RHEOLOGY OF A COAXIAL SHEAR ZONE IN THE VIRGINIA BLUE RIDGE: WET QUARTZITE DISLOCATION GLIDE AT ≤260°C AND LOW STRAIN RATES (Invited Presentation)

Cambrian quartzite along the western margin of the Blue Ridge near Front Royal, Virginia records significant penetrative strain under subgreenschist-facies conditions. Strain is localized in the hanging wall of an Alleghanian NW-vergent thrust fault system that includes a breccia zone several hundred meters wide. Detrital grains in the quartzite adjacent to the breccia zone record a general flattening strain and mean X:Z ratios of up to 5.7:1. Samples with mean X:Z ratios as low as 1.5:1 have moderately-developed crystallographic preferred orientations that record a dominance of basal <a> slip and patterns consistent with coaxial flattening. In most samples quartz has undergone minor amounts of bulging recrystallization with mean grain sizes consistently 4–5 µm. Locally, quartz grains are brittlely deformed, and recrystallized bulges are present along some fractures. Tuttle lamellae and fluid inclusions along deformation lamellae are abundant, and quartz FTIR analyses yield average H:10⁶Si values of ~1,500–2,700 (~225–400 ppm by weight), indicating high intragranular water content and fluid flow during deformation. While hydrolytic weakening likely facilitated deformation, strain does not appear to correlate with water content at the grain scale. Dissolution microstructures are rare in high strain samples, and strain magnitude does not correlate with the concentration of immobile elements, suggesting pressure solution was not an important deformation mechanism. Altogether these observations and data indicate that the quartzite deformed primarily via basal dislocation glide near the brittle-plastic transition. Thermal modeling of zircon (U-Th)/He data suggest peak deformation temperatures were ≤260°C, which is consistent with evidence that Cambrian units in this area were never buried to temperatures high enough to fully reset detrital zircon fission track ages. In addition, the most well-defined quartz c-axis opening angles suggest deformation at ~250 ± 50°C. These deformation temperature estimates are lower than what is typically considered the brittle-plastic transition in quartz. We attribute dislocation glide at these relatively low temperatures to significant hydrolytic weakening and a low strain rate estimated to be between 1 x 10^-15 s^-1 and 3 x 10^-16 s^-1.