PALEOTECTONIC RECONSTRUCTIONS, BACK-ARC EVOLUTION, AND THE SOUTHERN APPALACHIAN TACONIC OROGENY

Traditional models of orogenesis along the early Paleozoic Iapetan margin of Laurentia call for A-type subduction of continental crust and arc obduction during the Ordovician Taconic orogeny. Although the geologic history of the southern Appalachians clearly indicates Ordovician tectonism, a comparison of assembled terranes and timing of key tectonic events in Cenozoic arc-continent collisions (collisional orogens) of the western Pacific and the southern Appalachians reveals a number of important differences. One of the more significant differences between the southern Appalachian Taconic orogeny and collisional orogens in Taiwan, Timor, and western New Guinea is the structural position of extensive Early to Middle Ordovician back-arc rocks (Hillabee Greenstone-Dahlonega Gold belt) above rocks of the Laurentian margin. A model of accretionary orogenesis (B-type subduction) better explains the presence of these back-arc rocks, as well as many timing constraints on Ordovician tectonism in the southern Appalachians. Modern studies of back-arc dynamics offer important insights into the causes and evolution of Ordovician accretionary orogenesis in the southern Appalachians. Additionally, Ordovician paleotectonic reconstructions for the Laurentian-Iapetan realm provide corroborating evidence for models of accretionary orogenesis and back-arc development during Ordovician orogeny. Following subduction initiation, an Early Ordovician retreating Laurentian plate, coupled with subduction of mature Iapetan oceanic crust, resulted in back-arc extension outboard of the Laurentian hinge zone. A probable shift in plate motion and subduction of progressively younger lithosphere created the conditions necessary for back-arc compression by late Middle Ordovician. Collectively, these two events effectively account for most characteristics of the Taconic orogeny in the southern Appalachians.