THE BRINDLE CREEK THRUST: A MAJOR FAULT AND TERRANE BOUNDARY IN THE SOUTHERN APPALACHIAN INNER PIEDMONT, NORTH CAROLINA-SOUTH CAROLINA-GEORGIA

The Brindle Creek thrust (BCT) was originally described as a minor Type F thrust in the NC Inner Piedmont (IP), part of the Acadian core of the southern Appalachians. The BCT is now recognized as a terrane boundary in the IP, traceable for more than 300 km, and with a minimum displacement of 60-100 km. It terminates against the central Piedmont suture south of Athens, GA, and west of Winston-Salem, NC where it joins the Shacktown fault. It juxtaposes younger rocks of the Cat Square terrane/central IP against rocks of the western IP, part of the Tugaloo terrane. Metaigneous rocks are partitioned along this boundary in NC: Ordovician to the west and Devonian-Mississippian to the east. A zone of intense migmatization is present in the footwall of the BCT, in the Brushy Mountains and South Mountains, NC. Several unique structural patterns coincide with the fault. Rock units trend NE-SW proximal to the fault rotating to NW-SE trends eastward, away from the fault. This pattern is paralleled by axial trends of early folds and mineral lineations in the BCT sheet. Fold vergence in the BCT hanging varies from NW near the fault to W and SW in more easterly portions of the BCT sheet. Mineral lineations measured from aligned sillimanite, hornblende, quartz, etc., are interpreted to define flow lines, and display a similar pattern plunging gently NE or SW near the fault and rotating to SE eastward away from the fault. Minor faults within the BCT sheet have similar geometries and a significant dextral strike-slip component. The geometry of the BCT, vergence of folds, and mineral lineations suggest motion on the fault is oblique-slip. Trends observed in the Brushy Mountains are continuous throughout the BCT sheet, with some variations. The width of the western IP decreases near Lenoir, NC, possibly corresponding to an increase in displacement along the BCT. The steeper geometry of the fault near Lenoir is related to later folding, an increase in strike-slip movement, or both. The change in orientation of structures and fabrics may be the result of buttressing by the primordial Devonian Brevard fault zone.