Imaging the Lithosphere of the Colorado Plateau using Rayleigh Wave Phase Velocities

Abstract:

The Colorado Plateau in the southwestern US, with high elevation and low relief, is an enigmatic physiographic province with a subsurface lithospheric structure of contentious interpretation. Unlike the neighboring Rocky Mountain region, the Colorado Plateau shows no obvious signs of significant crustal shortening, making its high elevation confounding. Previous studies by other workers have suggested that the Colorado Plateau may have been formed as a result of a Rayleigh-Taylor instability associated with delamination and convective downwelling of the mantle lithosphere underneath the plateau (Levander et al., 2011). Others have argued that warming of the lithosphere, following the removal of the Farallon plate (Roy et al., 2009) or lower-crustal metasomatism (Levandowski et al., 2018) is the primary mechanism at play. Here we use Rayleigh wave phase velocities from over 130 earthquakes measured at USArray Transportable Array (TA) stations with inter-station cross-correlation to image the crust and lithospheric mantle underneath the Colorado Plateau at periods ranging from 20-100 seconds. We generate phase velocity maps using wavefront tracking (Lin et al, 2009) and use them to determine a 3-dimensional model of shear wave velocity. Additionally, the new maps are interpreted together with recent volcanism, topography, gravity anomalies and crustal thickness, in order to evaluate possible explanations for surface uplift of the Colorado Plateau. Our preliminary findings suggest that high elevation in the Colorado Plateau is supported by low density in the lithosphere rather than crustal thickness variations.