THE RELATIONSHIP OF GEOLOGIC STRUCTURE TO THE GILES COUNTY SEISMIC ZONE IN SOUTHWEST VIRGINIA, BASED ON FRACTURE MAPPING IN ALLOCHTHONOUS PALEOZOIC STRATA

The Giles County seismic zone in southwest Virginia is defined by a dense cluster of epicenters, clearly discernible from other seismicity of the southeastern US. The majority of its earthquakes have occurred within a 20-km-long, north-northeast-trending band that extends from 5-18 km depth and cuts across regional Valley and Ridge fabric. Based on a lack of shallow events and inferred oblique motion along focal planes that bear no relationship to surficial geology, it has been suggested that this seismic zone is confined to faults within Grenville basement that do not cut overthrust Paleozoic strata. However, empirical relationships between magnitude, displacement, and rupture dimension suggest that the largest historical event may have been produced by rupture through the entire upper crust. To better understand geologic controls on this seismicity, we tested whether previous ruptures may have broken to the surface. We characterized brittle fractures that occur along 7 km of discontinuous exposure along Route 100, which extends diagonally through the seismic belt. Brittle fractures were easily identified within the pristine exposures of undeformed, unweathered Cambro-Ordovician strata. We identified about 100 fractures, ranging from diffuse hairline cracks to throughgoing, clay-filled faults with cm's of subvertical displacement. The fractures fall into several orientation classes that bear no apparent relationship to local topography or karst-related subsidence. In particular, a set of northeast-trending extensional fractures cross-cuts the regional structural trend and may be a manifestation of rupture within the seismic zone. These are oriented properly to be a left-stepping en echelon shear zone associated with dextral slip in the subsurface. Although this illustrates that ruptures in the seismic zone may have affected bedrock at the surface, without age control on these fractures a strong conclusion can not yet be made.