TRANSVERSE FAULTS IN THE VIRGINIA BLUE RIDGE: A LEGACY OF MESOZOIC TECTONICS

Transverse faults have long been recognized in the central and northern Virginia Blue Ridge, but their kinematic and temporal significance has remained uncertain. Transverse faults strike north-northwest (330°-350°) and cross cut the regional structural grain (northeast-southwest) at high angles (60°-75°). These faults displace Mesoproterozoic basement, Neoproterozoic to Cambrian cover rocks, and Cambrian to Ordovician carbonates west of the Blue Ridge. In map view, transverse faults displace geologic contacts by up to 300 m with both apparent right- and left-lateral offset. Transverse faults commonly form right-stepping en-echelon sets and are likely underrepresented on existing maps as they are best recognized where contacts between cover rocks are offset. Although fault surfaces are not exposed, straight map patterns in areas with significant topographic relief indicate steep dips (>80°). Transverse faults cut previously folded contacts and the apparent strike-slip offset in map view is actually accomplished by dip-slip movement with maximum displacements of ~100 m. A number of transverse faults, including the recently recognized Simmons Gap and Sandy Bottom faults in Shenandoah National Park, correspond to well-developed topographic lineaments. At the outcrop-scale a set of subvertical extension fractures strike north-northwest (330°-350°) and cut all other fracture sets. Transverse faults and extension fractures are subparallel to a regional suite of Jurassic diabase dikes. We interpret Blue Ridge transverse faults as Jurassic structures. Collectively, all these structures are Mode 1 fractures that formed in the upper crust, record very modest amounts of east-northeast directed extension, and may have developed in a transtensional regime. We find no compelling evidence for neotectonic activity along these faults, but these zones of structural weakness have influenced the character of the western Blue Ridge landscape.