STRUCTURE AND STRATIGRAPHY OF THE BREVARD ZONE IN ALABAMA (USA): CONSTRAINTS ON KINEMATICS AND DISPLACEMENT

The Brevard zone of the southern Appalachians in Alabama (AL) consists of a coherent and mappable stratigraphic package of metasedimentary and metaigneous rocks comprising the Jacksons Gap Group (JGG). Separating the Ashland-Wedowee-Emuckfaw belt (eastern Blue Ridge) and Dadeville Complex (western Inner Piedmont), the Katy Creek fault marks the upper structural boundary of the JGG, separating it from rocks of the overlying Dadeville Complex. At the structural base of the JGG, on the northwestern flank of the Brevard zone, the Abanda fault has historically been mapped as a structural break between the Kowaliga Gneiss and Emuckfaw Group of the eastern Blue Ridge. The relatively straight trace of the Abanda fault, coupled with apparent differences in metamorphic grade between rocks of the eastern Blue Ridge and JGG have led workers to interpret the Abanda fault as a post-metamorphic thrust related to Alleghanian tectonism. Poor outcrop exposure along the mapped trace of the Abanda fault has left most workers with minimal structural evidence regarding the kinematics of the fault over its trace from the Coastal Plain unconformity in AL, northeast towards Georgia. In addition, recent geologic mapping in the JGG at the southwestern end of the Brevard zone suggests intrusive rocks of the eastern Blue Ridge cut across the position of the historically mapped Abanda fault, which would place a significant limit on displacement across this fault. Fortunately, extensive outcrop along the shores of Lake Martin, Alabama, which crosses the eastern Blue Ridge-Brevard zone-Piedmont boundary, provides an excellent opportunity for detailed mapping in the JGG and adjacent rocks. Abundant structural and lithologic data collected at varying levels within and adjacent to the Brevard zone allows different kinematic and tectonic models to be assessed. Preliminary work as part of this project indicates displacement along the Abanda fault tips out within ~30 km of the Coastal Plain unconformity. Igneous plutonic bodies similar to those in the eastern Blue Ridge that appear to cross the trace of the Abanda fault, coupled with mixed metasedimentary and mafic metavolcanic bodies within the JGG, indicates a genetic origin with rocks of the underlying eastern Blue Ridge, suggesting both formed within the Wedowee-Emuckfaw-Dahlonega back-arc basin.