Quantifying Shortening Across the Central Appalachian Fold- Thrust Belt, Virginia and West Virginia, USA: Reconciling Grain-, Outcrop-, and Map-Scale Shortening

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Recommended Citation Lammie, Daniel, Nadine McQuarrie, and Peter B. Sak. "Quantifying Shortening Across the Central Appalachian Fold-Thrust Belt, Virginia and West Virginia, USA: Reconciling Grain-, Outcrop-, and Map-Scale Shortening." Geosphere 16, no. 5 (2020): 1276–1292. https://pubs.geoscienceworld.org/gsa/geosphere/ article/16/5/1276/589664/Quantifying-shortening-across-the-central

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GEOSPHERE Quantifying shortening across the central Appalachian fold-thrust belt, Virginia and West Virginia, USA: Reconciling grain-, outcrop-,

GEOSPHERE, v. 16, no. 5 and map-scale shortening https://doi.org/10.1130/GES02016.1 Daniel Lammie1, Nadine McQuarrie1, and Peter B. Sak2,3 1Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA 6 figures; 2 plates; 2 tables; 2Department of Earth Sciences, Dickinson College, Carlisle, Pennsylvania 17013, USA 1 set of supplemental files 3Earth and Environmental Systems Institute, Pennsylvania State University, State College, Pennsylvania 16802, USA

CORRESPONDENCE: [email protected] ABSTRACT Measured magnitudes of LPS are highly variable, (Canada) to Alabama (USA), are among the most CITATION: Lammie, D., McQuarrie, N., and Sak, P.B., as high as 17% in the Valley and Ridge and 23% on recognizable and well-studied orogenic belts. 2020, Quantifying shortening across the central Ap- We present a kinematic model for the evolution the Appalachian Plateau. In the Valley and Ridge However, the deformation mechanisms and fault palachian fold-thrust belt, Virginia and West Virginia, of the central Appalachian fold-thrust belt (eastern province, the structures that accommodate short- kinematics that accommodated the shortening in USA: Reconciling grain-, outcrop-, and map-scale shortening: Geosphere, v. 16, no. 5, p. 1276–1292, https:// United States) along a transect through the west- ening vary through the stratigraphic package. In the this iconic range remain unresolved after more doi.org/10.1130/GES02016.1. ern flank of the Pennsylvania salient. New map and lower Paleozoic carbonate sequences, shortening than 150 years of investigations (e.g., Rogers and strain data are used to construct a balanced geologic is accommodated by fault repetition (duplexing) Rogers, 1843; Dana, 1866; Rodgers, 1949; Herman, Science Editor: David E. Fastovsky cross section spanning 274 km from the western of stratigraphic layers. In the interval between the 1984; Faill, 1998, Evans 2010). The central Appala- Associate Editor: Michael L. Williams Great Valley of Virginia northwest across the Burn- duplex (which repeats Cambrian through Upper chian fold-thrust belt has been described as a blind ing Spring anticline to the undeformed foreland of Ordovician strata) and Middle Devonian and younger fold-thrust belt with few emergent faults (Gwinn, Received 30 May 2018 Revision received 15 May 2020 the Appalachian Plateau of West Virginia. Forty (40) (Permian) strata that shortened through folding and 1964; Herman, 1984; Spraggins and Dunne, 2002). Accepted 1 July 2020 oriented samples and measurements of >300 joint LPS, there is a zone that is both folded and faulted. Shortening is accommodated through a series orientations were collected from the Appalachian Across the Appalachian Plateau, slip is transferred of duplexes that repeat a lower Paleozoic stiff Published online 10 August 2020 Plateau and Valley and Ridge province for grain-scale from the Valley and Ridge passive-roof duplex to sequence and an overlying cover sequence that dis- bulk finite strain analysis and paleo-stress recon- the Appalachian Plateau along the Wills Mountain plays the classic map-scale folds of the Appalachian struction, respectively. The central Appalachian thrust. This shortening is accommodated through Valley and Ridge province (Dunne and Ferrill, 1988). fold-thrust belt is characterized by a passive-roof faulting of Upper Ordovician to Lower Devonian Within the cover sequence, deformation manifests duplex, and as such, the total shortening accom- strata and LPS and folding within the overlying Mid- as folding and layer-parallel shortening (LPS). Axial modated by the sequence above the roof thrust dle Devonian through Permian rocks. The significant traces of map-scale folds trend at a high angle to must equal the shortening accommodated within difference between LPS strain (10%–12%) and cross the maximum shortening direction, and measured duplexes. Earlier attempts at balancing geologic section shortening estimates (18% shortening) high- LPS is oriented at small angles to the maximum cross sections through the central Appalachians lights that shortening from major subsurface faults shortening direction (Rutter, 1976; Herman, 1984; have relied upon unquantified layer-parallel shorten- within the central Appalachians of West Virginia Sak et al., 2012, 2014). ing (LPS) to reconcile the discrepancy in restored line is not easily linked to shortening in surface folds. Over the past several decades, studies have lengths of the imbricated carbonate sequence and Depending on length scale over which the variability sought to evaluate the structure of the Appalachians mainly folded cover strata. Independent measure- in LPS can be applied, LPS can accommodate 50% to using balanced cross sections (e.g., Gwinn, 1970; ment of grain-scale bulk finite strain on 40 oriented 90% of the observed shortening; other mechanisms, Herman, 1984; Dunne, 1996, Evans, 2010; Sak et al., samples obtained along the transect yield a tran- such as outcrop-scale shortening, are required to 2012; Ace et al., 2020). When drafting a kinematically sect-wide average of 10% LPS with province-wide balance the proposed model. viable and balanced cross section, shortening must mean values of 12% and 9% LPS for the Appalachian be conserved across the system, with unfaulted Plateau and Valley and Ridge, respectively. These units accommodating strain through folding and values are used to evaluate a balanced cross sec- ■■ INTRODUCTION LPS (Elliott, 1983; Geiser, 1988a; Woodward et al., tion, which shows a total shortening of 56 km (18%). 1989). Early attempts to construct sections through This paper is published under the terms of the The Appalachian Mountains, extending along the the central Appalachians highlighted a significant CC‑BY-NC license. Peter B. Sak https://orcid.org/0000-0003-2234-8721 eastern side of North America from Newfoundland discrepancy between the shortening described by

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faulted lower Paleozoic rocks and the overlying Virginia (USA) (Evans, 1989, 2010) (Fig. 1). Although both the distribution and magnitude of bulk grain- folded upper Paleozoic rocks (Herman, 1984; Hatcher the restored line lengths of the cover and carbon- scale strain is imperative in order to determine 1989). These and other studies (e.g., Nickelsen, 1988; ate sequences were equal in these studies, the LPS the magnitude of shortening and the geometry of Mount et al., 2017; Ace et al., 2020) argued for a values were not measured. Instead, they reflect the subsurface structures of the central Appalachians passive-roof duplex solution, where the structural magnitude required to balance the differences in (Sak et al., 2012). More recently, seismic surveys response to shortening varies with stratigraphic line lengths of the restored cross sections. at the Appalachian structural front have further position. Within the Cambrian–Ordovician carbon- In contrast, Sak et al. (2012) independently constrained the kinematics of how shortening is ate sequence, shortening is accommodated through measured LPS and used the measured values to transferred from the Valley and Ridge to the Appa- fault repetition (Herman, 1984; Evans, 2010; Sak et al., reconcile shortening magnitudes in the faulted lachian Plateau (Mount et al., 2017; Ace et al., 2020). 2012; Ace et al., 2020), and in the overlying cover and cover sequences. They showed that along the The goal of this study is to quantify grain-scale sequence, shortening is accommodated via map- Susquehanna River valley, folding in Silurian and LPS along the length of an orogen-scale transect scale folds and LPS. Proposed magnitudes of LPS younger rocks combined with 20% LPS in the Valley through the central Appalachians in Virginia and required to reconcile discrepancies in the restored and Ridge and 13% LPS in the Appalachian Plateau West Virginia and integrate these measurements line length of the cover and carbonate sequences accommodated the same amount of shortening as into a balanced geologic cross section that shows range from 28% in Pennsylvania (USA) (Herman, the duplex in Cambrian through Ordovician strata. permissible geometries and shortening distribu- 1984; Hatcher, 1989) to as much as 50%–60% in West In addition, they demonstrated that quantifying tions of folds and faults. Integration of grain-scale

