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.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.