Lithospheric Architecture and Mantle-Supported Topography of the Colorado Plateau Constrained by Receiver Function Imaging

After maintaining elevations near sea level for hundreds of millions years, the Colorado Plateau (CP) of the southwestern United States has a present average elevation of 2 km. However, the sources of buoyant support for this high elevation have long been unclear. We apply receiver function joint velocity analysis and imaging to data from the LA RISTRA experiment to construct a new seismic image of crust and upper mantle structure that constitutes the first complete seismic transect of its kind across the CP. We find that the central CP crust has variable thickness of 42-50 km that thins to 30-35 km at plateau margins. For the central CP, crustal Vp/Vs ratios average 1.71 to the northwest and 1.80 to the southeast of the Yavapai/Southern Yavapai province boundary. Higher Vp/Vs ratios to the southeast of the boundary indicates that the Southern Yavapai crust may on average be slightly more mafic which is consistent with the lower concentrations of incompatible elements found for this region. Isostatic calculations constrained by these observations show that only approximately 20% of central CP elevation can be explained by thickened crust alone, with the bowl-shaped, high-elevation edges of the CP requiring nearly total mantle compensation. We calculate an uplift budget which shows that the CP buoyancy arises from the combined effects of crustal thickening, Laramide/post-Laramide alteration of the lithospheric root, and dynamic support from mantle upwelling. Prior to the Laramide, thick crust and low-density uppermost mantle were balanced by a thick, dense Proterozoic lithospheric root that kept the CP near sea level. Laramide/post-Laramide tectonics altered this root most likely by a combination of metasomatism, post-Laramide thermal conduction and expansion, and perhaps minor tectonic erosion. This, combined with dynamic topography due to mantle upwelling produced by the foundering of the Farallon slab, resulted in buoyant CP uplift to the present high elevations. These buoyant effects are complemented significantly by edge effects where CP lithosphere is undergoing convective removal arising from edge-driven, small-scale convection in the hot asthenospheric mantle of the adjoining Great Basin and Rio Grande Rift.