The geology of the Harpers Ferry quadrangle records a diverse tectonic history of orogenies followed by episodes of rifting (table 3). Stratigraphy, structure, and metamorphism of the rocks represent several Wilson cycles of opening and closing ocean basins (Wilson, 1966) (fig. 25).
The oldest rocks are a suite of paragneisses that were assimilated by granitoid plutons during the Grenville orogeny. Isotopic data of granitoids in the region show that the plutonism lasted from 1144 to 1058 Ma (table 3). Rod and bleb perthite, almandine, hypersthene, hornblende, red biotite, and blue quartz in the gneissic foliation of these rocks record granulite-facies metamorphism that has a hornblende cooling age of 920 Ma (Kunk and others, 1993). These rocks show compositional and metamorphic layering (gneissic foliation) of granulite-facies mineralogy that often strikes northwest, parallel to unit contacts.
The Grenville orogeny was followed by a long period of Late Proterozoic extension. The Robertson River Igneous Suite (Tollo and others, 1991), found 14 km to the southwest, is the largest anorogenic pluton in the central Appalachian Blue Ridge province. The peralkaline granites indicate a period of incipient continental rifting that lasted from 730 to 700 Ma (Tollo and Aleinikoff, 1992).
Late Proterozoic metasedimentary rocks of the Swift Run Formation were deposited unconformably on an irregular surface of basement granitoids. They are correlative with rocks of the Fauquier (Espenshade, 1986) and Mechum River Formations (Gooch, 1958), as well as with rocks of the Lynchburg Group (Wehr, 1985). Exposures of the Fauquier Formation and Lynchburg Group are restricted to the east limb of the Blue Ridge-South Mountain anticlinorium and are interpreted to be shallow-water marine facies of a restricted basin (Conley, 1989). The Mechum River Formation occupies a long, linear synclinorium along the axial region of the anticlinorium that has been interpreted to be a Late Proterozoic graben (Gooch, 1958; Schwab, 1986). Fault bounded outliers of the Mechum River Formation can be traced to rocks of the Fauquier Formation of the east limb.
Late Proterozoic normal faults that were active during sedimentation of the Fauquier Formation have been recognized by Espenshade (1986) and Kline and others (1991) to the south. These high-angle faults strike northwest and offset the basement-cover contact. Cataclastic breccia (Espenshade, 1986) and extensional shear fabrics (Kline and others, 1991) are found in the Marshall Metagranite of the footwall. Elsewhere, clasts of granitoids in the basal Fauquier Formation have a greenschist-facies fabric that Kline and others (1991) and Kline (1991) interpret to be the result of Late Proterozoic extension.
The Middle Proterozoic and Late Proterozoic granitoids and Late Proterozoic metasedimentary rocks are intruded by hundreds of metadiabase dikes and some metarhyolite dikes. The metabasalt dikes are part of a major dike swarm that extends the entire length of the Appalachian orogenic belt and are related to the extensional event that preceded Iapetus rifting (Ratcliffe, 1987). They have also long been interpreted to be feeder dikes to the metavolcanic rocks of the Catoctin Formation that yield ages of 570 to 565 Ma (table 3) for the main phase of continental rifting (Rankin, 1976).
The metavolcanic rocks of the continental rifting are overlain by a fining-upward sequence of sedimentary rocks that record a marine-transgression of the newly formed Iapetus Ocean. Rocks of the Lower Cambrian Chilhowee Group represent the transition from rift to passive continental margin, beginning with conglomerate of the Loudoun Formation and quartzites of the Weverton Formation and ending with carbonate rocks of the Tomstown Formation. These rocks form the base of the Paleozoic Appalachian basin.
The Paleozoic tectonic history of the Appalachian orogen is complex (Hatcher, 1989), and the timing of events affecting the Blue Ridge-South Mountain anticlinorium remains an enigma (Rankin and others, 1989). The Short Hill fault provides evidence of extensional faulting that postdates Early to Middle(?) Cambrian deposition of carbonate rocks, but the tectonic setting is unclear (Southworth, 1993). There is strong evidence of Early and Middle Ordovician (480 to 420 Ma) deformation and metamorphism of the Taconic orogeny along the hinterland of the Appalachians, but evidence for this is lacking in the immediate region (Drake and others, 1989). The Taconic orogeny is thought to be the first major compressive event caused by one stage of the closing of the Iapetus ocean. The Middle Devonian Acadian orogeny of the northern Appalachian and Piedmont provinces was another compressional event during closing of the Iapetus Ocean (Hatcher, 1989), but deformation of this orogeny has not been documented in this region either. The Iapetus Ocean finally closed with the continental collision of North America and Africa during the late Paleozoic Alleghanian orogeny. The Alleghanian orogeny lasted from the Late Mississippian until the Late Permian (340 to 250 Ma) (Hatcher, 1989).
The Blue Ridge-South Mountain anticlinorium has long been interpreted to be an Alleghanian structure because the South Mountain cleavage, which is geometrically associated with it (Cloos, 1951), can be traced westward through rocks as young as Devonian (Mitra and Elliott, 1980). Since cleavage and folds of only one episode of deformation are recognized, Mitra and Elliott (1980) concluded that thrusting, folding, and cleavage were coeval. In general, cleavage on the east limb of the Blue Ridge-South Mountain anticlinorium is steeper than on the west limb, so cleavage fans with respect to the anticlinorium (Cloos, 1951; Mitra and Elliott, 1980). Local and regional variations in the orientation of the South Mountain cleavage have been related to later folding (Cloos, 1951; Nickelsen, 1956; Mitra and Elliott, 1980; Onasch, 1986; Mitra, 1987), the position of different order structures (Mitra, 1987), movement along the North Mountain fault above its footwall tectonic ramp (Mitra, 1987), and doming by antiformal duplex in the footwall of the Blue Ridge thrust sheet (Southworth, 1993).
The timing of deformation in the Blue Ridge-South Mountain anticlinorium can be determined only by its relation to stratigraphy and metamorphism (Drake and others, 1989). Evans (1991) and Bartholomew and others (1991) suggest that Taconian greenschist-facies foliation was simply transported during Alleghanian deformation and is parallel with Alleghanian cleavage that affected rocks of the foreland. Isotopic dating of muscovite in cleavage in the east limb of the Blue Ridge-South Mountain anticlinorium in this region shows a cooling age of 340 Ma with complex spectra (Burton and others, 1992b; Kunk and others, 1993).
The Alleghanian orogeny was followed by extension that led to the opening of the Atlantic Ocean in the Mesozoic. The continental rifting produced isolated half grabens of terrigenous sediments along Paleozoic faults (Root, 1989) on and across the east limb of the Blue Ridge. Syn- and postfaulting sedimentary and igneous rocks that dip to the west across the basins demonstrate listric normal motion along the main border faults. Later extension is shown by swarms of diabase dikes that cut across the basin into the Harpers Ferry quadrangle.
A new Wilson cycle began with the Cenozoic deposition of the Coastal Plain strata eroded from the uplifting Blue Ridge-South Mountain anticlinorium. High angle reverse faults that place Lower Cambrian(?) rocks on Cretaceous and Pleistocene strata in the Piedmont and Coastal Plain Provinces demonstrate an active compressive state of stress due to current plate motion (Prowell, 1989).
U.S. Geological Survey, U.S. Department of the Interior
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