Tracing crustal isotherms under the western margin of the Jemez Mountains using SAGE and industry magnetotelluric data
George R. Jiracek, Christopher L. Kinn, Clark L. Scott, Kuykendall, Martha, G., W. Scott Baldridge, Shawn Biehler, Lawrence W. Braile, John F. Ferguson, and Bernard Gilpin
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 present at these depths and is highly resistive (>1000 ohm-m) outside the caldera. However, inversion of MT data maps a deeper (>10 km), upper crustal conductive zone with a geometry that closely matches the pattern of projected isotherms. This change must occur in the ductile portion of the crust since the temperature would be over 450°C. A mechanism that can explain this conductive occurrence is intergranular aqueous fluid connectivity at porosities of less than 1%. The interconnectivity is controlled by the dihedral angle, or the angle subtended by fluid-filled, grain-edge intersections. Experimental results predict water connectivity coincident with an isotherm in a ductile crust under an overlying impermeable cap. The positive correlation between the depth of the upper crustal conductive zone and projected isotherms may have profound rheological implications since aqueous connectivity would result in higher strain rates. Thus, tracing this zone could contribute greatly to understanding of contemporary deformation in the Rio Grande rift.
- Jiracek, George R.; Kinn, Christopher L.; Scott, Clark L.; Kuykendall, Martha, G.; Baldridge, W. Scott; Biehler, Shawn; Braile, Lawrence W.; Ferguson, John F.; Gilpin, Bernard, 1996, Tracing crustal isotherms under the western margin of the Jemez Mountains using SAGE and industry magnetotelluric data, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 129-133.