The geological evolution of the western United States is an extensively studied problem with important consequences for understanding the nature of a host of Earth processes, including earthquake patterns and volcanic activity. The region is home to several famous case studies of Earth system processes occurring away from tectonic plate boundaries, including the substantial and ongoing extension of the Great Basin, and widespread hotspot volcanism across the Snake River Plain and Yellowstone regions. Other components of this complex system, such as the massive magmatic events expressed by the High Lava Plains of Oregon, and subduction of the Juan de Fuca plate system beneath North America that is responsible for the Cascade Volcanic Chain, are more poorly understood. Until recently, the only available seismic images of regional crust and mantle structure have been of limited resolution, with the exception of a few localized high-resolution studies. The lack of broad-scale, high-resolution seismic images across the entire region has limited our ability connect deep processes with their surface manifestations.
In this talk, I will show results from several recent studies that place new constraints on the structure and evolution of the western United States. I will focus primarily on high-resolution seismic investigations in which we have utilized data collected from the ongoing EarthScope Project (http://earthscope.org) and the High Lava Plains Project
(http://www.dtm.ciw.edu/research/HLP). I will present images of the structure and dynamics of the northwestern United States, and will concentrate on detailed studies of both the central Great Basin, where we find clear evidence for a newly-discovered zone of mantle downwelling, and the Snake River Plains / Yellowstone system, where we find no evidence for an upwelling deep mantle plume source. These new results demonstrate the need for revisions to models of the tectonic evolution of the western United States.