NEW TERRANE MAP FROM DETAILED MAPPING AND AIRBORNE GEOPHYSICS OF PARTS OF THE CENTRAL AND EASTERN PIEDMONT, VIRGINIA, USA

A preliminary new terrane map of a portion of the central and eastern Virginia Piedmont near the Fall Zone has been constructed. We compiled recent 1:24K geologic mapping with high-resolution airborne magnetic and radiometric data acquired by the USGS Earth Mapping Resources Initiative (Earth MRI). Along the Fall Zone southeast of the Spotsylvania fault, from west-to-east are the Goochland (Gt), Raleigh (Rt), Spring Hope (SHt), Triplet (Tt), Roanoke Rapids (RRt), and Dinwiddie (Dt) terranes. Bounding faults of the Eastern Piedmont fault system are the Macon (Rt from SHt), Hollister (SHt from Tt), Gaston Dam (Tt from RRt), Hylas (Gt from Dt), and Nottoway River (RRt from Dt) fault zones. In the Gt, a previously unrecognized dome cored by Mesoproterozoic State Farm Gneiss (SFG) was identified only after processing of the geophysical data. This dome lies south of and along strike with other mapped domes formed during dextral transpression along and west of the Hylas fault (Hyf). Terranes east of the HyF have a peri-Gondwanan affinity, and consist of amphibolite-facies (Gt, Rt, Tt, Dt) or greenschist-facies (SHt, RRt) metasedimentary, metavolcanic and metaplutonic rocks, and late Paleozoic granitoid plutons.

Terranes show relations to the geophysical data due to different rock types and boundary fault signatures. Faults are delineated as linear magnetic anomalies or contrasts in radiometric Th, U, or K content. Higher-grade metamorphic terranes show higher Th and lower K, but lower-grade terranes show lower Th and variable K. Some narrow lineaments correspond to magnetite-bearing rocks within the bounding fault zones. Granitoid plutons generally show high K, and either high magnetic signature + high Th or low magnetic signature + low Th. Structural domes appear as concentric oval rings in the radiometric data. In the Gt, magnetic highs correspond to amphibolite-rich Sabot gneiss and magnetic lows correspond to the SFG. The Mesozoic Richmond basin appears as a smooth magnetic low (due to increased source depth), with low Th and elevated K attributed to arkosic sandstone. Mesozoic diabase dikes form N-NW-trending linear magnetic highs that have no radiometric signature. For the easternmost terranes (RRt and Dt), these anomalies and corresponding structures can be traced eastward beneath Coastal Plain sediments.