CONSTRAINTS ON THE EMPLACEMENT CONDITIONS OF THE BLUE RIDGE THRUST SHEET: DEFORMATION TEMPERATURES FROM THE CATFACE AND IRON MOUNTAIN FAULTS, TN-VA

The Blue Ridge Thrust Sheet (BRTS) extends several hundreds of kilometers along the Southern Appalachians. The western edge is composed of imbricate low-angle thrust faults, which create a structurally complex thrust system that is punctuated by several tectonic windows. In northeastern Tennessee the Mountain City Window (MCW) exposes lower grade footwall rocks at the base of the BRTS. The Catface and Iron Mountain thrust faults frame the MCW, on the east and west respectively. The Catface fault emplaces Mesoproterozoic basement rocks and ~750 Ma Mount Rogers Formation rhyolites in the hanging-wall on top of younger sedimentary rocks in the footwall inside the MCW (Konnarock Formation, Chilhowee Group, Shady Dolomite and Rome Formation). The Iron Mountain fault places the Konnarock Formation and Chilhowee Group over similar rocks within the MCW (footwall). To resolve whether the deformation we observe in the leading edge of the BRTS is consistent with regional tectonic patterns, it is essential to determine the kinematic shear sense and deformation conditions along these faults.

We collected ten samples along a traverse across the Catface thrust sheet, through the MCW to the Iron Mountain fault and documented field and petrographic relationships and determined deformation temperatures using electron backscatter diffraction (EBSD). A pervasive SE-plunging mineral stretching lineation (149/25) and strong mylonitic fabrics occur along the Catface fault. Kinematic shear sense indicators (asymmetric porphyroclasts, S-C fabrics) are consistent with top to the NW motion, in agreement with regional thrusting. Microstructural observation including potassium-feldspar bookshelf-sliding structures, sigma clasts, and top to NW S-C fabrics also indicate NW-directed thrusting. Quartz grains in samples along the Iron Mountain fault display bulging recrystallization, kinking, subgrain boundaries, and undulatory extinction, characteristic of greenschist-facies deformation. Samples from the Catface fault are rhyolites and most of the strain has been accommodated by micas. The mylonitic fabrics and deformation temperatures from EBSD are consistent with optical observations and support greenschist facies conditions along the Catface and Iron Mountain Faults during NW directed thrusting.