Figure 1. Shaded-relief map of the cen- New York tral Appalachian Mountains, eastern United States. Black boxes delineate Pennsylvania C Ohio the study area, and the line of our cross section is shown as A-A′. Locations of West previous cross sections (B-B′ and C-C′) Virginia Virginia 42’ N are from Herman (1984) and Sak et al. (2012), respectively. The base map is a digital elevation shaded relief hillshade product from the SRTM Global Digital Appalachian Surface Model dataset with a 3 arc sec- Rs ond (i.e., 90 m) resolution. This dataset 1.30 Plateau B is publicly available at the OpenTopog- 1.25 raphy website (www.opentopography 1.20 .org). Pin symbols denote pin line 1.15 locations for section A-A’ and C-C’, re- Valley and 1.10 C´ spectively. Locations and magnitudes 1.05 Ridge B´ of bulk finite strain measurements re- 1.00 ported as ellipticity ratios (Rs) are shown AFA Great WCS as points: circles—grain-scale measure- 39°N A Valley ments from Sak et al. (2012) and this study; diamonds—locations of distorted crinoid ossicles used to measure bulk fi- BSA nite strain (Engelder and Engelder, 1977; A´ Sak et al., 2012); squares—bulk finite EVA strain constrained using calcite C-axes (Engelder, 1979; Evans and Dunne, 1991). WMA Dashed black line—Alleghanian structural front; black boxes—area of Figure 3; white box—area of Figure 6. BSA— Burning Springs anticline; AFA—Arches Fork anticline; EVA—Elkins Valley anticline; WMA—Wills Mountain anticline; WCS—Whip Cove syncline. 82°W 78°W

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strain into fold-thrust belt reconstructions eluci- Monongahela dates whether grain-scale shortening is a significant component of the overall shortening magnitude siltstone, sandstone, Conemaugh (e.g., Mitra, 1994; Yonkee and Weil, 2010; Sak et al., shale, 2012) or negligible (Eichelberger and McQuarrie, coal Allegheny

2015). Inclusion of grain-scale strain can alter the Pennsylvanian Pottsville kinematic path proposed in balanced cross sections (e.g., Mitra, 1994; Sak et al., 2012), and the magni- shale, Mauch Chunk sandstone, tude and orientation of grain-scale strain has been

Miss. limestone Greenbrier shown to systematically vary across curved orogens sandstone, Pocono (Yonkee and Weil, 2010). Marked differences in how conglomerate

and why grain-scale strain varies in fold-thrust belt Hampshire systems (e.g., Eichelberger and McQuarrie, 2015) sandstone, highlight the importance of spatially quantifying shale, siltstone, conglomerate and integrating estimates of grain-scale strain into Chemung orogen-scale geologic cross sections. siltstone, shale, mudstone, Brallier

Devonian sandstone, ■■ GEOLOGIC SETTING limestone Harrell Cover sequence Cover Mahantango/ Roof thrust for Marcellus the proposed duplex Deformation that accompanied the Allegh- limestone, shale Onondaga/Old Port any orogeny affected Cambrian to Permian strata Keyser/Tonoloway shale Wills Creek Detachment transferring slip throughout the central Appalachian region. Faulting Williamsport sandstone McKenzie to the Appalachain Plateau initiated along a basal décollement in shales at the limestone, shale Clinton Group base of the Cambrian Waynesboro Formation (Fig. 2) Silurian sandstone, shale Tuscarora (Kulander and Dean, 1986; Mitra, 1986), leading to conglomerate Juniata shortening preserved at the surface as predomi- Oswego nantly folded Silurian through Permian strata (Gray and Mitra, 1991, 1993). North and south of the study siltstone, shale, Reedsville/ area, regional-scale fold axes trend ~070° (Fig. 1) n limestone Martinsburg Roof thrust for i a c

i limestone Trenton (Whitaker and Bartholomew, 1999; Sak et al., 2012). Black River the duplex dolomite St. Paul Group

However, through the central Appalachians of West d o v r

Virginia, fold axes trend 030°–035° (Fig. 1). O Pinesburg Station Deformation styles vary between the Valley limestone and Ridge and Appalachian Plateau physiographic dolomite Beekmantown provinces, although both provinces are composed shale of unmetamorphosed foreland basin sequences limestone resting on passive-margin sedimentary rocks sandstone

and crystalline basement. In West Virginia, the n dolomite Stiﬀ sequence a i

boundary between the Valley and Ridge and the r limestone Conococheague Appalachian Plateau (the Alleghanian structural 1 km limestone Elbrook front) is delineated by the Wills Mountain anticline C a m b sandstone, shale (Fig. 1) (Perry, 1978; Mitra, 1987). To the east, in the limestone Waynesboro Main décollement Valley and Ridge, map-scale folds have 5–10 km Figure 2. Generalized stratigraphic column for the Valley and Ridge and Appalachian Plateau physiographic provinces of wavelength and 70–140 km axial traces (Cardwell the central Appalachian Mountains in West Virginia and Virginia (compiled from Orndorff et al., 1993; McDowell, 1995; et al., 1968). In contrast, the Appalachian Plateau is Dean et al., 2000, 2001; Dean and Kulander 2006a, 2006b, 2011). Décollement horizons marked by red arrows (modified characterized by broad 10–20-km-wavelength folds after Ryder et al., 2008; Cardwell et al., 1968; Ace et al., 2020) define the structural tiers. Miss.—Mississippian.

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with 20–100 km axial traces. The abrupt change in data, seismic interpretations, and geologic mapping. 40 oriented samples of the cover strata. The sam- structures exposed at the surface is attributed to These data are supplemented by detailed geologic ples came from outcrops distributed across the a series of horses of Cambro-Ordovician strata mapping in the immediate vicinity of the transect transect, with a single sample from the Great Val- (Kulander and Ryder, 2005; Ryder et al., 2008; Evans, and bulk finite strain estimates of grain-scale strain ley, 22 samples from the Valley and Ridge, and the 2010), defining the amplitude and wavelength of on samples spanning from the Great Valley of Vir- remaining 17 samples from the Appalachian Plateau the regional-scale folds within the Valley and Ridge ginia to the Appalachian Plateau in West Virginia. (Fig. 1; Table 1). Three mutually perpendicular thin province (Dunne and Ferrill, 1988; Herman, 1984; sections were cut: parallel to strike of bedding (A Evans, 2010; Sak et al., 2012). These faults form a plane), normal to strike of bedding (B plane), and passive-roof duplex system beneath the Reedsville Geologic Mapping in the plane of bedding (C plane) (Figs. 4A and 4B). Formation. Although the Wills Mountain anticline For each plane, three photomicrographs were taken is the western edge of this duplex system, faults Bedrock geology maps of West Virginia and Vir- with cross-polarized light: horizontal, −30°, and +30°. through the Cambro-Ordovician strata are also ginia were obtained through the U.S. Geological The three micrographs were merged into one image present under the Elkins Valley anticline in the Appa- Survey National Map Database, the West Virginia to highlight (primarily quartz) grain boundaries lachian Plateau (Ryder et al., 2008) (Fig. 3; Plate 1). Geological and Economic Survey, and the Virginia (Fig. 4C). Each compiled image was analyzed using The stratigraphy consists of four structural tiers: Department of Mines, Minerals and Energy, with the normalized Fry method (Erslev, 1988). The strain (1) a sedimentary and crystalline Neoproterozoic scales ranging from 1:500,000 to 1:24,000 (Cardwell analyses were conducted using a normalized Fry basement, (2) faulted Cambrian to Lower Ordovician et al., 1968; Orndorff et al., 1993; Rader and Evans, method script for Matlab (Eichelberger and McQuar- carbonates, (3) Middle Ordovician to Silurian faulted 1993; McDowell, 1995; Dean et al., 2000, 2001; Dean rie, 2015). Using this script, grain center locations shales and sandstones, and (4) an upper Silurian and Kulander 2006a, 2006b, 2011). These were then were marked along with the long and short axes for to Pennsylvanian folded siliciclastic cover sequence compiled within ArcGIS software and georeferenced ~150–250 individual grains for each sample (Fig. 4C). (Fig. 2). While basement is separated from the fold- using digital elevation models (DEMs) published by The normalized plotted grain centers define a ring thrust belt along a décollement found within the the West Virginia GIS Data Clearinghouse and the of high-density points surrounding a vacancy field Waynesboro Formation (Kulander and Dean, 1986; Radford University (Radford, Virginia) GIS center. representing the shape and orientation of the strain Mitra, 1986; Ryder et al., 2008), décollements are Field work targeted areas of limited coverage. ellipse (Figs. 4D and 4E). Axial lengths were used also identified within the Ordovician Reedsville For- Through the course of our field work, we col- to normalize the distance between grain centers, mation, Silurian Salina Formation salt, and Middle lected >500 structural measurements, including which are plotted through multiple iterations to Devonian shales. The Ordovician Reedsville For- joint measurements and measurements of bedding reveal a central vacancy field defined by the min- mation (0.5 km thick) transitions and thickens to orientation. The ~300 joint measurements were col- imum distance between centers. The normalized the east into the Martinsburg Formation (1–4 km lected using the selection method (Engelder and Fry method assumes that each sample had an thick), is the major décollement horizon in the Valley Geiser, 1980; Hancock, 1985; van der Pluijm and anti-clustered, isotropic distribution before defor- and Ridge (Drake and Epstein, 1967), and remains a Marshak, 2004), where three to five joint orienta- mation, consequently the central vacancy field key detachment horizon in West Virginia east of the tions were collected from visually identified joint represents the bulk finite strain ellipse. The best-fit Elkins Valley anticline (Fig. 1). The Silurian Salina sets. For each station, coordinates were obtained ellipse is then generated through a bootstrapping Formation (Salina salt) and the Middle Devonian using GPS. Bedding orientation measurements at approach (Eichelberger and McQuarrie, 2015). Using shales become major décollement horizons on the these reference points provide the bulk of the struc- this two-dimensional (2-D) evaluation, the elliptic-

