MESOZOIC DUCTILE TO BRITTLE DEFORMATION – A NEW PARADIGM IN THE SOUTHSIDE VIRGINIA PIEDMONT

The Danville Basin is one in a series of tilted fault – block basins developed along major Paleozoic listric detachment zones during Mesozoic Rifting of eastern North America. Until detailed mapping by Paul Thayer in 1970s, the paradigm established in the 1920s by J.K. Roberts modeled these basins as NE trending half grabens. The Danville basin was thought to be developed along a continuous north-east trending, southeast dipping Triassic normal fault (Chatham Fault) . The supposed half graben structure has been dismembered by post Triassic faulting to present day dimensions of 165 km by 2-10 km. A reticulate fracture system associated with a 300 m wide Jurassic ductile to brittle high strain zone along the NW side of the relict basin is host to the seventh largest undeveloped uranium ore deposit in the world on the historic Coles Hill Estate near Chatham.

Thayer’s detailed mapping coupled with systematic outcrop and thin section-based lithofacies analysis changed our understanding of the original basin architecture. This is apparent in the major change in basin structural geometry from North Carolina NE along strike into VA, where much of the basin was cut out by a major right step-over offset of nearly 5 km along the Chatham Fault . This post – Triassic fault truncation and uplift eliminated the central lacustrine beds, as well as most of the original western border conglomerate along a narrow 2 by 22 km long relict basin from Judy Byrd Mtn. northeast to Chatham. Modal analysis showed a high proportion of the lithic debris preserved in conglomerate beds in this narrow segment of the basin derived from metavolcanics only exposed along fault scarps that form the southeast boundary in Virginia, completely negating a half graben model there. Additional proof of a this relict basin geometry was supplied by a detrital zircon geochronology and provenance study by Voice and others (2008) that showed the zircons preserved in this portion of the basin are mostly Paleozoic in age and were probably derived from igneous sources to the southeast, not northwest of the preserved basin.