EVOLUTION OF THE LAURENTIAN RIFTED MARGIN IN THE SOUTHERNMOST APPALACHIANS

Southeast Laurentia underwent a protracted Neoproterozoic rift history recorded in rocks of the southern Appalachian western Blue Ridge (WBR) in Georgia. Here, several external Grenville basement massifs along the NW boundary of the WBR core map-scale, inclined-to-recumbent, west-vergent isoclinal anticlinoria formed during peak greenschist facies Paleozoic metamorphism. The massifs are nonconformably overlain by units of the Late Proterozoic Ocoee Supergroup. During initial continental rifting, the basal cover unit (Pinelog Fm.=Snowbird Gp.; >1,100 m thick) formed in fluvial-alluvial to shallow-water deltaic/intertidal environments adjacent to sharp basement uplifts, most probably along extensional fault scarps. A second, more extensive phase of rifting and subsidence formed the Great Smoky Group (GSG), a mostly deep water turbiditic sequence >7 km thick. During the 2nd rift cycle a large (>2,000 km²) continental block comprising most of the Georgia WBR and containing the deposits of the earlier rift basin, was tilted westward with the underlying basement along a SE-dipping normal fault system (stratigraphic offset >1 km) flanking the western margin of the WBR. GSG units unconformably above this block progressively cut deeper into the underlying cover sequence and then into the basement towards the SE. Both rift sequences are free of volcanic rocks. The Ocoee basin was bounded on the SW by a large NW-striking continental transfer (transform) fault (Cartersville transverse zone-CTZ), across which the polarity of faulting along the rifted margin was reversed. This transform was later converted to a large-scale lateral/oblique ramp during Alleganian thrusting events, producing the CTZ, a thin-skinned transverse zone extending from the Appalachian Plateau to the WBR, and separating the Alabama recess from the Tennessee salient. Southwest of the CTZ no Grenville basement or Ocoee units occur in the WBR (Talladega belt), but the basal unit, equivalent to the initial drift facies Chilhowee Gp. extents NE across the CTZ into the Murphy belt of the WBR, where it conformably overlies the Ocoee Supergroup composite rift sequence.