Geology of the southeastern South Mountains, North Carolina

Stratigraphic, geochemical, and structural data from across the North Carolina Inner Piedmont provide insight into the history of the southern Appalachian crystalline core. Detailed geologic mapping and compilation of over 500 km² (300 mi²) of previous mapping reveal a similar package of metasedimentary rocks to those found in the western Inner Piedmont. This sequence consists of the Tallulah Falls Formation, which includes both a sillimanite schist middle member and a metagraywacke-schist upper member. Isolated and disseminated bodies of the Walker Top orthogneiss (granite to granodiorite) and the Toluca Granite were also recognized throughout the central and eastern Inner Piedmont.

Crystallization ages, preliminary zircon and monazite ages, mineral assemblages, petrographic textures, garnet chemical zoning profiles, and temperature and pressure estimates of rocks in and around the study area suggest a single prograde metamorphic event that occurred as a result of the Acadian orogeny. Metamorphic conditions estimated from electron microprobe data using a garnet-biotite geothermometer and garnet-aluminumsilicate-silica-plagioclase and garnet-plagioclase-muscovite-biotite geobarometers. They range from 610-645°C and from 3.8 ± 0.8 kbar to 5.4 ± 0.8 kbar. These conditions are consistent with mineral assemblages present and regional isograds, and support a model of Barrovian metamorphism for the Inner Piedmont.

A deformational framework for this portion of the Inner Piedmont in North Carolina includes five deformational events. Rare D₁ intrafolial folds and foliation are attributed to the Taconic orogeny. Penetrative D₂ structures include foliation, lineation, tight to isoclinal and reclined to recumbent folds, sheath folds, and Type-F thrust sheets. D₃ folds and Type-C thrust faults probably constitute the latter portion of ductile Acadian deformation. D₄ broad open folds formed during the Alleghanian orogeny. D₅ fractures are a result of Mesozoic uplift and extension.

A portion of the North Carolina Inner Piedmont from the Brevard fault zone to the eastern Inner Piedmont is divisible into three fault-bounded domains. The easternmost domain is subdivided into five subdomains based on homogenous fabrics. Foliation reflects a regional change in strike from NE-SW in the western flank to N-S in the central Inner Piedmont back to NE-SW in the southeastern part of the terranes. Lineations are aligned NE-SW (orogen parallel) in the western Inner Piedmont and E-W in the central Inner Piedmont and probably indicate transport direction. The structural data and map patterns compiled in this study are best explained by crustal flow during oblique collision.