LATE CENOZOIC MANTLE DYNAMICS, INTRA-PLATE VOLCANISM, AND TOPOGRAPHIC EVOLUTION WITHIN THE WESTERN U.S

Both the geologic records and present mantle seismic images suggest a complex late Cenozoic history of mantle dynamics below western U.S. We try to understand the post-20 Ma dynamic history through numerical models with data assimilation. We developed a hybrid forward-adjoint geodynamic approach that takes advantage of both the high-resolution information from the forward subduction simulation and the regional scale mantle structure revealed by recent seismic tomography through the adjoint simulation. The iterative solver allows for a best match to both the past subduction and the present-day mantle image. Other geophysical constraints such as gravity anomalies, residual present topography, and seismic anisotropy are also taken into account in these models.

Our best-fit model provides important new insights on a variety of geologic and geophysical observations. The temporal evolution of hot mantle anomalies below the western U.S. suggests that the Yellowstone-related volcanic history was driven by the eastward advance of intruding hot oceanic asthenosphere, in response to slab-driven toroidal flows. The Yellowstone-Newberry hotspot tracks result from east to west bifurcating mantle flow above the sinking Farallon slab along the Snake River Plain. We also find that a deep-rooted mantle plume has a much more limited effect on the formation of intra-plate volcanism in the Pacific Northwest, contrary to the traditional thought. The model predicts three distinct phases of uplift in central Sierra Nevada, consistent with the stratigraphic inference. The dynamic topography results suggest a late Neogene growth of sharp topographies to the east of the Basin & Range Province, including the edges of the Colorado Plateau and the Central Rockies.