RECONCILING COLLISIONAL AND ACCRETIONARY OROGENIC MODELS FOR THE TACONIC OROGENY IN THE SOUTHERNMOST APPALACHIANS

Models for Taconic (i.e. Ordovician) orogenesis in the southern Appalachians incorporate both collisional and accretionary tectonism. Research over the past few decades has revealed lithotectonic terranes that contain unequivocal, pre-metamorphic, Ordovician and younger metasedimentary and igneous rocks that were not impacted by Taconic collisional tectonism. Lithotectonic terranes in Alabama (AL) and Georgia (GA) – the western Blue Ridge/Talladega belt and eastern Blue Ridge Ashland-Wedowee-Emuckfaw belt – include post-Ordovician, sedimentary successor basins (e.g. Silurian-Devonian Talladega and Mineral Bluff Groups, Jacks River Formation), as well as volcanic and plutonic rocks (e.g. Silurian Marble Hill Hornblende Schist, Zana and Kowaliga plutons, Mulberry Rock Gneiss) that carry the peak regional metamorphic isograds. These rocks are unequivocally linked to an extensive Ordovician-Silurian backarc basin on the seaward margin of the Laurentian plate (i.e. Wedowee-Emuckfaw-Dahlonega basin), with igneous crystallization and Ar-Ar cooling ages constraining peak metamorphism to the Upper Mississippian-Lower Pennsylvanian (330-320 Ma). The presence of these pre-metamorphic Ordovician-Silurian backarc rocks and younger successor basins are best explained using a Lachlan-style (i.e. accretionary) orogenic model for the southernmost Appalachians of AL-GA, with Taconic tectonism largely confined to the overriding (i.e. Laurentian) plate above subducting Iapetus oceanic crust. In contrast, other workers report U-Pb monazite ages and mineral thermobarometry from the western, central, and eastern Blue Ridge terranes of northernmost GA and North Carolina (NC) that indicate peak metamorphic mineral growth during the Ordovician. In this segment of the orogen, workers have traditionally modeled peak Barrovian metamorphism as a consequence of Taiwan-style, collisional orogenesis during subduction of the Laurentian margin beneath an accretionary wedge/subduction complex. Synthesizing the geologic record from the southern Appalachians of AL-GA-NC, we suggest these two competing hypotheses can be reconciled using one of two tectonic models – one based on intra-backarc subduction initiation and one based on a New Zealand-style, strike-parallel subduction polarity reversal.