Appalachian Plateau, separating different packages tural measurements used to constrain the balanced ity (Rs), or the axial ratio of the long to short axes, of faulted and folded strata (Perry, 1978; Davis and cross section. Within the hinterland portions of the as well as the angle of inclination (φ), or the angle Engelder, 1985; Nickelsen, 1988; Ryder et al., 2008; transect, road cuts are less abundant; and field data between the long axis and a horizontal reference, Sak et al., 2012; Mount, 2014; Ace et al., 2020). were supplemented by digitizing strike and dip data can be calculated for each image (Table S1 in the 1 1 Supplemental Material. Supplemental Data Table S1 from published 1:24,000-scale quadrangles (Fig. 3) Supplemental Material ). contains axial ratios for the best-fit vacancy field (with (Orndorff et al., 1993; McDowell, 1995). The 2-D strain data calculated using the normal- statistics) of grain-scale bulk finite strain from the ■■ METHODS ized Fry method for the A, B, and C planes (Fig. 4F) three mutually perpendicular cuts. Supplemental Data Table S2 contains orientations of the best-fit of the sample was then input into the Mathematica strain ellipsoid (with statistics) and ellipticity ratios This study presents a new balanced cross sec- Strain Analysis program Best-Fit Ellipsoid with Statistics (Mooker- and orientations on the bedding plane. Please visit tion along an east-west–trending transect across the jee and Nickleach, 2011) to determine the best-fit https://doi.org/10.1130/GEOS.S.12598586 to access the supplemental material, and contact editing@ central Appalachians of Virginia and West Virginia. Spatial variations in grain-scale bulk finite strain ellipsoid using the least-squares approach. geosociety.org with any questions. The cross section is constrained by pre-existing well strain across the transect were constrained using Measured axial ratios and angular orientation data

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Explanation

Pd Dunkard Gp Dmt Mahantango Fm *m Monongahela Gp Dm Marcellus Fm *c Conemaugh Gp Doo Onondaga and Old Port Fms *a Allegheny Fm DSkt Keyser and Tonoloway Fms *pv Pottsville Gp Swc Wills Creek Fm Mmc Mauch Chunk Fm Sw Williamsport and McKenzie Fms Mg Greenbrier Gp Smc Clinton Gp Mp Pocono Gp St Tuscarora Fm Dhs Hampshire Fm Ojo Juniata and Oswego Fm Chemung Gp Dch Om Reedsville Fm and Martinsburg Fm Db Brallier Fm O Trenton Gp and Beekmantown Gp, undivided Dh Harrell Shale _ Conococheague-Elbrook Fms, undivided _wy Waynesboro Fm

Note: 81.3° W Vertical color bars reflect groupings depicted in the accompanying cross section. 39.6° N AFA

78.4° W A 39.3° N WCS

39.1° N 79.0° W 78.1° W 275220 55 35 60 10 34 16 39.1° N 45 18 70 80 81.5° W 70 14 45 BSA 20 35 2449 72 20 50 14 50 78 35 16 10 80 40 9 20 8 65 27 6 13 A´ EVA

38.7° N WMA 38.6° N 78.4° W 79.1° W

20 10 0 20 km

Figure 3. Geologic map along the line of section (black line; see Fig. 1 for location, and Plates 1 and 2 for the cross section) (map data compiled from Cardwell et al., 1968; Orndorff et al., 1993; Rader and Evans, 1993; McDowell, 1995; Dean et al., 2000, 2001; Dean and Kulander 2006a, 2006b, 2011; our own mapping). Fm—Formation; Gp—Group; AFA—Arches Fork anticline; BSA—Burning Springs anticline; EVA—Elkins Valley anticline; WMA—Wills Mountain anticline; WCS—Whip Cove syncline.

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Explanation

Pd Dunkard Gp Mahantango Fm Monongahela Gp Marcellus Fm Conemaugh Gp Onondaga and Old Port Fms Allegheny Fm Keyser and Tonoloway Fms Pottsville Gp Wills Creek Fm Mmc Mauch Chunk Fm Williamsport and McKenzie Fms Section Mg Greenbrier Gp Clinton Gp Mp Pocono Gp Tuscarora Fm o set Dhs Hampshire Fm A. A´ Juniata and Oswego Fm Dch Chemung Gp H Appalachian D Reedsville Fm and Martinsburg Fm A (East) Db Brallier Fm Trenton Gp and Beekmantown Gp, undivided (West) Appalachian Plateau front Valley and Ridge Dh Harrell Shale J Great Valley Conococheague-Elbrook Fms, undivided J * * * * F/G * * G * Waynesboro Fm * * * * * I * * * * * * Note: * * * Vertical color bars reflect groupings depicted in the cross section. 10 3 4 12 1 2 7 8 9 11 14 5 6 13 15 B. E B A C A Appalachian Plateau Appalachian Valley and Ridge A´ (West) 10% LPS Great Valley front 25 % LPS 35% LPS (East)

1 2 3 4 5 6 7 8 9 10 11 12 20 km 13 14 15 C. Appalachian A Great Valley (West) front Valley and Ridge A´ (East) 10% LPS 25 % LPS 1 2 35% LPS 3 4 5 6 7 8 9 10 11 12 20 km 13 14 15

A. Thickness of the sedimentary package above the basement and 1.2° slope of basement (bottom of Cambrian Waynesboro Formation) constrained by map and well data (Ryder et al., 2008). B. Duplexing of Cambrian–Ordovician carbonates beneath the Valley and Ridge are inferred to ll space between the basal décollement and the roof thrust in the Ordovician Reedsville Formation. Horse geometries determined through the fault-bend fold model (Suppe, 1983). C. Duplexes 13, 14, and 15 ll space beneath the easternmost syncline in the Valley and Ridge. Deformation of the fold-thrust belt is dominated by extensive pressure solution and >50% LPS in the Ordovician Martinsburg Formation (Wright and Platt, 1982). D. Bedding thickness variations within the Reedsville Formation are inferred only when necessary to ll space beneath second-order features. Evidence of similar bedding thickness variation can be found in seismic reection lines in the southern Appalachians of Alabama (Thomas, 2001, 2007). E. Detachment at the base of the Silurian Wills Creek Formation based upon eld observations (Nickelsen, 1986; Klawon, 1994). The Wills Creek detachment feeds displacement from horses 3 and 4 onto the detachment in the Silurian Salina Formation and the Ordovician Martinsburg Formation on the Appalachian Plateau (i.e., Prucha, 1968; Wiltchko and Chapple, 1977; Davis and Engelder, 1985). F. Elkins Valley anticline. G. Location and depth of thrust faults in fault-propagation anticlines in strata between Salina detachment and Martinsburg Formation are inferred to accommodate shortening in cores of both anticlines and synclines consistent with regional * seismic and well data (Ryder et al., 2008; Kulander and Ryder, 2005). Well locations indicated by * . H. Bed thickness variations on the Appalachian Plateau vary in the plastic Salina evaporite group (Prucha, 1968; Wiltchko and Chapple, 1977). I. Bed thickness variations in the predominately folded sequence are constrained by previously published local stratigraphic studies and regional-scale correlations J. Location of wells used to constrain depth to basement (Ryder et al., 2008).

Plate 1. (A and B) Balanced (A) and restored (B) cross section A-A′ extending from the eastern Great Valley westward to the Burning Spring anticline (Fig. 1). Total deformed length (274 km) and undeformed restored length (346 km) are measured from a pin line east of the extent of documented map-scale shortening on the Appalachian Plateau, resulting in To view Plate 1 at full size, please visit 78 km (23%) total shortening. (C) As shown, shortening in Upper Devonian through Permian rocks assumes 10% layer-parallel shortening (LPS) in the Appalachian Plateau and across https://doi.org/10.1130/GEOS.S.12598616. the Appalachian front (to thick vertical bar) and 25% LPS in the Valley and Ridge (region between thick vertical bars). Shortening in the Great Valley requires 35% LPS, compared to the >50% LPS measured in that region (Wright and Platt, 1982). Cross sections drawn with no vertical exaggeration; circled numbers—duplex numbers; Fm–Formation; Gp—Group. To view Plate 1 at full size, please visit https://doi.org/10.1130/GEOS.S.12598616.

