ESTIMATES OF IN SITU DENSITY OF BLUE RIDGE AND VALLEY AND RIDGE ROCKS BASED ON GRAVITY SURVEYING IN SOUTHWEST VIRGINIA

In situ densities of rocks in the Blue Ridge and Valley and Ridge provinces are estimated using complete Bouguer anomalies (CBA) at 120 new gravity stations and densities of specimens from 140 locations near Roanoke, Virginia (latitudes 37.25°N to 37.5°N, and longitudes 79.5°W to 80.0°W).

CBA values projected to a N37W directed 16 km profile indicate a gravity gradient of -0.65 mgal/km, and mark the termination of the Appalachian Gravity Gradient which extends N37W from the eastern border of the North Carolina piedmont. Continuing N37W beyond the study area the gradient changes abruptly to less than 0.10 mgal/km. Seismic measurements indicate an increase in crustal thickness of approximately 10 km along this 170 km profile, sufficient to account for the regional gravity gradient.

Residual gravity values were calculated by subtraction of the regional gradient from CBA values. Conventional 2-D forward modelling is used to relate the residual gravity variation to a geologic cross section along the 16 km profile, which displays lithology from the land surface to a depth of 1 km. A density cross section consisting of polygonal units shaped to represent each of the 28 stratigraphic units displayed on the geologic cross section was prepared. Densities assigned these units were adjusted to produce gravity variation over the model that replicates, approximately, the residual gravity profile. Results indicate that densities of igneous and metamorphic rock specimens, which average 2700 kg/m³ appropriately explain the residual gravity variations over areas of igneous/metamorphic terrane.

A residual gravity anomaly of approximately -5 mgal is situated over Paleozoic sedimentary terrane in an area approximately 10 km by 5 km. Three-dimensional model calculations indicate that in situ density in the underlying sedimentary section could be as much as 100 kg/m³ lower than the average of 2700 kg/m³ determined from sedimentary rock specimens. Tectonic brecciation in large zones mapped in the Elbrook and Rome Formations is a probable cause of this reduced in situ density.

There is no obvious indication of gravity anomaly sources at intermediate depths in the crust.