MODELING THE DEFORMATION EVOLUTION OF THE WESTERN U.S. BASIN AND RANGE PROVINCE WITHIN THE PACIFIC-NORTH AMERICA PLATE BOUNDARY ZONE SINCE 36 MILLION YEARS AGO

The complex deformation history of the western U.S. since early Oligocene has dramatically altered the topography, and resulted in the exhumation of the metamorphic core complexes (MCCs) within the interior Basin and Range region as the tectonic setting shifted from compressional to extensional. We use position estimates from McQuarrie and Wernicke (2005) to determine a time-dependent lithospheric strain rates model with formal uncertainties. We integrate the strain rates to provide quatitative models of crustal thickness, surface elevation, and middle crustal exhumation since 36 Ma. The crustal thickness models enable us to predict the exhumation history of MMCs. Our crustal thickness model at 36 Ma shows a significant crustal welt along the eastern Nevada and northwestern Arizona border, similar to that proposed by Coney and Harms (1984). The timing and magnitude of exhumation in our models is measured by tracking the elevation change of middle crustal levels associated with the isostatic response to crustal extension through time. The timing of exhumation patterns for MCCs in the northern Cordilleran taphrogen in our model is mostly consistent with thermochronological ages, indicating an extensional history with two significant pulses of extension (36-28 Ma, and 18 Ma -present-day). We suggest that for the northern Cordilleran taphrogen roughly 50% of the total exhumation for MCCs occurred prior to the Miocene, and another 50% from Miocene to present-day, representing the opening of the Basin and Range. Our model for the southern Cordilleran taphrogen shows a rapid extensional pulse at ~15 Ma, consistent with thermochronological ages. In the southern Cordilleran taphrogen in western Arizona at least 80% of the total exhumation for MCCs occurred between ~18-14 Ma. The Eastern California shear zone shows four pulses of exhumation at 24, 19, 14 and 10 Ma, which is broadly consistent with total exhumation inferred from thermochronological studies. Our final integrated topography model shows a Nevadaplano of ~4.36 ± 0.4 km average elevation at 36 Ma. Our model, between 36 – 5 Ma, shows a Sierra Nevada adjacent to paleo-sea level, with a shoreline on the eastern edge of present-day Great Valley. Moreover, the northern Sierra Nevada (north of 36°N) shows little to no elevation change since at least 36 Ma.