Relations of Bouguer Gravity Anomalies to Geological Structure in the New River District of Virginia
Over 500 gravity observations were made at 2- to 3-km intervals along accessible roads in the New River District of Virginia, within an area defined by lat 37°009N., and 37°309N., and long 80°159W. and 81° 009W. The Bouguer anomalies of the data can be related to variation in crustal thickness, and folding and faulting in the Appalachian Valley. A quadratic polynomial surface fitted to Bouguer anomaly data by a least squares process reveals a broad anomaly trending north-northeasterly across the area. This anomaly correlates well with the gravity field associated with variation in crustal thickness determined from a seismic survey of the Middle Atlantic states. Residual gravity anomalies obtained from differences between the Bouguer anomalies and the quadratic polynomial surface indicate an east-northeasterly trending feature which parallels the regional strike of the Appalachian geological structure. This residual gravity anomaly of approximately 8 mgal can be explained by a structural model based upon attitudes of sedimentary units measured at the surface and density values measured from outcrop specimens. This interpretation suggests that the Blacksburg syncline consists of approximately 7 km of Paleozoic sedimentary rocks. There is no indication in the gravity data of any major decollement. Numerous local residual anomalies 2 to 3 mgal in amplitude are observed over the Pulaski and Saltville thrust faults. These anomalies may be associated with the extensive breccia zones exposed in the near surface material and with the variation in the thick-nesses of thrust plates.