A PRELIMINARY SEISMIC RECEIVER FUNCTION STUDY OF CRUSTAL COMPOSITION AND THICKNESS ACROSS THE BASIN AND RANGE-COLORADO PLATEAU TRANSITION ZONE IN ARIZONA

The juxtaposition of the highly extended Basin and Range (BR) and the minimally deformed Colorado Plateau (CP) provinces is a striking and poorly understood feature of Arizona. In places the provinces are in direct contact, elsewhere they are separated by the narrow, topographically intermediate Arizona Transition Zone (ATZ). Two fundamental questions are: 1) why have the BR and CP responded to the changing plate boundary conditions of the last 80 m.y. in such different ways and 2) what is the tectonic significance of the ATZ?

A knowledge of the crustal composition and thickness of the BR, CP, and ATZ is integral to answering these questions. For example, several authors have suggested that in locations where Precambrian continental assembly juxtaposed strong, mafic crust of the CP with weaker, more felsic crust of the BR, the CP has been better able to resist deformation associated with the region's 80 m.y. history of strong compression followed by extension (e.g. Zandt et al. 1995). A related proposal is that in areas where no such compositional contrast exists, the CP/BR boundary is gradual, marked by a zone of intermediate crustal thickness and topography, the ATZ (e.g. Parsons et al. 1996).

The 2006-2007 deployment of the USArray seismic network in and around Arizona provides the first opportunity to study regional lithospheric variations across the entire length of the CP/BR transition in Arizona using teleseismic techniques. We have performed a receiver function study to help constrain crustal thickness and composition (via the Vp/Vs ratio) under those stations. Our preliminary results show strong compositional contrasts between the CP and the BR where the ATZ is absent and little contrast where it is present, consistent with the above hypotheses. But they also indicate that the ATZ is underlain by crust of highly variable thickness, suggesting that its intermediate topographic expression is not due simply to an intermediate degree of stretching, contrary to the hypothesis.