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

Gravity methods are critical to understanding the geometry and structure of the San Luis Basin, the largest basin of the northern Rio Grande rift. The basin includes thick accumulations of Santa Fe Group sediments and syn-rift volcanic rocks, and basin-bounding and intrabasin faults. Gravity methods are useful for mapping basin geometry because the low-density sediments of the Santa Fe Group overlie relatively moderate- to high-density Proterozoic or pre-rift Tertiary volcanic and sedimentary rocks. We applied a three-dimensional gravity inversion 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 and associated sub-surface inference, interpretations of magnetotelluric data, and boreholes. A density-depth function developed from density logs in the central Rio Grande rift was used to model the increasing density with depth of the Santa Fe Group. In general, the greatest thicknesses of Santa Fe Group sediments exist in a 10-30 km wide zone along the eastern basin margin, adjacent to the basin-bounding Sangre de Cristo normal fault system. Preliminary sediment thickness estimates for this zone include more than 6 km in the Baca graben in the northern part of the basin, as much as 2 km under the structurally complex Costilla Plains-Sanchez graben-Sunshine Valley region in the basin center, and about 1.5 km in the Taos region near the southern basin margin. Shallower parts of the basin are estimated to contain thicknesses of less than 200 m of Santa Fe Group sediments, including most of the area of the Taos Plateau. 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. The model shows that the San Luis Basin is a series of subbasins separated by structural highs, in a more complex segmented pattern than suggested by the physiography of the basin surface. Moreover, the structural San Luis Basin is narrower, in many places shallower, and more structurally complex than previously modeled. The results suggest that the basin may therefore have experienced less cumulative, east-west extension than previously reported.