Imaging lithosphere structure beneath the Colorado Plateau using Rayleigh wave tomography

We have measured Rayleigh wave phase velocities to determine the shear velocity structure of the crust and upper mantle beneath the high-elevation (~2 km) and tectonically stable Colorado Plateau (CP), and adjacent Basin and Range Province (BR), the Rio Grande Rift (RGR), and the Southern Rocky Mountain (RM), using data from the USArray Transportable Array (TA) network. Approximately 60 teleseismic earthquakes with epicentral distance between 30°-120°, magnitude ≥ 6.0, and depth ≤ 70 km, were analyzed to extract the phase and amplitude information from the fundamental mode Rayleigh waveform using the two-plane-wave approach. We subsequently inverted the phase dispersion curves (20-100 s) for S- velocity structure to ~200 km on a 0.5°x0.5° grid, with Moho depths determined from the PdS receiver functions measurements, to produce a 3D lithospheric image, utilizing iterative linearized inversion. The entire study area was divided into four smaller subdivisions (~ 5° in longitude and latitude) to satisfy the two-plane wave assumption. The resulting S- velocity model reveals a complex lower crust and upper mantle structure beneath the CP and its adjacent areas. The topography of Moho derived from PdS receiver functions strongly correlates with the elevation of CP and its surrounding areas, from a fairly uniform crustal thickness of ~ 30 km in the southern BR, to a thickening crust of ≥ 42 km in the plateau. A smaller region with thinner crust (< 30 km) in the northern BR was found near the Wasatch front. Our results show slower upper mantle shear velocities than previous studies (Lastowka et al. 2001, West et al. 2004 etc.). We observe a consistent low velocity feature extending to ~200 km beneath Mt. Taylor along the NE trending Jemez lineament to the west of the RGR. Relative low velocity signatures in the upper mantle from ~50 to 100 km beneath the southernmost RM to the east edge of the plateau, and along the axis of the RGR, suggest warm upper mantle and small amount of partial melt. The central plateau shows a thicker crust and faster mantle lithosphere compared to the eastern, southwestern and northwestern transition zones towards the RM and BRP, indicating a cooler stronger lithosphere, consistent with the moderate surface heat flow and high Pn velocity compared to those of the plateau edges, where Pn velocity are relatively low and shallow upper mantle low velocity zones have been observed (Benz &McCarthy 1994, Henstock et al., 1998). We compare the surface wave results with the PdS and SdP receiver functions presented by Miller et al. (2009, this meeting).