A Billion Years of Deformation in the Central Appalachians: Orogenic Processes and Products

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The Geological Society of America Field Guide 40 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products

Steven J. Whitmeyer Department of Geology and Environmental Science, James Madison University, Harrisonburg, Virginia 22807, USA

Christopher M. Bailey Department of Geology, College of William & Mary, Williamsburg, Virginia 23187, USA

David B. Spears Virginia Department of Mines, Minerals, and Energy, Division of Geology and Mineral Resources, Charlottesville, Virginia 22903, USA

ABSTRACT

The central Appalachians form a classic orogen whose structural architecture developed during episodes of contractional, extensional, and transpressional deformation from the Proterozoic to the Mesozoic. These episodes include components of the Grenville orogenic cycle, the eastern breakup of Rodinia, Appalachian orogenic cycles, the breakup of Pangea, and the opening of the Atlantic Ocean basin. This ﬁ eld trip examines an array of rocks deformed via both ductile and brittle processes from the deep crust to the near-surface environment, and from the Mesoproterozoic to the present day. The trip commences in suspect terranes of the eastern Piedmont in central Vir- ginia, and traverses northwestward across the Appalachian orogen through the thick- skinned Blue Ridge basement terrane, and into the thin-skinned fold-and-thrust belt of the Valley and Ridge geologic province. The traverse covers a range of deformation styles that developed over a vast span of geologic time: from high-grade metamorphic rocks deformed deep within the orogenic hinterland, to sedimentary rocks of the foreland that were folded, faulted, and cleaved in the late Paleozoic, to brittle extensional structures that overprint many of these rocks. Stops include: the damage zone of a major Mesozoic normal fault, composite fabrics in gneiss domes, transpressional mylonites that accommodated orogen-parallel elongation, contractional high-strain zones, and overpressured breccia zones in the Blue Ridge, as well as folds, thrusts, and back thrusts of the Alleghanian foreland.

Whitmeyer, S.J., Bailey, C.M., and Spears, D.B., 2015, A billion years of deformation in the central Appalachians: Orogenic processes and products, in Brezin- ski, D.K., Halka, J.P., and Ortt, R.A., Jr., eds., Tripping from the Fall Line: Field Excursions for the GSA Annual Meeting, Baltimore, 2015: Geological Society of America Field Guide 40, p. 11–33, doi:10.1130/2015.0040(02). For permission to copy, contact [email protected]. © 2015 The Geological Society of America. All rights reserved.

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12 Whitmeyer et al.

INTRODUCTION continent collision followed by rifting and the formation of ocean basins have occurred here. Although situated on the trailing edge

The Appalachian Mountains form the backbone of east- of the North American plate, the 2011 Virginia earthquake (Mw = ern North America, and its underlying geologic structure is the 5.8) illustrates that deformation has not ceased in eastern North result of a long and complex tectonic history. For over 200 years, America and challenges the notion of passive-margin quiescence. geologists have studied the Appalachians, and many ﬁ rst-order This ﬁ eld guide focuses on a southeast-to-northwest, cross- geological concepts were formulated based on observations of orogen traverse of the central Appalachians in Virginia (Figs. 1 Appalachian structures. Some of these models have passed out and 2). The purpose of the trip is to examine the products of of favor (e.g., thermal contraction, geosynclines), replaced by crustal deformation and discuss orogenic processes at a range of new paradigms that have taken root (thin-skinned tectonics, oro- scales from the macro to the micro. The two-day traverse exam- genic cycles). In the 1960s, J. Tuzo Wilson recognized eastern ines rocks in the Piedmont, Blue Ridge, and Valley and Ridge North America as the type location for supercontinent assembly provinces, highlighting examples of the distinctive stratigra- and breakup (Wilson, 1966). At least two cycles of continent- phy and unique structural style of each province. We hope the

Figure 1. Generalized geologic map of north-central Virginia illustrating ﬁ eld-trip stops in yellow for days 1 and 2. Inset shows the location of the detailed map in reference to the geologic provinces of Virginia. NMT—Little North Mountain fault; BRF—Blue Ridge fault system; MRF—Mountain Run fault; ShFZ—Shores fault zone; CF—Chopawamsic fault; SpFZ—Spotsylvania fault zone; HFZ—Hylas fault zone. Downloaded from fieldguides.gsapubs.org on October 15, 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products 13

Figure 2. Simpliﬁ ed geologic cross section across the region covered by the ﬁ eld trip. From northwest to southeast: west- vergent folded and faulted rocks of the Valley and Ridge (surf green), basement gneisses (stippled blue) and cover rocks (dark green) of the Blue Ridge, suspect terranes of the Piedmont (tan), Triassic basins (stippled green), and the Coastal Plain onlap in the eastern Piedmont (thin yellow band at the surface). Major structural features indicated: LCD—Lower Carbonate duplex; NMT—Little North Mountain thrust; MS—Massanutten synclinorium; BRF—Blue Ridge fault system; SM—Brookneal/Shores Mountain Run zone; SZ—Spotsylvania zone; HZ—Hylas zone; T—Taylorsville basin.

structures exposed at each location provide fodder for lively sented by the extrusion of Catoctin basalts at ca. 570–550 Ma discussions concerning the geometry and processes related to (Aleinikoff et al., 1995; Southworth et al., 2009). crustal deformation in the Virginia Appalachians, and lead us to a The Early Cambrian rift-to-drift period is recorded in the Mid- better understanding of the orogenic history of the Mid-Atlantic Atlantic region by the siliciclastic Chilhowee Group (Simpson Appalachian region. and Eriksson, 1989; Smoot and Southworth, 2014). Cambrian- Ordovician carbonates formed along the divergent continental A BRIEF TECTONIC HISTORY OF MID-ATLANTIC margin of the Iapetus Ocean, which was followed by clastic sedi- NORTH AMERICA mentation and tectonism as the Taconic arcs approached the eastern margin of Laurentia in the Late Ordovician (Hatcher, 1989; Tectonism in the Virginia region commenced in the Meso- Glover, 1989). Surprisingly little evidence is preserved in the proterozoic during the Grenville orogenic cycle that culminated Virginia Appalachians of Taconic deformation fabrics, although with the assembly of the Rodinia supercontinent at ca. 950 Ma some of the Piedmont terranes may have been proximal to east- (Whitmeyer and Karlstrom, 2007; Fig. 3). Two pulses of igneous ern Laurentia during this time (Bailey et al., 2005; Hughes et activity are recorded in this region: an initial phase of magma- al., 2013). Silurian clastic rocks are preserved in the Massanutten tism and deformation between 1180 and 1080 Ma (Shawingian), synclinorium and farther west in West Virginia (Dennison and and a later phase of intrusion from 1065 to 1030 Ma (Ottawan; Head, 1975; Smosna et al., 1977). Devonian Acadian deforma- Bailey et al., 2006). The Robertson River intrusive suite (ca. 730– tion is largely absent from the Virginia Appalachians, although a 700 Ma) likely represents initial stages of the rifting of Rodinia signiﬁ cant Acadian clastic wedge blankets western Virginia and (pre-Iapetus rifting; Fig. 3), the main phase of which is repre- West Virginia (Woodward, 1943; Dennison, 1970).

Millions of years 1200 1100 1000 900 800 700 550600 500 400450 350 200250300 150 100 050 Meso Neoproterozoic Paleozoic Mesozoic Cz Sten. Ton. Cryo. PIPMDSOCEd. R JT K Pg Ng

? ? Iapetus Atlantic rifting rifting Mineral, Va, pre-Iapetus earthquake Sh Ot rifting TS AC NA AL Grenvillian Appalachian orogenies orogenies

Figure 3. Temporal summary of tectonic events in the Piedmont, Blue Ridge, and Valley and Ridge provinces in north- central Virginia. Orogenic events indicated: Sh—Shawingian; Ot—Ottawan; TS—Taconic-Salinic; AC—Acadian; NA— Neo-Acadian; AL—Alleghanian. Downloaded from fieldguides.gsapubs.org on October 15, 2015

14 Whitmeyer et al.

Neo-Acadian greenschist-facies metamorphism and ductile Tucker, 2003). The State Farm gneiss is overlain by discontinu- deformation fabrics in the Blue Ridge (Bailey et al., 2006) may ous bands of amphibolite and an areally extensive, heteroge- represent the earliest phases of the Alleghanian collision of the neous gneissic unit (the Maidens Formation/gneiss) (Fig. 4A). Gondwana supercontinent with eastern Laurentia during the for- The Mesoproterozoic Montpelier anorthosite is a small alkalic mation of Pangea. The majority of the deformation fabrics (e.g., anorthositic pluton that intruded Maidens-like gneiss (Aleini- predominantly northwest-directed folding and thrust faulting) koff et al., 1996). A suite of 650–590 Ma A-type granitic plutons in the Valley and Ridge province of Virginia are Alleghanian. intrude the State Farm gneiss, but are absent from the overlying This region is considered a type locality for thin-skinned tecton- Maidens-like gneiss (Owens and Tucker, 2003). Early granulite- ics associated with foreland fold-and-thrust belts, with major facies metamorphism in older Goochland rocks is overprinted by northwest-directed faults (Blue Ridge thrust system, Little North a regionally extensive amphibolite-facies event, and recent geo- Mountain thrust; Fig. 2) that accommodated tens of kilometers of chronological studies suggest that both of these events are likely westward transport and bracket asymmetric west-vergent folds at Paleozoic (Owens et al., 2010). a variety of scales (Evans, 1989). Surface structures are under- The similarity between the Mesoproterozoic Blue Ridge lain by duplexes of Cambrian to Ordovician carbonate and clas- basement complex and rocks in the Goochland terrane has led tic rocks (Kulander and Dean, 1986; Mitra, 1986; Evans, 1989; many workers to conclude that the Goochland terrane is Lauren- LCD in Fig. 2). tian (Farrar, 1984; Glover, 1989; Owens and Samson, 2004). Bar- Evidence for the breakup of Pangea and opening of the tholomew and Tollo (2004) suggested that the Goochland terrane Atlantic Ocean includes several Late Triassic to Early Jurassic is a rifted and translated block of Laurentian crust that originated fault-bounded basins to the east of the Blue Ridge, and Jurassic in the northern part of the central Appalachians. However, other maﬁ c dikes and sills that occur primarily in the Piedmont and workers posit a peri-Gondwanan afﬁ nity for the Goochland ter- eastern Blue Ridge provinces. Cretaceous to Quaternary sedi- rane (Rankin et al., 1989; Hibbard and Samson, 1995; Bailey et ments and sedimentary rocks overlie Piedmont terranes in the al., 2005). Both the provenance of the Goochland terrane and the east and form the broad Coastal Plain of the Atlantic divergent origin of its distinctive gneiss domes remain controversial. margin (Fig. 2). Eocene volcanic plugs (e.g., Mole Hill near Har- The early Paleozoic Chopawamsic terrane is a composite risonburg), faults that cut Neogene Coastal Plain strata, and the terrane of metamorphosed volcanic, plutonic, and sedimentary

