4, 5). In the competent
units, this is primarily accomplished by conjugate
strike-slip (“wrench”) and dip-slip (“wedge”) faults.
Joint sets perpendicular to the strike of bedding also
formed (figs. 4, 5). The orientations of these joints
suggest a similar shortening required to form the
conjugate fault sets. Another set of joints, parallel
to the strike of bedding, is also recognized in many
of the competent layers. Both of these orthogonal
joint sets are in orientations consistent with the J1
and J2 joints sets of Engelder et al. (2009), so both
may have formed during the very early stages of
folding.
In the incompetent layers, bed-perpendicular
cleavage is most pronounced (figs. 4, 5). The cleavage orientation is consistent with the shortening
directions required to form the structures preserved
in the competent layers. This set is preserved as
fanned cleavage, suggesting that it is passively rotated as the layers folded.
Late Stage. As the beds began to fold, younger
conjugate fault sets formed in the competent layers
(figs. 4, 5). The acute bisector of these younger conjugate sets is oriented subperpendicular to the
hinge surfaces of the first-order folds, with a subhorizontal bisector plane. Slickenlines plunge
down the dip of these conjugate faults. Extensional
fractures are preserved within fold hinge zones, accommodating tangential longitudinal strain (figs. 4,
5). Slickenfibers are commonly found on bedding
Journal of Geology
D E F O R M AT I O N W I T H I N T H E P E N N S Y LVA N I A S A L I E N T
373
Figure 4. Representative structures used to determine maximum shortening directions in the Juniata culmination.
a, Early-stage structures form before folding, in flat-laying strata; b, late-stage structures form during or after folding
of strata. LPS p layer-parallel shortening.
surfaces, with lineations perpendicular to the hinge
and hingeward directed shear sense, suggesting bedparallel flexural slip assisted folding. En-echelon
fractures and sigmoidal veins are sometimes found
in the competent layers in orientations consistent
with bed-parallel, flexural slip. In the incompetent
layers, higher-order parasitic folds and a second (or,
third, in the Anthracite region) generation of cleavage surfaces further indicate bed-parallel flexural
slip (figs. 4, 5). With continued folding, low-angle
faults formed in the steep limbs. These faults propagated through the competent and incompetent
layers (figs. 4, 5).
Time-Independent Structures. Subvertical strike-
slip faults that strike approximately perpendicular
to the first-order fold hinges formed throughout the
folding history, and cut through the competent and
incompetent layers (figs. 4, 5). These strike-slip
faults often form conjugate sets. Folded slickenlines indicate that the faults formed prefolding,
while other strike-slip faults overprint all the other
structures, indicating that they are among the
youngest structures formed. The strike of these
faults supports a shortening direction comparable
to the shortening direction that formed the other
structures throughout the study sites.
Data Summary. Although the structures preserved at the 22 sites represent different stages in
374
Figure 5. Representative field examples of structures from early and late stages of deformation.
375
376
P. B . S A K E T A L .
the deformation history of the Pennsylvania salient, they all suggest consistent shortening oriented 320⬚–340⬚, that is, subparallel to bisector of
the Eocambrian cratonic corner (table 2). The stages
temporally overlap, which is consistent with Nickelsen’s (1979) observations from the Valley and
Ridge Province. The clockwise and counterclockwise maximum shortening directions (MSDs)
within the NE and SW segments, respectively, suggest that the deformation was continuous during
rotation of the MSDs, which is consistent with the
single-phase model (Gray and Mitra 1993). Both of
these observations are critical in distinguishing this
model from others that call for a pause in deformation between phases.
Discussion
Early- and late-stage MSDs are consistently oriented to the northwest across the Juniata culmination (fig. 3; table 2). The mean orientations of the
early-stage and late-stage MSD are 336⬚ Ⳳ 16.3⬚ (1j)
and 343⬚ Ⳳ 8.5⬚ (1j), respectively (fig. 3; table 2).
There are no systematic variations in MSD orientation as a function of position within the culmination. These findings differ from those reported
by Wise and Werner (2004) to the southeast in the
Piedmont province.
Discrepancies in the orientation of MSDs between the Piedmont and rocks exposed in the
Valley and Ridge and southernmost Appalachian
Plateau likely reflect the fact that the two regions
expose rocks of different age, and consequently, record different deformational histories. Rocks in the
Piedmont are older, ranging from Precambrian to
Ordovician, whereas rocks exposed in the Valley
and Ridge and southern Appalachian Plateau range
in age from the Ordovician to Pennsylvanian. The
older rocks of the Piedmont may have been deformed during the Taconic orogeny in the middle
to late Ordovician as well as the during the Alleghany orogeny in the early Permian. This may
complicate the Piedmont data, which are the basis
for the two-phase model. However, the younger depositional age of the selected field sites in the
Valley and Ridge and Appalachian Plateau strata
preclude Taconic deformation. The deformation
histories in these post-Taconic orogeny strata only
record Alleghanian deformation, eliminating the
potential complications produced by overprinting
orogenies.
