3D GRAVITY MODELING OF THE SAN LUIS BASIN, NORTHERN RIO GRANDE RIFT, COLORADO AND NEW MEXICO

Gravity methods have critical importance for understanding basin geometry and structure of the San Luis Basin of Colorado and New Mexico, the largest of the basins comprising the northern Rio Grande rift. The basin includes thick accumulations of Santa Fe Group rift sediments, syn-rift volcanic deposits, and basin-bounding and intrabasin faults, both active and inactive. Gravity methods are useful for mapping basin geometry in the San Luis Basin because the low-density sediments of the Santa Fe Group overlie relatively moderate- to high-density Precambrian or pre-rift Tertiary volcanic and sedimentary rocks. We use a three-dimensional gravity inversion of the entire basin to estimate Santa Fe Group thickness. The inversion accounts for variable densities of the pre-rift rocks and is constrained by independent data, including geologic mapping, seismic interpretations, and limited well data. A density-depth function based on density logs from the central Rio Grande rift is used to model the increasing density with depth of the Santa Fe Group. 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. Santa Fe Group sediments in the Taos region near the southern basin margin are estimated to be 1.5 km thick, half the thickness estimated by previous gravity studies. Shallower parts of the basin have less than 1 km of Santa Fe Group sediments, including parts of the western margin of the San Luis Basin. Small 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 San Luis Basin segment of the Rio Grande rift is narrower, reflecting lesser cumulative extension, and more structurally complex than previously modeled.