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TABLE 1. MICROSCOPIC STRAIN SAMPLES LISTED BY LOCATION, AND ASSOCIATED ANALYSIS RESULTS

† § # †† Sample* Latitude Longitude Province Interval Fm Bedding orientation Orientation of Rs** φ (°N) (°W) (°) semi-minor axis (°) (°) 15-19 38.9739 78.3465 GV 16 Om 235, 85 22, 358 1.14 −42.16 15-15 39.0792 78.3811 V&R 16 Db 054, 80 20, 163 1.07 −25.60 15-13 39.0824 78.4263 V&R 16 Dmt 054, 15 10, 302 1.06 19.74 15-12 39.0984 78.4724 V&R 16 Ojo 046, 54 45, 340 1.04 −16.66 15-11 39.0854 78.5110 V&R 16 Ojo 045, 60 08, 218 1.09 −79.37 15-20 39.0919 78.5214 V&R 16 Dmt 063, 34 16, 114 1.13 −34.08 15-25 39.0644 78.6780 V&R 15 DSkt 050, 35 24, 349 1.05 −42.64 15-26 39.0429 78.7151 V&R 15 St 038, 14 04, 130 1.14 −1.97 15-27 39.0447 78.7186 V&R 15 St 231, 18 28, 015 1.09 −46.60 15-29 39.0493 78.7514 V&R 14 Dch 212, 78 37, 098 1.04 −20.55 15-30 39.0519 78.7680 V&R 14 Dhs 205, 55 11, 135 1.03 −28.16 15-32 39.0720 78.8028 V&R 14 Dch 035, 70 28, 156 1.05 −6.64 15-33 39.0720 78.8028 V&R 14 Dch 217, 27 40, 329 1.13 −22.60 15-34 39.0722 78.8031 V&R 14 Dch 216, 52 11, 180 1.05 −74.04 15-10 39.0939 78.8067 V&R 14 Dhs 012, 24 35, 135 1.08 −26.88 15-9 39.1004 78.8208 V&R 14 Dhs 055, 20 06, 174 1.17 −27.48 15-8 39.0989 78.8318 V&R 14 Dhs 065, 20 41, 331 1.05 −10.53 15-7 39.1021 78.8455 V&R 14 Dch 114, 07 02, 128 1.17 75.88 15-4 39.1025 78.8474 V&R 14 Dhs 105, 10 03, 146 1.16 49.13 15-5 39.1450 78.8468 V&R 14 Dhs 218, 52 05, 320 1.08 −32.43 15-2 39.0990 78.8692 V&R 14 Dch 315, 15 08, 200 1.07 24.86 13-24 39.0794 78.9086 V&R 14 Dhs 040, 45 34, 323 1.02 −27.83 13-20 38.9991 79.0953 V&R 14 Dm 215, 50 18, 329 1.14 −28.53 13-9 38.9856 79.2364 AP 13 St 040, 20 16, 307 1.09 8.11 13-11 38.9859 79.2602 AP 13 St 023, 72 08, 164 1.06 −62.70 13-19 38.8420 79.4077 AP 13 Mp 250, 10 20, 300 1.08 36.52 13-5 38.896 79.6494 AP 12 Dhs 205, 20 34, 147 1.10 −34.81 13-4 38.9068 79.6532 AP 11 Mg 200, 15 08, 332 1.14 −41.02 13-3 38.9075 79.7662 AP 11 Dhs 215, 09 24, 198 1.10 −77.27 13-2 38.9154 78.8038 AP 10 Dch 025,15 08, 173 1.17 −57.13 13-8 38.9345 79.8935 AP 10 Dch 200, 40 08, 338 1.20 −48.57 15-42 38.9408 79.9410 AP 9 pv 348, 14 38, 209 1.06 56.38 15-43 38.9626 80.0350 AP 8 c 041, 16 20, 150 1.11 −18.44 15-47 39.0129 80.3201 AP 7 m 269, 10 16, 311 1.10 48.87 15-46 38.9792 80.4482 AP 6 m 321, 06 33, 287 1.09 −73.03 15-48 39.0484 80.4929 AP 5 c 145, 05 51, 311 1.13 73.42 15-51 39.0351 80.6460 AP 4 c 178, 05 03, 299 1.09 −30.96 15-52 39.0573 80.8574 AP 3 m 328, 03 02, 172 1.23 66.08 15-53 39.0496 80.9950 AP 2 m 265, 04 18, 348 1.13 6.38 15-55 39.0882 81.4189 AP 1 Pd 150, 10 17, 303 1.13 29.22 *Samples are arranged as a function of position along the line of section from the hinterland to foreland. †GV—Great Valley; V&R—Valley and Ridge; AP—Appalachian Plateau. §Formation—see Figure 3 or Plate 1 for abbreviations. #Bedding orientations (strike and dip) are reported using right-hand rule notation.

**Rs—(ellipticity) axial ratio of long to short axial lengths of best-fit ellipse measured using normalized Fry method (Erslev, 1988). ††φ—angle of inclination of the long axis of the best-fit strain ellipse. Values are positive when the slope is positive and negative when it has a negative slope following the established sign conventions of Mookerjee and Nickleach (2011). Notes: Intervals—spatially contiguous grain-scale bulk finite strain data that are grouped together to optimize the amount of shortening accommodated by the measured LPS values (see text for further explanation). Semi-minor axis orientation is reported as plunge and trend.

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are subtracted from the axial ratio and angular ori- bedding plane entation of a general ellipsoid defined in terms of A thin section six unknown matrix elements. The differences are W Strike E squared and summed, with the sum minimized to A thin section photo C thin section calculate the best-fit ellipsoid (Figs. 4G and 4H). N N E Error for each ellipsoid is obtained through the sim- B thin section photo B thin section ulation of 1000 best-fit ellipsoids drawn randomly N Horizontal S Thin section in bedding plane within the standard deviation of 2-D input data C thin section (Rs, φ, and orientation) (Mookerjee and Nickleach, photo 2011; Mookerjee and Peek, 2014). The resulting three-dimensional (3-D) strain ellipsoid can then be used to calculate the final orientation of princi- E pal strain axes. Best-Fit Ellipsoid then rotates the calculated ellipsoids into the geographic reference 1 W A ellipse E 1 0.8 Horizontal frame and gives the trend and plunge of the three 0.6 C ellipse N Bedding plane principal axes (Fig. 4I; Table S2 [footnote 1]). 0.5 0.4 E 0.2 In 3-D space, because each sample has three 0 0 mutually perpendicular cuts, any given plane

Position -0.2 y Position B ellipse shares an axis with another, allowing the com- y N S -0.4 -0.5 Horizontal -0.6 parison of axial ratios along the shared axis. For -0.8 example, because the “C” plane is cut parallel -1 -1 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 to bedding, this plane quantifies the bulk finite -1 -0.5 0 .5 1 x Position strain between the along-strike and the dip direc- x Position tions. In the majority (32) of the 40 samples, the G A ellipse H longest axes of measured strain ellipsoids (and 3-D strain ellipsoid: East X therefore of the grains) parallel the strike of the X > Y > Z Rs(YZ) = r2/r3 bedding. The axial ratio in the bedding-parallel Rs in Z direction = 1.0 N North r3 Z section (C plane) records the magnitude of LPS W E due to volume loss in the B direction assuming r2 φ φ Z E W there are no overgrowths on grains and no filled R (XZ) = r /r Y Bedding plane fractures within grains that increase the length of s 1 3 N S φ C ellipse the A plane (parallel to strike of bedding). These Z r3 Z Up features were not observed in the 40 samples eval- N S E X uated for this study. We resolve the 3-D ellipsoid r 1 Y onto the C (bedding) plane to quantify the amount B ellipse of LPS (Table S2 [footnote 1]). The rest of this

