Magnetotelluric mapping of upper crustal isotherms beneath the Valles Caldera hydrothermal system

Magnetotelluric (MT) soundings across the western margin of the Valles Caldera hydrothermal system in New Mexico have revealed a striking correlation between depth of the electrically conductive portion of the upper crust and isotherms projected from deep geothermal test wells. The crust is highly resistive (approximately greater than 1000 ohm-m) in the range of 2 to 4 km depth where temperatures of over 300 C have been logged in test wells. However, at greater depths the upper crust becomes increasingly conductive; in this range, the conductivity contours closely match the projected isotherms. The inferred geoelectric section was obtained by two-dimensional inversion after shallow three-dimensional distortion corrections were applied to the data. Inversions were constrained to depths as great as 4 km by borehole electric logs which highly compatible with the nearby MT soundings. Possible reasons for the positive correlation between the conductive depth patterns and the isotherms are: (1) fluid accumulation within the thermally controlled brittle-ductile transition and (2) a halo of conductive mineralization (e.g., connected sulfide films) surrounding a still cooling, shallow intrusive.