CRUSTAL STRUCTURE AND ACCRETIONARY HISTORY OF THE SOUTHERN AND CENTRAL APPALACHIANS

The Appalachians are narrow in the central (C) Appalachians, and widen into the southern (S) and New England Appalachians related to processes that built the orogen. Appalachian crust is the product of several Paleozoic collisional/accretionary orogenies spaced between the breakup of Rodinia and amalgamation of Pangea. The Taconic orogeny is the product of diachronous Ordovician arc collision with Laurentia, closing Iapetus ocean, and the Alleghanian orogeny is the product of zippered N-to-S closing of the Theic(?) ocean and Late Carboniferous-Permian collision of Gondwana with composite Laurentia and Paleozoic crust to form Pangea.

The cause of the (Neo-) Acadian orogeny, however, is still debated despite growing evidence supporting this event as the product of N-to-S zippered collision of Avalon (S New England) and Carolina superterrane (in the S-C Appalachians) from Devonian into early Mississippian (412-345 Ma) time, closing the Rheic ocean. Detrital zircon ages helped identify the Cat Square terrane in the S Appalachian Inner Piedmont (IP), with provenance from both Carolina and Laurentia, including a Silurian detrital zircon suite with a likely C Appalachian source, and a signature that closely matches the S New England Putnam-Nashoba terrane. 412-355 Ma U-Pb zircon ages from granitic and mafic plutons in Carolina and Cat Square terranes provide additional evidence of subduction of a small ocean and the E Laurentian margin. SW-directed thrust sheets and migmatization (at sill II) in most of the IP at burial depths of >20 km in the IP, provide additional evidence for SW transport of the Cat Square terrane. Dominance of Late Devonian metamorphic ages in the IP and absence of Taconian metamorphism east of the Eastern Blue Ridge confirm mid-Paleozoic docking of Carolina superterrane. Narrowing of the C Appalachians is best explained by SW terrane transport during the Acadian and Alleghanian collisions.