BRITTLE-DUCTILE TRANSITION EXPOSED ALONG STRANDS OF THE BLUE RIDGE THRUST SHEET IN THE MOUNTAIN CITY WINDOW, NORTH CAROLINA–TENNESSEE–VIRGINIA

The Blue Ridge thrust sheet (BRTS) extends for ~1000 kilometers and transported crystalline rocks northwestward during the Alleghanian orogeny. In northeastern Tennessee and adjoining North Carolina and Virginia, the western leading edge of the BRTS frames the Mountain City Window (MCW). The east side of the MCW is bounded by the Catface fault, a ~240 m thick mylonite zone. The Catface fault juxtaposes Proterozoic basement rocks and the volcanogenic Neoproterozoic Mount Rogers Formation over dominantly Cambrian sedimentary rocks in the MCW (footwall). The western side of the MCW is defined by the Iron Mountain fault, a discrete <1m wide fault that emplaces Proterozoic and Cambrian sedimentary rocks in the hanging wall over younger Cambrian rocks in the footwall. These two faults are interpreted to represent a single folded strand of the BRTS.

Rocks along the Iron Mountain fault are characterized by highly localized deformation zones that display evidence of ductile to brittle deformation, and display random crystallographic preferred orientations. Within these deformation zones there are abundant fine quartz grains, which commonly display blurry subgrain and grain boundaries indicative of bulging recrystallization as well as less common angular grains indicating brittle fracture. Quartz grains outside of the deformation zones display undulatory extinction, kink bands, subgrain boundaries, deformation lamellae, and widespread fluid inclusions. In contrast, rocks from the Catface fault display widely distributed deformation zones, with clear top-to-the-NW kinematic indicators in some samples, and minimal brittle deformation of quartz. Samples from mylonites of the Catface fault record ubiquitous bulging recrystallization in quartz, minor evidence for subgrain rotation recrystallization, fracturing in feldspars, and abundant sericite; these support greenschist facies deformation conditions. Samples from the Catface fault hanging wall have crystallographic preferred orientations that record basal <a> slip, consistent with greenschist facies deformation. Differences in deformation style and apparent conditions of faulting along the Catface and Iron Mountain faults suggest motion along the BRTS was part of a progressive event that traverses the brittle-ductile transition.