A geologic and potential field investigation of the central Virginia Piedmont

Abstract

The State Farm Antiform is a major structural feature of the eastern Piedmont of Virginia. Exposed in the core of this structure is the easternmost known outcrop of Grenville basement, the State Farm Gneiss. The eastern limb of the antiform coincides with a zone of late Paleozoic mylonitization, the Hylas Zone, and the western border fault of the Richmond Triassic Basin. The crest of a major gravity anomaly, the Piedmont gravity high, is coincident with the hinge of the State Farm Antiform. In addition, this region is in the eastern portion of the Central Virginia Seismic Zone. Detailed geologic mapping and potential field modeling shows that the character of the Piedmont gravity high is a result of both crustal thickness variation and density contrasts between the surficially exposed lithologies. Geologic, gravity and magnetic field modeling has determined the structure of the antiform and the Triassic basin to a depth of 5 km. Microearthquake activity is shown to be concentrated along a planar zone that extends to a depth of 20 km, which is concordant to regional structure. This concordancy implies that any major sole thrust in the crust is either very shallow or deeper than 20 kilometers.