EARLY MESOZOIC PALEOGEOGRAPHY AND TECTONIC EVOLUTION OF THE WESTERN UNITED STATES: INSIGHTS FROM DETRITAL ZIRCON U-PB GEOCHRONOLOGY, BLUE MOUNTAINS PROVINCE, NE OREGON

Detrital-zircon ages from Triassic and Jurassic sandstones in the Blue Mountains Province of NE Oregon provide important information about provenance linkages and tectonic models for the western U.S. Two end members of detrital zircon age distributions are represented in samples from the Blue Mountains. Triassic samples were derived from the Baker terrane offshore accretionary-subduction complex and are dominated by Paleozoic, Late Paleoproterozoic (~ 1.7 to 2.1 Ga), and Late Archean (~ 2.5 to 2.7 Ga) detrital-zircon grains that suggest a pre-Triassic genetic affinity with other Cordilleran accretionary-subduction complexes of the western U.S. Jurassic samples are dominated by Mesozoic detrital ages that span ~ 228 to 159 Ma and include significant quantities of Neoproterozoic (~ 576-675 Ma) and Mesoproterozoic (~ 1030-1457 Ma) detrital-zircon grains with lesser amounts of Late Paleoproterozoic grains (~ 1740-1840 Ma). We interpret these ages to reflect southwestern U.S. enrichment of an Ouachita-Appalachian provenance. Jurassic sediment was derived from (1) orogenic highlands in Nevada that yielded recycled sand from uplifted Triassic backarc-basin deposits, and/or (2) a NW-flowing, arc-parallel axial river that flowed along the NW-deepening Cordilleran arc. Our data suggest that during Jurassic time, far-traveled sediment was delivered to Cordilleran marginal basins that were located proximal to the craton margin. Similar detrital-zircon age distributions in Jurassic turbidites of numerous Cordilleran terranes suggests that (1) transcontinental and recycled orogenic sediment dispersal pathways were ubiquitous across the Mesozoic landscape, (2) continental sediment was commonly delivered to the Cordilleran arc and forearc region, and (3) many Jurassic basins of the Cordillera were pericratonic and are not accreted fragments of “exotic” oceanic crust. The two end-member zircon populations recognized in this study appear ubiquitous in the Cordilleran system. Paleoproterozoic-Archean-enriched signatures appear to be derived from mobile, tectonically translated (ex situ) crustal fragments of subduction-accretionary origin, whereas southwestern U.S.-enriched, Ouachita-Appalachian-derived signatures represent an in situ cratonal provenance.