BUILDING A UNIFIED TECTONIC MODEL FOR THE TACONIC OROGENY IN THE SOUTHERNMOST APPALACHIANS

In the southernmost Appalachians three Ordovician tectonic entities, the Blountian foreland basin (BB), the Wedowee-Emuckfaw-Dahlonega back-arc basin (WEDB) (Talladega belt/E. Blue Ridge/Inner Piedmont), and the Dadeville volcanic arc complex (DC) (Inner Piedmont), place important constraints on the nature and tectonic evolution of the Taconic orogeny in this region. Significantly dismembered and deformed during the Alleghanian, these terranes contain similar-aged strata and Ordovician volcanic and/or plutonic rocks. Several factors suggest that the BB is a retroarc basin: compositional and structural contrasts with foreland Taconic basins to the north; Laurentian affinity, Ordovician volcano-sedimentary back-arc basin successions (WEDB) which formed concurrently with, but palinspastically SE of the BB; large explosive felsic ash flow sheets within the more distal WEDB, which likely sourced BB K-bentonites; elimination of the frontal Alleghanian metamorphic allochthon as a source of crystalline rock fragments in the BB; and the presence of rift-related marginal/intraplate basement faults that may have been reactivated/inverted during the Middle Ordovician, providing marginal uplifts and the subsidence and deposition of the BB. The >4,000 km², >10 km thick WEDB, contemporaneous with the BB, contains bimodal volcanics and associated base metal deposits intercalated with thick sequences of deep-water flysch-like metasedimentary rocks. Lithogeochemistry of WEDB metavolcanic rocks (480–460 Ma) indicates formation in a supra-subduction (back-arc) setting. Detrital zircons in metasedimentary units are dominantly Mesoproterozoic and Paleozoic (474-460 Ma), suggesting both Laurentian basement and contemporaneous arc sources. The Ordovician DC is nested in a large (>4,300 km²), ~15 km thick synformal klippe along the western flank of the Inner Piedmont, structurally above and palinspastically outboard of the WEDB. It consists primarily of Ordovician mafic igneous rocks, with lesser volumes of ultramafic and felsic intrusions, all of arc affinity, and minor metasedimentary rocks. We argue that the origins of these terranes can be linked to a common tectonic scenario: an Ordovician accretionary orogenic complex resulting from B-type subduction along the seaward margin of Laurentia.