LITHOSPHERIC BOUNDARIES, MAGMATIC PROCESSES AND CRUSTAL SCALE FLUID CONNECTIONS OF THE GREAT BASIN AND ITS TRANSITION TO THE COLORADO PLATEAU AS TRACED BY ELECTRICAL RESISTIVITY STRUCTURE

The contrast in geological processes across the long-lived Wasatch Line in west-central Utah remains strong today, with active extension, magmatism, seismogenesis and geothermal systems in the Great Basin juxtaposed against a nearly imperturbable Colorado Plateau. Deep magnetotelluric (MT) geophysical profiling across 38.5N latitude projects this contrast through the crust and upper mantle, revealing probable intense magmatic underplating of Great Basin crust extending east through the High Plateaus to Thousand Lake Mtn and western Capitol Reef NP. A pronounced resistive keel to the Colorado Plateau truncates this underplating and may reflect the ancestral Yavapai-Mojave lithospheric boundary. Upper mantle melt source regions in the eastern Great Basin appear anisotropic in fabric with fissuring elongate N-S, in concert with seismic SKS anisotropy. Basaltic underplating with magma hybridization and crystallization produce magmatic fluids that traverse crustal scale fault zones mixing with the meteoric regime and contributing to geothermal systems as evident in He isotopes and thermal constraints. Magmatism and deformation is more unsteady and distributed over the central and western Great Basin, but zones of active underplating some of which correspond with seismic velocity and attenuation anomalies also are revealed in the MT. A fundamental crustal-scale fluidized break links the Buena Vista seismic/electrical bright spot with the Dixie Valley thermal area contributing heat and magmatic fluids to the geothermal system. Several other such large-scale breaks are identified across the province; some correlate with known geothermal systems while others still are cryptic to the surface. Fluid infiltration of stressed lithologies is a well-recognized trigger of earthquakes and this is supported by observed seismicity often immediately surrounding low-resistivity fluid-based structures.