ANDEAN SUBDUCTION INITIATION ALONG THE ALABAMA PROMONTORY CONTINENTAL MARGIN

The >10 km thick, middle to upper amphibolite facies Ashland-Wedowee belt (AWB) of the Alabama eastern Blue Ridge (EBR) consists predominantly of pelitic rocks and minor orthoamphibolite. Stratigraphic and geochemical data support interpretations of the AWB as deep water sediments intercalated with tholeiitic ocean-floor basalts and associated sills which formed along the Alabama promontory, an amagmatic segment of the rifted margin of Laurentia. The Upper Cambrian Elkahatchee and Middle-Upper Ordovician Zana and Kowaliga suprasubduction plutons that intrude the AWB suggest the presence of a Late Cambrian-Ordovician Andean-type arc developed on the outboard edge of the Laurentian trailing margin. The presence of a continental arc forming above attenuated Laurentian continental crust along the SE edge (referenced to the modern Appalachian orogenic belt) of the rifted margin Alabama promontory dictates subduction polarity toward the Laurentian craton. Initiation of the Alabama promontory subduction zone most likely occurred during the Middle to Late Cambrian, and may be the direct result of juxtaposition of old and young oceanic lithosphere associated with the Cambrian passage of the Ouachita rift along the Laurentia-bounding Alabama-Oklahoma transform. The likely presence of serpentinized peridotite in the oceanic-continental transitional crust outboard of the attenuated margin provides a zone of weakness that may have been exploited during the earliest stages of subduction initiation and would have accommodated evolving slab pull and subduction polarity toward the SE side of the Alabama promontory. Geochemical analysis of the Middle-to Late Ordovician Hillabee Greenstone suggests the influence of continental lithosphere, and its tectonic emplacement above rocks of the Laurentian shelf is best explained by a backarc position to a Laurentian continental margin arc associated with the Alabama promontory subduction zone. The presence of an Andean-type margin outboard of this promontory explains the lack of a significant Taconic clastic wedge and Taconic deformation in the southernmost Appalachian Talladega belt, as well as some Ordovician K-bentonites preserved in the eastern and central North American platform-foreland basin successions.