UPDATED LITHOSTRATIGRAPHY OF THE ALLIGATOR BACK METAMORPHIC SUITE, FOCUSED ON MAPPING IN SPARTA, NORTH CAROLINA

Recent cooperative mapping efforts by the USGS, NCGS, and UNC-CH (EDMAP) have resulted in more detailed maps of eastern Blue Ridge lithostratigraphy in southwestern Virginia to northwestern North Carolina. Previously, the contact between the Ashe Metamorphic Suite (AMS) and overlying Alligator Back Metamorphic Suite (ABMS) has been described as stratigraphic and tectonic. The ABMS has been correlated with part of the Lynchburg and Evington Groups in Virginia.

Detailed mapping in response to the 2020 Sparta, NC earthquake provides insight into the ABMS lithologic units and kinematic relations along this contact from northwestern NC and into southwestern VA. The ABMS is composed of interlayered pinstripe-mica gneiss, phyllite, mica schist, amphibolite, and mylonite with a generalized Paleozoic foliation that strikes NE-SW and dips SE. The ABMS mica gneiss was originally defined by a “pinstripe” texture, referring to thin layers of mica interlayered with quartz and feldspar. Following recent field work a more descriptive term, “truncated pinstripe,” was introduced to describe the polydeformed mica layers within the gneiss of the ABMS. Abundant magnetite in the ABMS is also used as a field identifier for the unit. Along the contact near Sparta, moving closer to the contact with the AMS, the ABMS gneiss becomes strongly magnetic and strongly foliated, losing its truncated pinstripe texture. A similar relationship was documented near Boone, NC. Additionally, the presence of mylonites, SW-plunging mineral lineations, and macro shear indicators at the AMS-ABMS contact suggest the contact is a fault in northwestern NC. In southern VA, the contact is mapped as the Rock Castle Creek fault marked by a tectonite zone with a down-dip mineral stretching lineation and top-to-NW motion. In Boone, the Grassy Creek shear zone overprints part of the AMS-ABMS contact. In outcrop and in thin section, shear sense indicators show top-down-to-the-SSE motion along the Grassy Creek fault. Based on the shear sense indicators, the contact between the AMS and ABMS in the vicinity of Sparta appears to be a dextrally transpressive fault, indicating multiple generations of movement with differing kinematics along the same structure.