Description
The Jemez volcanic field (JVF) has been the site of nearly continuous volcanic activity for 13 million years (Ma). It is located at the intersection of the northeast trending chain of Cenozoic volcanoes, called the Jemez lineament, and the north-south trending zone of continental extension, called the Rio Grande rift. The Valles caldera in the JVF was the site of explosive rhyolitic volcanism 1.12 Ma. Forty-one magnetotelluric (MT) soundings were collected by Unocal and SAGE (Summer of Applied Geophysical Experience) to understand the structure of the Valles caldera and its relation to the Rio Grande rift. Subcrustal temperatures greater than 300°C are documented at depths of 2 to 4 km beneath the western portion of the Valles caldera by some of the nearly 60 geothermal wells in the area. Two-dimensional (2-D) inversion of the MT data reveal that in the upper 5 km, Precambrian basement is highly resistive (>1000 ohm-m) when compared with the conductive (~30 ohm-m) materials within the caldera. A ~600 m thick conductive zone (~10 ohm-m) near the surface beneath the Los Alamos National Laboratory Hot Dry Rock Project at Fenton Hill equates with the Mesozoic and Paleozoic sedimentary formations from drilling results. A correlatable ~600 m conductive zone is found further to the east and is at a depth of 500 m from the surface, the approximate amount of down-to-the-east displacement across the Canada de Cochiti fault zone. The conductive zone is not detectable in the Valles caldera where its continuity has been disturbed by faulting and the conductor is isolated on down-dropped fault blocks. Inversion results also show that a deep crustal conductor, dipping to the west, follows the pattern of projected crustal isotherms derived from borehole results. Therefore, the top of the prominent crustal conductor is assumed to be an isotherm. Just east of the Valles caldera, the deeper crustal conductor is nearly horizontal at a depth of 7 km. A similar conductor is observed deeper (~15 km) in the Rio Grande rift further to the east. A likely mechanism to explain this conductor that is clearly in the ductile portion of the crust is intergranular aqueous fluid flow controlled by the dihedral angle, the angle subtended by adjacent crystal faces. It is also apparent that the geoelectric structure of the crust and upper mantle east of the Valles caldera is influenced by the enhanced thermal effects of the Rio Grande rift. Three-dimensional (3-D) forward results generally reproduce the observed TE and TM mode split around period of 1 s in the apparent resistivity curves. These results suggest that the deeper, geoelectric expression of the western margin of the Rio Grande rift has remained constant for millions of years, while the surface expression of the western margin the rift has moved east. The thermal effects of volcanism have caused the midcrustal conductor to shallow, but seem to have no discernible effects on its continuity.
