Downloaded from fieldguides.gsapubs.org on October 15, 2015 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 defor- mation 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 fore- land 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. 11 Downloaded from fieldguides.gsapubs.org on October 15, 2015 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 sys- tem; 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 east- ern 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.