EXTENSION IN THE COLORADO PLATEAU/BASIN AND RANGE TRANSITION ZONE, CENTRAL UTAH: AN ACTIVE OR PASSIVE PROCESS?

The Colorado Plateau-Great Basin transition zone (TZ) in central Utah is a presently extending lithospheric block composed of previously stable Proterozoic lithosphere. TZ extension may be driven by high topography resulting from overthickening during the Laramide Orogeny and passive plate boundary forces similar to the Great Basin. However, high TZ topography coupled with a thinned lithosphere may indicate dynamic mantle upwelling and active processes acting within the TZ. To investigate the active vs. passive rifting hypotheses we have collected 56 new broadband and 9 long period MT sites to merge two existing MT lines for a combined length of 400 km (124 sites) covering the eastern Great Basin into the Colorado Plateau at a latitude of ~38.5°N. We have also collected over 300 new heat production and thermal conductivity measurements across the southwest to develop a thermal model of the eastern Basin and Range and Colorado Plateau. MT data show a highly conductive body that is semi-contiguous in the lower crust of the eastern Great Basin that rises to a shallow depth of <20 km beneath the TZ. This conductive layer appears to be connected to the surface by a series of symmetric rift related normal faults mapped at the surface. These normal faults may be acting as pathways for large scale fluid connection between the upper and lower crust. MT inversion results suggest an electrical anisotropy factor >3 in the upper mantle with an enhanced conductivity in a N-S orientation that is in agreement with observations of fast seismic SKS split direction. We propose that anisotropy observations may be due to small degrees of interconnected partial melt aligned with N-S geologic strike beneath the Basin and Range. An in progress thermal model combining measured thermophysical properties with existing heat flow data, MT measurements and estimates of seismogenic depth is used to estimate the thermal structure and lithospheric thickness of the mantle along the MT profile by a Monte Carlo estimation. Lithospheric thicknesses estimated from a preliminary thermal model suggest a thick, ~150 km, lithosphere beneath the core of the Colorado Plateau and thin, ~60 km, in the Great Basin. Geodynamic modeling using thermal and resistivity constraints may reveal insight into the active/passive nature of the rift.