Modes of Extension, Magmatism and Crustal Scale Fluid Pathways in the Great Basin province, USA, from a Magnetotelluric Transect

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Phil Wannamaker from the University of Utah is producing and interpreting a high-quality section view of electrical resistivity structure through the central and western Great Basin, and the southeastern Modoc Plateau. This is being achieved by the addition of 55 wideband magnetotelluric soundings to an existing transect of 179 sites, plus installation of 35 long-period magnetotelluric soundings, to create a transect some 650 km in length span-ning Nevada and northeastern California into westernmost Utah. The magnetotelluric station data will span the range of 0.005 to 10,000 second wave periods, with corresponding imag-ing depths from a few hundred meters to about 300 km, and perhaps greater. The profiling covers fully 100 kilometers of the Modoc Plateau, the actively extending northwestern Great Basin, the Central Nevada Seismic Belt, the relatively quiescent east-central Great Basin of higher elevation, and onlaps the active eastern Great Basin of western Utah. These data are undergoing modern methods of processing, trend analysis, and regularized inversion to pro-duce a well-resolved model with minimal artifacts. The overall goal of the project is to im-prove our understanding of the process of distributed continental rifting. The consequences of rifting can be profound, including the potential for large earthquakes, development of sedimentary basins and, ultimately, breakup of continents to form new ocean basins. Many important hydrocarbon, geothermal and mineral resource occurrences can arise from the structural, magmatic and hydrothermal processes involved in extension. The project specifi-cally is addressing first-order problems regarding vertical strain distribution, magmatic crustal growth, domains of crustal scale fluid flow, and coupling between the lithosphere and asthenosphere remain unsolved. These include the prevalence of crustal scale fault zones which host the largest earthquakes; under what circumstances upper and lower-crustal thin-ning are decoupled; where mantle lithosphere is weak or strong during rifting; whether volumetrically significant magmatic input occurs during the early to middle stages of conti-nental rifting; and how mantle-sourced fluids make their way to the shallow ground water regime. The project also addresses societal benefits including broader understanding of the controls on fault strength, crustal scale hydrology, and the ability to build up damaging stresses. The link between deep geothermal systems and epithermal gold deposits is being illuminated as well. A graduate student is being supported for education into state of the art geophysical instrumentation, parameters of field survey design, practical logistical matters in remote settings, modern methods of inversion, and the relationship between electrical resis-tivity and physico-chemical state in the Earth. Linked companion projects have the potential to provide the first results approaching regional to national scales, and public interest is ex-pected in section views through diverse regions provided by a potential megatransect from near the California Pacific coast to the Colorado Plateau interior at Utah?s eastern border. In addition to the research, the project is supporting the graduate training of a Ph.D. student from an underrepresented group in the earth sciences in magnetotelluric methods. Funding for the research is being provided by the NSF Geophysics and Tectonics programs.