The variations in MSDs between the Piedmont
and the Valley and Ridge and Appalachian Plateau
provinces may also reflect the setting of the basement thrust sheets within the Piedmont. The Read-
ing Prong and Blue Ridge thrust sheets define a
basement corner that is positioned south of the Juniata culmination in the Valley and Ridge and Appalachian Plateau provinces. The corner is projected to lie along a distinctive bend within the Blue
Ridge. The axis of this bend trends parallel to the
axis of the Juniata culmination. This basement corner likely served as a structural barrier within the
Piedmont, thus establishing boundary conditions
within the Piedmont that are distinctive from those
in the Valley and Ridge and Appalachian Plateau
Provinces.
It is important to note that while the data collected in the Valley and Ridge and Appalachian Plateau are consistent with the predictions of the onephase model, the Piedmont may reflect a different
history given the age of the rocks, their closer proximity to the core of the orogenic belt, and the location and orientation of the basement uplifts.
Therefore, the kinematic histories are not expected
to be similar across the entire salient, and so both
models may be valid, but for different provinces.
Distinction between the single- and two-phase
models is useful in classifying the Pennsylvania salient along the gradient of orogenic curves from
primary arcs, which form in a curved shape from
the onset of deformation, to secondary arcs, which
are originally linear mountain belts that have later
been bent by another deformation event. Because
the Pennsylvania outer arc of the salient appears
to have been produced by a single tectonic event,
it is improbable that the salient is a secondary arc.
However, rotation of MSDs with time implies that
primary inheritance of curvature is also unrealistic.
Most likely, the Pennsylvania salient is an intermediate between these primary and secondary end
members and can be classified as a progressive arc,
where curvature is acquired continuously throughout the evolution of the Alleghany orogeny (Marshak 2004).
The progressive arc model for the Pennsylvania
salient was originally developed by Gray and Stamatakos (1997) and is sometimes referred to as a
three-dimensional spreading wedge model (fig. 2a).
In this model, shortening and gravitational spreading occur simultaneously under a single transport
direction. It is a single-phase model that simply and
elegantly integrates the structural and paleomagnetic data preserved throughout the Valley and
Ridge Province of the Pennsylvania salient.
However, proponents of the two-phase model
suggest that a progressive arc requires tangential
extension around the arc (cf. Wise 2004). The Pennsylvania salient has a notable lack of tangential extension features, and this is used to argue that a
Journal of Geology
D E F O R M AT I O N W I T H I N T H E P E N N S Y LVA N I A S A L I E N T
one-phase model is not possible. Nickelsen (1979)
noted a few small late-stage cross-grabens on the
“whaleback” anticline of the Bear Valley mine, and
Faill (1981) reported a series of minor conjugate
strike-slip faults just west of the Alleghany front,
but these account for only a small fraction of the
tangential stretching that some suggest would be
required for progressive bending of the Pennsylvania salient (cf. Wise 2004). But, a progressive arc
does not require tangential extension—it would be
necessary only if shortening and gravitational
spreading did not occur simultaneously.
Although a progressive arc does not require extension around the arc, it does not preclude that
extension can take place and may help assist formation of the salient. Difficult-to-observe microscale processes, such as grain-boundary sliding and
microscopic finite bulk strain (Sak et al. 2012), may
have diffused extensional strain (Gray and Stamatakos 1997), and strike-slip faults common
throughout the Pennsylvania salient (Nickelsen
2009) may assist curvature of the salient (Gray and
Stamatakos 1997).
The consistent orientation of the MSD throughout the Alleghanian orogeny progressive deformation sequence in the Pennsylvania salient’s hinge
zone has implications for the construction of balanced geologic cross sections through the orogen.
Couzens et al. (1993) argued that it is not possible
to construct balanced geologic cross sections
through the southern part of the central Appalachians because of the overprinting of at least two
episodes on noncoaxial deformation. Because the
Juniata culmination and overall hinge zone appear
377
to have been subjected to a single MSD, this is a
suitable location through which to draw a balanced
cross section oriented parallel to the direction of
maximum shortening.
Conclusions
Our analyses of progressive Alleghanian deformation in within the hinge zone of the Pennsylvanian
salient permit us to draw four important conclusions: (1) All 22 field sites exhibit structures that
have elements of the Bear Valley sequence (Nickelsen 1979) of progressive Alleghanian deformation. (2) In general, structures spanning the Alleghanian orogeny exhibit an MSD of ∼340⬚ in, and
adjacent to, the Juniata culmination, Pennsylvania
salient. (3) The Gray and Stamatakos (1997) singlephase model agrees best with the data in the Pennsylvania Valley and Ridge. (4) It is possible that the
MSD data from the Great Valley and Piedmont differ from those in the Valley and Ridge because some
of the deformation in the internal portions of the
salient likely predates the Alleghanian.
ACKNOWLEDGMENTS
This work was partially supported by USGS
EDMAP grant 08HQAG088 awarded to P. B. Sak,
with additional support from the Dickinson College Research and Development Committee. M.
Wills, J. Herbert, E. Spiker, S. Lunde, and the Dickinson College 2010 Field Methods course assisted
in the field. We benefited from reviews by G. Mitra,
D. Wise, and D. Rowley.
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