TERRANES OF THE SOUTHERNMOST APPALACHIANS, ALABAMA AND GEORGIA

Recent geologic findings in Alabama have significance for understanding the evolution and amalgamation of terranes in the southern Appalachians. Starting in the foreland and working toward the hinterland, the chlorite-grade Talladega belt was thrust upon the foreland platform. The Talladega belt is of key interest because it preserves some of the youngest fossiliferous and metamorphosed rocks in the orogen. Some Talladega belt units are biostratigraphically tied to the distal Laurentian margin but the tectonic affinity of the structurally highest unit, the Hillabee Greenstone, is suspect. Rocks lying above the Hillabee and beneath the Brevard zone generally are correlated with the eastern Blue Ridge (EBR; i.e., Jefferson terrane). A unique assemblage of lower-metamorphic grade siliciclastics, cherts, carbonate-bearing pelites, and volcanics, the Lake Martin terrane (i.e., Jacksons Gap Group), corresponds to the Brevard zone. This fault bounded package (duplex) contains lithologies and structures that contrast sharply with those above and below it. The Dadeville and Opelika terranes lie above and east of the Lake Martin terrane and have been considered part of the Inner Piedmont (i.e., Tugaloo terrane). Recent field and isotopic work indicate, however, that the Opelika terrane likely corresponds to an arkosic/plutonic complex of the EBR. The southern margin of Grenville continental basement, the basement-cored Pine Mountain terrane (PMT), lies structurally beneath the Opelika terrane. U-Pb dating of detrital zircons from a basal cover unit within the PMT suggests a partly Gondwanan (2.2-2.4 Ga) source, implying a suspect history distinct from other Appalachian massifs. Wide mylonite zones (i.e., Goat Rock and Bartletts Ferry) separate the PMT from the overlying Uchee terrane. New U-Pb (SHRIMP-RG) zircon dates (ca. 620 and 640 Ma) indicate that this most internally exposed terrane has a Gondwanan or peri-Gondwanan origin (i.e., Carolina zone). Finally, the Gondwanan Suwannee terrane, buried beneath Coastal Plain sediments, is juxtaposed above the Uchee terrane along the proposed Suwannee suture. We explore the origin and Paleozoic amalgamation of these terranes, how they compare to other Appalachian terranes, and implications for pre-Appalachian (Rodinian) continental evolution.