DOI: 10.5066/p13xjcps
title: Luminescence and Radiocarbon data for: 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 9, 2020, Mw 5.1 earthquake in Sparta, NC

description: 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 Lynchburg Group, Wills Ridge Formation, Ashe Metamorphic Suite (AMS) and Alligator Back Metamorphic Suite (ABMS); (5) the nature of the contact between Lynchburg Group, 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 (~740 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 in the eBR separates interlayered graphitic mica schists (Zlp), metagraywacke and graphitic schist (Zl), metaconglomerate and metagraywacke (Zlc), and metagabbro (Zlg) into the Lynchburg Group, Wills Ridge Formation. A SHRIMP U-Pb zircon age of ~455 Ma was obtained on a ~1 m thick metagabbro dike that intrudes metagraywacke in the Lynchburg Group. The Lynchburg Group rocks are separated from metaultramafic-bearing rocks of the AMS by an unnamed fault. The AMS consists of metagraywacke and schist (_Za), muscovite schist and metagraywacke (_Zas), metaconglomerate and metagraywacke (_Zac), amphibolite (_Zaa) and metaultramafic schists and rocks (_Zau and _Zaud). 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 (_Zab). 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. This report consists of a surficial and bedrock geologic map with cross sections, correlation of map units, and description of map units, and a GeMS level 3 geodatabase. Mapping was supported by the National Cooperative Geologic Mapping Program and Earthquake Hazards Program; a EDMAP Grant supported mapping by A. Lynn and K. G. Stewart.