UPDATE ON 1:24,000 GEOLOGIC MAPPING OF THE BREVARD ZONE AND RELATED STRUCTURES IN EAST-CENTRAL ALABAMA

The Brevard zone (BZ) is an enigmatic polyphase Paleozoic shear zone in the southern Appalachians that serves as, at least in part, the boundary between the eastern Blue Ridge (EBR) and the Inner Piedmont (IP) terranes. In east-central Alabama, the BZ is lithologically defined by the Jacksons Gap Group (JGG), a distinctive group of relatively low-grade and locally weakly strained metasiliciclastic and metapelitic rocks structurally bounded between the Abanda (northwest) and Katy Creek faults (southwest), occurring between the Emuckfaw Group (EG) of the EBR and Dadeville Complex (DC) of the IP. Recent detailed geologic mapping at 1:24,000 scale, in conjunction with geochronologic, geochemical, lithotectonic, and structural data provide new insights into the nature and significance of this regional geologic feature. We present a compilation of detailed geologic maps tracing the limbs of the late-stage Tallassee synform in Alabama and report field, structural, petrographic, and isotopic data related to the development of the BZ and its associated structures. Major findings include: (1) locally the JGG is a distinctly lower grade (greenschist-facies) and lower strained package of metasedimentary rocks than the bounding EG and DC (both amphibolite-facies); (2) recent geological mapping in the Roanoke East, Alabama, quadrangle has revealed the presence of an igneous intrusion within the JGG that we correlate to the Long Island Creek Gneiss in neighboring Georgia, which has implications for the relationship between the EG and JGG; (3) the detrital zircon age spectra of some rocks from the EBR, JGG, and Opelika Complex (OC) are similar to those of the western Blue Ridge in the foreland and are thus interpreted to be earliest Laurentian margin drift facies; (4) the DC is a Late Cambrian- to Early Ordovician-aged exotic volcanic arc complex cradled between Laurentian units of the EBR (e.g., the JGG and OC) and Pine Mountain Window (PMW); (5) the Stonewall line fault juxtaposes the DC and OC in the southeast and likely corresponds, at least in part, to the Katy Creek fault in the northwest; (6) the early Paleozoic Alabama-Georgia Laurentian margin may have originated as an embayment with marginal EBR strata to the west and PMW basement/cover rocks to the east, a shape roughly corresponding to the regional Tallassee synform.