2011 Louisa County (Virginia) earthquake (Mw 5.8; Horton et al., rocks that forms part of an arc complex built upon continental 2015), however, provide clear indications that this region is not crust (Fig. 4A). In central and northern Virginia, the Chopawam- exactly a “passive margin” in the present day. sic terrane is composed of a suite of mafi c to felsic metavolcanic rocks interlayered with metamorphosed volcaniclastic and GEOLOGIC SETTING clastic layers. U-Pb zircon ages from metavolcanic rocks yield Ordovician ages of 450–470 Ma (Coler et al., 2000; Horton et Piedmont al., 2010; Hughes et al., 2013). A suite of felsic to intermediate plutons intrudes the Chopawamsic volcanic sequence. In central The central Appalachian Piedmont province is a region of Virginia, the main bodies are the Columbia and Ellisville plutons, modest topographic relief, forming the igneous and metamorphic which yield U-Pb zircon ages of 457 and 444 Ma, respectively hinterland of the Appalachian orogen. Modern paleotectonic (Wilson, 2001; Hughes, et al., 2013). Successor basin deposits models recognize that the Piedmont is an amalgam of terranes, of the Arvonia and Quantico Formations unconformably overlie some with North American or Laurentian affi nity. Other terranes both the volcanic and plutonic rocks (Fig. 4A). Based on their are demonstrably exotic with respect to Laurentia, likely of peri- fossil assemblages, both the Arvonia and Quantico Formations Gondwanan affi nity (Secor et al., 1983; Horton et al., 1989; Hib- have been considered Late Ordovician to Silurian deposits (Wat- bard et al., 2002). In central and northern Virginia, the Piedmont son and Powell, 1911; Stose and Stose, 1948; Tillman, 1970; includes three terranes: the Goochland, Chopawamsic, and West- Pavlides, 1980). However, detrital zircons from the base of this ern Piedmont (Hardware/Potomac) terranes (Fig. 4A). Terranes sequence in central Virginia are as young as 390 Ma, requiring a are intruded by a suite of variably deformed Neoproterozoic to Devonian age for deposition of these units (Bailey et al., 2008). late Paleozoic plutons, and are separated by a system of north- The Chopawamsic terrane experienced one episode of east-striking faults and ductile high-strain zones (Figs. 1 and 2). regional metamorphism that ranged from upper greenschist facies These structures served as a locus for subsequent regional defor- in the western extent of the terrane to amphibolite facies across mation events throughout the Mesozoic and Cenozoic. much of the central and eastern extent of the terrane. 40Ar/39Ar The Goochland terrane in the eastern Piedmont contains a cooling ages from amphibole and muscovite range from 310 to variety of gneiss, amphibolite, granite, and anorthosite. The struc- 285 Ma (Burton and Armstrong, 1997; Jenkins et al., 2012; Bur- turally lowest, and presumably oldest, unit in the Goochland ter- ton et al., 2015). Most workers consider the Chopwamsic terrane rane is the Mesoproterozoic State Farm gneiss, a heterogeneous to have accreted to Laurentia; whether it is a marginal Laurentian coarse-grained granitoid gneiss exposed in a series of dome-like or peri-Gondwanan terrane is still debated (Glover, 1989; Spears structures (Fig. 1; Glover et al., 1982; Farrar, 1984; Owens and and Bailey, 2002; Hughes, et al., 2013). Downloaded from fieldguides.gsapubs.org on October 15, 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products 15

The Western Piedmont terrane includes a Neoproterozoic to grade metamorphism and cooling ages range from 410 to 452 Ma early Paleozoic sequence of metaclastic rocks and mafi c to ultra- (Jenkins et al., 2012; Burton et al., 2015). mafi c bodies interpreted as either a marginal Laurentian terrane Piedmont terranes are separated by regionally signifi cant (Glover, 1989; Pavlides et al., 1994; Hughes et al., 2013) or an faults and high-strain zones. From east to west these include: exotic terrane with a Gondwanan affi nity (Horton et al., 1989; the Hylas, Spotsylvania, Chopawamsic, Brookneal/Shores, Hibbard et al., 2002) (Fig. 4A). These rocks experienced low- and Mountain Run zones (Figs. 1 and 2). Some zones form

Figure 4. Composite and simpliﬁ ed stratigraphic diagrams of (A) the Blue Ridge province and Piedmont terranes (Western Piedmont, Chopawamsic, Goochland), and (B) Cambrian–Devonian units of the Valley and Ridge provinces in north-central Virginia. RRIS—Robertson River Intrusive Suite. Downloaded from fieldguides.gsapubs.org on October 15, 2015

