UNDERSTANDING MAGMATIC-HYDROTHERMAL RELATIONSHIPS IN GEOTHERMAL SYSTEMS OF THE GREAT BASIN THROUGH INTEGRATED MAGNETOTELLURICS, ISOTOPE GEOCHEMISTRY AND STRUCTURAL GEOLOGY

Successful pursuit of deeper, larger, higher enthalpy geothermal systems could have a significant impact on national energy needs. Many of these will have magmatic sources but verification of such and delineation of fluid flow pathways from the sources has been a challenge when only surface manifestations are at hand. Recent reconnaissance magnetotelluric (MT) profiling of the Great Basin shows numerous highly conductive, quasi-tabular zones in the lower crust interpreted to represent magmatic underplating and hydrothermal fluid exsolution, corroborated by seismic surveying where coincident. These zones commonly have steep, dike-like conductors extending surfaceward, several of which appear to feed into recognized high-temperature geothermal systems. Anomalous ³He and CO₂ fluxes, and C and O isotope trends, in southwestern U.S. hot springs and exhumed terranes similarly point to magmatic geothermal contributions from the deep crust and upper mantle and, where overlapping, correlate with the MT structures. A prominent example is the Dixie Valley system of west-central Nevada where coincident large historic earthquakes with steep failure planes nucleated just above the brittle-ductile transition. Such failure characteristics are typical of larger Great Basin events. Propagation of the failure plane downward into the ductile domain can temporarily embrittle it, tapping overpressured magmatic fluids and allowing their release upward to mix with meteoric waters. Focusing of geothermal fluids to the prospect scale typically occurs in favorable structural settings which are intrinsically 3-D involving relays, horse-tailing, opposing overlaps or dilational intersections. Frequently, high fluid productivity seems to lie on the margins of the most fluidized volumes where stress and brittle fracture permeability can persist.