Bouguer gravity anomalies at 1870 locations on the central Piedmont
of Virginia from 37° 3TN 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 WNW 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 Granitoid-2.75
gm/cc (tonalite) and 2.73 gm/cc (pegmatite), Chopawamsic Volcanics-
2.77 gm/cc (felsic units), and 2.79 gm/cc (mafic units), Catoctin/
Lynchburg-2.815 gm/cc, Maidens Gneiss-2.775 gm/cc, Grenville Basement-
2.71 gm/cc. Gravity and seismic data are consistent with the
existence of a major thrust fault at depths between 9 km and 16 km
that separates Grenville Basement rocks from younger Catoctin/Lynchburg
rocks. The slight eastward dip of this thrust fault beneath the
western part of the area increases significantly east of 78° 05' W.
Gravity anomalies suggest the existence of several mafic inclusions
within the Columbia Granitoid that were not identified by geologic
mapping.
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