The crustal structure of the Arizona Transition Zone and southern Colorado Plateau from multiobservable probabilistic inversion

The Arizona Transition Zone is a narrow band that separates two of the main and most contrasting tectonic provinces in western US, namely the southern Colorado Plateau and the southern Basin and Range provinces. As such, the internal crustal structure and physical state of this transitional zone hold clues for understanding (i) the amalgamation of these provinces, (ii) the partitioning of deformation due to both past and present-day stress fields, and (iii) the role of thermal versus compositional effects in controlling surface observables. Here we employ and expand a novel multiobservable probabilistic inversion method and jointly invert fundamental mode Rayleigh phase velocities, receiver functions, surface heat flow, geoid height, and absolute elevation to obtain an internally consistent 3-D model of the temperature, density, V-s, and V-p of the Arizona Transition Zone and the southern portions of the Colorado Plateau and Basin and Range. Our results confirm a significant crustal thickening from similar to 28 km in the SW of the Arizona Transition Zone and southern Basin and Range to similar to 48 km beneath the southern Colorado Plateau. Inverted temperatures agree well with the location of recent volcanism and indicate that the lithosphere-asthenosphere boundary is not deeper than similar to 70 km in most of the region. We find that major pre-Cambrian surface structures and/or shear zones separate crustal domains with distinct bulk properties, suggesting that the juxtaposed crustal blocks still retain, at least in part, their original characteristics. However, widespread intrusions of significant volumes of mafic magmas have affected these blocks at different depths, locally overprinting their original compositions and creating highly heterogeneous crustal sections. A dominant and large-scale internal crustal pattern of SW dipping planes/structures is evident in our models, coinciding with the orientation of deep faults previously inferred from earthquake focal mechanisms. While we cannot categorically corroborate the presence of melt or aqueous fluids within the crust, our results are compatible with these scenarios beneath some parts of the Basin and Range, the Mogollon-Datil, and Springerville volcanic fields.