INNER PIEDMONT TIMING RELATIONSHIPS REVEALED THROUGH DETAILED GEOLOGIC MAPPING, BRUSHY MOUNTAINS, NC

Map-scale relationships from the northeastern IP provide fundamental information on the relative timing of IP tectonism. Six person-years of EDMAP-supported geologic mapping in the Brushy Mountains, NC, has produced a ~700 km² detailed geologic map essential to addressing fundamental questions regarding Inner Piedmont (IP) structure, metamorphism, and tectonics.

Four polydeformed, moderately SE–dipping type F thrust sheets are recognized and expose high–grade IP gneisses and schists. The Brindle Creek fault zone separates Laurentian affinity rocks of the eastern Tugaloo terrane (western IP) from mixed Laurentia–Gondwanan affinity rocks of the Cat Square terrane (eastern IP). Western IP thrust sheets include the Marion, Tumblebug Creek, and Mill Spring thrust sheet from lowest to highest. Within the northwesternmost Marion thrust sheet, the Tumblebug Creek fault (TCF) truncates map-scale folds by placing Ordovician Henderson Gneiss (~470 Ma) over the Tallulah Falls-Ashe Formation. The largely dextral strike-slip Mill Spring fault zone places Ordovician metavolcanics of the Mill Spring sheet over folded mylonites of the Tallulah Falls Formation and truncates the TCF. The folded BCFZ juxtaposes polydeformed Siluro-Devonian metasediments and Devonian-Carboniferous anatectic granitoids of the eastern IP Cat Square terrane against Ordovician-Silurian metavolcanics of the Mill Spring sheet and Cambrian(?) metasediments of the Marion sheet (Tugaloo terrane). Furthermore, the BCFZ truncates the Poplar Springs shear zone (~409 Ma) within the Cat Square terrane and juxtaposes it against the ~427 Ma Brooks Crossroads pluton of the western IP Tugaloo terrane. Fabrics in a ~15 km wide zone in the NW part of the study area are characterized by NE-SW trending moderately SE-dipping mylonitic foliation. Mineral lineations, mantled porphyroclasts, S-C fabrics, asymmetric folds, sheath fold axes, boudins, and snowball garnets consistently indicate SW-directed transport. These data suggest a Neoacadian (360-340 Ma) emplacement of the BCFZ and accretion of the composite Cat Square-Carolina terrane. Mapped relationships showing truncation and transposition of early fabrics and lithologies by later structures and units are further supported by geochronologic data.