GEOLOGIC MAP OF THE SOUTHERN HALF OF THE MOUTH OF WILSON AND SOUTHEAST CORNER OF THE GRASSY CREEK 7.5-MINUTE QUADRANGLES, NC-VA: ASHE-BASEMENT CONTACT REVISITED

Geologic mapping in the Blue Ridge of NW NC and SW VA, part of a USGS project to map the Mount Rogers National Recreational Area, focused on the contact between the Mesoproterozoic granitoid basement rocks and the overlying Ashe Metamorphic Suite (AMS) in the southern parts of the Mouth of Wilson and Grassy Creek 7.5’-quadrangles. Basement rocks consist of deformed felsic meta-igneous rocks including weakly foliated to gneissose biotite granitoids, megacrystic gneisses, and lesser amphibolite. A network of anastomosing shear zones in the basement marks the Fries fault (FF). The Neoproterozoic to Cambrian AMS is juxtaposed against basement rocks by the Gossan Lead fault (GLF). The AMS was metamorphosed to the staurolite-kyanite grade and consists of interlayered metagraywacke, graphitic muscovite phyllite and schist, amphibolite, chlorite-amphibole schist, and minor quartz pebble metaconglomerate. Two elongate bodies of chlorite-amphibole schists, the Rocky Ridge and Peden bodies, have been traced for ~7.7 km. Amphibolites in the AMS vary from laminated hornblende schists to garnet-plagioclase-hornblende gneisses.

FF shear zones vary from meters to ~1 km wide and are composed of basement-derived porphyroclastic mylonites with the assemblage chlorite + epidote + garnet + biotite + muscovite + quartz + plagioclase. These shear zones trend NE-SW, dip 45-60^o SE with a moderately ESE plunging lineation, and contain top-to-NW kinematic indicators. The GLF trends N 65-70^o E, dips moderately SE, and is parallel to the regional foliation in the AMS, basement rocks, and FF shear zones. AMS and basement rocks near the GLF are mylonitic, locally truncated and contain moderately plunging ESE mineral lineation. In the AMS the GLF is marked by mylonitic graphitic phyllite to muscovite schist and quartz pebble metaconglomerate with rare lithic clasts, which could be traced ~500m. Recent Ar/Ar ages (Stokes et al., 2010) from the FF and AMS suggest the faults and related structures developed during a Dev.-Miss. event.

Quaternary terrace deposits of unconsolidated sands, with quartz and lithic pebbles to cobbles occur ~500’ above the present level of the New River. The largest terrace deposit underlies a gentle rolling surface (~7 sq. km) near in Piney Creek, NC and is at least 40 feet thick.