NORTH AMERICAN AFFINITY OF SOUTHWEST CORDILLERAN MESOZOIC TERRANES IMPLIED BY DETRITAL ZIRCON U/PB AGES

The stratigraphy and provenance of siliciclastic assemblages is probably the most important key to establishing the genetic significance of lithic packages within terranes and the genetic linkage between terranes. Recent development of laser ablation multicollector ICPMS methods for U-Th-Pb geochronologic analyses of detrital zircon is having a significant impact on provenance studies and terrane analysis. Large, statistically robust detrital zircon data sets can now be assembled with unprecedented throughput. Detrital zircon ages (n = 2438) from Colorado River sands (modern to ~5.5 Ma) establish a reference age-probability distribution characteristic of the southwestern US Cordillera. The distinctive Colorado River distribution matches closely the detrital zircon age distributions of Late Jurassic strata of the Great Valley Group and Vizcaino Peninsula of Baja California. This observation conflicts with models that accrete the Vizcaino Peninsula as part of an exotic Guerrero terrane in the late Early Cretaceous. Further, extensive prebatholithic Triassic-Jurassic turbidite shelf/slope sequences within the Peninsular Ranges of southern and Baja California contain zircon detrital age distributions that match closely those of broadly coeval fluvial deposits of the Colorado Plateau region. This match suggests that the Peninsular Ranges wall-rock assemblages represent at least part(s) of the ultimate deep marine sediment burial site of major west-flowing transcontinental river systems carrying Appalachian-derived detritus. The new detrital zircon data thus support an autochthonous origin for Mesozoic terranes along this sector of the Cordilleran margin.