A GEOPHYSICAL MODEL OF THE ESPANOLA BASIN, RIO-GRANDE RIFT, NEW-MEXICO

A model of the subsurface structure of the eastern part of the Espanola Basin in the northern Rio Grande rift of New Mexico was constructed from geophysical data obtained since 1983 by the Summer of Applied Geophysical Experience (SAGE) field course. Approximately 742 new gravity observations, 1276 ground magnetic stations, 30 km of seismic refraction lines, 19 km of seismic reflection lines, 22 magnetotelluric stations, and several Schlumberger and dipole-dipole resistivity lines were established. Our studies provide new information on one boundary of a major continental rift and on the depositional and structural style of an extensional basin within the rift. Integration of these data sets into a single transect indicates that the Espanola Basin is asymmetrical with approximately 2 to 3 km of sediments and sedimentary rocks near the center, thinning eastward to the Precambrian outcrop of the flanking Sangre de Cristo uplift. Several minor faults with throws of less than 200 m were found, but no major eastern bounding fault was observed. Thus, the Espanola Basin could be an asymmetrical, west-dipping half-graben. However, major fault offset, down toward the basin axis, may occur within Precambrian rocks of the Sangre de Cristo uplift. In either case, the geometry of the basin does not agree well with current models for the structural evolution of continental rifts, which emphasize low-angle detachment faults which create asymmetrical, hinged half-grabens. These models predict that major shoulder uplift should occur adjacent to the side of the graben bounded by a listric master fault rather than adjacent to the hinged side. In contrast, for the Espanola Basin major uplift occurred adjacent to the eastern side, which could be the hinged side of the basin. A thick wedge of older sedimentary rocks with high P-wave velocity (4.4 km/s) and low electrical resistivity (5-OMEGA.m) was discovered under the younger Tertiary sediments and sedimentary rocks near the center of the basin. This wedge has maximum thickness of 1.2 km at the western end of the profile and thins eastward. The physical properties suggest this layer could be older Tertiary, or possibly a Mesozoic-Paleozoic, section of rocks. If the latter, it has potential economic importance because of the possible presence of a Creaceous section which is known to produce oil and gas in the Albuquerque Basin to the south and the San Luis Basin to the north. However, based on data from the Yates La Mesa no. 2 well, 10 km south of the transect, this wedge is likely middle Tertiary lacustrine deposits (NMOCD, 1986). The great thickness of lake deposits may represent a major lacustrine facies of the Eocene Galisteo and El Rito formations, exposed around the southern, southwestern, and northwestern margins of the basin. Magnetotelluric data suggest the crystalline basement underlying the central Espanola Basin may be more conductive than near the eastern margin. The entire Espanola Basin is also underlain by a highly conductive layer of about 1-OMEGA.m at a depth of 15 km. Both the shallow and deep low-resistivity zones may result from hot, saline fluids. Such fluids deep within the crust may reduce the shear strength of the crust significantly and concentrate crustal extension on the west side of the rift.