Abstract
Bouguer gravity anomalies at 1,870 locations on the central Piedmont of Virginia from 37°37′N to 37°52′N and 77°44′W to 78°23′W display patterns of variation produced by upper-crustal density contrasts and thickening of the crust in a west-northwest direction. No other deep sources are evident. Upper-crustal density contrasts are associated with rock units known from geologic mapping. The subsurface distribution of these rock units, interpreted from seismic-reflection data, was confirmed by measured variations in gravity. A two-dimensional model analysis indicates the following average in situ density values for the principal formations: Arvonia Formation, 2.77 gm/cc; Columbia Granite, 2.75 gm/cc (tonalite) and 2.73 gm/cc (pegmatite); Chopawamsic volcanics, 2.77 gm/cc (felsic units) and 2.79 gm/cc (malic units); Catoctin/Lynchburg/Evington, 2.82 gm/cc; Maidens Gneiss, 2.78 gm/cc; and Grenville Basement, 2.71 gm/cc. Gravity anomalies are consistent with the existence of a major thrust fault, at depths between 9 km and 16 km, that probably separates Grenville Basement rocks from younger, Catoctin/Lynchburg rocks. These anomalies confirm that the slight eastward dip of this thrust fault beneath the western part of the area increases significantly east of 78°05′W.
