QUATERNARY FAULTING IN THE CENTRAL VIRGINIA SEISMIC ZONE AND POSSIBLE INFLUENCE OF NEARBY ANOMALOUS CRUST

Recorded seismicity and geologic field evidence indicate that active faulting in the Central Virginia seismic zone (CVSZ) has occurred on multiple, sub-parallel, NE-striking structures in a ~100 km-wide belt extending WNW >100 km from the Fall Line at Richmond, back in time to at least the Late Pleistocene. Recorded earthquake hypocenters range from <2 to ~15 km in depth and magnitudes have been <M6.0; no known surface ruptures have been found. P-axis determinations are mixed ENE and ESE orientations. Three probable paleoliquefaction sites in the CVSZ indicate Holocene to Late Pleistocene localized strong ground shaking; recent C-14 and OSL dates suggest ages between 2000 and 900 ybp. Faults and fractures exposed at 3 excavation sites near the epicenter of the August, 2011 M5.8 Mineral earthquake cut surficial deposits, some of which are dated by OSL as Late Pleistocene in age. At these sites are two styles of NE-striking Quaternary thrust faulting: an older style reactivating Ordovician-age NE-trending fabric that dips steeply (75-90˚) to the SE, and a younger faulting that cross cuts bedrock fabric, has moderate to shallow SE dips (45-0˚), and typically flattens upward within a few meters of the surface. Both styles show displacements of <1m on individual faults and deform Quaternary residual soil. Locally, the younger thrusts cut surficial deposits of Late Pleistocene age. At the Everona fault site along the Mountain Run fault zone (MRFZ), a N55E-trending, 25-50˚ NW-dipping thrust fault shows ~2m of offset and flattens upward—it places undated gravel over bedrock phyllite, and is antithetic to the MRFZ. Slip indicators on all Quaternary faults in the CVSZ are consistent with ESE-WNW compression. The CVSZ is anomalous for the east coast of the U.S. in its concentration of seismicity and known Quaternary faults. We suggest that anomalous crust of the Chesapeake Bay impact structure could perturb regional stress patterns to produce the mixed P-axis orientations, and increase horizontal stress on CVSZ faults, all of which strike tangentially to the impact structure. The impact structure has a post-Eocene history of behaving isostatically different from the surrounding region according to Coastal Plain stratigraphy.