STRAIN ANALYSIS IN THE EASTERN BLUE RIDGE AND ADJACENT BREVARD FAULT ZONE, SOUTHERN APPALACHIANS OF ALABAMA AND GEORGIA

The southern Appalachian Brevard fault zone (BFZ) separates the eastern Blue Ridge (EBR) from the western Inner Piedmont along a nearly linear trace for >600 km from eastern Alabama (AL) to North Carolina (NC). Kinematic studies along the length of the BFZ suggest a polyphase history, with some authors interpreting the structure as an orogenic channel accommodating hundreds of kilometers of dextral strike-slip offset. Strain analysis (Rf-phi) on ductilely-deformed feldspars from the Kowaliga Gneiss in AL and Austell Gneiss in Georgia (GA) on the northwestern flank of the BFZ was used to quantify strain gradients along the margins of the proposed BFZ “orogenic channel”, but with conflicting results. In the AL section, bulk strain (Rs) values increased with proximity to the BFZ, while similar analyses in GA suggest ductile deformation associated with regional dynamothermal metamorphism and folding had a more profound effect than potential BFZ-associated shearing. In both cases, calculated bulk strain for rocks along the margin of the BFZ “orogenic channel” was significantly less than concentrated bulk strain internal to the EBR, especially in proximity to lithologic contacts with significant mechanical contrasts (e.g. orthogneiss-schist contact). It is not clear, therefore, that intense ductile deformation attributed to shearing within the BFZ in NC (e.g. Henderson Gneiss) is present in the BFZ of western GA and eastern AL. In an attempt to reconcile these conflicting interpretations, we present the results of Rf-phi strain analysis on ductilely-deformed feldspars in pre-kinematic orthogneiss and quartzofeldspathic metasedimentary rocks both adjacent to and within the BFZ from near the Coastal Plain onlap in eastern AL, northeast to Atlanta, GA. If the orogenic channel model proposed for the BFZ does, in fact, accommodate hundreds of kilometers of strike-slip offset as proposed, rocks within and adjacent to the AL and GA BFZ should record that deformation as a significant strain gradient associated with the orogenic flow. For comparison, we also provide data from Rf-phi analysis of feldspar-bearing rocks from a ductile shear zone along the northwestern flank of the peri-Gondwanan Uchee belt that accommodated emplacement of that terrane atop the Laurentian Pine Mountain belt during Alleghenian orogenesis.