Structural geology and deformation history of the Brevard fault zone, Chauga Belt, and Inner Piedmont, northwestern South Carolina and adjacent areas

The rocks in the Brevard fault zone, Chauga belt, and adjacent Inner Piedmont, southern Appalachians have experienced polyphase deformation, magmatism, and metamorphism related to the orogenic cycles that overlap both temporally and spatially. The Chauga belt and adjacent Inner Piedmont consist of a stack of thin thrust sheets from NW to SE, including the pre-metamorphic peak Stumphouse Mountain thrust and Cedar Creek thrust, and post-metamorphic peak Six Mile thrust. Emplacement of the thrust faults was from NW to SE. The earliest verifiable deformation occurred between 509 Ma, indicated by the age of the Henderson Gneiss, and the end of the Acadian event, about 350-320 Ma. The Acadian orogeny produced much of the ductile deformation in this region, including isoclinal folds and subsequent mylonite overprint that formed NE-trending subhorizontal lineations in the Brevard fault zone and Chauga belt. Application of thermobarometers yielded a temperature of 490-550°C in the Chauga belt below the Cedar Creek thrust, and 608- 648°C in the Six Mile thrust. Pressure during the metamorphism was approximately 5.3-6.3 kbar. The deformation near the Acadian metamorphic peak led to large-scale mylonitization of granitic rocks in the Brevard fault zone and Chauga belt, accompanied by prograde metamorphism reaching garnet grade in the Brevard and Chauga belt and sillimanite grade in the Cedar Creek and Six Mile sheets in the Inner Piedmont further SE. Deformation style observed in the Inner Piedmont formed in the middle crust in a compressional regime. Alleghanian deformation is characterized by two major phases: 1) Earlier dextral strike-slip. It was mainly recorded by the formation and subsequent distortion of shear bands, reverse-slip crenulations, rotated porphyroclasts, and other asymmetric fabrics. These structures were essentially established on and controlled by preexisting anisotropies in the rocks of the southern Appalachian internides. Normal-slip crenulations are often accompanied by reverse slip crenulations that accommodated the slip on the former. The formation of dextral shear fabric in the Brevard fault zone was accompanied by retrograde metamorphism. In the Chauga belt and farther to the SE, retrograde effects are minor although the shear structures were still well developed. 2) Later NW-directed brittle thrusting. In the Brevard fault zone, dextral shear fabrics were overprinted by structures such as faultH^elated folds and fractures that formed dining the motion of the Rosman fault. Shear strain was largely accommodated by thickening of shear zones during the early dextral strike-slip, and by large displacement along the Rosman fault during the later thrusting. The inversion of the temporal order of major phase Alleghanian thrusting and dextral strike slip from the western to eastern Piedmont resulted from the diachronous nature of the Alleghanian thrust front advancing northwestward and the southeastward relocation of the strike-slip system into the eastern Piedmont. Detailed study on the spatial crossover of these two processes may reveal the roles of the strike-slip systems in the entire orogenic cycle, from mountain building to mountain collapse. The complex deformational history of the study area recorded by superposed fabrics of at least five generations primarily at two crustal levels provided valuable evidence for the mode of the continental growth through island-arc and probably microcontinent accretion, through final collision between continents, and through related magmatism.