RE-EXAMINATION OF THE GREENBRIER FAULT: POSTMETAMORPHIC SLIP ON A FAULTED STRATIGRAPHIC CONTACT

The Greenbrier fault as originally defined fails to explain many aspects of Great Smoky and Snowbird Group geology, and introduces unnecessary complexities for the tectonic history of the southern Appalachians. Early workers admitted limitations in assumptions that were the basis for their interpretation that the Greenbrier is a premetamorphic fault accommodating at least 23 km of shortening. Truncation of bedding in footwall and hanging wall rocks near the mapped contact is the only observation that requires displacement, be it pre- or postmetamorphic. We document structural, metamorphic and petrofabric features at and near the fault, and throughout the eastern and central Great Smoky Mountains supporting a predominantly postmetamorphic deformation history. Great Smoky hanging wall and Snowbird footwall exhibit contrasting behavior during deformation due to marked differences in detrital composition, bedding thickness, and competency. The absence of a regional axial planar foliation associated with pre- or synmetamorphic folding of bedding makes it difficult to correlate phases of deformation across the region. All foliations in the Snowbird below biotite grade are completely overprinted within meters of the fault by a ductile shear band cleavage. This is the youngest fabric observed; it postdates prograde and early retrograde mineral assemblages, indicating postmetamorphic slip. Premetamorphic fault fabrics are absent even in the lowest grade rocks. Rock and mineral chemistry support the inference that the Metcalf phyllite is the highly tectonized equivalent of the Pigeon Fm. We correlate the shear band cleavage at the type locality of the fault with the penetrative shear band cleavage that characterizes the Metcalf. Evidence supports the simpler interpretation that the Greenbrier is a faulted stratigraphic contact between units of markedly different competency that does not require tens of kilometers of displacement of an inferred younger Great Smoky on older Snowbird, or deposition of Ocoee units in separate basins. Postmetamorphic, Middle to Late Paleozoic shortening of the entire Great Smoky Mountain basement-cover sequence, and folding and extrusion of the great mass of competent Great Smoky group rocks relative to underlying Snowbird group could result in km-scale offset or less.