THE TACONIC OROGENY: COLLISIONAL VS. ACCRETIONARY OROGENESIS IN THE SOUTHERN APPALACHIANS

The type region of the Taconic orogeny in the northern Appalachians records the obduction of one or more volcanic arcs onto the eastward-dipping Ordovician Laurentian (Iapetan) margin, an event incorporated into many tectonic models for the central and southern Appalachians. However, the western and eastern Blue Ridge of Alabama records late Cambrian initiation of a westward dipping subduction zone and Ordovician development of an arc-backarc system along the Laurentian margin, but no evidence for arc obduction associated with Taconic orogenesis. Tectonic models which synthesize the geology of both the Talladega and Ashland-Wedowee belts suggest an Andean-type margin along the Alabama promontory and extensional, not collisional, orogenesis for this portion of the margin during the Ordovician. The limit of this Middle Ordovician extensional regime is currently unknown, but determining its northeastern extent is important in paleotectonic reconstructions of the Laurentian margin for the early Paleozoic. Distinguishing between collisional and extensional orogenesis is paramount to unraveling the along-strike extent of the Taconic orogeny in the northern Appalachians and backarc extension in the southernmost Appalachians. Additionally, distinguishing between these two types of orogenic systems plays a critical role in recognizing one or more transform faults accommodating changes in subduction polarity along the Paleozoic margin. Extensional accretionary orogens should be distinguishable from collisional orogens in their magmatic and metamorphic character, as well as the nature of assembled terranes following cessation of orogenesis.