GEOPHYSICAL FRAMEWORK OF THE COLORADO PLATEAU/BASIN AND RANGE TRANSITION: GEOINFORMATICS IN ACTION

We are constructing an ArcGIS database to constrain the geodynamics and tectonic evolution of the southern regions of the Colorado Plateau (CP)/Arizona Transition Zone (ATZ)/Basin and Range (BR) in Arizona and New Mexico (http://www.geoinformaticsnetwork.org/swgeonet). The CP and BR have responded differently to a broad range of tectonics over the past 100 Ma. Our database includes topography, complete Bouguer gravity anomaly (CBA), heat flow, seismicity, shear wave splitting, and receiver functions, with planned additions of seismic tomography and active source seismic data.

Integrated analysis characterizes the structure and dynamics of the crust and mantle beneath the BR and CP. For example, gravity, receiver functions, and heat flow all suggest relatively thicker crust in the CP and relatively thinner crust in the BR. Interpolated CBA data clearly show a relatively thickened crust beneath the CP and ATZ (~ -275 to -200 mgal) in northern Arizona and New Mexico, and a relatively thin crust of the BR (~ -100 to -50 mgal) in Arizona. The Rio Grande Rift (RGR) also appears as a ~100 mgal local increase. Receiver function data show a thickening of the crust from ~35 km in NW Arizona to ~45 km in south-central Colorado. Differences in heat flow values suggest thicker crust beneath the CP relative to the BR: heat flow is lower near the southern CP (<50 mW/m²), and higher in the southern BR and the RGR (>150 mW/m²). While changes in lithospheric thickness likely contribute to the heat flow variations, they suggest thicker crust beneath the CP relative to the BR.

Another example shows the relationship between topographic gradients, regional seismicity, and the inferred mantle flow field. The CP has a surface elevation of >1500 m and drops by ~1000 m over a lateral distance 50-100 km into the ATZ. This pronounced relief contrast is associated with enhanced seismicity in the southern CP. Active fault data show E-W extension in the RGR and southern Arizona and NE-SW extension in the southern CP. These observations correlate with the limited body of shear wave splitting measurements which exhibit ~NE-SW fast polarization directions across Arizona. Nearly N-S fast polarization directions for the RGR suggest a change in the relationship between active surface deformation and mantle fabric across the region.