DEFORMATION CONDITIONS, KINEMATICS, AND THERMOCHRONOMETRIC CONSTRAINTS ALONG THE GOSSAN LEAD FAULT, THE BOUNDARY BETWEEN THE WESTERN AND EASTERN BLUE RIDGE IN NORTHWEST NORTH CAROLINA

The fault boundary between the western and eastern Blue Ridge (WBR-EBR) in the southern Appalachians separates Mesoproterozoic basement rocks and their cover, from Neoproterozoic to Paleozoic accreted rocks. Several northeast-striking faults delineate the boundary, including the Gossan Lead fault (GLF) in northwestern North Carolina. Varying tectonic interpretations of the WBR-EBR boundary include a pre-metamorphic fault, an Acadian dextral strike-slip fault, or an Alleghanian fault. We use field-based, microstructural, and theromochronometric analyses to determine the conditions, kinematics, and timing of deformation, in order to distinguish among competing hypotheses for the GLF.

Detailed mapping and microstructural analysis of the GLF document a several km-wide mylonitic zone with kinematic indicators that record dominantly top-to-the-NW thrust motion, and local strike-slip and normal sense indicators. Microstructural observations of recrystallized quartz and feldspar constrain deformation conditions. Quartz is characterized by either subgrains and new grains with irregular boundaries, or subgrains and new grains with similar orientations to the host. These observations, and preliminary electron backscatter diffraction data, indicate bulging grain boundary migration (BLG) and subgrain rotation recrystallization in quartz, typical of deformation at greenschist to lower amphibolite-facies conditions. Feldspar has core and mantle structures with irregular boundaries on subgrains and new grains indicating BLG recrystallization and deformation within the amphibolite-facies, but is variably epidotized suggesting a greenschist-facies overprint. Unaltered lineation-forming amphibole from a mylonitic amphibolite records an ⁴⁰Ar/³⁹Ar cooling age of ~340 Ma, indicating that amphibolite-facies deformation occurred prior to the Alleghanian orogeny. This indicates lower grade deformation recorded elsewhere in quartz and feldspar is either due to this same protracted event or a discrete Alleghanian event. These data are consistent with dominantly transpressional amphibolite- and greenschist-facies deformation during the Neoacadian and possibly Alleghanian orogenies along this portion of the complexly deformed WBR-EBR boundary.