KINEMATIC AND PETROFABRIC ANALYSIS OF THE GREENBRIER DISCONTINUITY, GREAT SMOKY MOUNTAINS, NORTH CAROLINA AND TENNESSEE

The nature of the contact between the Great Smoky (GSG) and Snowbird (SG) Gps. has long been an enigma for interpretation of the tectonic evolution of the southern Appalachian Blue Ridge. The contact has been interpreted as a premetamorphic fault (“Greenbrier Fault”), a lithologic contact, or a fault-reactivated contact. Prograde and retrograde metamorphism and deformation complicate the interpretation. Hanging wall (thick bedded GSG coarse-grained metasandstones) and footwall (laminated to thin bedded SG metasiltstone) exhibit different styles of folding of S0 (open vs. tight). S1 produced by D1 is weak (SG) to absent (GSG). D1/F1 folds in the footwall are truncated by the contact. Peak metamorphic (M1) Bt and Grt are rarely associated with a preferred fabric. Bt in the SG is overprinted by Chl-grade D2 (S2); this Bt, and Bt and Grt in the GSG hanging wall, are overprinted by a late disjunctive/shear band cleavage [slip cleavage of Hadley and Goldsmith] (S3). Samples from an outcrop scale shear zone (presumably the “Greenbrier fault”) in the footwall near Greenbrier Pinnacle (Bt grade) reveal mylonitic SG displaying ductile (S/C) microstructures that overprint regional metamorphic assemblages and foliations. The mylonitic fabric is overprinted by the disjunctive cleavage (S3). Shear zone footwall microstructures include S/C fabric, dynamic recrystallization of quartz, (quartz veins?) and mica fish. Sense of shear based on asymmetry of footwall microstructures is top-to-NW. Microstructures associated with the disjunctive cleavage include dynamic recrystallization of quartz, feldspar, chlorite, and mica fish. Assuming Bt-grade metamorphism is Taconian, the fabrics imply a period of deformation along the contact during late Taconian, Acadian, or early Alleghanian orogenesis. Exposures near Big Creek, where basal GS Elkmont/upper SG(?) Rich Butt Fm. is comprised of feldspathic conglomerates with shale/slate clasts, are consistent with a gradational transition from SG to GSG, suggesting the contact was lithologic. We cannot resolve any premetamorphic fault-related microstructures in GSG or SG at the type locality. A viable explanation for the Greenbrier discontinuity is that it is a faulted lithologic contact that doesn't require the 23 km of displacement proposed by previous workers.