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Magnetotelluric Transect of the Sevier Overthrust Belt in Southwestern Utah and Eastern Nevada
An electrical resistivity cross section through the Sevier orogenic belt in southwestern Utah and easternmost Nevada has been derived from a detailed profile of magnetotelluric (MT) soundings. The resistivity model was produced using two independent interpretation algorithms, a finite element forward program and a minimum-structure inversion code. Common to the results of both approaches, and of principal interest to this study, are two nearly subhorizontal layer segments of low resistivity in the upper crust. One, with an average depth of about 2 km, underlies the entire Wah Wah Mountains and Valley. The second, more westerly segment underlies the Mountain Home Range and eastern Hamlin Valley at a depth of 4-5 km. The poor correlation of these structures with aeromagnetic anomalies and surface igneous activity suggests that they do not result from hydrothermal processes. No low-resistivity stratigraphic units which could directly account for the layer segments were encountered in the entire stratigraphic interval of the overlying Wah Wah plate from the Middle Mississippian to the Late Proterozoic. Multiplexing of thin shaly units in these older rocks at major thrust ramps may be one cause of the low resistivity, but would require massive repetition of thin conducting lithologies. Late Mississippian and younger rocks with large shale or graphitic components could provide the necessary conductivity, but in turn lead to the controversial hypothesis that rocks this young lie underthrust as far west as the Utah-Nevada border. If true, however, it would suggest new opportunities in exploration for deep hydrocarbon source or reservoir rocks in this environment. At present, no completely satisfactory explanation for the relative geometries of the resistivity structures exists, either in terms of overthrust motion or of later normal detachment faulting.
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