GRAVITATIONAL COLLAPSE OF THE WESTERN INNER PIEDMONT DUCTILE THRUST STACK, SOUTH CAROLINA AND ADJACENT NORTH CAROLINA

Southwest- to southeast-directed gravitational collapse of an oversteepened ductile thrust stack produced numerous greenschist-grade muscovite-quartz schist-phyllonite zones in western Inner Piedmont rocks of South Carolina and adjacent North Carolina. Collapse occurred subsequent to a complex tectonic history involving Ordovician supra-subduction zone granitic intrusion, early-middle Paleozoic polyphase folding episodes (regional NW-, W-, and SW-vergent folds), amphibolite-facies metamorphism, compressional ductile thrusting (emplacement of the Seneca fault-Six Mile and Eastatoee fault-Walhalla crystalline thrust sheets), and regional Type-1 interference folding. Ductile features produced by collapse include S-C’ and S-C-C’ fabrics, sheared asymmetric quartzo-feldspathic lenses in schist, and meso-scale normal faults. These extensional structures developed in Chauga River Formation garnet schist, Table Rock gneiss, Poor Mountain Formation amphibolite, and Tallulah Falls Formation schist and paragneiss units. Widespread extensional shear zones lie at the base of the collapsing stack (within the Chauga River Formation, near a rheological contrast with overlying more rigid Poor Mountain amphibolite) and at diverse infrastructural levels within the presumably warm, plastic orogenic wedge of thickened crust. Sense of shear on these dextral extensional structures is everywhere consistently top-to-the-southwest, although locally it is to the south and southeast. Subsequent folding and brittle thrusting episodes deformed these older shear zones.

Regional relationships suggest the timing of western Inner Piedmont gravitational collapse and development of associated shear fabrics occurred prior to Alleghanian orogenesis and prior to Acadian Brevard zone orogenesis. The Brevard zone later incorporated the older Eastatoee fault and the collapse-related, extensional fabrics of the Chauga River Formation into its own complex array of thrust and fold structures.