MANTLE STRUCTURE OF THE COLORADO PLATEAU AND ITS MARGINS REVEALED BY LA RISTRA

During the past 30 Ma much of interior western North America has been in an extensional stress regime, resulting in the formation of the Basin and Range and Rio Grande Rift provinces. However, extensional processes have left the Colorado Plateau (CP) relatively undeformed, and it has emerged as a distinct tectonic province. New images of the lithosphere-scale structure constructed from the first complete deep seismic imaging transect of the CP in the "LA RISTRA" experiment show that CP crustal thickness exhibits highly variability (42-50 km) near its center and thins to 30-35 km thick at the edges, where the highest elevations are observed. Isostatic calculations constrained by these results show that only approximately 25% of central CP elevations can be explained by thickened crust alone, CP margins requiring nearly total mantle compensation. Travel-time body wave velocity inversions show a sharp, large magnitude contrast across the CP-Great Basin transition extending from the base of the crust to about 200 km depth. Also imaged is a fast seismic anomaly from 300 to 600 km depth beneath the NW portion of the model. Very slow P and S velocities beneath the Great Basin require the existence of partial melt and/or higher than average water content. We propose that the sharp contrast in mantle seismic properties across the western CP edge corresponds to differential lithospheric modification, during and subsequent to Farallon subduction and foundering across a boundary marking the western extent of unmodified Proterozoic mantle lithosphere. We interpret the deep fast anomaly as a remnant of thickened Farallon plate and/or detached continental lithosphere that has sunk to transition zone depths. Shear-wave splitting observations suggest a system dominated by mantle flow around and deflected by a wedge-shaped lithospheric root beneath the CP. These results indicate strongly dynamic convecting mesoscale mantle beneath the southwestern U.S.