CRUST AND MANTLE STRUCTURE ACROSS THE BASIN AND RANGE COLORADO PLATEAU BOUNDARY AT 37-DEGREES-N LATITUDE AND IMPLICATIONS FOR CENOZOIC EXTENSIONAL MECHANISM

We present new evidence on the seismic velocity and density of the cru st and upper mantle along a 200-km-long transect across the eastern Ba sin and Range and western Colorado Plateau at 37 degrees N latitude. R eceiver functions computed from the P waveforms recorded with 10 porta ble broadband stations deployed along the transect were used to estima te crustal thickness variations. The crust is 30-35 km thick within th e eastern Basin and Range and increases over a distance of similar to 100 km at the western edge of the Colorado Plateau, reaching a maximum of approximately 45 km east of the Hurricane fault. The timing of cru stal multiples within the reciever functions were used to estimate the V-p/V-s of the crust along the profile, and we found that the western Colorado Plateau crust is characterized by a high Poisson's ratio (0. 28-0.29) indicative of a crust with an average mafic composition. We e stimated the upper mantle lid thickness along our profile based on tel eseismic P wave travel times and constraints provided by gravity data. Our data and available geophysical constraints are most consistent wi th a lithosphere that thickens from an average thickness of 60 km bene ath the Basin and Range to 100 km beneath the western Colorado Plateau , although the Basin and Range lithosphere may have significant thickn ess variations. The thick, strong mafic crust and thicker mantle lid u nder the Colorado Plateau can account for the relative geologic stabil ity and subdued magmatism of the plateau during Laramide compression a nd Cenozoic extension compared to surrounding regions. The crustal and lithospheric thinning across the tectonic boundary occurs over a shor t distance (similar to 100 km), suggesting it is a geologically young feature produced by a predominantly mechanical response to late Cenozo ic extension. Our new lithosphere model at 37 degrees N latitude is co nsistent with the existence, in early Cenozoic time, of a flat subduct ed slab at 100 km depth and a relict Sevier-Laramide 50-60 km thick cr ustal welt, and 60-100% pure shear extension (beta values of 1.6-2.0) during the late Cenozoic.