"Stacking of about 15,500 P-to-S receiver functions recorded at 72 USArray and other broadband seismic stations placed on the Colorado Plateau and the SBRP in Arizona revealed systematic spatial variations in crustal Vp/Vs, crustal thickness and amplitude of P-to-S converted phases. Our results reveal that the BRP is characterized by a thin crust (28.2 ± 0.5 km), a mean Vp/Vs of 1.761 ± 0.014 and a mean amplitude (R) of P - to - S converted wave (relative to that of the direct P wave) of 0.181 ± 0.014 that are similar to a typical continental crust, consistent with the model that the thin crust was the consequence of lithospheric stretching during the Cenozoic. The CP is characterized by the thickest crust (42.3 ± 0.8 km), largest Vp/Vs (1.825 ± 0.009) and smallest R (0.105 ± 0.007) values in the study area. The Colorado Plateau in the study area has the thickest crust and its southern part revealed two layers crustal structure having 12 km thickness of the lower layer. Crust beneath the southern Basin and Range Province is less mafic but thinner than normal continental. The simplest model for the observation is that the Colorado Plateau crust is underlain by a mafic layer which increases both the thickness and Vp/Vs, and decreases the amplitudes of the converted phases by reducing the velocity contrast between the crust and mantle. The results are in general agreement with previous studies (e.g. Frassetto et al., 2006; Gilbert et al., 2007). We hypothesize that the lower crustal layer, which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines, contributed to the long-term stability and pre-uplift low elevation of the Colorado Plateau in the study area"--Abstract, leaf iv.