Geology of the northwestern South Mountains near Morganton, North Carolina

Detailed mapping in the South Mountains has revealed a structurally complex sequence of high-grade gneisses with a mappable stratigraphy. The Walker Top gneiss has an igneous origin and was originally mapped in 1988 by Goldsmith et. al. as a less deformed version of the Henderson Gneiss. Geochemical and petrologic data support the conclusion that the Henderson Gneiss and Walker Top gneiss are genetically related. Contact relationships and field observations are consistent with the interpretation of the Walker Top gneiss as a pluton or as a depositional platform for the overlying metasedimentary units (i.e., a basement unit). Metamorphic data and geochronologic data, however, favor an intrusive interpretation of the Walker Top gneiss-metasedimentary unit contact. The surrounding metasedimentary sequence consists of the Tallulah Falls and the Poor Mountain Formations that have been subjected to upper-amphibolite facies, sillimanite-grade metamorphic conditions. Electron microprobe analyses of mineral phases yield minimum equilibration temperatures between 425°C and 675°C. The upper end of these temperature estimates are consistent with the mineral assemblage present and record peak- or post-peak Acadian metamorphic conditions. Prior to peak Acadian metamorphism, this gneiss sequence was folded into a tight synform-antiform pair (the Cane Creek synform and the Buzzards Roost antiform). With progressive deformation, the common limb between these two folds became attenuated and a discrete fault surface developed (the Brindle Creek fault). The magnitude of displacement on this fault is <5 km. Alleghenian open folding of the axial surfaces of the major antiform-synform pair and the Brindle Creek fault surface produced a dome-and-basin interference pattern. Pervasive fracture sets also developed after the peak-deformational episode. This area was uplifted during the Miocene as part of the major uplift that created the modern Appalachian Highlands (Hack, 1982). The South Mountains became a highland in the gently rolling hills of the Piedmont. Due to the South Mountains relatively high relief, rock type, foliation, and fracture orientation have locally become controlling factors in landscape evolution. Resistant units (Walker Top gneiss and the sillimanite-mica-schist) tend to hold up areas of high relief, while less resistant units (Poor Mountain Amphibolite) tend to underlie areas of low relief. Fracture sets have controlled drainage development as the dissection of the South Mountains progresses.