LITHOSPHERE STRUCTURE BENEATH THE COLORADO PLATEAU DETERMINED BY RECEIVER FUNCTIONS AND SURFACE WAVE TOMOGRAPHY USING USArray TELESEISMIC DATA

Using USArray Transportable Array teleseismic data we have made common conversion point (CCP) stacked PdS and SdP receiver function image volumes along with a Rayleigh wave inversion to determine, in more detail and higher resolution than previously obtained, the crustal thickness and the depth to the Moho and lithosphere-asthenosphere boundary (LAB) beneath the Colorado Plateau. Individual receiver functions have been converted to depth and laterally “migrated” to their conversion point using a 3D reference velocity model, (Schmandt and Humphreys, unpublished) with redundant signals stacked for signal enhancement. We have also used fundamental mode Rayleigh wave tomography to image the shear velocity lithosphere structure using the two-plane wave approach.   Interpreted together, receiver functions and surface wave tomography have imaged an unusually complex crust-mantle boundary region beneath the Colorado Plateau in comparison to most other parts of the western U.S. From the south the Moho deepens significantly from the southern Basin and Range, from ~ 30 km, into the Colorado Plateau where it is ≥ 40 km. The feature identified as the Moho broadens considerably and in places separates into two distinct positive amplitude events.  This character appears throughout the plateau, particularly along its northwestern periphery. Positive amplitudes, which appear to be related to the base of the crust and Moho, extend to depths greater than previous estimates of CP thickness (> 50 km). These complications in the Moho are correlated with low upper mantle velocities observed in P and S body wave tomography and S-velocity structure determined from Rayleigh wave inversion.  In the receiver function images the lithosphere-asthenosphere boundary (LAB) appears as bright negative amplitudes in the center of the Colorado Plateau at approximately 110 km depth, but have much weaker amplitudes around the edges of the plateau and again are split into two distinct negative amplitudes.  The topography of the LAB varies considerably from 85-120 km beneath the region and appears to be related to the transition between tectonic regimes on each side of the plateau.