RETHINKING THE BLUE RIDGE – VALLEY AND RIDGE INTERFACE: THE STANLEY FAULT AS A LATE ALLEGHANIAN THRUST ARRAY

Recent mapping in the Big Meadows 7.5' quadrangle, Page Valley, Virginia, suggests that the late Alleghanian Stanley Fault, originally depicted by King (1950) as a single, curving N-S and E-W trending fault, is more likely a complex system of smaller thrust splays. This research builds on work done by previous JMU undergraduate students in the adjacent Stanley and Tenth Legion quadrangles. Previous mapping by King (1950), Radar and Gathright (1976), Sarros (1995), and others show the Stanley fault following a contorted path that trends northeast across Page Valley between the Conococheague Limestone and Elbrock Dolomite, then turning east and cutting across the carbonate units, and finally turning north along the western margin of Proterozoic basement rocks of the Blue Ridge. Regional stratigraphy includes Mesoproterozoic gneiss (basement) overlain by Neoproterozoic Catoctin greenstone, Cambrian clastic units and Cambro-Ordovician carbonate rocks. Stratigraphy is upright in the southwestern portion of the quadrangle, but at least partly over-turned in the center of the quadrangle. Direct outcrop evidence of the Stanley fault system is lacking throughout much of the Big Meadows quadrangle, with the notable exception of small-scale, west-directed thrust surfaces in restricted outcrops in the northwestern corner of the quadrangle. Indirect evidence of thrusting includes lateral offset of units in the southern section of the quadrangle relative to the overturned units in the central portion of the quadrangle, which suggests fault truncation – perhaps from a splay of the Stanley fault system.

Evans (1989) showed an imbricated thrust duplex of Cambro-Ordovician carbonate units underlying Blue Ridge basement just east of Page Valley, with the Stanley fault as an out of sequence thrust that truncates the duplex system. At present it is unclear whether the overturned units in the center of the Big Meadows quadrangle are western components of the duplex system, or rather fault-bend folds related to splays of the Stanley fault. Continued mapping in the area will focus on improving constraints on the locations of Stanley Fault thrusts and the relative temporal relationships between folding and the thrust system.