EMPLACEMENT CONDITIONS OF THE BLUE RIDGE THRUST SHEET: COMPARISON OF QUARTZ MICROSTRUCTURAL DATA FROM THE CATFACE AND IRON MOUNTAIN FAULTS, NC-VA-TN

The Mountain City Window (MCW) in the western leading edge of the Blue Ridge Thrust Sheet (BRTS) exposes deformation fabrics attributed to Alleghanian aged emplacement of the BRTS approximately 340 million years ago. Notably, the western and eastern bounding faults of the MCW display contrasting deformation conditions over a short (< 2 km distance), providing an opportunity to observe the brittle-ductile transition across the BRTS. The Catface fault, which bounds the east side of the MCW, contains ductile deformation fabrics, whereas the Iron Mountain fault, which bounds the west side of the MCW, contains dominantly brittle deformation fabrics. We analyzed samples using a petrographic microscope to characterize ductile and brittle microstructures, dynamic recrystallization, and fluid-related features. We also used Electron Backscatter Diffraction (EBSD) to constrain deformation conditions and preferred orientations in quartz grains. Quartz grains in the Catface fault samples display scalloped and blurry edges with small, irregular grain shapes, and some polygonal grains with a strong preferred orientation, which are associated with deformation via bulging recrystallization and subgrain rotation, respectively. Samples from the Catface fault have pole figures with distinct patterns that display some combination of basal, rhomb, and/or prism <a> slip, indicative of deformation temperatures between 300-450 °C. Iron Mountain fault samples, in contrast, show some quartz grains with undulatory extinction, but more widespread fractures and veining, and all samples notably lack bulging recrystallization and other features associated with greenschist facies deformation conditions. Interestingly, EBSD c-axis pole figures from Iron Mountain fault samples, which contain primarily brittle deformation fabrics, display a weakly developed, but distinct, foliation-parallel central girdle not associated with any common slip system in quartz.