Late Cenozoic Mantle Upwelling at the Western and Southern Margins of the Colorado Plateau?

The Colorado Plateau of the southwestern United States is noted for its tectonic stability relative to its neighboring provinces, yet in the late Cenozoic has undergone magmatism and tectonism at its margins. Two observations suggest that upper-mantle activity may be contributing to on-going tectonic evolution of the Plateau, 1) the uplifted and topographically rough western and southern edges of the Plateau, and 2) the recent magmatic activity (distinct from earlier, mid-Tertiary magmatism) along the edges of the Plateau. To understand the dynamics of the Colorado Plateau, we present results of numerical modeling and propose that the recent “bloom” of magmatism and the differential elevation of the Plateau edges are attributable to small-scale convection along the margins of the Plateau.

Our models are driven by middle and late Cenozoic thinning of the lithosphere in the adjacent Basin and Range and Rio Grande rift provinces, leading to formation of the Plateau as a distinct tectonic entity. The resulting differences in lithospheric thickness trigger small-scale convection whereby upward flow of mantle occurs beneath the extended terrains and the margins of the Plateau. Convective upwelling causes uplift of the Plateau margins and magmatism on the edges of the Plateau. Our modeling results are consistent with new tomographic results that reveal a significant seismic velocity gradient between the Basin and Range and the Colorado Plateau. Our models imply that small-scale flow in the upper mantle is an important factor contributing to tectonic deformation in the western US.