Origin and Tectonic Evolution of the Southern Appalachian Neoacadin Crystalline Core: Evidence from the Geology of the Gilreath 7.5-Minute Quadrangle, North Carolina

Detailed geologic mapping of the Gilreath 7.5-minute quadrangle recognizes the northernmost continuation of the Brindle Creek fault, a terrane boundary that separates Neoproterozic-Ordovician metasedimentary and metavolcanic rocks of the western Tugaloo terrane (western Inner Piedmont) from Silurian-Devonian metasedimentary rocks and Devonian-Mississippian anatectic granitoids of the Cat Square terrane (eastern Inner Piedmont). The Brindle Creek fault is folded in the study area and exposes the Ordovician Brooks Crossroads Granite in a reentrant. Low high-field strength element concentrations and a flatter rare earth element patterns typical of western Inner Piedmont granitoids, support a footwall setting for the Brooks Crossroads Granite.

New geochemical analyses of Inner Piedmont amphibolites reveal a mixed volcanic arc and E-type MORB signature for eastern Inner Piedmont amphibolite. These preliminary data suggests the Cat Square terrane may have originated in a back-arc basin influenced by E-type MORB volcanism during the waning stages of Middle Ordovician-late Silurian volcanic arc magmatism in the western Inner Piedmont.

Subduction of the Inner Piedmont beneath a W- to NW-advancing Carolina superterrane resulted in possible early kyanite-grade metamorphism (7.0 to 8.7 kbars and 630 to 695°C), followed by sillimanite I and II grade metamorphism. High-temperature, upper amphibolite facies metamorphic conditions, in conjunction with pervasive migmatization, the presence of meso- and map-scale sheath folds, ubiquitous top-to-the SW shear-sense indicators, and protomylonitic to ultramylonitic deformation fabrics, suggests Inner Piedmont thrust sheets deformed as a viscous mass during SW-directed flow in a sub-simple shear to extreme simple shear stress regime. These data support models that reconcile the SW-directed arcuate trend of Inner Piedmont stretching directions, with a tectonically forced flow path resulting from oblique subduction of Inner Piedmont thrust sheets beneath the W- to NW-advancing Carolina superterrane. Flow was deflected to the southwest as Inner Piedmont thrust sheets buttressed against a primordial Brevard fault zone. Truncation of the Popular Springs and Big Warrior sheath folds against the Brindle Creek fault indicates SW-directed displacement of ductile, Inner Piedmont crystalline thrust sheets was prior to final emplacement of Cat Square terrane rocks.

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