Figure 4. Diagrams illustrating strain calculation techniques (see text). (A) Orientations of A, B, and C thin-section cuts paper discusses the axial ratio obtained from this relative to bedding planes. (B) Thin sections are oriented mutually perpendicular relative to the bedding plane. (C) Plot of projection onto the C plane as the ellipticity ratio

grain centers (red dots), long axes (yellow lines), and short axes (blue lines) on an oriented thin-section photomicrograph. (Rs), and assumes that the loss of material in the (D) Fry plot of the distribution of normalized grain centers produced using the normalized Fry method (Erslev, 1988). (E) shortening (B) direction indicates a percent short- Best-fit strain ellipse for five fitting iterations. In each successive iteration the elliptical fit (blue, red, orange, purple, and green) are constrained by fewer points. Green crosses denote points indicate those used to achieve the best-fit ellipse. ening that can be applied over the distance the Blue and red lines represent the semi-major and semi-minor axes, respectively of the corresponding elliptical fits. Axial sample is assumed to represent. This distance over units in (D and E) are arbitrary and used to determine ellipse aspect ratios. (F) Strain ellipses on A, B, and C thin sections which the strain measurement is valid is unknown, from normalized Fry analysis. (G) Rs(XZ) is ratio of longest and shortest axes (r1/r3), and Rs(YZ) is ratio of middle and as is the true variability in LPS. We attempted to shortest axes (r /r ). Angle between horizontal and the long axis of the ellipse is φ (down to north or south in X cuts; 2 3 quantify the length scale over which a strain mea- down to east or west in Y cuts). (H) Three-dimensional (3-D) strain ellipsoid resulting from combining two-dimensional surement is applicable by taking multiple samples X, Y, and Z strain ellipses; Rs in Z direction = 1.0. (I) 3-D oriented strain ellipsoid produced with the Mathematica program Best-Fit Ellipsoid with Statistics (Mookerjee and Nickleach, 2011; Mookerjee and Peek, 2014). Modified after Sak et al. (2012). from the same outcrop. Where outcrop-scale folds

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were present, we collected samples in the core of outcrop-scale shortening may be significant. For the ductile duplexes in accommodating deforma- and along the limbs of a fold. These samples indi- example, Hogan and Dunne (2001) demonstrated tion (e.g., Thomas, 2001, 2007). cated no change in principal strain orientation or that outcrop-scale shortening may account for as In the hinterland, the southeasternmost 23 km significant change in magnitude. These data are much as 10% shortening along local transects in of the cross section are in the Great Valley at the shown in the Valley and Ridge portion of Plate 2 as Valley and Ridge of West Virginia. Similarly, Sak front of the Blue Ridge Mountains. The exposed two or more strain measurements tied to the same et al. (2012) documented 13% outcrop-scale short- strata in the Great Valley have undergone exten- location. In these areas, the range in measured LPS ening along an 11.5 km section of the transect C-C′ sive pressure solution (Wright and Platt, 1982). We is 0%–3%. However as seen in Plate 2, LPS values (Fig. 1) in the north-central portion of the Valley collected one strain sample from the Great Valley can change by 9%–12% over a distance of 1–2 km. and Ridge of Pennsylvania. We assume that there (Fig. 1; Plate 2), however Wright and Platt (1982) is a potential for as much as 10% shortening by collected 14 samples from 10 localities through outcrop-scale structures east of the Elkins Valley the Martinsburg Formation in Pennsylvania, Mary- Cross-Section Methodology anticline and through the Valley and Ridge. While land, and West Virginia that show high variability spaced cleavage is a ubiquitous hand sample– to in strain (7%–56%), with nine out of 14 samples The 274-km-long transect for the geologic cross outcrop-scale structure within incompetent shale showing >28%. The three samples closest to our section was selected to bisect field data collected and siltstone lithologies throughout the central study area vary from 16% to 50% LPS. The exten- along an east-west–trending corridor through West Appalachians (Engelder and Geiser, 1979; Geiser sive and variable LPS complicates applying these Virginia and Virginia (Fig. 1). The transect extends and Engelder, 1983; Sak et al., 2014), it is not pres- LPS estimates to our cross section. Because of this, from the Great Valley of Virginia in the hinterland ent in competent sandstone to quartzite lithologies we focus on applying our measured strain data to to a pin line in the foreland northwest of the Burn- where equivalent shortening is accommodated 251 km of the cross section that extends through ing Springs anticline (Figs. 1 and 3) (Cardwell et al., through quartz dissolution along grain boundar- the Valley and Ridge and across the Appalachian 1968; Davis and Engelder, 1985). The line of section ies (LPS) and wedge faulting (Sak et al., 2012, 2014; Plateau; the northern 23 km of the 274 km cross is offset 30 km along the axis of the Whip Cove Eichelberger and McQuarrie, 2015; Ace et al., 2020). section shows a geometry of faulted Cambrian syncline (Figs. 1 and 3) to more closely align with As with previously published cross sections of through Ordovician strata in the Great Valley that field observations. the Valley and Ridge province in the central Appa- is balanced by 35% LPS, the mean value of the The basal décollement for the central Appala- lachians, a passive-roof duplex solution is invoked recorded strain through the region. chians is gently dipping (~1°), constrained from the to fill space beneath the first-order folds (Herman, As mentioned in the previous section, we use

depth to basement, in well log data, of 6200 m 1984; Sak et al., 2012; Ace et al., 2020). The passive- the ellipticity ratio (Rs) of the 3-D strain resolved in the Wills Mountain anticline and 6600 m at the roof duplex deformation model assumes that the onto the C plane (bedding plane) to determine Arches Fork anticline (Fig. 1) (Ryder et al., 2008). Cambrian to Ordovician carbonates are a stiff layer a percent shortening of the strata before folding All unit thicknesses are based upon stratigraphic that shortens through fracturing and faulting with- and faulting. The lack of overgrowths and frac- columns, well logs, and previously published geo- out additional grain-scale shortening (Herman, ture-filled veins in the long-axis (strike-parallel) logic cross sections of the region (Cardwell et al., 1984; Hatcher, 1989; Evans 2010), consistent with direction supports our assumption that the LPS was 1968; Patchen et al., 1985; Ryder et al., 2008; Evans, the low values of LPS (<5%) measured in these car- accommodated by grain dissolution, while consis- 2010). The topographic profile was extracted from bonates (Evans and Dunne, 1991; Sak et al., 2012). tent LPS orientations that do not fan after folds are a 30 m DEM using MOVE software (Midland Valley) The overlying Ordovician to Pennsylvanian cover restored indicate that dissolution preceded folding. with surficial geology based upon the composite strata accommodate shortening through folding Due to the high variability in measured LPS values map. All structural data were then transposed from and LPS. We also assume that the major décolle- (Table 1), the balanced cross section was divided the map to the cross section using apparent-dip ment horizons on the Appalachian Plateau (Salina into 16 intervals (Plate 2). Intervals were chosen to calculations. The cross section was drafted and Formation) and in the Valley and Ridge (Reedsville group spatially related LPS data together and opti- balanced by hand at a 1:100,000 scale, and then and Martinsburg Formations) have mobility to flow mize the amount of shortening accommodated by digitized (Plate 1). As a result, this cross section and deform ductilely. Significant thickness changes measured LPS values. For each interval, an average focuses upon the first-order features of the orogen, in the Salina Formation on the plateau are imaged LPS is calculated from the range in measured values, with insufficient resolution to capture outcrop-scale in industry seismic data (Mount, 2014; Ace et al., producing an estimate of the measured shorten- features, which for the purposes of this study are 2020). In general, locally thick zones of strata that ing accommodated through LPS in the overlying defined as 0.01–100-m-scale features in competent are ductilely thickened along décollement hori- cover sequence over the specified distance. This rocks including mesoscale folds and outcrop-scale zons have been imaged in seismic data through value was subtracted from the percent shortening faults, particularly wedge faults (Fig. 5). The effect the Appalachians, emphasizing the importance of described by the geometry of faulting in the stiff

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A. A A´ Section o set Appalachian Plateau Valley and Ridge Great Valley

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

14 6 9 13 9 4 6 7 20 8 7 17 9 14 5 17 10 14 8 5 8 13 13 10 14 2 16 5 3 4 13 23 9 13 9 10 11 6 17 5

9 10 12 14 3 4 7 8 11 15 1 2 5 6 13 Burning Springs Elkins Valley Wills Mountain Whip Cove anticline anticline anticline syncline

Assume 10% outcrop-scale Not included due to extensive shortening pressure dissolution (35–50% LPS) (12.5 km)

B. A Section A´ o set Appalachian Plateau Valley and Ridge Great Valley

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

6 9 7 14 9 7 8 13 4 6 20 8 17 9 14 5 17 10 10 14 2 8 5 13 4 13 9 10 6 14 16 5 5 3 13 23 9 13 11 17

9 10 12 14 3 4 7 8 11 15 1 2 5 6 13 Burning Springs Elkins Valley Wills Mountain Whip Cove anticline anticline anticline syncline

Horizontal Scale Assume 5% outcrop-scale Not included due to extensive Explanation shortening pressure dissolution (35–50% LPS) 10 20 km Pd Dunkard Gp Dmt Mahantango Fm (6.25 km) *m Monongahela Gp Dm Marcellus Fm 10 *c Conemaugh Gp Doo Onondaga and Old Port Fms *a Allegheny Fm DSkt Keyser and Tonoloway Fms *pv Pottsville Gp Swc Wills Creek Fm Mmc Mauch Chunk Fm Sw Williamsport and McKenzie Fms Vertical Scale Vertical 20 km Mg Greenbrier Gp Smc Clinton Gp Mp Pocono Gp St Tuscarora Fm Dhs Hampshire Fm Ojo Juniata and Oswego Fm Chemung Gp Dch Om Reedsville Fm and Martinsburg Fm Db Brallier Fm O Trenton Gp and Beekmantown Gp, undivided Dh Harrell Shale _ Conococheague-Elbrook Fms, undivided _wy Waynesboro Fm

Note: Vertical color bars reflect groupings depicted in the cross section.