16 Whitmeyer et al.

pronounced geophysical lineaments (Neuschel, 1970; Zietz et Catoctin Formation (Figs. 1 and 4A). The Swift Run Formation al., 1977). Individual high-strain zones range from 1 to 2 km in is heterogeneous and discontinuous, with a highly variable thick- width up to zones >10 km wide with diffuse boundaries. Lower ness. The Catoctin Formation originally covered a large area amphibolite- to greenschist-facies mylonites are common; folia- (>10,000 km2), and was likely generated from mantle-derived tion in these zones invariably strikes northeast-southwest, and tholeiitic magmas (Badger and Sinha, 2004; Johnson et al., 2014). typically dips moderately to the southeast. Mineral elongation Metadiabase dikes of similar composition to Catoctin metaba- lineations, where present, plunge gently to both the southwest salts intrude the basement complex (and older Neoproterozoic and northeast. Some workers describe distinct thrust sheets and rocks) and are likely feeder dikes for the overlying Catoctin lava nappes in the Virginia Piedmont, suggesting that terranes were fl ows. Badger and Sinha (1988) report an Rb-Sr isochron age of stacked by southeast-to-northwest directed thrusting (Glover, 570 ± 36 Ma, and zircons from metarhyolite tuffs and dikes in 1989; Pratt et al., 1988, 2015). Ductile fabrics in Piedmont rocks the Catoctin Formation yield U-Pb zircon ages between 570 and however, consistently preserve dextral asymmetric structures that 550 Ma (Aleinikoff et al., 1995; Southworth et al., 2009). do not record a simple northwest-directed thrust stacking; rather, The Early Cambrian Chilhowee Group overlies the Catoctin these penetrative fabrics record general shear that occurred dur- Formation, and in north-central Virginia includes the Weverton, ing bulk transpression (Bobyarchick et al., 1979; Bobyarchick, Harpers, and Antietam Formations (Figs. 1 and 4A). The contact 1981; Gates and Glover, 1989; Bailey et al., 2004) (Fig. 3). Bai- between the underlying Catoctin metabasalts and the overlying ley et al. (2004) estimate that the Spotsylvania zone, which sepa- Chilhowee Group has traditionally been interpreted as an uncon- rates the Chopawamsic and Goochland terranes, translated the formity (King, 1950; Gathright, 1976; Southworth et al., 2009). Goochland terrane between 80 and 300 km southwestward dur- The Weverton Formation includes quartz metasandstone, meta- ing the Alleghanian orogeny. conglomerate, laminated metasiltstone, and quartzose phyllite. The Harpers Formation is dominated by phyllite and thinly bed- Blue Ridge ded metasandstone with minor well-cemented quartz arenite and ferruginous metasandstone. The Antietam Formation consists of In north-central Virginia, the Blue Ridge province forms well-cemented quartz arenite with abundant Skolithos. Collec- a thick-skinned massif underlain by Mesoproterozoic granit- tively, the Chilhowee Group records shallow-marine sequences oid basement with a Neoproterozoic to Early Cambrian cover along the Laurentian margin during the Cambrian (Simpson and sequence of metasedimentary and metavolcanic rocks (Figs. 1, 2, Eriksson, 1989; Smoot and Southworth, 2014). and 4A). Regionally, the Blue Ridge forms an anticlinorium com- Unmetamorphosed diabase dikes cut both Mesoproterozoic posed of imbricated basement thrust sheets that were emplaced basement and the Neoproterozoic to Early Cambrian cover rocks. over Paleozoic rocks of the Valley and Ridge along a series of The olivine-normative, low-titanium diabase is similar to mafi c related, low-angle thrusts (Mitra, 1986; Evans, 1989). dikes and sills exposed in the Mesozoic Culpeper basin east of The Blue Ridge basement complex is composed of granitic, the Blue Ridge. A diabase dike along the Potomac River yielded charnockitic, and leucogranitic rocks that formed between 1.2 an Ar-Ar age of 200 Ma (Kunk et al., 1992). and 1.0 Ga. The suite includes three temporally distinct groups Blue Ridge rocks are commonly foliated. A distinctive of granitoids; the oldest group crystallized between 1190 and coarse foliation or compositional banding is developed in the 1150 Ma, the middle group between 1120 and 1110 Ma, and the older Mesoproterozoic basement units (>1150 Ma) and formed at youngest group between 1090 and 1020 Ma (Aleinikoff et al., upper amphibolite- to granulite-facies conditions. Younger Meso- 2000; Tollo et al., 2004; Southworth et al., 2010). Felsic rocks proterozoic basement units (group 3, <1090 Ma) typically lack a of the 730–700 Ma Robertson River plutonic/volcanic complex high-temperature fabric. A younger, more pervasive foliation is intrude the Mesoproterozoic basement (Tollo and Aleinikoff, variably developed in both the basement and cover sequence; this 1996). These igneous rocks have a distinctive A-type geochemis- foliation characteristically strikes northeast, dips to the southeast, try (Tollo and Aleinikoff, 1996). and is defi ned by aligned greenschist-facies minerals. Ar-Ar geo- In the eastern and central Blue Ridge, a 1 to ~5–km-thick chronology indicates that pervasive deformation in the basement sequence of Neoproterozoic metasedimentary rocks (Lynchburg, and cover sequence occurred between ca. 380 and 320 Ma (Late Fauquier and Mechum River Formations) overlies the basement Devonian to Pennsylvanian) (Wooton et al., 2005; Bailey et al., complex (Wehr, 1985; Bailey et al., 2007b) (Figs. 1 and 4A). This 2007a; Jenkins et al., 2012). A suite of north-northwest (330°– sequence includes both marine and non-marine arkoses, con- 350°) and west-northwest (280°–300°) transverse faults cut the glomerates, and wackes formed during an episode of continental regional structural grain and may be related to transtensional rifting that created signifi cant accommodation space, but did not stresses during Mesozoic rifting (Bailey et al., 2006). generate oceanic crust (Wehr, 1985; Bailey et al., 2007b). A thick Mylonitic high-strain zones in the core of the Blue Ridge sequence (1–3 km) of metabasalts (Catoctin Formation) overlies anticlinorium occur as km-thick anastomosing zones; asymmetric Neoproterozoic clastic units in the eastern Blue Ridge. structures in these rocks consistently record a top-to-the- northwest In the western Blue Ridge, the cover sequence includes the sense of shear (i.e., hanging wall up movement) (Bailey and clastic Swift Run Formation and the basaltic greenstones of the Simpson, 1993). Displacement across Blue Ridge mylonite zones Downloaded from fieldguides.gsapubs.org on October 15, 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products 17 accommodated crustal contraction, enabling the relatively stiff tems). Additional shortening was also accommodated by folds basement complex to shorten while cover rocks were folded (Bai- and small-scale thrusts in Page Valley and the Massanutten syn- ley et al., 1994; Bailey et al., 2007a). Folds in western Blue Ridge clinorium (Whitmeyer et al., 2012). cover rocks are typically asymmetric, northwest-verging structures. Axial-planar foliation (cleavage) is well developed in fi ne-grained Mesozoic to Present-Day Features rocks and dips gently to moderately southeast (Cloos, 1971). The frontal Blue Ridge fault system truncates early fold structures and Early Mesozoic rift basins cut Proterozoic to Paleozoic rocks penetrative fabrics in both the hanging-wall and footwall rocks; the of the Piedmont (Figs. 1 and 2), and northwest-striking Jurassic deformation style is typically brittle and breccias are well developed mafi c dikes occur in the Piedmont and Blue Ridge and to a lesser at a number of localities (Bailey et al., 2006). degree in the Valley and Ridge provinces (Rader and Gathright, 2001; Whitmeyer et al., 2012). Northwest-striking, high-angle Valley and Ridge transverse and oblique faults cross-cut predominantly northeast fabrics throughout the region, frequently marked by saddles The Valley and Ridge of west-central and northwestern Vir- (gaps) in the Blue Ridge and Massanutten mountain ridges. Sub- ginia is characterized by northeast-trending alternating mountain parallel, northwest-striking joint sets are also pervasive, often ridges and valleys that extend from the base of the Blue Ridge paralleling bends in the Shenandoah River system (Drummond thrust system in the east to the Allegheny Plateau in eastern West et al., 2011b; Whitmeyer et al., 2012). Other brittle structures Virginia (Fig. 1). Lithologic units immediately west of the frontal found in the region include northeast-trending faults that appear Blue Ridge fault system include Cambrian and Ordovician car- to have reactivated older Paleozoic structures. Collectively, these bonate rocks (primarily Elbrook Fm., Conococheague Fm., Beek- late overprinting features may represent crustal extension related mantown Gp., Lincolnshire Fm., and New Market Fm.; Fig. 4B) to the breakup of the Pangea supercontinent and the opening of that were deposited on a continental margin as the Iapetus Ocean the Atlantic Ocean in the Mesozoic. Cretaceous to Quaternary opened after the eastern breakup of Rodinia (Whitmeyer and Karl- sediments and sedimentary rocks of the Coastal Plain uncon- strom, 2007). In Page Valley, west of Elkton, micritic carbonate formably overlie Piedmont rocks in the Fall Zone (Figs. 1 and 2), rocks that represent deepening conditions (Edinburg Formation) and are themselves offset along recent faults. The central Virginia transition to primarily clastic sequences (Martinsburg Formation) seismic zone, located between Richmond and Charlottesville, is related to the Mid- to Late Ordovician Taconic orogeny (Cooper a zone of moderate, but persistent seismic activity (Bollinger and and Cooper, 1946; Diecchio, 1993). Page Valley is bound to the Sibol, 1985; Çoruh et al., 1988; Horton et al., 2015). west by the Massanutten synclinorium, defi ned by high ridges underlain by the Silurian Massanutten Sandstone (King, 1950; ROAD LOG AND STOP DESCRIPTIONS Allen, 1967; Fig. 4B). The central core of the Massanutten synclinorium consists of Devonian black shales (Millboro and Mah- DAY 0—PRELIMINARY TRAVEL DAY antango Formations; Rader and Biggs, 1975, 1976) that underlie Fort Valley (dark-gray region at Stop 2-8 on Fig. 1). 2015 trip participants will be picked up at the Hilton Balti- The Virginia Valley and Ridge province is a type example of more (401 W Pratt St., Baltimore, Maryland 21201) and driven a foreland thrust belt, representing the foreland of the Allegha- south on I-95 (take I-495 around Washington, D.C.) ~125 miles nian collision of the African part of Gondwana with Laurentia to exit 104 (Rt. 207) at Ruther Glen. We will stay overnight at the (Hatcher, 1989). West-vergent folds predominate throughout Days Inn Carmel Church/Kings Dominion. The fi eld trip log starts Page Valley and the Massanutten synclinorium, locally truncated from that point the next morning (day 1). All latitude and longitude by smaller scale west-directed thrust faults (Evans, 1989; Rader coordinates for fi eld trip stops are given in the WGS 84 system. and Gathright, 2001; Drummond et al., 2011a; Fig. 2). Struc- Many stops are on private property and permission is required tural features are apparent throughout the region at many scales, to visit these stops. from outcrop-scale folds and thrusts with centimeters to meters of displacement, to folds with a wavelength of several kilome- DAY 1—PIEDMONT TO BLUE RIDGE ters. The scale and geometry of structural features are controlled, The road log for fi eld-trip day 1 starts just off of I-95, exit 104: in part, by mechanical aspects of alternating strong (sandstone, Ruther Glen, Virginia (N37.9352°, W77.4797°; mile 0.0). dolomite) and weak (shale, limestone) lithologies. Signifi cant westward transport likely occurs along weak, shale-dominated Cum. Point-to-point Directions units (Waynesboro and Martinsburg Formations), with larger mileage mileage and comments folds best defi ned by the more massive clastic rocks (e.g., Mas- sanutten Sandstone). Tens of kilometers of westward transport 0.0 0.0 Turn right onto VA Rt. 207. were accommodated along the Blue Ridge fault system (Evans, 0.2 0.2 Turn left (south) onto U.S. Rt. 1. 1989; Bailey et al., 2006; Hatcher et al., 2007) and other major 2.9 2.7 Turn right (west) onto Oxford Rd. thrusts (e.g., the Staunton-Pulaski and Little North Mountain sys- (Rt. 689). Downloaded from fieldguides.gsapubs.org on October 15, 2015

