NOT YOUR FATHER’S TACONIC OROGENY: SIGNIFICANCE OF AN IAPETUS-FACING, LAURENTIAN PLATE ORDOVICIAN ARC-BACKARC SYSTEM IN THE SOUTHERN APPALACHIANS

The southernmost Appalachian eastern Blue Ridge (EBR) and western Inner Piedmont (WIP) terranes of Alabama (AL), Georgia (GA) and North Carolina (NC) consist of metasedimentary units with mixed Grenville and Ordovician sources, intercalated with bimodal metavolcanic units that formed in a suprasubduction system. This paired Dadeville complex arc (AL-GA) and Wedowee-Emuckfaw-Dahlonega backarc (WEDB) basin (AL-GA-NC) was subsequently intruded by Ordovician to Carboniferous silicic plutons prior to ca. 330 Ma metamorphism. Contrary to traditional models for the Ordovician Taconic orogeny, which call for collisional orogenesis driven by obduction of exotic/peri-Laurentian arc terranes onto the subducting Laurentian margin; the age, geochemical character and geometry of assembled lithotectonic terranes indicate the southern Appalachian Taconic orogeny occurred on the overriding plate (Laurentia) in an evolving accretionary orogen, similar to the Paleozoic Lachlan orogen in southeastern Australia or modern paired Sea of Japan backarc and Japan arc in the western Pacific. Tectonic switching between extensional and contractional phases of this accretionary orogen explains both the development of expansive Ordovician basins and formation of convergent margin features (e.g. eclogite) in the southern Appalachians. Modern geochronology, trace element tectonic discrimination, and map relationships indicate transition from a passive to active Laurentian margin during the latest Cambrian-earliest Ordovician (>480 Ma), with the oldest preserved backarc lithologies in stratigraphic (AL Wedowee Group) or faulted contact (GA Cane Creek Gneiss) atop rocks of the distal Laurentian slope-rise (e.g. AL Ashland Supergroup, GA Great Smoky Group). Ordovician backarc extension, sedimentation and igneous activity in the Laurentian plate WEDB developed concurrently with arc volcanism-plutonism and sedimentation (e.g. Ropes Creek amphibolite, Camp Hill Gneiss, Agricola Schist) preserved in the WIP Dadeville complex (AL-GA). The current structural position of the Dadeville complex suggests both the EBR and WIP have Laurentian affinities, originating between the EBR (WEDB) back-arc basin and Iapetus-facing WIP Dadeville complex arc above westward-subducting Iapetus lithosphere.