A MOBILIST ARGUMENT FOR THE APPALACHIAN ORIGIN OF INBOARD CORDILLERAN TERRANES

The Cordilleran collage includes a belt of suspect terranes composed for the most part of mid to late Paleozoic assemblages, which lies inboard of exotic Mesozoic arc and accretionary wedge terranes and outboard of the Neoproterozoic, Paleozoic and Mesozoic autochthonous continental margin sedimentary wedge. In this belt, the Yukon Tanana, Dorsey, Yreka, Eastern Klamath, and Northern Sierra terranes, the Rapid River tectonite of the Slide Mountain terrane, and the Acatlan complex of the Mixteca terrane share distinctive Devono-Mississippian granitoids and/or rhyolites, in some cases deformed and metamorphosed, and mid to high-grade metamorphic and mylonitic host rocks of the same or greater age. The tectonic setting implied by these characteristics is incompatible with the mid Paleozoic geologic history of western NAm but has striking correlations with the southern Appalachian orogen. The Kootenay, Barkerville, Yreka, and Northern Sierra terranes and the Harper Ranch and Okanagan basement subterranes of Quesnellia contain late Precambrian to mid Paleozoic sedimentary sequences that are not homologous with those of the autochthonous Cordilleran continental margin but are correlative with Appalachian margin stratigraphy. All share Cambrian platformal sandstones and carbonates, but only in the Cordilleran suspect terranes and the Appalachians are these followed by deep-water lithologies indicating profound subsidence. Early Paleozoic sandstones of the Cordilleran suspect terranes lie outboard of the coeval shale-out of the Cordilleran continental margin. The geologic evidence suggests that this belt of terranes originated in the southern Appalachian orogen, where its distinctive characteristics were forged by Taconic subsidence and Acadian orogenesis. Post-Acadian transtension along the Appalachian margin displaced the terranes south and resumed subduction in the Rheic Ocean moved them across southern NAm, transferring them to the Pacific realm at approximately the latitude of “Baja BC”. During the Permian and Triassic these terranes were swept northward along the Cordilleran margin, closing the Golconda-Slide Mountain-Angayucham Ocean sequentially from south to north by their advance. In this process, a variety of arc related assemblages was built across the terrane belt.