MIDDLE TO LATE PALEOZOIC ACCRETION OF THE CAROLINA TERRANE: EVIDENCE FROM THE BRINDLE CREEK THRUST SHEET, INNER PIEDMONT, NORTH CAROLINA

Field, petrologic and geochronologic data from the South and Brushy Mountains, NC support a high-grade Middle to Late Paleozoic tectonothermal event in the Inner Piedmont (IP). The Brindle Creek fault juxtaposes an unique sequence of Siluro-Devonian metagraywacke and sillimanite schist intruded by Devonian granitoids of the Cat Square terrane against the Cambrian(?) Ashe/Tallulah Falls Formation, Middle Ordovician Poor Mountain Formation, and various granitoids of the eastern Tugaloo terrane. Rocks of the Brindle Creek thrust sheet and its footwall are pervasively migmatitic, and reached sillimanite I and II grade metamorphism. Sillimanite II rocks contain the assemblage garnet + biotite + sillimanite + quartz + K-feldspar + plagioclase + ilmenite + rutile and are enveloped by sillimanite I rocks. P-T estimates yield peak conditions of 800–850º C and 5–6 kb consistent with 800–880º C zircon saturation temperatures for the Toluca (TOL), ~375 Ma, and Walker Top (WT), ~366 Ma Granites, and retrograde conditions of 500–560º C and 2.5 to 5 kb. Map patterns and P-T estimates suggest that the Brindle Creek thrust sheet followed a clockwise P-T path. U-Pb age dates of zircon rims from migmatitic amphibolite in the footwall of the Brindle Creek fault indicate metamorphism and coeval migmatization at ~350 Ma. Dismembered TOL and WT bodies outline map-scale NW– and SW–directed sheath folds. Coaxial relationship between fold axes, mineral lineations and map-scale sheath fold axes support NW– rotating to SW–directed crustal flow in the IP at ~360-350 Ma under peak metamorphic conditions. Detrital zircons indicate Cat Square terrane rocks have both Laurentian and Gondwanan affinities, with a maximum age of ~430 Ma. The Smith River allochthon has Gondwanan affinities and a structural position above the IP suggesting both the Cat Square and Tugaloo terranes were subducted beneath the Carolina terrane. IP metamorphic conditions require burial to 15-20 km, consistent with subduction beneath the Carolina terrane. Curved high-temperature NW– to SW–directed IP flow patterns are consistent with oblique convergence at ~350 Ma and middle to late Paleozoic docking of the Carolina terrane. Loading of the Carolina terrane from NE to SW is consistent with SW–directed escape and orogenic channel flow of the IP and eastern Blue Ridge.