INNER PIEDMONT TIMING RELATIONSHPS AND NEOACADIAN CRUSTAL FLOW, BRUSHY MOUNTAINS, NC: RESULTS FROM DETAILED GEOLOGIC MAPPING, STRUCTURAL AND PETROLOGIC ANALYSES

Recent detailed geologic mapping of an ~700 sq. km area in the North Carolina Inner Piedmont (IP), between Wilkesboro and Lenoir, NC has provided important limits on the timing of crystalline thrust sheet emplacement and crustal flow in the southern Appalachian Neoacadian (~350 Ma) orogenic core. The Brindle Creek fault (BCF) is a terrane boundary juxtaposing eastern IP polydeformed Siluro-Devonian sillimanite I and II grade aluminous schist and metagraywacke intruded by Devonian-Carboniferous anatectic granitoids, against western IP Cambrian(?) Tallulah Falls/Ashe Formation metasedimentary rocks, migmatitic Middle Ordovician Poor Mountain Formation rocks, ~470 Ma Henderson Gneiss, and other Ordovician-Silurian granitoids as young as ~427 Ma. Recognition of the BCF is based on a map-scale shear zone composed of annealed mylonitic to ultramylonitic porphyroclastic biotite gneiss located along the NW front of the Brushy Mountains. The ~427 Ma Brooks Crossroads pluton is exposed in a reentrant in the BCF thrust sheet, indicating mid-Paleozoic or younger emplacement of the BCF. Furthermore, map-scale sheath folds cored by ~366 Ma Walker Top orthogneiss are truncated by the BCF, limiting emplacement to Late Devonian or younger. These observations are consistent with 350–360 Ma metamorphic zircon rims in the IP, providing evidence for Late Devonian high-grade metamorphism. Flow patterns interpreted from mesoscopic sheath fold axes, mineral lineations, and rotated porphyroclasts indicate NW- and W- to SW-directed mid-crustal flow. Curved IP flow paths may reflect NW- and W-advancing IP thrust sheets buttressed against an oblique footwall structure (eastern Blue Ridge?) and deflected SW along the Neoacadian Brevard fault zone. These observations support Neoacadian thrust sheet emplacement coeval with ductile crustal flow and upper amphibolite facies metamorphism in the central Inner Piedmont. 3-D kinematic analysis of IP flow paths provides a model for mid- to lower crustal deformation during orogenesis.