SHARP CONTRASTS IN CRUSTAL CHARACTERISTICS BETWEEN THE SOUTHERN BASIN AND RANGE PROVINCE AND THE COLORADO PLATEAU IN ARIZONA

Stacking of about 13500 P-to-S receiver functions recorded at about 60 USArray and other broadband seismic stations placed on the Colorado Plateau and the Basin and Range Province in Arizona revealed systematic spatial variations in crustal Vp/Vs, crustal thickness and amplitude of P-to-S converted phases(relative to the first P-arrival). The Colorado Plateau in the study area has the thickest crust with an average of about 47 km. In the northern part of Arizona, a two-layer crustal structure is revealed beneath the Colorado Plateau with a 30 km thick upper layer and a 20 km thick lower layer. The Colorado Plateau part is dominated by higher than normal Vp/Vs, with a mean value of about 1.81 and smaller stacking amplitude with a mean value of 0.12. The Basin and Range Province in southern Arizona has the thinnest crust with an average of about 28 km and a Vp/Vs of about 1.71 and higher amplitude with a mean value of 0.26, which represent a sharp contrast of crustal characteristics with the Colorado Plateau. Small stacking amplitudes of the converted phases indicate that the Colorado Plateau in Arizona has a blurred Moho relative to a less modified craton (e.g., the Kaapvaal craton in southern Africa). 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. Crust beneath the southern Basin and Range Province is more felsic and thinner than normal continental crust which could imply delamination of the lower continental crust.