Influence of ancient thrust faults on the hydrogeology of the Blue Ridge Province

The Blue Ridge Province contains ubiquitous northeast-southwest-trending thrust faults or smaller thrust "slivers" that greatly impact the nature and character of ground water flow in this region. Detailed investigations at a field site in Floyd County, Virginia, indicate that high-permeability zones occur in the brittle crystalline rocks above these thrust faults. Surface and borehole geophysics, aquifer tests, and chlorofluorocarbon and geochemical data reveal that the shallow saprolite aquifer is separated from the deeper fault-zone aquifer by a low-fracture permeability bedrock confining unit, the hydraulic conductivity of which has been estimated to be six orders of magnitude less than the conductivity of the fault zones at the test site. Within the Blue Ridge Province, these fault zones can occur at depths of 300 m or more, can contain a significant amount of storage, and yield significant quantities of water to wells. Furthermore, it is expected that these faults may compartmentalize the deep aquifer system. Recharge to and discharge from the deep aquifer occurs by slow leakage through the confining unit or through localized breach zones that occur where quartz accumulated in high concentrations during metamorphism and later became extensively fractured during episodes of deformation. The results of this investigation stress the importance of thrust faults in this region and suggest that hydrogeologic models for the Blue Ridge Province include these ancient structural features. Faults in crystalline-rock environments may not only influence the hydrology, they may dominate the flow characteristics of a region.