THE SEA OF JAPAN AS A CENOZOIC ANALOGUE FOR THE ORDOVICIAN WEDOWEE-EMUCKFAW-DAHLONEGA BACK-ARC BASIN: SOUTHERN APPALACHIANS

Early-Middle Ordovician metamorphosed bimodal volcanic sequences (e.g. Hillabee Greenstone, Pumpkinvine Creek Formation) and intercalated metasedimentary units (e.g. Wedowee-Emuckfaw Groups) in the southern Appalachian Talladega belt, eastern Blue Ridge and Inner Piedmont terranes formed at the seaward margin of the Laurentian plate during B-type subduction of Iapetus lithosphere beneath the Alabama promontory and Tennessee embayment. Trace element geochemical analyses of mafic rocks within these bimodal suites typically display geochemical trends intermediate between volcanic arc and MORB, typical of back-arc volcanic sequences. This marginal basin (Wedowee-Emuckfaw-Dahlonega back-arc: WEDB) experienced active extension between 480 and 460 Ma and developed outboard of the Laurentian continental hinge zone on attenuated continental crust and/or transitional crust. Although deformed and segmented into different thrust sheets by Acadian and Alleghanian orogenesis, the original dimensions of the WEDB was at least 500 km X 100 km. Of more significant extent, the Sea of Japan (>1200 km X >600 km) back-arc basin between the Japanese arc system and Asian mainland experienced ~20 m.y. of extension during the Oligocene-Miocene and formed in attenuated continental crust of the Asian margin. Divided into three principal basins, the Sea of Japan consists of thick (2-4 km) sequences of hemipelagic-turbiditic sediment derived, in part, from the Asian mainland interlayered with extensive bimodal volcanic sequences erupted during back-arc rifting. Individual basins (e.g. Ulleung basin) were opened via asymmetric detachment faulting with both volcanic and non-volcanic segments and produced volcanic-sedimentary sequences extending to the margin of the Asian continental slope. The Sea of Japan may, therefore, provide not only an excellent Cenozoic analogue for development of the Ordovician WEDB basin, but also a model for the ideal crustal architecture for emplacing at least one of the WEDB units (Hillabee Greenstone) atop the Laurentian shelf with little or no accompanying deformation.