Crustal thickness and Vp/Vs ratio variations beneath the Colorado Plateau

Stacking of about 35000 P-to-S receiver functions recorded at about 400 USArray and other broadband seismic stations placed on the Colorado Plateau 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 northern part of the state of Colorado has the thickest crust with an average of about 50 km, and Utah and Arizona have thinner crust with an average of about 35 km. The northern part of New Mexico has a crust of about 45 km thick and the southern part is about 35 km thick. The Colorado Plateau is dominated by higher than normal Vp/Vs, with a mean value of about 1.85. The Basin and Range Province in southern Arizona has the thinnest crust in the study area with an average of about 28 km and a Vp/Vs of about 1.71. Small stacking amplitudes of the converted phases indicate that the majority of the Colorado Plateau has a blurred Moho relative to a less modified craton (e.g., the Kaapvaal craton in southern Africa). The simplest model for the observations 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.