MICROSTRUCTURAL AND QUARTZ CRYSTALLOGRAPHIC ORIENTATION DATA DOCUMENT LOW-TEMPERATURE COAXIAL FLATTENING IN THE CENTRAL BLUE RIDGE, VIRGINIA

In central Virginia, orthoquartzite of the Cambrian Antietam Formation is exposed in a large north-plunging fold within the Blue Ridge anticlinorium. Most of the quartzite preserves modest strains, but significant penetrative deformation developed in a high-strain zone associated with a NW-vergent fault system near Front Royal, Virginia. Detrital quartz grains here exhibit mean aspect ratios of ~3:1 to 6:1, and microstructures include fractured grains, undulose extinction, deformation lamellae, and bulging recrystallization. Fluid inclusions associated with deformation lamellae are common, and quartz FTIR analyses demonstrate high intragranular water content (200-500 ppm by weight) during deformation. We explore the microstructure using detailed maps of quartz crystallographic orientation. Quartz c-axes are clustered near the observed maximum shortening direction, indicating plastic strain dominated by basal <a> slip. Most quartz grains exhibit Dauphiné twinning, which can be described as a 180° rotation around the quartz c-axis that swaps the positive and negative rhomb faces. Both untwinned grains and the larger component of twinned grains have a positive rhomb plane oriented perpendicular to the shortening direction. This observation is consistent with the idea that Dauphiné twinning often forms as a response to stress, with the more elastically compliant positive rhomb aligned perpendicular to the maximum compressive stress direction. A coaxial flattening deformation is indicated by both oblate strain analyses as well as crystallographic orientation data, in which quartz c-axes are distributed in small circle girdles around the maximum shortening direction. Quartz opening-angle thermometry suggests low deformation temperatures of ~260 ± 50°C, values typically considered too low to enable plastic deformation in quartz. We infer that plastic deformation at these temperatures was possible because of significant hydrolytic weakening and a low strain rate.