Reinterpretation of the Greenbrier fault, Great Smoky Mountains: New petrofabric constraints and implications for southern Appalachian tectonics

Abstract

The Greenbrier fault in the southern Appalachian Blue Ridge was defined as a premetamorphic younger-over-older thrust fault with a minimum 23 km of displacement. Subsequent studies accepted the original interpretation and further proposed that the fault represents a Taconian foreland style ramp-flat system, the sole evidence for a foreland fold-and-thrust belt in the southern Appalachians. New structural and petrofabric observations in the Great Smoky Mountains region permit reassessment of the original interpretation. All major foliations (mylonitic S–C′ fabric and shear-band cleavage in phyllonites; S-tectonites in the foot-wall; steep, disjunctive, shear-band cleavage in footwall and hanging wall; crenulation cleavage and schistosity above garnet grade) are postmetamorphic and have similar NE strike and SE dip. All foliations in the foot-wall are overprinted within meters of the fault by the S–C′ shear-band cleavage. This fabric represents the youngest deformation event observed and postdates prograde and retrograde mineral assemblages, indicating postmetamorphic slip along the contact. No premetamorphic fault fabrics are present along the fault, even in the lowest grade rocks. Postmetamorphic foliations near the fault may be accounted for by differential slip during folding of hanging-wall and foot-wall lithologies due to competency contrasts. Reconstitution of Snowbird footwall meta-siltstones into a phyllitic S–C′ tectonite correlates with similar lithologies and fabrics that define the footwall along strike and that yield Carboniferous ages for noncoaxial shear deformation. The evidence permits the alternative interpretation that the Greenbrier is a faulted stratigraphic contact that does not require tens of kilometers of displacement or deposition in separate basins along the extending Laurentian margin, and does not represent a foreland ramp-flat system. The general lack of premetamorphic folds and foliations in Ocoee metaclastic rocks (i.e., relatively “static” Taconic Barrovian regional metamorphism) correlates with the absence of regional deformation in the southern Appalachian foreland at a time (Middle Ordovician) when central and northern Appalachian Taconic-type fold-and-thrust belts developed. Postmetamorphic, middle to late Paleozoic shortening of the entire Great Smoky Mountain basement-cover sequence, and folding and extrusion of the competent Great Smoky Group rocks relative to the underlying Snowbird Group, may have produced km-scale offset along the fault.