SUBDUCTION INITIATION AND EVOLUTION OF THE ALABAMA PROMONTORY VOLCANIC ARC

The Talladega and Ashland-Wedowee belts of the southernmost Appalachians record the presence of a Cambrian-Ordovician continental arc (Elkahatchee-Zana-Kowaliga plutons) and backarc (Hillabee Greenstone) built on Grenville basement and rift-drift deposits of the Neoproterozoic-Paleozoic Laurentian margin. The dramatic shift from rift-drift passive margin sedimentation to subduction tectonics and arc volcanism is probably a direct result of the juxtaposition of young, thermally buoyant oceanic crust in the evolving Ouachita embayment (along the Ouachita rift) and older Iapetan oceanic-transitional crust southeast of the Alabama promontory. Collapse of dense Iapetan lithosphere at the locus of Ouachita ridge termination and the seaward extension of the Alabama-Oklahoma transform could have resulted in large-scale lithospheric sinking of Iapetan seafloor beneath the attenuated Laurentian margin and the establishment of an Andean-type arc along the southernmost early Paleozoic Laurentian margin. Cratonic response to this subduction initiation event may be present in Middle-lowermost upper Cambrian stratigraphy of the Conasauga Formation which records destabilization of the Laurentian cratonic interior and may be associated with developing dynamic topography on the overriding plate. Establishment of the Alabama promontory arc is temporally bracketed by the oldest suprasubduction pluton (Elkahatchee) and must have been established by latest Cambrian (~490Ma), remaining continuously or intermittently active until at least Late Ordovician (~450Ma). Some of the widespread Ordovician K-bentonites in the eastern and central North American platform-foreland basin successions may have sourced this continental margin arc. Following establishment of the subduction zone and arc, rollback in the subducting Iapetan plate resulted in Middle Ordovician (460-470Ma) backarc extension and eruption of bimodal volcanics (Hillabee Greenstone). Accretionary orogenesis associated with contractional and extensional phases of the Hillabee backarc and potential equivalents to the northeast (e.g. Pumpkinvine Creek) may be responsible for Blue Ridge Taconian metamorphism, deformation, and Taconic foreland clastic wedge deposition.