Correlation of magnetotelluric data and geothermal drilling in the Valles caldera region, New Mexico

The Jemez volcanic field in northern New Mexico is a natural laboratory to study the magnetotelluric (MT) response of sharply-varying, subsurface temperatures across a continental magmatic system. over 60 geothermal wells provide unusually complete subsurface temperature data. Deep boreholes document that the 300°C isotherm occurs at only 2.0 to 4.5 km depth across the western margin of the Valles caldera. Precambrian basement is found at these depths and is highly resistive (>1000 ohm-m) outside the caldera. Within the caldera the effects of hot water and alteration products reduce the basement resistivity to less than 100 ohm-m. Inversion of MT data collected by Unocal Geothermal Division and SAGE (Summer of Applied Geophysical Experience) yield excellent correlation with borehole results in the upper 5 km. Inversion also maps a deeper (>10 km), uppercrustal conductive zone whose geometry closely matches the pattern of isotherms projected from the boreholes. A mechanism that can explain this deep conductive occurrence is intergranular (not fracture controlled) aqueous fluid connectivity at porosities of less than 1%.