Imaging crustal and upper mantle structure beneath the Colorado Plateau using finite frequency Rayleigh wave tomography

Abstract

A new 3-D shear velocity model of the crust and upper mantle beneath the Colorado Plateau and surrounding regions of the southwestern United States was made with finite frequency Rayleigh wave tomography using EarthScope/USArray data. The goal of our study is to examine the Colorado Plateau lithospheric modification that has resulted from Cenozoic tectonism and magmatism. We have inverted for the isotropic Vs model from a grid of Rayleigh wave dispersion curves obtained by a modified two-plane wave method for periods from 20 to 167 s. We map the lithosphere-asthenosphere boundary under the Colorado Plateau by identifying the middle of the shallowest upper mantle negative Vs gradient. The depths of the lithosphere-asthenosphere boundary inferred here agree well with receiver function estimates made independently. The strong lateral heterogeneity of shear velocity can be mainly attributed to 200–400 K variations in temperature together with ∼1% partial melt fraction in the shallow upper mantle. The resulting Vs structures clearly image the upper mantle low-velocity zones under the Colorado Plateau margins that are associated with magmatic encroachment. These upper mantle low-velocity zones resulted from the convective removal of the Colorado Plateau lithosphere that had been rehydrated by subduction-released water, refertilizing and destabilizing it. This convective erosion by the asthenosphere at the low-viscosity part of the lithosphere is driven by the large step in lithospheric thickness and the thermal gradient across the boundary between the plateau and the extended Basin and Range since the Mid-Cenozoic at a rate similar to that of magmatic migration into the plateau from the southeast, south, and northwest. Moreover, the Rayleigh wave tomography model images parts of a high-velocity drip in the western Colorado Plateau and thus provides additional seismic evidence for ongoing convective downwelling of the lithosphere that was initially suggested by receiver functions and body wave tomography. The widespread edge convective erosion, which the regional delamination-style downwelling processes are a 3-D manifestation of, could provide additional buoyancy sources to support the excess uplift at the margins of the plateau.