Rheology of a coaxial shear zone in the Virginia Blue Ridge: Wet quartzite dislocation creep at ~250–280 °C

Abstract

Cambrian quartzite along the western margin of the Blue Ridge near Front Royal, Virginia records significant penetrative strain under subgreenschist-facies conditions. Strain analyses of the quartzite indicate general flattening and octahedral shear strain values up to 1.25 (X:Z up to 5.7:1). Samples with X:Z as low as 1.5:1 have 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 dynamic recrystallization with mean grain sizes ~4–5 μm. Locally, the quartzite is brittlely deformed, and subgrains and recrystallized bulges are present along fractures and extinction bands. Fluid-inclusion planes and fluid inclusions along deformation lamellae are abundant, and quartz FTIR analyses yield average H:106Si values of 1100–3300, indicating fluid flow and high intragranular water content during deformation. Higher strain samples generally record lower water contents, which may indicate that fluids migrated to grain boundaries during deformation. Dissolution microstructures are rare in high strain samples, and we interpret most strain to have been accommodated by dislocation creep. Thermal modeling of zircon low-temperature thermochronology data coupled with quartz c-axis opening angle thermometry suggest deformation temperatures were likely ~250–280 °C. These deformation temperatures are lower than what is typically considered the brittle-plastic transition in quartz. We attribute localization of this high strain zone and dislocation creep at these temperatures to hydrolytic weakening and a relatively low strain rate estimated to be ~10−15 s−1.