18 Whitmeyer et al.

4.1 1.2 Park on the side of Oxford Rd., hike striking extension fractures occurs throughout the outcrop; this ~0.5 mi. to Stop 1-1. fracture set is parallel to a regional suite of Jurassic basaltic/diabase dikes. Narrow zones (0.5–2 cm) of cataclasite are also present, and Stop 1-1: North Anna River Fault Zone Rapids composed of fi ne-grained quartz, epidote, and chlorite. (Fork Church Fault Damage Zone) Arrays of pseudotachylyte veins are present at the outcrop; Mile 4.1 (N37.8913°, W77.4924°) these include both fault and injection veins (up to 5 cm wide) that cut both metamorphic rock types (Fig. 5A). Pseudotachylyte is fi ne- This large outcrop occurs at a dramatic set of rapids along the grained with angular clasts of feldspar in a glassy matrix. Some fault North Anna River in the Fall Zone of the eastern Piedmont. Most veins are listric and record normal fault movement. Narrow (mm- Atlantic-draining streams have signifi cant knick zones as they scale) veins of quartz, calcite, and zeolite cut the pseudotachylyte. transit from the Piedmont to the Coastal Plain geologic provinces; The age of pseudotachylyte formation is uncertain, but we hypoth- these rapids formed barriers to upstream navigation and conse- esize that it is the product of seismogenic faulting during the early quently numerous colonial towns (e.g., Baltimore, Georgetown/ Mesozoic. Our research efforts are focused on quantifying the fault Washington D.C., Richmond, Raleigh) developed into mercan- kinematics, temperature and pressure conditions during formation, tile centers at this geomorphic boundary. Here the North Anna and age of pseudotachylyte generation. River splashes over a vertical drop of ~3 m in ~100 m. Cum. Point-to-point Directions Although the river, outcrop, and surrounding forest are mileage mileage and comments suitably tranquil today, on 24 May 1864 this was a hectic and horrible scene during the Battle of North Anna. In the spring of 4.1 0.0 Turn around and backtrack on 1864, Ulysses S. Grant’s Overland Campaign was grinding south Oxford Rd. toward the Confederate capital in Richmond; Robert E. Lee’s 5.3 1.2 Turn right (south) on U.S. Rt. 1. Confederate troops occupied entrenched positions along the high 7.2 1.9 Turn right (west) onto Verdon Rd. terrain to the west and south of the North Anna River. Union (Rt. 684). troops crossed the North Anna River both up- and downstream 14.9 7.7 Turn right (northeast) onto Hewlett from these rapids, and continued onward into heavy fi re from Rd. (Rt. 601). the dug-in Confederates. Ultimately, the Battle of North Anna 16.4 1.5 Cross the North Anna River, turn resulted in thousands of causalities and no conclusive victor; left, and park. Walk back across the Grant disengaged and swung the Federal troops to the southeast, bridge and ~50 m into the woods to continuing the bloody campaign that would eventually end the Stop 1-2. Civil War nearly a year later. The Fall Zone rapid outcrop exposes both metamorphic and Stop 1-2: Mossy Rock, 50 m Downstream from Butler fault rocks from the footwall damage zone of the Fork Church Bridge (Goochland Terrane K-feldspar Gneiss) fault (Weems, 1980). The Fork Church fault is a major structure Mile 16.4 (N37.9361°, W77.5620°) that bounds the west-northwest side of the Mesozoic Taylorsville basin, a large Mesozoic rift basin that extends from east-central This outcrop exposes a medium- to coarse-grained gneiss Virginia northeastward for ~115 km into Maryland. Approxi- composed of quartz, biotite, plagioclase, K-feldspar, kyanite, mately 90% of the Taylorsville basin lies buried beneath younger garnet, and rutile. The main foliation/banding is gently dipping Cretaceous and Cenozoic Coastal Plain deposits. LeTourneau and forms a series of visually striking recumbent folds (Fig. 5B). (2003) estimated that the Taylorsville basin is fi lled with an A weak, but pervasive, mineral lineation consistently plunges ~4.5-km-thick sequence of Late Triassic fl uvial and lacus- gently to the southwest. A set of undeformed and subhorizontal trine deposits. Arkosic cobble to boulder conglomerates of the K-feldspar rich veins cuts the folded foliation. Farrar and Owens Taylorsville basin are exposed along the west side of the river, (2001) refer to rocks exposed near Butler Bridge as the Hewlett ~100 m downstream from the rapids. metapelite of the Maidens Formation. Two rock types are exposed at the outcrop: a medium- Farrar (1984) interpreted the Maidens Formation to be the grained, hornblende-rich gneiss and a medium- to coarse-grained, youngest unit in the enigmatic Goochland terrane. Some rocks K- feldspar-rich granitoid gneiss/mylonitic gneiss. The cross- cutting in the Goochland terrane experienced an early granulite-facies relationship between the two rock types is not clear, although the metamorphic event that is overprinted by a regionally extensive hornblende-rich gneiss occurs as boudins surrounded by the amphibolite-facies event (Farrar, 1984). Farrar (1984) and Glover K- feldspar-rich gneiss. Foliation typically dips gently to moderately (1989) interpreted the granulite-facies metamorphic event as to the northwest, and is clearly folded at some locations. Mesoproterozoic; however, electron-microprobe dating of mona- Metamorphic rocks at the North Anna rapids are cut by numer- zite and U-Pb TIMS (thermal ionization mass spectrometry) zir- ous fracture sets, including a set of NE-striking normal faults that con ages from Maidens-like gneisses yields ages between ca. 404 parallel the map-scale Fork Church fault, and are commonly orna- and 387 Ma (Shirvell et al., 2004; Owens et al., 2010). The sta- mented with down-dip slickenlines. A set of steeply dipping, NNW- ble K-feldspar and kyanite assemblage from these gneisses is Downloaded from fieldguides.gsapubs.org on October 15, 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products 19

Figure 5. (A) Pseudotachylyte vein from Stop 1-1. (B) Recumbent folds in K-feldspar-kyanite–bearing gneiss at Stop 1-2. (C) Polydeformed hornblende gneiss on limb of map-scale anticline at Stop 1-3. (D) Stellate hornblende clusters in garbenschiefer at Stop 1-3.

distinctive; Owens et al. (2012) used both conventional geother- 22.9 1.4 Continue straight on Rt. 715. mometry and pseudosection analysis and estimated maximum 27.7 4.8 Turn left onto Rt. 601. P-T (pressure-temperature) conditions of 760 °C and 10 kb for 32.1 4.4 Turn right onto Rt. 601/618. these rocks. This part of the Goochland terrane experienced high 37.5 5.4 Turn right onto U.S. Rt. 33. P conditions, yet the question remains as to when that event 40.0 2.5 In Cuckoo, turn left onto occurred. There is a compelling need for modern thermochronol- U.S. Rt. 522. ogy across the eastern Piedmont. 40.1 0.1 Turn right onto Rt. 643. 40.5 0.4 Turn left onto the farm road. Cum. Point-to-point Directions 40.8 0.3 Park in the ﬁ eld and proceed to mileage mileage and comments Stop 1-3. 16.4 0.0 Turn right (south) on Hewlett Rd. (Rt. 601). Stop 1-3: Polydeformed Gneiss SW of Cuckoo 17.9 1.5 Turn right (northwest) on Verdon (Chopawamsic Terrane, Chopawamsic Formation) Rd. (Rt. 684). Mile 40.8 (N37.9456°, W77.9182°) 21.0 3.1 Turn left (south) on Rt. 738. 21.4 0.3 Continue straight onto Rt. 800. The Chopawamsic terrane is a Middle to Late Ordovician 21.5 0.1 Turn right onto Rt. 739. volcanic-plutonic arc that was docked to Laurentia during the Downloaded from fieldguides.gsapubs.org on October 15, 2015