Plate 2. Geologic cross section divided into 16 sequentially numbered intervals (circled numbers above the cross section) spanning from the western limb of the Burning Springs anticline eastward to the Great Valley. Locations of each of the 40 samples used to constrain grain-scale layer-parallel shortening (LPS) are shown as small white dots projected into the line of section; calculated LPS (as a percentage) are shown above each sample. Mean LPS values for each interval are summarized in Table 2. (A) Cross section constructed to To view Plate 2 at full size, please visit minimize the amount of unit thickness variation in the Reedsville-Martinsburg Formations. Balancing this section requires 10% outcrop-scale shortening between the Elkins Valley https://doi.org/10.1130/GEOS.S.12598616. anticline and the boundary between the Valley and Ridge and Great Valley. (B) Cross section constructed to minimize contributions from outcrop-scale shortening. Balancing this section requires 5% outcrop-scale shortening between the Elkins Valley anticline and the boundary between the Valley and Ridge and Great Valley. Cross sections drawn with no vertical exaggeration; circled numbers—duplex numbers; Fm—Formation; Gp—Group. To view Plate 2 at full size, please visit https://doi.org/10.1130/GEOS.S.12598616.

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Cambrian through Ordovician section to obtain the net value representing the difference between shortening amounts in the folded and LPS strata and the faulted strata. For simplicity, we assume that shortening due to duplexing is locally accommodated in the overlying strata within the cover sequence. If the difference between LPS and cross section shortening is positive, then the magnitude of LPS is greater than that needed to balance shortening by folding and faulting as shown on the cross section, and indicates that the net positive value (within that specific interval) can be used to accommodate shortening elsewhere in the section. In contrast, if the value is negative, it indicates that there is miss- ing shortening in the cover sequence that must be accommodated by either excess LPS from other intervals or other contributions to shortening such as outcrop-scale deformation. After this method is applied across all 16 intervals, a total net balance of measured and proposed LPS can be used assess whether the strain data set across this portion of the Appalachians can account for the magnitude of LPS needed to balance the proposed cross section.

■■ RESULTS

Joint Orientations Figure 5. Field photos illustrating outcrop-scale deformation (A and B). Examples of outcrop-scale folds with rounded and subangular hinge zones, respectively. Inset A is a view to the north of ~ 30 m high outcrop located at 39.10°N, 78.84°W and (B) is a view to the northeast of an outcrop located at 39.07°N, 78.80°W. (C) Photograph of wedge-faulting Raw joint orientations across the transect show within competent sandstone beds. View to the northeast of a ~1.5 high and the exposure located at 39.04°N, 78.72°W. a wide scatter with two dominant joint sets trending (D) A close-up of an anticlinal core of the fold shown in inset B highlighting how the mechanisms of accommodating 321° and 227°. A narrower subset of data (n = 132) shortening in the cores of outcrop-scale folds vary as a function of composition, with wedge faults common in sandstone that includes both joint orientation and bedding ori- beds and cleavage development in shales. entation at the same location allows for the data to be restored to an original orientation with respect to horizontal bedding (Fig. 6). Restored data show Strain Measurements directions, exhibit a predominant west-northwest a dominant joint orientation of 035° ± 5°, parallel mean trend of short axes (Fig. 6). The calculated 3-D

to fold axis orientations of 030°–035°, suggesting Calculated bulk finite strain values (Rs) for a total strain ellipsoids show a maximum shortening direc- outer-arc extension or post-tectonic unloading or of 40 oriented samples collected from siliciclastic tion axis with low-angle (<40°) plunge magnitudes release joints (Engelder, 1985). The next most-dom- cover-sequence rocks range from 1.02 to 1.23 (Fig. 1; and 25 of 40 samples with plunge magnitudes ≤20° inant joint set fans from 300°–350° with a dominant Table 1) with an average of the 10% LPS. The errors with respect to horizontal bedding (Table S2). Low orientation between 300°–330°. This joint set is in strain magnitude are ≤0.01 (Table S1 [footnote 1]), angles between the plunge of the short axis and interpreted as cross joints, which are broadly par- and the error in ellipsoid orientation is provided bedding strongly support LPS preceding folding. allel to the shortening direction and consistent with in Table S2. The samples display strain ellipsoids The 3-D ellipsoids show 68% of maximum shorten- measured joint orientations through this portion with scattered orientations. However, axial orien- ing directions falling between 250° and 330° (Fig. 6). of the Appalachian orogen (Nickelsen and Hough, tations in the bedding-parallel (C) plane, which The mean preferred shortening direction is 304° ± 5° 1967; Engelder, 1985, 2004). directly compare strain along the strike and dip (Fig. 6). The mean maximum shortening direction

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82°W fault system (Dahlstrom, 1969; Boyer and Elliott, 78°W Figure 6. Rose diagrams show- 1982; Geiser, 1988b; Woodward et al., 1989). In an ing measured orientations of admissible cross section, structural styles depicted the maximum shortening direc- in the cross section are the same as those observed tion of the strain ellipses (white in the field (Elliott, 1983). For fold-thrust belts char- 304° rose diagrams) from this study (304°) and results from Whita- acterized by blind thrusts and LPS, this requires that ker and Bartholomew (1999) as the fault displacement decreases toward the fault 39°N (308°). Black rose diagram shows tip in the direction of transport, equivalent amounts restored joint orientations of shortening are accommodated by folding or LPS. (n = 132). Extent of map is shown by the white box in Figure 1, the The balanced cross section presented here black boxes and blue lines define (Plate 1) was constructed using the sinuous bed extent of Figure 3 and the line method (Dahlstrom, 1969). In the Valley and Ridge of section, respectively. The base province of Pennsylvania and West Virginia, pre- map is a digital elevation shaded vious workers have documented that most of the 38°N relief hillshade product from the Ohio Pennsylvania SRTM Global Digital Surface folds are flexural-slip kink folds with planar limbs 308° West Model dataset with a 3 arc sec- and narrow hinges (Faill, 1969, 1973; Orndorff et al., Virginia ond (i.e., 90 m) resolution. This 1993; Sak et al., 2012, 2014). We observe both nar- dataset is publicly available at Virginia the OpenTopography website row anticlinal hinges adjacent to broad synclines as 0525 0 100 (www.opentopography.org). well as narrow synclinal hinges adjacent to broad- km topped anticlines (Plate 1). Concentric folding is maintained on the Appalachian Plateau portion of the cross section, consistent with field obser- is perpendicular to the general 030°–035° structural and higher across the Appalachian Plateau (12%). vations. The low-magnitude dips measured in the trend of map-scale structures and similar to the These values are significantly lower than the ~50% Appalachian Plateau do not create significant space findings of Whitaker and Bartholomew (1999) who LPS previously proposed as necessary to balance problems between the surface exposures and the documented 10% LPS parallel to a 308° preferred cross sections through the region (Evans, 1989, mobile salt accommodating the folding (Wiltschko shortening direction (Fig. 6) based upon deformed 2010). Average LPS measurements in the Appala- and Chapple, 1977; Mount, 2014). crinoid ossicles, ooids, and chert nodules preserved chian Plateau (12%) compare favorably with the The process of balancing the cross section on bedding surfaces in southern West Virginia. 13% LPS measured to the north and east in the begins in the foreland by restoring the folded and Grain-scale bulk finite strain is highly variable, Appalachian Plateau of Pennsylvania and New York faulted bed lengths to horizontal, including short- ranging from 2% to 23% (Table 1). Eleven (11) of the (Engelder and Engelder, 1977). Average LPS mea- ening in Cambrian through Silurian rocks between 40 samples produce LPS values between 10% and surements in the Valley and Ridge (9%) are notably the Elkins Valley anticline and the Appalachian 15%, and 17 out of 40 samples have LPS values of lower than the 20% reported by Sak et al. (2012) in structural front (Plate 1). For our proposed model, 5%–9%. Although the largest percentage of LPS Pennsylvania (Fig. 1). the total amount of shortening experienced by the values are similar to previous Appalachian strain cover strata of the Appalachian Plateau is 24 km studies, the measured variability is significantly (11%), with ~2 km (1%) accommodated through the different than previously found. Sample variability ■■ BALANCING THE SECTION gentle folding of the Upper Devonian rocks. This does not correspond with stratigraphic position suggests that the 22 km difference in shortening (Table 1), geographic location (Fig. 1), or position A balanced cross section must be both via- between the folded Upper Devonian strata and with respect to map-scale structures in the fold- ble and admissible. Embedded in viability is the Lower Devonian strata needs to be accommodated thrust belt (Fig. 1; Plate 2A). The high variability of assumption that little or no motion has occurred through 10% LPS. Applying the same approach LPS complicates incorporation of grain-scale bulk into or out of the plane of the section (Dahlstrom, through the Valley and Ridge province, the faulted finite strain into the geologic cross section. 1969; Elliott, 1983; Woodward et al., 1989). In addi- Cambro-Ordovician section accommodates 31 km The average amount of LPS determined from tion, for a section to be viable, the displacement (33%) of shortening, and folding in the Silurian

