Quantifying Shortening Across the Central Appalachian Fold-Thrust Belt, Virginia and West Virginia, USA: Reconciling Grain-, Outcrop
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Research Paper 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 © 2020 The Authors GEOSPHERE | Volume 16 | Number 5 Lammie et al. | Quantifying shortening across the central Appalachian fold-thrust belt Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/5/1276/5151179/1276.pdf 1276 by guest on 25 September 2021 Research Paper 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 struc- tural front; black boxes—area of Figure 3; white box—area of Figure 6. BSA— Burning Springs anticline; AFA—Arches Fork anticline; EVA—Elkins Valley anti- cline; WMA—Wills Mountain anticline; WCS—Whip Cove syncline.