Constraints on residual topography and crustal properties in the western United States from virtual deep seismic sounding

We use virtual deep seismic sounding (VDSS) and data from similar to 1000 broadband seismic stations to provide high-resolution estimates of crustal structure in the western Cordillera of the United States (U.S.). The most robust result is the geographic distribution of residual topography (that is, the difference between observed elevation and that expected from crustal buoyancy alone) and, by implication, thermal or petrologic anomalies in the mantle. Overall, residual topography of the western U.S. Cordillera varies considerably; with contrasts of up to about 3km across distances of 200km or less. High residual topography, indicating large mantle effects, is evident along the periphery of the Colorado Plateau and the surroundings of the Great Basin. In contrast, the central Colorado Plateau and the Wyoming Basin show low residual topography, close to what is expected of a geologically stable lithosphere. Overall, in regions to the east of the Wasatch hinge line (the eastern limit of significant extension in the North American cratonic basement) patterns of high residual topography and anomalies of low seismic wave speeds in the upper mantle are similar, suggestive of a common, thermal origin. In contrast, such a similarity is absent in regions to the west of the hinge line, suggesting substantial effects of petrological heterogeneities in the mantle. Finally, joint analyses of VDSS and conventional receiver functions reveal a wide range of crustal P wave speeds, locally as high as 6.7km/s, perhaps indicating magmatic modification of the crust.