GRAVITY INVESTIGATION OF THE THREE-DIMENSIONAL GEOMETRY OF THE SAN LUIS BASIN, NORTHERN RIO GRANDE RIFT – PRELIMINARY RESULTS

The San Luis Basin of Colorado and New Mexico, the largest of the basins comprising the northern Rio Grande rift, is a natural laboratory where ground-based gravity methods have critical importance for understanding basin geometry and structure. The basin includes thick accumulations of rift sediments called the Santa Fe Group, syn-rift volcanic flows, and active faults. Gravity methods are useful for mapping basin geometry in the San Luis Basin because the low-density sediments of the Santa Fe Group variably overlie relatively moderate- to high-density Precambrian or pre-rift Tertiary volcanic and sedimentary rocks. We use a three-dimensional gravity inversion to estimate Santa Fe Group thickness that accounts for variable densities of the pre-rift rocks and is constrained by independent information, such as seismic interpretations, limited well data, and magnetotelluric soundings. A density-depth function based on density logs from the central Rio Grande rift is used to model the increase in density with depth of the Santa Fe Group, with modifications made to account for variable thicknesses of syn-rift volcanic rocks. The general pattern revealed is that the greatest thicknesses of Santa Fe Group sediments tend to occur in a 10-30 km wide zone along the eastern portion of the basin, bounded on the east by the Sangre de Cristo fault system. Preliminary thickness estimates for this zone include more than 6 km of Santa Fe Group sediments in the Baca graben in the northern part of the basin and up to 2 km under the structurally complex Costilla Plains-Sanchez graben-Sunshine Valley region in the basin center. Estimated 1.5 km thicknesses for Santa Fe Group sediments in the Taos region near the southern basin margin are half the values commonly cited for this area based on previous gravity studies, an enigmatic result that requires more detailed study. Other portions of the basin have smaller accumulations (generally not exceeding 1 km) of Santa Fe Group sediments, including the region along the western margin of the San Luis Basin. Small apparent structural basins in the Tusas Mountains of New Mexico are revealed along the same north-south structural trend as the Monte Vista graben in Colorado. These results show that the Rio Grande rift is narrower and more structurally complex in the San Luis Basin than previously modeled.