20 Whitmeyer et al. middle Paleozoic. Outcrops in this pasture represent intermedi- followed by a later, retrograde greenschist-facies event. Because ate to mafi c-composition metavolcanic rocks of the Chopawam- the hornblende clusters lie in the plane of foliation, they likely sic Formation that record deformation from at least two tectono- formed during the earlier, prograde, amphibolite-facies event and thermal events. This stop is ~11 km northeast of the epicenter prior to the actinolite-producing greenschist event. of the August 2011 M 5.8 Virginia earthquake, which occurred w Cum. Point-to-point Directions along a blind, previously unrecognized fault in the Chopawam- mileage mileage and comments sic terrane. The quake caused extensive damage to unreinforced masonry structures as far away as Washington, D.C. (Horton et 40.8 0.0 Turn around and retrace route al., 2015). to Cuckoo. This location is on the east limb of a map-scale anticline that 41.6 0.8 Turn left on U.S. Rt. 33. bifurcates the Columbia-Quantico syncline (Fig. 1). The syncline 68.7 27.1 Turn left on Rt. 678. preserves graphitic mica schist of the Quantico Formation, which 69.7 1.0 Arrive at Barboursville Vineyards unconformably overlies the Chopawasmic Formation. The main for lunch. body of the syncline is about a kilometer to the west, and a nar- 70.7 1.0 Turn around and retrace route to row belt of Quantico Formation lies in a subsidiary arm of the U.S. Rt. 33, then turn left. main syncline about half a kilometer to the east. This bifurcated 78.3 7.6 Turn right on U.S. Rt. 29. geometry of the Columbia-Quantico syncline, with anticlines of 87.5 9.2 Turn left (west) on VA Rt. 230. the older Chopawamsic Formation splitting the outcrop belt of 91.1 3.6 Turn right (northwest) in Wolftown the younger Quantico Formation, was fi rst identifi ed by Marr on Rt. 662 (Graves Mill Rd.). (2002). Recent mapping (Spears et al., 2013), aided by high- 92.7 1.6 Turn left (west) on Rt. 665 resolution aeromagnetic and aeroradiometric data acquired after (Garth Run Rd.). the earthquake (Shah et al., 2012), has refi ned the geometry. 93.4 0.7 Turn right (north) and continue on Second-generation interference folds evident in these out- Rt. 665 (Garth Run Rd.). crops (Fig. 5C) are a product of deformation of the earlier com- 94.8 1.4 Park on the right side of Rt. 665, positional layering and foliation during refolding of the main proceed to outcrop. Columbia-Quantico structure. Structural analysis indicates a nearly horizontal fold axis trending 015°. The main foliation dips Stop 1-4: Garth Run High-Strain Zone (Mylonitic Blue 70° west-northwest, indicating that the anticline is asymmetric Ridge Granitoids) and verges to the southeast, in contrast to the general southeast- Mile 94.8 (N38.3911°, W78.3894°) dipping orientation of most structures in the Chopawamsic terrane. Other steeply northwest-dipping structures are present This classic exposure of Blue Ridge mylonitic rocks was southeast of the main Columbia-Quantico syncline in this part created along the channel of Garth Run during an intense storm of the Chopawamsic terrane, and may be related to south- and in June 1995 that generated hundreds of debris fl ows in the southeastward-directed backthrusting linked to the refolding of steep slopes of the eastern Blue Ridge Mountains. Although two the syncline (Spears et al., 2013). decades of revegetation has taken its toll on the outcrop, there is Another interesting feature fi rst identifi ed in these outcrops still plenty to see. This exposure reveals much concerning the by Bailey and Owens (2012) is the presence of garbenschiefer deformation processes associated with Blue Ridge high-strain or “feather amphibolite.” This unusual texture is defi ned by stel- zones, and has been visited by a number of previous fi eld trips late and featherlike hornblende clusters in the plane of foliation (Bailey et al., 2004; Tollo et al., 2004; Bailey et al., 2006). (Fig. 5D). Owens and DeCourt (2014) identifi ed more than a The Garth Run high-strain zone strikes north-northwest to dozen occurrences of garbenschiefer in the Chopawamsic Forma- north-northeast; connects to other, more regionally extensive, tion, all of which are on the limbs of map-scale anticlines. Steffen high-strain zones to the north; and tips out 1–2 km to the south. et al. (2001) described similar rocks in the Greiner shear zone The high-strain zone is ~125 m thick and dips moderately to the in the Eastern Alps, where they are interpreted to have formed east. In the footwall to the west, the zone is bound by weakly during ductile deformation. Rapid intergranular diffusion aided deformed to massive ca. 1050 Ma charnockite and, in the hang- by the presence of small quantities of hydrous fl uid enables the ing wall, to the east, by ca. 1180 Ma leucogranitic rocks (Bailey growth of large hornblende crystals, effectively strain-hardening et al., 2003; Tollo et al., 2004). the mafi c layers and resulting in strain concentration and grain Rocks exposed in the Garth Run high-strain zone are pri- size reduction in plagioclase+quartz layers. marily porphyroclast-bearing protomylonites and mylonites. Bailey and Owens (2012) identifi ed three amphibole species Concordant to slightly discordant tabular to lenticular bodies of here. Hornblende and cummingtonite dominate the mineralogy fi nely layered leucogranite gneiss, coarse-grained leucogranite, of the mafi c layers; actinolite is present as late stage growths and well-foliated fi ne-grained greenstone (0.2–2 m thick) are overprinting the earlier fabric. The two-stage amphibole growth abundant. Foliation strikes north-northwest to north-northeast is evidence for an early amphibolite-facies metamorphic event and dips to 30°–50° to the east. A mineral elongation lineation, Downloaded from fieldguides.gsapubs.org on October 15, 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products 21 not everywhere discernible, plunges downdip to obliquely hanging wall up). Sections normal to both foliation and linea- downdip. Sheath folds occur in the fi nely layered leucogra- tion also preserve asymmetric structures (with a sinistral sense nitic gneiss (Fig. 6A). Coarse-grained leucogranite bodies dis- of shear) (Fig. 6B). We interpret these structures to indicate that play pinch-and-swell structures, and are commonly isolated as Garth Run mylonites experienced a triclinic deformation sym- lozenge-shaped boudins. At many locations, the leucogranite metry. Folded boudins (Fig. 6C) may record a non–steady-state bodies are elongated both parallel to and normal to foliation, deformation path that changed from simple shear dominated to consistent with fl attening strain. Leucogranite dikes are locally pure shear dominated over time. folded. In places, the mylonitic foliation is kinked into narrow The mineralogy and microstructures evident in Garth Run bands. At the northern end of the outcrop, a very fi ne-grained mylonites are consistent with deformation at greenschist-facies rock contains angular fragments (up to 2 cm) of leucogranitic temperatures of ~400 °C. An 40Ar/39Ar cooling age from white rocks; the texture and geochemical character of this rock is con- mica in these mylonites yields a broad plateau at 333 ± 8 Ma sistent with a devitrifi ed pseudotachylyte. (Wooton et al., 2005). This cooling age is similar to other ages in Asymmetric structures are common in the Garth Run high- the central Virginia Blue Ridge, and places a minimum constraint strain zone. Strain and vorticity analysis indicates that Garth Run on the timing of mylonite formation in these rocks. Penetrative mylonites generally record fl attening strains (K = 0.05–0.5) and deformation in the Blue Ridge basement and cover sequence pre- general shear (Wm = 0.4–0.6) (Bailey et al., 2004). Shear-strain dates the classic Alleghanian orogen (Valley and Ridge folding integration across the zone yields a total displacement of ~500 m. and thrusting); contractional deformation and metamorphism Sections normal to foliation and parallel to lineation consistently in the Virginia Blue Ridge is consistent with Neoacadian (320– preserve structures with a top-to-the-west asymmetry (reverse/ 350 Ma) activity.

Figure 6. (A) Sheath fold in leucocratic mylonite at Stop 1-4, view is parallel to elongation lineation. (B) Folded boudins of leucogranitic dikes in mylonite. (C) Weakly asymmetric structures in porphyroclastic mylonite, view is parallel to elongation lineation/YZ section. (D) Granitic and gneissic clasts in Lynchburg Group metaconglomerate at Stop 1-6; note the grain-to-grain contacts and truncations. Downloaded from fieldguides.gsapubs.org on October 15, 2015

22 Whitmeyer et al.

Cum. Point-to-point Directions Discrete, mm- to dm-scale high-strain zones cut all base- mileage mileage and comments ment units at this outcrop. Typically, foliation strikes to the 94.8 0.0 Turn around and retrace route to northeast and, where discernible, mineral elongation lineations Wolftown. plunge obliquely downdip. The apparent offset, as illustrated on 98.5 3.7 Turn right (east) on VA Rt. 230. the subhorizontal outcrop surface, is dextral, but out-of-section 101.2 2.7 Turn left (north) on Rt. 657 movement occurred as well. The mineral assemblage and micro- (Thrift Rd.). structures in the high-strain zones are consistent with formation 102.4 1.2 Continue straight on Rt. 658 under greenschist-facies conditions, and we interpret these as (Ruth Rd.). Paleozoic contractional structures. 106.5 4.1 Turn left (north) on Rt. 652 (Gaar Cum. Point-to-point Directions Mtn. Rd.). mileage mileage and comments 108.8 2.3 Turn left (west) on Rt. 651 (Aylor Rd.). 109.4 0.0 Turn around and proceed back 109.4 0.6 Park in fi eld on the right and pro- to junction. ceed to outcrop. 110.0 0.6 Turn left (north) onto Rt. 651 (Aylor Rd.). Stop 1-5: Aylor Farm, 1 km West of Rt. 651 and Rt. 652 112.9 2.9 Turn right (south) onto VA Rt. 231. (Leucogranitoids with Dikes of Biotite-Bearing Granitoid 117.2 4.3 Turn left onto U.S. Rt. 29 Business and Porphyritic Metagabbro) (in Madison). Mile 109.4 (N38.4246°, W78.3231°) 118.0 0.8 Turn left onto U.S. Rt. 29. 120.9 2.9 Turn right onto Rt. 647. This large outcrop was exposed by debris fl ows in June 1995 121.1 0.2 Turn right (south) onto Rt. 607 (Lil- and has proven an important exposure for understanding of the liards Ford Rd.). Blue Ridge basement complex. Rocks at this exposure are part 122.1 1.0 Turn left (east) onto Rt. 629 (Spring of a composite felsic pluton that includes: (1) strongly foliated, Branch Rd.). coarse-grained to megacrystic, leucocratic granite; (2) weakly 124.2 2.1 Turn left (south) onto Rt. 632 foliated, medium-grained leucogranite; and (3) leucogranite peg- (Beahm Town Rd.). matite. At the base of the outcrop, an ~10–15-cm-thick dike of 126.8 2.6 Turn left (east) onto Rt. 618 (Fords leucogranite pegmatite is deformed into a series of tight folds Shop Rd.). with rounded hinges. The surrounding medium-grained leuco- 127.5 0.7 Park in fi eld on the left and proceed granite has a weak foliation that strikes ~070°, dips steeply to the to outcrop. north, and is axial planar to the folded dike. The medium-grained leucogranite is also deformed, although the fabric is only weakly Stop 1-6: Lynchburg Group Conglomerate (Conglomerate developed, indicating that static recrystallization at high tempera- with Granitic Clasts) tures occurred after deformation. The high-temperature foliation Mile 127.5 (N38.3911°, W78.1423°) is well developed in the coarse-grained megacrystic granite and is commonly overprinted by a foliation defi ned by aligned micas. Outcrops of metaconglomerate and arkosic metawacke are At the larger exposure, 20 m uphill from the base of the debris exposed in both the pasture and streambed. Clasts range from fl ow scar, a 30–50-cm-thick dike of fi ne- to medium-grained, granules to boulders and include granitoid gneiss, granite, quartz, biotite-bearing granitoid intrudes all of the leucocratic granit- and phyllitic mudstone. Some layers are dominated by a frame- oids, postdating the high-temperature deformation recorded in work of well-rounded clasts, whereas other layers are matrix- the leucogranitoids. The mineral assemblage of this dike is simi- dominated with angular clasts. The most intriguing clasts are lar to the widespread ca. 1030 Ma biotite-bearing monzogranite subangular phyllitic mudstones (some >50 cm in diameter) that mapped nearby (Bailey et al., 2003). The high-temperature defor- enclose outsized granitoid clasts. In the pebble to cobble meta- mation fabric visible here in the leucogranitoids is characteris- conglomerate, many clasts are bluntly terminated and/or indented tic of the older suite of Blue Ridge Grenvillian granitoids. The against other clasts, consistent with deformation by pressure dis- basement units here are cut by a northeast-striking, ~5-m-wide solution processes (Fig. 6D). At this stop, bedding is upright and dike of porphyritic hornblende metagabbro. The metagabbro is dips moderately to the southeast. These exposures are ~2 km part of a suite of mafi c to ultramafi c igneous rocks that intrude upsection from the basement-cover contact to the west. Finer Blue Ridge basement and Neoproterozoic metasedimentary units grained layers have a penetrative greenschist-facies foliation in the central and eastern Blue Ridge. The dike may be related that also dips to the southeast. Glover (1989) and Kasselas and to ca. 570 Ma Catoctin volcanism or an older pre-Catoctin pulse Glover (1997) postulated that the penetrative foliation in eastern of Neoproterozoic mafi c magmatism that affected rocks in the Blue Ridge cover rocks developed during the Late Ordovician eastern Blue Ridge. (Taconic); however, 40Ar/39Ar cooling ages for white micas in the Downloaded from fieldguides.gsapubs.org on October 15, 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products 23

