The Central Piedmont Shear Zone of the Southern Appalachian Piedmont

Allen Dennis, Univ South Carolina Aiken

The central Piedmont shear zone in western South Carolina, sometimes called the Cross Anchor fault, juxtaposes Ediacaran-Cambrian rocks of Carolinia with Silurian Cat Square (CS) paragneiss. The most recent major movement on the fault is Pennsylvanian, and is related to emplacement of a large composite crystalline thrust sheet. Lower metamorphic grade Carolina terrane rocks are in the hanging wall, and higher grade CS rocks are in the footwall. The Charlotte t of Carolinia is an exotic peri-Gondwanan arc terrane that experienced peak metamorphic conditions (upper greenschist to amphibolite facies) between 538-535 Ma. The Wenlock-Ludlow protolith of CS paragneiss included sediments derived from both Carolinia and Laurentia, and was metamorphosed to upper amphibolite facies at the Devonian-Mississippian boundary. There is no strain gradient associated with the CPSZ; generally the fault crosscuts the foliation, and the fault and foliation were subsequently folded. In central South Carolina the CPSZ is rotated to a near horizontal orientation, and footwall rocks of the CS t are exposed within the Whitmire reentrant.

The Central Piedmont shear zone is not the suture between Carolinia and Laurentia. Several lines of evidence suggest that Carolinia accreted to Laurentia in the Middle Ordovician, and subsequently rifted from the continent. Sedimentary rocks of the CS basin were deposited during this rifting event, accepting detritus from both the accreted terrane and Laurentia and covering their suture. The structure mapped as the Brindle Creek fault originally formed as the unconformity beneath the CS basin. When southern Appalachian crystalline rocks southeast of the Burnsville fault and Brevard zone are restored to their Devonian position at the New York Promontory after removing ≥ 500 km of dextral motion, it is clear that the CS basin is a southern extension of the system of Salinic basins that covers much of New England and the Canadian Maritimes. Asymmetric lithospheric thinning was coeval with CS basin deposition, and a suite of 424-414 Ma lower crustal melts (e.g., Newberry granite, Lake Murray gneiss, Clouds Creek pluton) intrudes Carolinia. These magmas were followed by a linear array of ca. 400 Ma mantle-derived alkalic-subalkalic gabbros and ring syenites. Frasnian-Fammenian collapse of the CS basin and metamorphism up to granulite facies is recorded as the Neoacadian orogeny. Subsequent dextral strike-slip motion along the Burnsville/Brevard faults during the Mississippian brought these terranes south where oblique-slip emplacement of the crystalline thrust sheet resulted in their current juxtaposition.