KINEMATIC AND AGE CONSTRAINTS ON THE ALEXANDER CITY FAULT, EASTERN BLUE RIDGE, ALABAMA

Within the Ashland-Wedowee-Emuckfaw belt of the eastern Blue Ridge (EBR) in Alabama, the Alexander City fault (ACF) has traditionally been mapped on the southeastern margin of the Upper Devonian Elkahatchee batholith, separating it along most of its upper intrusive contact with metasedimentary rocks of the structurally higher Ordovician Wedowee Group. In this region, the fault displays a nearly linear northeast trace over a distance of >30km from Jordan Lake, where it emerges from beneath Cretaceous sedimentary rocks of the Gulf Coastal Plain. Units on either side of the fault are at the same metamorphic facies (middle-upper amphibolite) and share a common metamorphic fabric. Many workers have suggested late-stage, brittle, predominantly normal slip kinematics for the fault in this region. The extent and kinematic history of the ACF at the northeastern terminus of the Elkahatchee batholith, however, have been a topic of debate for decades. Southeast of Alexander City, AL, the ACF has been interpreted differently by a number of researchers. Recently, it has been proposed that the fault has a complex kinematic history, with an initial (Devonian) ductile phase overprinted by later (latest Paleozoic to Early Mesozoic) brittle slip. Both regional map relationships and our detailed investigations on the presence and nature of the ACF on the western edge of Lake Martin, where seasonal low water levels provide extensive outcrop from the southeastern margin of the Elkahatchee batholith through the Wedowee and Emuckfaw Groups, provide critical constraints on the extent of the proposed brittle and ductile shear zones associated with the ACF. Additionally, isotopic age constraints from pre-, syn- and post-metamorphic granitoids in the EBR and stratigraphic-structural relationships allow for assessment of the timing for proposed ductile and brittle shear along this boundary. Our mapping near the northeastern extent of the Elkahatchee batholith suggests the brittle ACF terminates south-southwest of Alexander City, consistent with models suggesting a relay ramp relationship with the more southeasterly Abanda fault. Although evidence for a ductile shear zone along the southeastern margin of the Elkahatchee is present in the study area, age relationships suggest a Carboniferous, not Devonian, age for this shear zone.