Blue Ridge cover sequence range from 355 to 325 Ma, and likely 28.1 1.2 Turn right (north) at the Thorn- refl ect Neoacadian deformation (Bailey et al., 2007a). ton Gap entrance to Shenandoah End of day 1 fi eld stops; proceed northeast back to U.S. Rt. National Park. 29 and turn right onto Rt. 29 north. Drive ~8 miles to the Rt. 299 29.2 1.1 Pass through the park entrance gate- (Business Rt. 29) exit. Drive 0.25 miles north on Rt. 299 to the house and then proceed south on Holiday Inn Express. Skyline Dr. to Mary’s Rock tunnel. 29.4 0.2 Pause at the north end of Mary’s DAY 2—BLUE RIDGE TO VALLEY AND RIDGE Rock tunnel to admire the Catoctin feeder dike, then proceed through The road log for day 2 starts at the Holiday Inn Express on Rt. the tunnel to the parking lot at the 299 just south of Culpeper (N38.4555°, W78.0199°; mile 0.0). south end. Cum. Point-to-point Directions Stop 2-2: Mary’s Rock Tunnel Parking Lot mileage mileage and comments (Foliated Charnockite Gneiss) 0.0 0.0 Drive northeast on Rt. 299 Mile 29.4 (N38.6525°, W78.3116°) through Culpeper. 2.1 2.1 Turn left (west) onto Rt. 522 (W. On the north side of the Skyline Drive tunnel, a dike of Neo- Evans St.). proterozoic Catoctin metabasalt intrudes foliated charnockite 21.1 19.0 Turn left (west) onto Rt. 211, just gneiss, part of the Blue Ridge basement sequence (Fig. 7). The north of Sperryville. dike is probably a feeder dike for Neoproterozoic basalt fl ows 26.9 5.8 Turn right and park on the grass at that once covered the basement complex. Note that the present- the old quarry on the north side of day elevation of this dike outcrop is ~200 m above the Catoc- Rt. 211. tin quarry of the previous stop. Therefore, if this outcrop is a “feeder” for fl ows, such as those at the previous stop, there is Stop 2-1: Small Abandoned Quarry on the North Side of likely a normal fault located between the two stops (e.g., such as Rt. 211 (Catoctin Fm.) the one mapped at Thornton Gap; Southworth et al., 2009). Mile 26.9 (N38.6625°, W78.3075°) The south side of the tunnel has several large boulders of megacrystic charnockite gneiss (Ycf of Tollo et al., 2004, dated This old quarry exhibits massive to well-foliated metaba- close to this location at 1159 ± 14 Ma) that exhibit 1–5 cm θ salts (greenstones) of the Catoctin Formation (ca. 570 Ma, Alein- and σ plagioclase porphyroclasts within a well-defi ned foliation. ikoff et al., 1995). Abundant spherical to ellipsoidal vesicles and Unfoliated charnockite (Ygr of Tollo et al., 2004), dated at 1060 ± amygdules are typically fi lled with quartz ± epidote. This outcrop 5 Ma in the Old Rag Mountain quadrangle to the south (Tollo et is an example of regionally extensive Catoctin basalts that are al., 2004), intrudes the foliated megacrystic charnockite. Thus, interpreted as fl ood basalts that extruded and blanketed the surface during the eastern breakup of Rodinia and opening of the Iapetus Ocean. Catoctin Formation basalts exhibit evidence for both submarine (pillow structures) and subaerial extrusion. Cloos (1971) documented moderately plunging, ESE-trending, elongated amygdules in this general area. Southeast- dipping shear bands with slickensided surfaces are abundant and indicate east over west transport. The shear bands and foliation likely occurred during later-stage brittle deformation of the Alleghanian orogenic cycle, which juxtaposed Blue Ridge basement rocks and cover sequences over Cambrian–Ordovician carbonates in Page Valley. Greenschist-facies metamorphism is relatively unconstrained, but likely happened prior to Alleghanian deformation. Note that this quarry and the next stop (Mary’s Rock tunnel) are both within Shenandoah National Park, so no sample collect- ing is allowed without a permit. Cum. Point-to-point Directions mileage mileage and comments Figure 7. Steeply dipping dike of metabasalt (Catoctin Formation) in- truding Mesoproterozoic charnockitic gneiss on the west side of the 26.9 0.0 Turn right (west) on Rt. 211 and north portal to Mary’s Rock tunnel at Stop 2-2. Note well-developed continue uphill. columnar joints in the metabasalt. Downloaded from fieldguides.gsapubs.org on October 15, 2015

24 Whitmeyer et al.

the gneissic and protomylonitic fabrics observed at this location Blue Ridge thrust system, inﬁ ltration of highly fractured quartz- likely formed in the period between 1159 Ma and 1060 Ma in ite by manganese-saturated waters enabled replacement of clay an early phase of the Grenville orogeny, the last episode in the matrix with manganese oxide cement. Elsewhere, manganese assembly of the Rodinia supercontinent. deposits have been mined within residual clays of the Waynes- boro and Tomstown (Shady) Formations (King, 1950), but there Cum. Point-to-point Directions has not been extensive study on the ore-bearing deposits of the mileage mileage and comments Antietam Formation in the region of the Jeremys Run outcrops. 29.4 0.0 Reverse direction and travel north Cum. Point-to-point Directions on Skyline Dr. to exit Shenandoah mileage mileage and comments National Park at Thornton Gap. 30.7 1.3 Turn right (west) onto Rt. 211. 46.2 0.0 Reverse direction and travel south 39.2 8.5 Turn right on exit to north Rt. 340. and west on Vaughn Summit Rd. to 39.4 0.2 At end of exit ramp, turn right onto Rt. 340. Rt. 340 north. 48.9 2.7 Turn left (south) on Rt. 340. 43.5 4.1 Turn left on Vaughn Summit Rd. (Rt. 53.2 4.3 Go under Rt. 211 bridges to the 654, changes to Rt. 611 at Hulse Rd.). entrance to the parking lot for 46.2 2.7 Cross railroad tracks at 1.8 miles; Luray-Hawksbill Greenway. cross bridge over Jeremiah Run at 53.3 0.1 Turn right and park in the lot for 2.4 miles, and park on the left (west) Luray-Hawksbill Greenway. Out- side of road next to yard at house. crops are to the north along the path Outcrop is in the stream to the west on the bank of the Hawksbill River of the road—get permission to visit under the Rt. 211 overpass. at the nearby house. Optional Stop 2-4: Hawksbill River Greenway under Stop 2-3: River Cliff of Antietam Breccia (Breccia of Rt. 33 (Beekmantown Fm.) Antietam Fm.–Vaughn Fault) Mile 53.3 (N38.6727°, W78.4579°) Mile 46.2 (N38.7262°, W78.3886°) Outcrops along the bank of the Hawksbill River under the This section of Jeremys Run exposes alternating beds of Rt. 211 overpass show paleokarst layers in limestones and dolo- Early Cambrian Antietam Formation (Chilhowee Group) quartz mites of the Beekmantown Formation. Paleokarst layers near the arenite/quartzite and siltstone in the streambed. The cliffs near upper contact of the Beekmantown Formation occur throughout the house along the outside bend (cut bank) of the stream con- Page Valley (Kirby et al., 2008), and are interpreted as karst col- sist of 20–30 m vertical exposures of Antietam derived breccia. lapse features related to the regionally extensive erosional surface These are some of the largest exposures of Antietam breccia that of the Knox unconformity (Mussman et al., 1988; Smosna et al., typically occur in discontinuous outcrops along the frontal Blue 2005). At this location, however, we may not be all that close to Ridge thrust. The Jeremys Run exposures are likely a continu- the top of the Beekmantown Formation, as the contact with the ation of the Vaughn fault, as mapped by Allen (1967), and are interpreted as part of the Blue Ridge thrust system (Drummond et al., 2011a). subvertical fractures Brecciated outcrops of Antietam Formation quartzites are sometimes ore-bearing and vary structurally and mineralogically. Composed of angular to sub-angular fragments, exposures are not laterally extensive—typically occurring as meters to decime- ters high subvertical outcrops. Alleghanian west-directed thrust faulting likely generated fractures, and hydraulic dilation of incipient breccia, as well as more chaotic clast rotation occurred within collapsed voids. This mechanism has been shown to pro- duce breccias of similar character in the Dent fault, NW England (Woodcock et al., 2006). Evans (1992) proposed an analogous breccia mechanism for formation of Antietam breccia outcrops in the Elk- basal thrust surface ton area in which west-directed thrusting produced breccia along Figure 8. Breccia genesis mechanism as proposed by Evans (1989), the low-angle principal thrust surface, with associated subvertical whereby breccias generated by frictional processes along a subhorizon- fractures facilitating the injection of breccia as discrete columns tal basal thrust are injected via overpressurized siliceous ﬂ uids into in- above the basal thrust (Fig. 8). In other locations along the frontal cipient subvertical fractures that form above the principal thrust surface. Downloaded from fieldguides.gsapubs.org on October 15, 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products 25

