UNDERSTANDING ORDOVICIAN TECTONICS OF THE APPALACHIANS FROM PACIFIC RIM OROGENIC MODELS AND ANALOGUES

Studies from orogenic belts around the Pacific Rim have been vital in the development of collisional and accretionary orogenic models. For orogenic belts whose geologic histories have been complicated by multiple orogenies and/or continental collision, studies from the Pacific Rim have provided valuable insight into how these complex orogens may have evolved through different stages of their tectonic histories. The Appalachians of eastern North America experienced a series of Paleozoic collisional and accretionary orogenic events followed by terminal continental collision, which has complicated interpretations of tectonic development. However, using orogenic models and analogues from the Pacific Rim, unraveling the tectonic history of complex orogens such as the Appalachians can be more easily accomplished. In the northern Appalachians, the Ordovician Taconic orogeny shares many characteristics with Cenozoic Pacific Rim arc-continent collisions, such as those of Taiwan and Timor. Timing of volcanism, metamorphism and foreland basin deposition in the northern Appalachians, as well as the assemblage of lithotectonic belts, fits well with Pacific Rim models of collisional orogeny. Historically, this tectonic model of Ordovician arc-continent collision developed in the northern Appalachians has been extended along the entire length of the orogenic belt. However, along the Alabama promontory in the southernmost Appalachians, the timing of key tectonic events and arrangement of lithotectonic belts does not resemble those observed in Pacific Rim collisional orogens, but instead shares many characteristics with accretionary orogens such as those of the Paleozoic Lachlan of southeastern Australia. Differences in the timing of key tectonic events between the Taconic orogeny in the northern and southern Appalachians suggest a fundamental tectonic break between the two segments of the orogen. In light of the along-strike tectonic discontinuities present around the Pacific Rim, however, it should not be surprising that attempts to extend tectonic models developed in one segment of an orogen to the entire orogen may be unrealistic.