the axial ratio (Rs) of bedding-plane surfaces is 10% path of each structure must be known such that the through Devonian strata accounts for 8 km (4%) of along the entire transect. Average LPS values are structures can be restored to an unstrained state, shortening. This 23 km discrepancy would need slightly lower through the Valley and Ridge (9%) and that fault slip is conserved through the entire to be reconciled by 25% LPS, or smaller values of

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LPS combined with permissible outcrop- or small- At the Appalachian structural front, the struc- from 4 to 46 km for which an average LPS is cal- er-scale shortening features (Plate 1). tural relief increases to ~3 km, and well log data culated (Plate 2). These intervals were chosen to Across the Appalachian Plateau, the deformation from the core of the Wills Mountain anticline represent structural domains and to maximize within strata overlying the Cambrian–Ordovician indicate the presence of a fault-propagation fold the applied amount of LPS shortening. The aver- carbonate sequence is intrinsically linked to the originating from a basal décollement within the age of the measured LPS values defined for each transfer of 24 km of slip from faults in the Valley and Waynesboro Formation (Ryder et al., 2008). We pro- interval is then compared to the magnitude of Ridge and faults coring the Elkins Valley anticline pose that the Wills Mountain anticline represents LPS required to balance the shortening accom- into décollements found in the Ordovician Reeds- the western extent of the passive-roof duplex sys- modated by faulting and folding. Net positive LPS ville Formation, Silurian-aged Salina salt, and the tem where the Ordovician Reedsville Formation values indicate that the measured grain-scale LPS Mahantango Formation (Ace et al., 2020). Of this separates faulted Cambro-Ordovician rocks from is greater than the magnitude of LPS needed to slip, 16 km originates from horses 3–5 (Plate 1), the overlying folds of the Valley and Ridge province balance the shortening in the duplexed strata. Net directly east of the Wills Mountain anticline. The (Perry, 1978; Evans, 2010). East of the Wills Moun- positive values are calculated for seven of the 16 remaining 8 km is transferred onto the Appala- tain anticline (Fig. 1), first-order folds are inferred subdivisions (Plate 2, sections 7, 8, 11, 12, 14, 15, chian Plateau through horses 1 and 2 within the to be a result of Cambro-Ordovician horses in order and 16), while the remaining nine domains yield Elkins Valley anticline. The Appalachian Plateau is to fill space below the cover strata. The geometry net negative results. Without accounting for other dominated by broad, gentle folds with 10–20 km of these horses is inferred to follow a fault-bend- mechanisms of shortening, our model suggests wavelengths and 500 m of structural relief. West of fold geometry (Suppe, 1983). Above the horses that including our calculated LPS values, the pro- the Elkins Valley anticline, these folds do not have of the duplex system, smaller second-order folds posed cross section solution requires an additional sufficient amplitudes or structural relief to indicate with 5 km wavelengths and 100–300 m amplitudes 12.66 km of shortening that is 4% greater than the the presence of underlying Cambrian–Ordovician observed in the cover strata are a result of defor- measured LPS values (Table 2). However, if an horses. Instead, we infer that these folds result from mation-induced thickness variations within the average of 10% outcrop-scale shortening (Hogan detachments at the base of the Tully Limestone weak and deformable Reedsville Formation. While and Dunne, 2001) is applied to the portion of the within the Mahantango Formation and at the top of thickness variations in the Reedsville Formation are transect between the Elkins Valley anticline (inter- the Salina salt, combined with wedge-style faulting not directly observed in outcrops along the tran- val 10), where outcrop-scale shortening becomes of the Middle Devonian shale and carbonate rocks sect, variation in thickness is assumed only where significant, eastward to the boundary of the Great between these detachment horizons. Repetition of necessary to fill space. The Reedsville Formation Valley (interval 16) (Plate 2), the proposed model the Middle Devonian shale and carbonates along serves as the upper detachment for the underlying would have an deficit of only 0.7 km. Thus 10% individual faults with offsets <1 km is recognized passive-roof duplex, and thickness variations have outcrop-scale shortening combined with shorten- in seismic lines straddling the Alleghanian front in been observed along weak décollement horizons in ing accommodated by folding and layer-parallel central Pennsylvania (Ace et al., 2020) and in well other regions of the Appalachians, including within dissolution is necessary to balance the proposed cores at the western limit of the Appalachian Plateau this formation (e.g., Thomas 2001, 2007; Ace et al., geometry and resulting shortening of the faulted (Ryder et al., 2008). East of the Elkins Valley anticline, 2020). The importance of the Reedsville-Martins- Cambrian through Ordovician section. the structural relief of the overlying cover sequence burg Formations in accommodating shortening and indicates that wedge faulting in the Middle Devo- filling space increases to the east in the Great Valley nian shales alone is insufficient to fill space below region; here, an additional 15 km of shortening is ■■ DISCUSSION the cover strata, and available seismic lines through accommodated by Cambro-Ordovician horses and the region do not support faulting of the underlying is balanced by 35% LPS (Wright and Platt, 1982). The cross section that we present assumes that Ordovician–Cambrian section. Instead, we suggest The shortening required by the proposed cross slip transferred from horses 1–5 (Plate 1) in the that this space is filled by faulting in the Williams- section through the Appalachian Plateau and Valley Ordovician–Cambrian strata to the Appalachian port through Reedsville Formations (Plate 1). Similar and Ridge can be compared to that calculated by Plateau is accommodated in the strata exposed at duplexing of the correlative stratigraphic interval is measurements using the normalized Fry method the surface solely through folding and LPS. How- recognized in seismic lines in the Valley and Ridge (Table 2). As stated previously, measured LPS ever, the Middle Devonian organic-rich shales and in central Pennsylvania (Ace et al., 2020). These two across this portion of the Appalachians is highly the Silurian–Ordovician siliciclastics are decoupled systems, inferred within the Appalachian Plateau, variable. To compare measured values to the val- from this cover sequence and accommodate slip accommodate 24 km of shortening and need to be ues needed to balance the cross section, we have through faulting within the two separate packages. matched by equal amounts of shortening within the averaged spatially related LPS samples across the It is commonly assumed that the detachment at the cover rocks for the cross section to balance. transect, producing 16 intervals ranging in length base of the Salina Formation on the Appalachian

