KINEMATIC HISTORY OF BRITTLE DEFORMATION IN THE PETERSBURG GRANITE, RICHMOND, VIRGINIA

The Petersburg granite forms a large Carboniferous pluton in the eastern Piedmont of Virginia and is well exposed in the James River along the Fall Zone in Richmond. These expansive outcrops were studied to characterize the fracture geometry and understand the kinematic history of brittle deformation in the Petersburg granite. Previous workers have suggested some fractures in the Richmond area are radial fractures associated with the 35 Ma Chesapeake Bay impact crater in eastern Virginia. We mapped fractures at five locations along an 8 km transect of the James River.

The Petersburg granite in Richmond is cut by two dominant fracture sets. The older set strikes NE to ENE, dips steeply to the north, and commonly displays gently plunging mineralized slickensides that record dextral slip. P- and T-axes for these shear fractures are clustered and consistent with a subhorizontal σ1 oriented WNW-ESE. Quartz, muscovite, and biotite indicate mineralization and slip occurred under greenschist facies conditions. The younger set strikes NNW to NNE, is subvertical, and rarely mineralized. Cross cutting relations and surface ornamentation indicate the younger set are extension fractures, although rare fractures were reactivated as reverse faults.

We interpret the older set to have formed during WNW-directed contraction in the Alleghanian orogeny. The orientation of the younger set records E-W extension and is parallel to extensional faults in the Richmond Triassic basin, exposed 15 km to the west. Reactivation of these extension fractures may have occurred under the late Cenozoic compressional stress field. Regardless, there is no tenable evidence linking fractures in the Petersburg granite to deformation associated with the Chesapeake Bay impact crater.