GEOLOGIC MAP OF THE SPARTA EAST, SPARTA WEST, AND PARTS OF THE GLADE VALLEY AND WHITEHEAD 7.5-MINUTE QUADRANGLES, NORTH CAROLINA AND VIRGINIA, AND THE EPICENTRAL AREA OF THE AUGUST 2020 MW 5.1 EARTHQUAKE NEAR SPARTA, NC

New bedrock and surficial geologic mapping in the Blue Ridge of NW NC and SW VA covers the Sparta East, Sparta West, and parts of the Glade Valley and Whitehead 7.5-minute quadrangles and includes the epicentral area of the 9 August 2020 Mw 5.1 earthquake near Sparta, NC. Mapping documented (1) the co-seismic surface rupture from the 2020 earthquake and brittle structures in the bedrock; (2) the fault contact between the western Blue Ridge (wBR) and eastern Blue Ridge (eBR); (3) lithostratigraphy in the Ashe Metamorphic Suite (AMS) and Alligator Back Metamorphic Suite (ABMS) and the nature of the contact between AMS and ABMS; and (4) surficial deposits.

The wBR consists of Mesoproterozoic granitoid gneiss (1.3–1.0 Ga) intruded by the Neoproterozoic Striped Rock pluton (~730 Ma). These rocks are overprinted by a late Paleozoic greenschist facies foliation that intensifies into several anastomosing high-strain shear zones of the Fries fault zone; rocks within the shear zones are white-mica phyllonites to ultramylonites. Kinematic indicators consistently document top-to-NW thrust motion. To the southeast, the polydeformed eBR is juxtaposed over the wBR along the Gossan Lead fault, a ~1 km-wide shear zone. Lithostratigraphy within the AMS includes interlayered graphitic mica schists, metagraywacke, metaconglomerate, amphibolite, and ultramafic schists. NE-SW trending structures dominate the eBR: relict S₀ bedding, S₁ foliations and intrafolial F₁ folds are transposed into a regional S₂ foliation (mean 063/52). Map-scale isoclinal F₂ folds are overprinted by an S₃/F₃ crenulation. The structurally higher ABMS consists of pinstriped mica gneiss and schist. The contact between the AMS and ABMS is a dextral shear zone. In the epicentral area, the Little River fault (~110/45) is mapped for ~4 km and similarly oriented brittle faults occur in the Bledsoe Creek valley up to 4 km to the NW. Manganese-coated, striated brittle faults and surfaces are common; manganese cemented breccias occur locally. Terrace deposits are mapped above the New River, Little River, Bledsoe Creek, and major tributaries. Terrace deposits above Bledsoe Creek in the epicentral area overlie a brittle fault and yield cosmogenic burial ages of ~500 Ka. Potential paleoliquefaction structures were identified at 4 locations.