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Plateau separates the unstrained units (below the TABLE 2. BALANCING THE CROSS SECTION THROUGH THE USE OF A SHORTENING detachment) from those deformed via LPS and fold- BUDGET USING LAYER-PARALLEL SHORTENING (LPS) CONSTRAINED BY THE AXIAL RATIO OF THE BEST-FIT ELLIPSE ON THE BEDDING PLANE ing above the detachment (Evans, 2010; Sak et al., 2012). However, newly acquired high-resolution Interval Cover sequence Stiff sequence Proposed LPS Average LPS Net length (km) length* (km) (cross section, km) (siliciclastic samples, km) balance† 3-D seismic data across the Alleghanian structural front in central Pennsylvania (Ace et al., 2020) com- 1 4.00 4.00 0.00 0.52 −0.52 2 31.54 35.20 3.66 4.58 −0.92 bined with pre-existing well data from the core of 3 6.50 6.50 0.00 1.50 −1.50 the Burning Springs anticline (Ryder et al., 2008) 4 8.13 8.13 0.00 0.73 −0.73 reveal wedge-style faulting within the Middle Devo- 5 9.20 9.20 0.00 1.20 −1.20 nian organic-rich shales, forming the cores of the 6 13.75 13.75 0.00 1.24 −1.24 low-amplitude folds of the Appalachian Plateau. We 7 23.93 27.32 3.39 2.73 0.66 incorporate these geometries in the balanced cross 8 15.36 17.32 1.96 1.91 0.05 section (Plate 1) and propose that the cover strata 9 7.77 7.77 0.00 0.47 −0.47 10 21.61 21.61 0.00 4.00 −4.00 of the Appalachian Plateau are detached from the 11 11.61 17.05 5.44 2.05 3.39 underlying stiff sequence along a décollement at 12 22.14 30.18 8.04 3.02 5.02 the base of the Devonian Tully Limestone. 13 11.70 11.70 0.00 0.90 −0.90 In a previous study, Evans (2010) invoked a 14 39.82 46.16 6.34 4.10 2.24 double stack of Cambrian to Ordovician carbon- 15 10.45 14.02 3.57 1.31 2.26 ate thrust sheets directly east the Wills Mountain 16 20.45 33.93 13.48 3.00 10.49 anticline to explain the higher structural eleva- tions between the Wills Mountain anticline and *Spatial extents of individual intervals are shown in Plate 2. For intervals 1–13 in the Appalachian Plateau, length is measured at the base of the Marcellus Shale. Length for intervals 14–16 is measured for the the Whip Cove syncline (Plate 2). However, the underlying passive-roof duplex of Ordovician–Cambrian carbonates at the top of the Waynesboro Formation. proposed 32.5% to 63% shortening magnitudes †Net balance is the difference between the proposed LPS and the average LPS constrained by the siliciclastic required to balance these sections (Evans, 2010) samples. Note: Cover and Stiff sequences are defined in Figure 2. are not compatible with the strain values obtained through this study or previous studies of grain- scale bulk finite strain in the central Appalachian Mountains (e.g., Dunne, 1996; Whitaker and Bar- documented along the 18.6 km transect of Devo- is consistent with estimates of Hogan and Dunne tholomew, 1999; Sak et al., 2012). Even using the nian-aged strata in the western Valley and Ridge (2001), where a majority of studied transects exhibit more conservative geometries proposed here, of northeastern West Virginia (Hogan and Dunne, 4%–6% outcrop-scale shortening. the calculated shortening budgets indicate more 2001). Our proposed model’s reliance upon out- shortening than is accounted for by the measured crop-scale shortening can be minimized if 2.2 km grain-scale LPS. Consequently, the model we pro- and 2.8 km less displacement is assumed for ■■ CONCLUSION pose does not assume a “double stack”, but instead horses 6 and 9, respectively (Plate 2B), resulting proposes a series of faults all originating from the in 5 km less LPS to balance the section. This, in Integration of bulk grain-scale finite strain detachment at the base of the Waynesboro Forma- turn, reduces the magnitude of outcrop-scale short- measurements in balanced cross sections is a crit- tion. Furthermore, the 18% shortening required to ening required to balance the section from 10% ical component for documenting total amount of balance the section is plausible only when the max- to 6.5%. Reductions in outcrop-scale shortening shortening and elucidating the kinematics of defor- imum permissible measured LPS values for each require greater variations in the thickness of the mation. The strong variability in measured LPS interval (Plate 2A) are combined with the maximum ductile Reedsville Formation through the Valley values from this study suggests caution in assum- permissible outcrop-scale shortening (Hogan and and Ridge. Additionally, the incorporation of out- ing magnitudes of LPS even between transects Dunne, 2001). crop-scale shortening can further be reduced from across the same portion of an orogen. The LPS In order to balance the cross section, this 6.5% to 5% if 2.2 km less displacement is assumed measurements we present here, when combined study relies upon 10% outcrop-scale shortening on horse 4 (Plate 2B), adjacent faults in the Ordo- with other Appalachian measurements, document over a 125 km portion of the eastern Appalachian vician to Silurian packages, and wedge faulting significant variations in both location and magni- Plateau and the Valley and Ridge. This is equal to of the Middle Devonian shales east of the Elkins tude of grain-scale shortening within the system the upper limit (10%) of outcrop-scale shortening Valley anticline. The 5% outcrop-scale shortening and provide insight into the partitioning of strain

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between shortening mechanisms. Although LPS is Wojtal, an anonymous reviewer, and Associate Editor Mike Wil- Society of America Bulletin, v. 127, p. 87–112, https://doi liams for their detailed and comprehensive reviews that greatly .org/10.1130/B30968.1. a critical component of shortening within the West improved this paper. Elliott, D., 1983, The construction of balanced cross sections: Virginia portion of the Appalachian fold-thrust belt, Journal of Structural Geology, v. 5, p. 101, https://doi.org the cross section remained unbalanced without /10.1016/0191-8141(83)90035-4. Engelder, T., 1979, The nature of deformation within the outer the additional estimates of outcrop-scale shorten- REFERENCES CITED limits of the central Appalachian foreland fold and thrust ing. The 40 siliciclastic samples collected across this Ace, A., McQuarrie, N., Sak, P.B., Grundy, B., and Lavergne, belt in New York State: Tectonophysics, v. 55, p. 289–310, transect reveal an average LPS value of 10% with a B., 2020, Documenting the geometry and magnitude of https://doi.org/10.1016/0040-1951(79)90181-1. mean trend of 304° ± 5°. Complementary fracture shortening at the Allegheny Front, Lycoming County, Penn- Engelder, T., 1985, Loading paths to joint propagation during sylvania: American Association of Petroleum Geologists a tectonic cycle: An example from the Appalachian Pla- analyses of outcrops highlight a strike-parallel joint Bulletin (in press), https://doi.org/10.1306/04282017325. teau, U.S.A.: Journal of Structural Geology, v. 7, p. 459–476, set and cross joints that are broadly parallel to the Boyer, S.E., and Elliott, D., 1982, Thrust systems: Ameri- https://doi.org/10.1016/0191-8141(85)90049-5. shortening direction. 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Overall, the total proposed shortening along Dean, S.L., Kulander, B.R., McColloch, G.H., McColloch, J.S., inia to Pangea: The Lithotectonic Record of the Appalachian a transect from Burning Springs anticline of West Kulander, C.S., and Lessing, P., 2001, Bedrock geology of the Region: Geological Society of America Memoir 206, p. 477– Sector quadrangle, Hampshire and Hardy Counties, West 552, https://doi.org/10.1130/2010.1206(21). Virginia to the Great Valley of Virginia is 56 km (18%); Virginia: West Virginia Geologic and Economic Survey Evans, M.A., and Dunne, W.M., 1991, Strain factorization and additional shortening from the Great Valley region Open-File Report 0005, scale 1:24,000. portioning in the North Mountain thrust sheet, central may increase this estimate to 71 km (Plate 1). Drake, A.A., Jr., and Epstein, J.B., 1967, The Martinsburg Forma- Appalachians, U.S.A.: Journal of Structural Geology, v. 13, tion (Middle and Upper Ordovician) in the Delaware Valley, p. 21–35, https://doi.org/10.1016/0191-8141(91)90098-4. Pennsylvania–New Jersey: U.S. Geological Survey Bulletin Faill, R.T., 1969, Kink band structures in the Valley and Ridge 1244-H, 16 p., https://doi.org/10.3133/b1244H. province, central Pennsylvania: Geological Society of Amer- ACKNOWLEDGMENTS Dunne, W.M., 1996, The role of macroscale faults in the defor- ica Bulletin, v. 80, p. 2539–2550, https://doi .org /10 .1130 /0016 This research was partially funded by a Stephen Pollock Under- mation of the Alleghanian roof sequence in the Central -7606(1969)80[2539:KBSITV]2.0.CO;2. graduate Research Program Grant from the Northeastern Appalachians: A re-evaluation: American Journal of Sci- Faill, R.T., 1973, Kink-band folding, Valley and Ridge province, Section of Geological Society of America to DL, with additional ence, v. 296, p. 549–575, https://doi .org /10 .2475 /ajs .296 .5.549 . Pennsylvania: Geological Society of America Bulletin, v. 84, support provided by a Summer of Undergraduate Research Dunne, W.M., and Ferrill, D.A., 1988, Blind thrust systems: p. 1289–1314, https://doi.org/10.1130/0016-7606(1973)84 Fellowship from the College of Arts and Sciences at the Univer- Geology, v. 16, p. 33–36, https://doi.org/10.1130/0091-7613 <1289:KFVARP>2.0.CO;2. sity of Pittsburgh to DL. PBS graciously acknowledges funding (1988)016<0033:BTS>2.3.CO;2. Faill, R.T., 1998, A geologic history of the north-central Appala- from U.S. Geological Survey EDMAP program and additional Eichelberger, N., and McQuarrie, N., 2015, Three-dimensional chians: Part 3, The Allegheny orogeny: American Journal support from the Dickinson College Research and Development (3-D) finite strain at the central Andean orocline and impli- of Science, v. 298, p. 131–179, https://doi.org/10.2475/ajs Committee. Finally, we are grateful to Mary Beth Gray, Steve cations for grain-scale shortening in orogen: Geological .298.2.131.

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