Teleseismic data recorded across the western U.S. provide information on the structural nature of the mantle beneath North America, a volume of Earth's interior that has been shaped by a complex history of subduction of oceanic lithosphere. Tomography images high-velocity structures that are commonly attributed to subducted slabs but which are not as simple as expected from our current understanding of the geologic history of subduction. Hence, such images hold key information on the actual subduction history, the fate of young subducted slabs, and convective flow in the upper mantle and across the transition zone to the lower mantle. The area affected by the Laramide orogeny is now experiencing small-scale convection beneath most of the western U.S., and a presumed Yellowstone plume is (probably) interacting with subducted slab and small-scale downwellings. Understanding this richness of geological processes and resolving the individual structures from one another requires careful and accurate seismic imaging. The goal of this project is to test specific hypotheses about the complex history of the western U.S. by creating a comprehensive 3-D P-wave velocity model extending from the base of the crust to ~1000 km depth. The seismic modeling will combine all available local, regional, and global travel-time datasets with newly available constraints on crust and transition zone structure. In addition, it uses finite frequency techniques (where useful) and 3-D wave propagation techniques. This work supports two graduate students.
Most geologic processes are driven by activity deep within the Earth. We use seismic tomography to image the structures within the Earth to depths of 1000 km, being particularly able to determine variations in temperature that relate to the subduction of oceanic plates and the ascent of mantle plumes. We use our images to help resolve the geologic activity of the western U.S. that has occurred since about 50 million years ago. This is a time when the subducting Farallon ocean plate flattened and rode at the base of the North American plate as far east as the Great Plains, creating the Rocky Mountains; this event was followed by times when volcanism was very vigorous across much of the western U.S., which was a consequence of the Farallon plate coming off the base of North America. The imaged structures have provided important clues to allow reasoned interpretation of many important western U.S. geologic events. Through these studies we conclude: (1) There are many fragments of Farallon plate still attached to North America, dangling in a curtain-like fashion. Other portions of the Farallon plate have descended and are found at depths of about 600 km and below. (2) The Farallon plate that subducted after the Rocky Mountain event has fragmented into pieces. This includes the currently-subducting portion of Farallon plate now called the Juan de Fuca plate, as it subducts beneath the Pacific Northwest. (3) Yellowstone is imaged as a mantle plume ascending from 1000 km depth or more. (4) A large fragment of Farallon plate accreted to the Pacific Northwest about 50 million years ago, exciting strong volcanism in most of Idaho and controlling subsequent tectonics in the Oregon and Washington. (5) The Columbia River flood basalt event was greatly intensified by the triggering of Farallon plate delamination from the base on NE Oregon by arrival of the Yellowstone plume about 17 million years ago. (6) Delamination of lower continental plate has been "caught in the act" beneath the SW Colorado Plateau.