underlying Conococheague Formation is nearby in downtown on the north side of U.S. 211 (Patterson and Whitmeyer, 2012). Luray, whereas contacts with overlying formations (Lincoln- Detailed mapping in this area suggests that the Lincolnshire and shire, New Market, Edinburg Fms.) are several miles to the east New Market limestones are absent (Fig. 9). The narrow regions (see Stop 2-5). between Edinburg and Beekmantown outcrops (e.g., directly east of the quarry) are grassy with several low, water-ﬁ lled depres- Cum. Point-to-point Directions sions. We suggest that this zone of no outcrop represents the sur- mileage mileage and comments face expression of a cryptic west-directed thrust fault that locally 53.3 0.0 Exit the parking lot and travel back has juxtaposed Beekmantown dolostones and limestones against north on Rt. 340 to the entrance Edinburg limestones (Fig. 9), thereby facilitating the removal ramp for Rt. 211 west. of the Lincolnshire and New Market limestones at the surface 53.6 0.3 Turn left (west) on Rt. 211. through subsequent weathering and denudation. The fault does 58.4 4.8 Turn left (south) on Gander’s Dr. not extend far to the south, as Edmundson (1945) and Cooper and (just before the bridge over Shenan- Cooper (1946) document both New Market (Mosheim) and Lin- doah River, South Fork). colnshire (Lenoir) limestones near Leaksville, less than a mile 58.7 0.3 Park in the pullout on the right south of the quarry. In our interpretation, the folds in the quarry (west) side of the road next to the initially formed as upright, open folds similar to geometries river. The quarry is on the opposite seen elsewhere in Page Valley. The cryptic, west-directed thrust (east) side of the road. occurred later in the Alleghanian orogenic cycle and tightened and rotated the preexisting footwall folds to horizontal. Stop 2-5: Quarry on the Shenandoah River, South Fork Cum. Point-to-point Directions (Edinburg Fm.) mileage mileage and comments Mile 58.7 (N38.6432°, W78.5303°) 58.7 0.0 Travel back north on Gander’s Dr. to This quarry is on the eastern bank of the South Fork of the Rt. 211. Shenandoah River (Fig. 9) and exposures along the road display 59.0 0.3 Turn right (east) on Rt. 211. limestone beds of the Edinburg Formation, which are recum- 60.6 1.6 Turn left (north) on Airport Rd. bently folded in the northeastern quarry wall (Fig. 10). The Edin- 62.6 2.0 Turn left (north) on Bixler’s Ferry Rd. burg Formation is the uppermost Ordovician carbonate in this 64.9 2.3 Turn right and park in the riverside area, deposited at the time of the arrival of the Taconic arc system lot at the junction with S. Page Val- offshore of the eastern margin of Laurentia. The unit consists of ley Rd. (Rt. 684). black (weathering gray), amalgamated micrite, sometimes inter- bedded with thin black shale. Stratigraphic and sedimentary fea- Stop 2-6: East of Bixler’s Bridge (Lower Martinsburg Fm.) tures are best observed along the road just south of the quarry, Mile 64.9 (N38.8621°, W78.4007°) where the beds are sub-horizontal with minor folding and faulting. Beds are generally a few centimeters thick, but some reach Thin-bedded argillaceous rocks have absorbed a great a thickness of half a meter or more. The Edinburg Formation is deal of deformation in the Appalachian foreland, both as pen- interpreted to have been deposited in a deep-water anoxic envi- etrative strain and mesoscopic folding and faulting. Here, along ronment by mass transport processes from the west ( present-day the South Fork of the Shenandoah River, extensive exposures orientation) during subsidence of the Taconic foreland basin. of the Martinsburg Formation typically display outcrop-scale Though not as obvious at this stop, other locations have good folds and faults, as well as strongly developed slaty cleavage. examples of scours, slump, and soft sediment deformation fea- The cuts along this stretch of South Page Valley Road, extend- tures (e.g., Lowry and Cooper, 1970; Pritchard, 1980; Read, ing in a west-northwesterly direction, are nearly perpendicular 1980) that suggest down-slope mass movement. to regional strike and provide an excellent cross-sectional view Folding and development of horizontal axial-planar cleavage of these structures. (Fig. 10) likely occurred during one or more shortening episodes The Martinsburg Formation is an ~1-km-thick sequence during the Alleghanian orogeny. Structural features at a variety of turbiditic shale and sandstone that represents burial of the of scales are apparent on the quarry walls. These include large- Cambrian–Ordovician carbonate shelf beneath the Taconic clas- scale recumbent folds; note that the left and right quarry walls are tic wedge during the Middle to Late Ordovician (Spears, 1983). dipping in opposite directions. The right quarry wall contains the Here in the lowermost Martinsburg, thin, argillaceous limestone large-scale fold hinge. beds persist in the eastern, downsection part of the outcrop. The Folds throughout Page Valley are generally upright with limey beds disappear as the formation becomes dominated by sub-vertical axial planes. In contrast, the large-scale folds in this shale and thin sandstone beds upsection to the west. quarry have sub-horizontal axial planes (Fig. 9). This fold pattern Drummond et al. (2011b), in their geologic map of the Luray has been traced to the northeast in ﬁ elds of the White House Farm quadrangle, mapped a laterally extensive northeast-striking thrust Downloaded from fieldguides.gsapubs.org on October 15, 2015

26 Whitmeyer et al. A A′ 1000 ft Oln 7 -> 007 900 ft

800 ft Ob Oe 700 ft Oln Ob 600 ft

3 -> 021 A

A′ Oe

9 -> 005

4 -> 203

Ob Ob

(Edinburg) (Beekmantown) Thrust fault

Figure 9. Geologic map of White House Farm and quarry region at Stop 2-5. Inset cross section shows west-directed thrust fault that juxta- poses Beekmantown Formation over Edinburg Formation. Stereonets show poles to bedding and interpreted fold axes. Downloaded from fieldguides.gsapubs.org on October 15, 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products 27

dding beddingbe bedding bedding bedding

cleavagecleavag e axialaxial cleavagecleavage

Figure 10. (A) Photo of northeastern quarry wall at Stop 2-5 showing recumbently folded beds with horizontal axial-planar cleavage. (B) Detail of bedding and refracted cleavage indicating ﬂ exural slip during folding.

fault cutting through this outcrop. The unnamed fault places the Cum. Point-to-point Directions Edinburg Formation over the Martinsburg Formation. Strati- mileage mileage and comments graphically, the Martinsburg lies directly above the Edinburg, so 64.9 0.0 Exit the parking lot and turn left the fault is likely to have relatively little displacement compared (east) on N. Egypt Bend Rd. to regionally signifi cant thrust faults such as the Little North (Rt. 684). Mountain fault in the western Shenandoah Valley. 65.4 0.5 Turn right (north) on Fort Valley Rd. Despite the apparently minor displacement on the mapped (Rt. 675). fault, these rocks are quite deformed and at least seven intrafor- 68.1 2.7 Outcrops of Massanutten Sandstone mational faults can be identifi ed in this outcrop (Fig. 11). Most of are visible on left (east) side of road. the bedding dips steeply to the southeast, and bedding-cleavage 68.2 0.1 Park at the top of the hill and walk relationships indicate that in some of the fault-bounded blocks, back down the road to the outcrop. the bedding is overturned. At least two anticlines are exposed; synclinal axes appear to be faulted out. Optional Stop 2-7: Large Roadcut on the West Side of Shale beds display a well-developed slaty cleavage that fans Fort Valley Rd. (Massanutten Sandstone) slightly around the fold hinges (Fig. 12A). Limestone beds dis- Mile 68.2 (N38.7026°, W78.4917°) play spaced cleavage. In some of the steep to overturned limestone beds, the spaced cleavage has been reactivated as slip This extensive roadcut along the east fl ank of Massanutten planes accommodating subhorizontal contraction and vertical Mountain provides excellent exposures of cross-bedded Mas- extension during late deformation (Fig. 12B). sanutten Sandstone. The Massanutten Sandstone consists of Downloaded from fieldguides.gsapubs.org on October 15, 2015

28 Whitmeyer et al.

W E

Covered

0 10 20 30 40 FEET

0 3 6 9 12 METERS

Figure 11. Outcrop sketch of structures in lower Martinsburg Formation along South Page Valley Road east of Bixler’s Bridge at Stop 2-6. Heavy lines represent faults; light lines represent bedding; dashed lines represent cleavage. Approximately 2× vertical exaggeration.

