This project studies the causes and consequences of mantle flow beneath the western United States (U.S.). Several, interacting spatio-temporal scales of geologic activity need to be considered: (1) circulation related to North America plate motion, (2) sinking of old Farallon plate in the lower mantle beneath the eastern U.S., (3) sinking of younger, more recently subducted plate in the upper mantle beneath the western U.S., and (4) small-scale convective dripping of the lithosphere. Related deep structure has recently been mapped to unprecedented detail thanks to EarthScope; the consequences of the resulting mantle flow are geologically important for the forces applied to the lithosphere, and for melting anomalies caused by mantle ascent and decompression. Induced flow is also important because it results in observable predictions that can be used as a constraint for physics-based models, such as seismic anisotropy, and the uplift and deformation of the continental crust.
The project analysis method involves mapping seismic images of the mantle into density structure that can then be used to drive computer models of mantle flow. Global seismic images and flow modeling are used to account for the large-scale processes, and high-resolution seismic images and regional flow modeling is used beneath the western U.S. EarthScope GPS data provide a highly resolved crustal strain-rate field and EarthScope teleseismic data provide detailed seismic images used to infer density, temperature, and anisotropy structure. Those parameters that affect resolvable constraints are explored in a search for the models that best account for the observations, leading to an improved understanding of issues such as the strength and mechanical behavior of plates.
Of particular interest are: (1) the magnitude of coupling between the large-scale flow patterns and the North American plate (with special emphasis on the effects of a cratonic root that penetrates deeply into the asthenosphere); (2), the importance of the sinking young slab beneath western U.S. on Yellowstone plume ascent and regional-scale flow that may be concentrated beneath the active northern Basin and Range province; (3), the lithospheric drips that are imaged adjacent to many of the young western U.S. uplifts and volcanic fields, and, (4), the general role such processes may play for the long-term tectonic and thermal evolution of the Earth.
