DETRITAL ZIRCON LINKAGES BETWEEN SE SIBERIA AND SW NORTH AMERICA

A reconstruction of Mesoproterozoic supercontinent Nuna joins SW North America basement terranes to matching ones in SE Siberia and places the Death Valley lower Pahrump Group against strikingly similar and correlative rocks of the Sette-Daban basin of SE Siberia. Mesoproterozoic Uy Group sandstones of the Sette-Daban basin contain large percentages of detrital-zircon grains that correlate with Grenville, Granite-Rhyolite, and Mojave-Yavapai-Mazatzal terranes. Furthermore, gaps in Uy Group detrital-zircon ages coincide with age gaps in SW North American source rocks. Paleocurrents indicate that the Uy Group sandstones were derived from outboard of the Siberian craton to the SE, consistent with sediment sources in SW North America. By contrast, post-breakup Neoproterozoic and younger sandstones in the Sette Daban and Death Valley regions contain mutually exclusive detrital-zircon age suites that only correlate with basement sources on their respective cratons. This indicates that the cratons were too isolated for sedimentary processes to transfer zircon-bearing sand between them. SE Siberia and SW North American both exhibit grand cycles that evidently record correlative Neoproterozoic and Cambrian eustatic sea level changes phased with passive continental-shelf subsidence. We conclude that by Early Cambrian time, separation of the SE Siberian and SW North American cratons along a rift-transform system was sufficient to isolate segments of their rift margins from intercontinental sediment mixing. Clastic sediment from North America was trapped in kms-thick deposits on its new western continental shelf, but the Siberian shelf was largely covered with carbonate and received only limited sand from its interior. The orientations of faults would allow Siberia to rotate clockwise around Canada on margin-parallel transform faults toward late Paleozoic collision with the Ural Mountains. The ocean basin that opened between the diverging continents may have cradled explosive Early Cambrian metazoan evolution. Trilobites might have originated in Siberia and migrated to nearby North America within 2 m.y. Widening of the ocean basin may have increased evolutionary branching rates as animal populations became isolated on the diverging continents and Early Cambrian transgressions across the continental shelves created more faunal habitat.