indurated, medium- to coarse-grained quartz arenite beds with meandering fl uvial system, are absent. Overall, this suggests a planar and trough cross-bedding. Rare beds with granules in proximal braided system in the north and a more distal braided the cm range are also present. system to the south. Alternatively, the Massanutten Sandstone Pratt et al. (1978) and Pratt (1979) proposed that the Mas- may refl ect a low sinuosity river or a coarse-grained meander belt sanutten Sandstone represents a Silurian braided river system. transitioning from a classic braided river to a classic meandering Supporting evidence for the braided river interpretation includes: river (Whitmeyer et al., 2012). This interpretation is supported by Trough and planar cross-bedding (transverse bars) at this loca- trough cross-bedding (lunate ripple migration) mixed with planar tion, and gravel beds (longitudinal bars) at the northern end of cross-beds (transverse bars). Massanutten Mountain that become cm-scale granule beds here. Paleocurrent data from the Massanutten Mountain region Signifi cant silts and shales, which would otherwise indicate a suggest north (Yeakel, 1962) to northeast transport (Roberts and

Figure 12. (A) Anticline with axial-planar slaty cleavage in shale and thin limestone beds, lower Martinsburg Formation at Stop 2-6. Note- book is 18 cm tall. (B) Spaced cleavage in nearly vertically dipping, thin-bedded limestone at Stop 2-6. The spaced cleavage has been reactivated as slip surfaces accommodating vertical extension during late deformation. Coin is 2.2 cm in diameter. Downloaded from fieldguides.gsapubs.org on October 15, 2015

A billion years of deformation in the central Appalachians: Orogenic processes and products 29

Kite, 1978) during the period of deposition of the Massanutten Cum. Point-to-point Directions Sandstone. Sedimentary transport subparallel to regional strike mileage mileage and comments (NE) suggests the presence of a tectonic trough in this part of 68.2 0.0 Exit the parking lot and continue the Appalachian foreland during deposition of the Taconic clas- on Rt. 675 basically north, winding tic wedge. Both the Martinsburg Formation (Rader and Biggs, down the mountain toward 1976) and the Massanutten Sandstone (Roberts and Kite, 1978) Fort Valley. are more than twice as thick in the Massanutten synclinorium 69.9 1.7 Turn right (north) on Camp Roos- than their stratigraphic equivalents in outcrop belts west of the evelt Rd. (still Rt. 675). Shenandoah Valley, further evidence of a syndepositional trough 73.2 3.3 Bear right onto Fort Valley Rd. during Late Ordovician to Middle Silurian time. Cloos (1971), (Rt. 678). however, maintains that the stratigraphic section in the proximal 81.8 8.6 Turn left on Rt. 770 and left into the foreland “has doubled in thickness and reduced its width to half” gravel driveway of Trinity Church. (p. 111) due to penetrative deformation. Cloos’s evidence is from deformed carbonate rocks, but shales in the Massanutten syn- Stop 2-8: Trinity Church, Fort Valley (Mahantango Fm.) clinorium bear evidence of similar magnitudes of subhorizontal Mile 81.8 (N38.7025°, W78.4909°) shortening and subvertical elongation (see discussion for Stop 2-8). Although it is not likely that the brittle quartz arenites of the The Massanutten synclinorium is a deep structural trough Massanutten Sandstone are tectonically shortened and thickened that bisects the Shenandoah Valley. The syncline preserves the to the same degree, an analysis of the quartz fabric, gash veins, youngest rocks in the entire region, including the ridge-forming wedge faults, and intraformational folds is needed to quantify the Massanutten Sandstone, the eastern equivalent of the Tuscarora degree to which its measured thickness is tectonically inﬂ uenced. Formation. The ﬂ oor of this valley is underlain by the Middle

Figure 13. Strain geometry determined from ﬁ brous chlorite pressure fringes on diagenetic framboidal pyrite in the Martinsburg Formation,

Massanutten synclinorium. (A) XZ section with S1 slaty cleavage oriented vertically indicates extension in the X direction and contraction in the Z direction. (B) XY section indicates extension in the X direction (vertical) and no extension in the Y direction (horizontal). YZ sections show no chlorite fringes. Together, these observations indicate a state of plane strain. From Spears (1983). Scale bars represent 100 µm in length. Downloaded from fieldguides.gsapubs.org on October 15, 2015

30 Whitmeyer et al.

End of ﬁ eld trip! Return to Baltimore by continuing north on Rt. 678 11.3 miles to the intersection with Rt. 55 (Strasburg Rd.). Turn right (east) on Rt. 55 and follow for 5.1 miles to Rt. 340. Turn left (north) on Rt. 340 and follow for 1.0 miles to the entrance ramp for I-66 east. Take I-66 east for 58 miles to I-495 north (toward Tyson’s Corner/Baltimore). Follow I-495 north for 21 miles to I-95 north (toward Baltimore). Take I-95 north for ~26 miles to I-395 north (toward Inner Harbor).

ACKNOWLEDGMENTS

This ﬁ eld guide was substantially improved by comments and suggestions from Matthew Heller and Brent Owens.

REFERENCES CITED

Figure 14. Deformed brachiopods and crinoids at Stop 2-8, Middle Aleinikoff, J.N., Zartman, R.E., Walter, M., Rankin, D.W., Lyttle, P.T., and Bur- Devonian Mahantango Formation, Fort Valley, Shenandoah County. ton, W.C., 1995, U-Pb ages of metarhyolites of the Catoctin and Mount Rogers Formations, central and southern Appalachians: Evidence for two pulses of Iapetan rifting: American Journal of Science, v. 295, p. 428– 454, doi:10.2475/ajs.295.4.428. Devonian Mahantango Formation, a fossiliferous siltstone that Aleinikoff, J.N., Horton, J.W., Jr., and Walter, M., 1996, Middle Proterozoic is so poorly bedded it commonly exhibits spheroidal weathering. age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia: Geological Society of America Bulletin, v. 108, p. 1481–1491, At this location, bedding, which is defi ned only by layers of dis- doi:10.1130/0016-7606(1996)108<1481:MPAFTM>2.3.CO;2. articulated fossils, dips gently to the southeast. Weak penetrative Aleinikoff, J.N., Burton, W.C., Lyttle, P.T., Nelson, A.E., and Southworth, C.S., cleavage dips steeply southeast. 2000, U-Pb geochronology of zircon and monazite from Mesoprotero- zoic granitic gneisses of the northern Blue Ridge, Virginia and Mary- One persistent problem in interpreting foreland structures land: Precambrian Research, v. 99, no. 1–2, p. 113–146, doi:10.1016/ and balancing cross sections is quantifying the amount of pen- S0301-9268(99)00056-X. etrative deformation recorded in the rocks. Cloos (1947, 1971) Allen, R.M., Jr., 1967, Geology and Mineral Resources of Page County: Vir- ginia Division of Mineral Resources Bulletin 81, 78 p. used deformed quartz-fi lled vesicles in the Catoctin Formation Badger, R.L., and Sinha, A.K., 1988, Age and Sr isotopic signature of the and deformed ooids in the carbonate rocks of the Great Valley Catoctin volcanic province—Implication for subcrustal mantle evolution: to quantify penetrative strain in Maryland and northern Virginia. Geology, v. 16, p. 692–695, doi:10.1130/0091-7613(1988)016<0692 :AASISO>2.3.CO;2. Spears (1983) and Woodward et al. (1986) used chlorite-fi lled Badger, R.L., and Sinha, A.K., 2004, Geochemical stratigraphy and petrogenesis of pressure shadows on framboidal diagenetic pyrite grains to quan- the Catoctin volcanic province, central Appalachians, in Tollo, R.P., Corriveau, tify strain in cleaved Ordovician and Devonian shales in the Mas- L., McLelland, J., and Bartholomew, M.J., eds., Proterozoic Tectonic Evolu- tion off the Grenville Orogen in Eastern North America: Geological Society of sanutten synclinorium. The geometry of the pressure shadows America Memoir 197, p. 435–458, doi:10.1130/0-8137-1197-5.435. indicates that the bulk deformation has a plane strain geometry Bailey, C.M., and Owens, B.E., 2012, Traversing suspect terranes in the cen- (Fig. 13). Using the strain integration technique of Hossack tral Virginia Piedmont: From Proterozoic anorthosites to modern earth- quakes, in Eppes, M.C., and Bartholemew, M.J., eds., From the Blue (1978), Spears (1983) determined that over 40% shortening had Ridge to the Coastal Plain: Field Excursions in the Southeastern United occurred within the shale units of the Massanutten synclinorium. States: Geological Society of America Field Guide 29, p. 327–344, Another classic strain marker is deformed fossils. The Mah- doi:10.1130/2012.0029(10). Bailey, C.M., and Simpson, C., 1993, Extensional and contractional deforma- antango Formation contains abundant crinoids and brachiopods; tion in the Blue Ridge province, Virginia: Geological Society of America here, near the central axis of the Massanutten synclinorium, fos- Bulletin, v. 105, p. 411–422, doi:10.1130/0016-7606(1993)105<0411 sils are clearly deformed (Fig. 14). Measurement of the axial :EACDIT>2.3.CO;2. Bailey, C.M., Simpson, C., and De Paor, D.G., 1994, Volume loss and tectonic ratios of 31 deformed crinoid segments yield values (long axis/ fl attening strain in granitic mylonites from the Blue Ridge Province, cen- short axis) ranging from 1.03 to 1.44 with a mean of 1.18 and a tral Appalachians: Journal of Structural Geology, v. 16, p. 1403–1416, standard deviation of 0.11. Assuming plane strain, as indicated doi:10.1016/0191-8141(94)90005-1. Bailey, C.M., Berquist, P.J., Mager, S.M., Knight, B.D., Shotwell, N.L., and by the evidence from the pressure shadow study, and no vol- Gilmer, A.K., 2003, Bedrock Geology of the Madison 7.5′ Quadrangle, ume loss, the axial ratio of the deformed crinoid segments is a Virginia: Virginia Division of Mineral Resources Publication 157, 22 p. proxy for YZ strain. 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