SOUTHERN APPALACHIAN UNCONFORMITIES, FAULTS, AND FAULTS REINTERPRETED AS UNCONFORMITIES

The origin of unconformities, like the top-of-Knox and Grenville basement-cover, has never been controversial. Large southern Appalachian faults like the Blue Ridge, Valley and Ridge, and the eastern Piedmont fault system have been known for decades, and their nature has similarly been accepted. The Brevard fault has been debated as a thrust, strike-slip fault, suture, etc., but remains a fault. The Brindle Creek fault (BCF) in NC–SC–GA, and the Talladega fault in AL, have been reinterpreted as unconformities reactivated as small-displacement faults. The geometry and kinematics of the BCF are well known from detailed geologic mapping in NC-SC-GA revealing thick mylonite zones and from near horizontal to near vertical dip. Abundant zircon geochronologic and geochemical data confirm that it also is a terrane boundary separating the Cat Square terrane from the Tugaloo terrane. Exposure of Tugaloo terrane rocks in the Newton window in NC suggests > 200 km of SW-directed transport on the BCF, making it unlikely to have originated as an unconformity. The Talladega fault (TF) in AL separates Talladega belt rocks from low-grade, penetratively deformed rocks of the SE foreland, truncating rock units in both the hanging wall and footwall. The TF, like the Blue Ridge fault farther NE, was the indenter that drove Valley and Ridge–Plateau deformation in front of it. Mesoscopic deformation along the fault extends >200 m into the hanging wall and >1 km into the footwall. It consists of S–C fabrics, sheath folds, and asymmetric boudins in highly strained footwall carbonate rocks. These data suggest that the TF has a NW displacement at least equal to the sum of the displacements on the Valley and Ridge and Plateau faults to the NW (>100 km). Regional structural relationships, stratigraphic data, and mesoscopic data suggest that the TF is also not a reactivated unconformity. All data (facts) must be considered before suggesting alternative origins for well-studied geologic features.