LITHOSPHERIC STRUCTURE OF WESTERN NORTH AMERICAN

The continental-scale EarthScope program allows for the study of the lithosphere and upper mantle of the United States at details that have not been previously attainable. Broadband seismic observations from the Transportable Array component of USArray have illuminated features in the crust and upper mantle that provide insight into the evolution of North America. Patterns of crustal thickness across expansive regions, as presented here, allow for the crustal response to specific tectonic processes to be determined while still imaging small-scale structures that can be more difficult to identify by less detailed studies. The thickness of the crust varies from less than 30 km in the southern Basin and Range and along the west coast to more than 50 km thick beneath the Rocky Mountains in Colorado and Wyoming. Distinct crustal structures characterize the Basin and Range, Snake River Plains, the Sierra Nevada, and the active Cascade volcanic arc suggesting that the recent tectonic processes that affected the region have shaped the crustal structure. The northern Basin and Range generally exhibits thin crust. A zone of magmatic underplating thickened the crust along the Snake River Plains that is distinct from the High Lava Plains and the surrounding Rocky Mountains and Basin and Range. The impedance contrast across the Moho within the Colorado Plateau appears to be diminished, supporting ideas that it possesses a high velocity lower crust that provides strength and allows it to resist internal deformation compared to surrounding areas. Comparing the crustal thickness of the Colorado Plateau to its elevation and Bouguer gravity anomaly suggests the need for a source of buoyancy from the mantle to support the elevation of the plateau, especially around its perimeter. Observations presented here also indicate that the Rockies possess thin crust in much of Montana. The Laramide uplifts along the eastern bounds of the Rockies in Wyoming also possess a large zone of reduced impedance contrast across the Moho resulting from fast lower crust towards the southern portion of Archean craton, which suggests that preexisting structures influence more recent processes. This continent-scale study provides a context for other detailed studies to understand how observed crustal structures relate to the evolution of western North America.