MICROSTRUCTURAL AND QUARTZ CRYSTALLOGRAPHIC PREFERRED ORIENTATION EVIDENCE FOR SPATIAL VARIATION IN THE CHARACTER OF DEFORMATION NEAR INDEPENDENCE, VA

The Fries Fault exposed near Independence, VA, is a part of a regionally significant northeast-southwest trending fault system that stretches from northern Virginia through North Carolina and southwestward as far as Georgia. The southeast-dipping faults in the region, such as the Fries, are typically associated with Paleozoic compressional orogenesis and described as thrusts, though some contend the Fries Fault preserves an earlier history of late Precambrian extension. We have analyzed thirteen samples collected along a strike-perpendicular transect from the Fries Fault and adjacent 690 ± 10 Ma Striped Rock Pluton to document variations in microstructure and quartz crystallographic preferred orientation (CPO) fabrics. Petrographic observations and quartz fabrics from many samples suggest asymmetric deformation in greenschist facies, such as fractured and extended feldspar clasts, occasionally fractured quartz porphyroclasts, and quartz CPO patterns characteristic of basal <a> slip in quartz. However, other samples contain evidence for prism [c] slip in the central parts of the Striped Rock Pluton. Quartz in these samples commonly displays a “chessboard” pattern of extinction, and preliminary quartz CPO fabrics show c-maxima near the mesoscopic extension direction, as is typically observed for prism [c] slip. Previous experimental and observational studies indicate that c slip is favored when water content is high; this suggests that fluids variably affected the actively deforming rocks of the Striped Rock Pluton, locally activating prism [c] over slip systems more typically associated with lower temperature deformation.