Superposed Fault Systems of the Southernmost Appalachian Talladega Belt

Two major thrust systems are located along the eastern Blue Ridge – western Blue Ridge boundary in the Alabama and Georgia Appalachians. The pre-metamorphic Hillabee thrust is near the trailing edge of the Talladega belt thrust sheet and is associated with emplacement of the Hillabee Greenstone backarc volcanic suite atop rocks of the Paleozoic Laurentian shelf Talladega Group. The post-metamorphic Hollins Line fault system emplaces metasedimentary rocks of the Neoproterozoic Laurentian rifted margin, the Ashland-Wedowee belt, atop the amalgamated shelf-volcanic terrane. These three fault bounded lithotectonic belts are important for unraveling the geologic history of this part of the Appalachian orogen and have widespread implications for the tectonic history of the southern Appalachians in general. Following Neoproterozoic rifting of the Rodinian supercontinent and deposition of a thick metasedimentary package of rocks along an amagmatic rifted margin, now comprising the Ashland-Wedowee belt in the eastern Blue Ridge of Alabama and Georgia, shelf strata of the Kahatchee Mountain and Sylacauga Marble Groups (Talladega belt) indicate establishment of a passive margin along the Cambrian trailing edge of this portion of Laurentia. Intrusion of continental slope-rise deposits within the Ashland-Wedowee belt by latest Cambrian(?) to Middle Ordovician arc related plutons was immediately preceded by subduction initiation outboard of the trailing margin continental hinge zone as a result of passage of the Ouachita mid-ocean ridge along the southern margin of the Alabama promontory and foundering of Iapetan oceanic lithosphere in the weak transitional crust at the continent-ocean boundary. Rollback of the newly-formed, westward-dipping- subduction zone resulted in extension within the overriding Laurentian plate and backarc spreading, leading to formation of the bimodal metavolcanic Hillabee Greenstone (Alabama-Georgia), Pumpkinvine Creek formation (Georgia), and other backarc affinity volcanic suites along structural strike of the Appalachians to the northeast. Continued westward subduction of Iapetan seafloor beneath the attenuated Laurentian margin resulted in extensive suprasubduction plutonism intruding the marginal rift basin and distal trailing margin sediments of the eastern Blue Ridge of Alabama and Georgia and farther to the northeast. Uplift of Cambrian and Ordovician shelf strata in the evolving backarc resulted in deposition of the southern Appalachian Blount-Taconic clastic wedge, which sourced intrabasinal uplifts along the westward facing (craton-facing) side of the Hillabee backarc, and perhaps localized thrust belts at the continental edge of the backarc associated with lithospheric compression during contractional phases of backarc evolution. Following Ordovician accretionary orogenesis (Taconic orogeny) in Alabama, Georgia, and farther northeast, deposition of the Talladega Group in the Silurian(?) to earliest Mississippian(?) indicates stabilization of the most outboard portion of the Laurentian shelf margin. Latest Devonian-Mississippian collision along the Alabama promontory, however, resulted in emplacement of the Hillabee Greenstone terrane atop the adjacent Devonian-earliest Mississippian(?) shelf strata of the Talladega Group along a thin-skinned thrust fault, the Hillabee thrust, possibly as a result of collision with the Ouachita arc. Following movement along the pre-metamorphic Hillabee thrust, progressive dynamothermal metamorphism affected both the amalgamated Talladega belt (Hillabee and Talladega-Sylacauga Marble-Kahatchee Mountain Groups) and Ashland-Wedowee belt. Alleghanian terminal orogenesis (continental collision), culminating in the late Paleozoic Pangean supercontinent, resulted in telescoping of the Laurentian margin and emplacement of rifted margin metasediments and associated arc plutons atop the Talladega belt along the post-metamorphic Hollins Line footwall duplex thrust system.