EXTENSIONAL FAULTING IN A CONVERGENT OROGEN, WESTERN NORTH CAROLINA

The southern Appalachian Blue Ridge province is dominated by thrust faults formed during three Paleozoic orogenies; however, our recent mapping has revealed the existence of two ductile normal faults in the Eastern Blue Ridge (EBR). The Grassy Creek fault and the Idlewild fault are the first identified extensional faults within the southern Appalachians unrelated to a known rifting event.

The Grassy Creek fault consists of discontinuous mylonite zones typically less than 200 meters thick and corresponds to the previously mapped contact between the Ashe and Alligator Back Metamorphic Suites near Boone, NC. The Idlewild fault is ~5 km east within the hanging wall of the Grassy Creek fault. The Idlewild shear zone is locally ~700 meters thick and continuous along the currently mapped extent of ~10 km. In outcrop and in thin section, shear sense indicators show top-down-to-the-SSE motion for the Grassy Creek fault and top-down-to-the-S motion for the Idlewild fault. Mineral-deformation mechanisms observed within the Grassy Creek fault and the Idlewild fault suggest shearing occurred at mid-crustal levels and temperatures of ~500-600 ̊C. Chlorite replaces biotite within and outside the shear zone, indicating a later greenschist facies overprint.

The Grassy Creek and Idlewild faults accommodated top-down-to-the-SSE extension at similar metamorphic conditions, indicating the two faults are associated with the same tectonic event. The extent and thickness of the Idlewild fault implies it was likely the principal shear zone and experienced more extension than the Grassy Creek fault. The EBR was locally deformed under amphibolite facies conditions during the Devonian-Mississippian Acadian/Neoacadian orogenies. At amphibolite facies deformation, the Grassy Creek and Idlewild faults may represent pre-collisional or synorogenic extension associated with the Acadian/Neoacadian orogeny. Extensional faults in other orogenic belts are associated with the exhumation of high-pressure rocks, such as eclogite, and the extension within the EBR may have facilitated exhumation of eclogite bodies in the footwall of the fault.