Lithospheric structure beneath the Colorado Plateau determined by receiver functions

Using a combination teleseismic data from the USArray Transportable Array, previous PASSCAL experiments, and the COARSE array in Arizona we have produced images of the lithospheric structure beneath the Colorado Plateau and the surrounding tectonic provinces. We have made common conversion point (CCP) stacked PdS and SdP receiver function image volumes 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“migrated” to their conversion point using a 3D P- and S-wave tomography velocity models, (Schmandt and Humphreys, unpublished) with redundant signals stacked for signal enhancement. Both S and P receiver functions have imaged an unusually complex crust-mantle boundary region beneath the Colorado Plateau in comparison to most other parts of the western U.S. Positive amplitudes in the receiver functions, 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). Progressing southwards the Moho shallows significantly from an average of ≥ 40 km to the southern Basin and Range, where the crust is ~ 30 km. This positive amplitude feature, identified as the Moho, broadens considerably and in places separates into two distinct positive amplitude events, which we suggest may be active detachment of the lower crustal portion of the lithosphere. This character appears most predominately along the northwestern edge of the plateau. 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. We will present different geologic scenarios that can explain these structures. At greater depth in the receiver function images the lithosphere-asthenosphere boundary (LAB) appears as bright negative amplitudes in the center of the Colorado Plateau at ~110 km depth, but have much weaker amplitudes around the edges of the plateau and again 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.