REGIONAL CONTACT BETWEEN THE ASHE AND ALLIGATOR BACK METAMORPHIC SUITES IN NORTHWESTERN NORTH CAROLINA CORRESPONDS TO AN AMPHIBOLITE FACIES NORMAL FAULT

The Ashe Metamorphic Suite (AMS) and the Alligator Back Metamorphic Suite (ABMS) are the two principal components of the Eastern Blue Ridge belt that accreted onto Laurentia likely during the Taconic orogeny. The ABMS was originally mapped as the AMS, but was later separated from the AMS, by a non-faulted lithologic contact, based on the distinguishing ‘pin-striped’ texture of the ABMS gneiss. Carter and Merschat (2014) and Cattanach et al. (2016) identified ductile thrust faults along the AMS and ABMS contact in southeast Virginia and central North Carolina, respectively. During new mapping in northwestern North Carolina, supported by the GSA Student Research Grant, we found that the contact between the AMS and the ABMS corresponds to a ductile normal fault near Boone, NC. The shear zone continues approximately 50 km NE of Boone in Sparta, NC, though we have not yet determined the shear sense at this location. We collected oriented samples for thin section analysis in both locations, but only those near Boone, NC have been analyzed. In thin section, shear sense indicators including S-C and C’ fabric, mica fish, and rotated garnets, show top-down-to-SE motion. Microstructures, indicative of mineral-deformation mechanisms in quartz and feldspar, provide information on temperature conditions during shearing. Recrystallization of quartz and feldspar, lobate grain boundaries, and mostly recovered, strain free grains are evidence of grain boundary migration. These characteristics indicate temperatures of ~500-600 °C and amphibolite facies deformation, which differs from the greenschist facies mylonites of the nearby Alleghanian faults. At these temperatures, this fault is likely pre-Alleghanian and one of the first amphibolite facies normal faults identified in the southern Appalachians.