JOINT AND LINEAMENT ANALYSIS OF GRANITES IN THE VIRGINIA PIEDMONT TO DETERMINE EXPANSE OF RADIATING FRACTURES FROM THE CHESAPEAKE BAY IMPACT CRATER

Joint and lineament analyses were conducted in the Virginia Piedmont to determine whether radiating fractures within the 35.4 km radius fracture zone of the Chesapeake Bay impact crater extend across Virginia. Joints were measured in the Petersburg Granite (as mapped on the 1963 state map) at four sites along the Rappahannock, James, Appomattox, and Nottoway Rivers. On the 1993 state map, the Petersburg Granite is subdivided into two different rock types--granite and granite gneiss. Lineaments were identified and measured within a 34,000 km2 area on a LANDSAT image, that covers the Virginia Piedmont, including the four field sites, and parts of the adjacent Blue Ridge and Coastal Plain. Joints were compiled on rose diagrams and stereo nets; lineaments were compiled on rose diagrams.

Joint and lineament patterns were analyzed by relating them to possible genetic mechanisms. Tested models included: joints related to the intrusion of a pluton (vertical in the center and sub-vertical toward the edges); joints related to the Central Virginia Seismic Zone, which trend northeast and northwest, consistent with the earthquake focal mechanisms; and radial or concentric joints related to a meteorite impact in the Chesapeake Bay. Elements of each genetic model were interpreted to be consistent with the data, but the pluton model was least consistent. Overall, lineament and joint orientations appeared to coincide adequately for the expected radiating pattern at the James and Appomattox sites and only moderately at the Rappahannock and Nottoway sites.