An Ordovician Accretionary Orogen in the Southern Appalachians – Part 2: Tectonic Model

Arc-continent collisions in the western Pacific (Taiwan-Papua New Guinea-Timor) are often used as modern analogues for ancient collisional orogens. In each of these modern collisions, subduction of the leading edge of the continental margin results in cessation of arc volcanism, shortening of the forearc, fault emplacement of accretionary prism/mélange rocks between obducted arc terranes and rocks of the deformed continental margin, and exhumation of low temperature-high pressure rocks. In fact, this geometry of terranes (continental margin–accretionary prism–arc) should be typical of simple arc-continent collisions, as subduction of continental margin sediments and basement promotes the growth of extensive accretionary prisms and limits subduction erosion of the overriding arc plate. In the northern Appalachians, a model of arc obduction is frequently used to explain Taconic orogenesis, where deformed rocks of the Laurentian margin are separated from obducted arc complexes by extensive bodies of rock interpreted as accretionary prisms. In contrast, the southernmost Appalachians of Alabama and Georgia includes Ordovician bimodal volcanic sequences of backarc affinity (Hillabee Greenstone and Pumpkinvine Creek Formation) structurally emplaced atop strata of the trailing, passive Laurentian margin, with no intervening accretionary prism or low temperature-high pressure rocks. Related rocks of the adjacent Ashland-Wedowee belt include suprasubduction plutons of Cambrian-Ordovician age intruding Neoproterozoic to Ordovician(?) Laurentian derived sediments, and indicates the presence of a continental margin arc in southernmost Laurentia. The hallmark features of arc-continent collision in this segment of the southern Appalachians are absent, and westward directed subduction of oceanic lithosphere beneath the Laurentian margin better explains the origins and tectonic assembly of terranes in this region. A model of Taconian-aged accretionary orogensis for the southernmost Appalachians also provides a plausible mechanism for the origin and early Paleozoic tectonic evolution of southern and central(?) Appalachian terranes traditionally explained by Taconian arc obduction and collisional orogenesis.