NEOPROTEROZOIC-EARLY CAMBRIAN PULL-APART BASINS ALONG CORDILLERAN TRANSFORM MARGIN, NORTH AMERICA, AND IMPLICATIONS FOR SIBERIAN CONJUGATE CONNECTION

Neoproterozoic-Early Cambrian facies and isopachs of the North American Cordilleran cratonic margin are consistent with deposition in a series of four pull-apart basins along a right-lateral, coast-parallel transform system against a conjugate Siberian Verkhoyansk cratonic margin. Five transform fault segments follow concentric arcs that parallel small circles about a potential Euler pole of rotation located in southern Greenland (present co-ordinates). The transform faults right-step across syndepositional extensional faults. The transform system was active during Windermere Supergroup deposition and was re-activated during latest Neoproterozoic to Early Cambrian time. The Cordilleran sides of the pull-apart basins captured most of the clastic sediments, while carbonate dominated on the conjugate Siberian sides. The earliest trilobites evolved along the conjugate margins of the pull-apart basins, and Siberian and Cordilleran localities share endemic Early Cambrian genera. Archeocyathan reefs first appeared on the Siberian craton and crossed the transform boundary to the Cordilleran margin near Death Valley. They establish an Early Cambrian bathymetric piercing point. The transition to seafloor spreading in the pull-apart basins may be recorded by divergence of Siberian and Cordilleran trilobite genera, as well as by the rapid subsidence of archeocyathan reefs and their eventual burial beneath thick, black, pyritic shale in late Early Cambrian time. The Cordilleran margin then evolved as a passive shelf, while the Siberian craton continued its clockwise rotation on the transform system until its terminal Permian collision with the Ural Mountains.