14-8 Intra-Backarc Subduction Initiation as a Driving Mechanism for the Southern Appalachian Taconic Orogeny

Session: New Research in the Appalachian-Ouachita Orogen: Integrated studies from the Foreland to the Hinterland

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Abstract:

Decades of southern Appalachian research have produced a dichotomy in our understanding of the timing and driving mechanisms for the Taconic orogeny. Using structural and stratigraphic constraints, as well as geochronology, numerous geologists have historically argued for peak metamorphism in the southern Appalachian Blue Ridge and western Inner Piedmont (WIP) during the Ordovician, which was then partially overprinted by the effects of younger (e.g., Neoacadian, Alleghanian) orogenic pulses. However, the presence of Silurian and younger rocks overprinted by peak metamorphic isograds, as documented by paleontological and igneous isotopic ages, in these same belts contrasts with these earlier interpretations. In the former, Ordovician subduction of the Laurentian continental margin resulted in granulite to eclogite facies metamorphism in the central (CBR) and eastern Blue Ridge (EBR) terranes, as well as the growth of prograde metamorphic mineral assemblages in the western Blue Ridge (WBR) between 460 and 440 Ma as a result of collisional orogenesis. In the latter interpretation, researchers note the presence of Ordovician to Silurian, metamorphosed mafic and bimodal backarc igneous-sedimentary rocks in the Blue Ridge and WIP terranes that span and postdate reported metamorphic ages. In this accretionary orogenic model, the southern Appalachian outer margin was flanked by a backarc basin spanning the Alabama promontory and Tennessee embayment throughout the Ordovician and Silurian, which was not closed until the Devonian. The most robust models for the southern Appalachian Taconic orogeny, therefore, must accommodate all of the geologic constraints placed on Blue Ridge and WIP terranes during the Paleozoic, including: subduction of Laurentian margin rocks at ca. 460 Ma as recorded by rocks in the CBR and along the EBR-WBR boundary; the presence of an Ordovician-Silurian paired back-arc and arc immediately outboard of the Laurentian margin as recorded by rocks in the Talladega belt (TB), EBR and WIP; Ordovician-Silurian growth of monazite in the WBR, CBR, and EBR; the presence of post-Ordovician successor basin rocks in the TB/WBR that carry the peak metamorphic isograds; and Silurian premetamorphic “OIB-like” alkaline mafic volcanism along the distal Laurentian shelf in the WBR. We argue that all of these constraints can be accommodated by modeling the southern Appalachian Taconic orogeny as the result of an aborted subduction initiation event within the Ordovician-Silurian Wedowee-Emuckfaw-Dahlonega backarc basin at ca. 460 Ma.

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