GEOPHYSICAL INSIGHTS INTO THE GEOMETRY AND STRUCTURE OF THE SOUTHERN SAN LUIS BASIN, NORTHERN NEW MEXICO

The San Luis Basin is the largest of the basins comprising the northern Rio Grande rift; the southern part of the basin underlies mainly Pliocene rocks of the Taos Plateau volcanic field of northern New Mexico. The fault-bounded basin includes thick accumulations of Santa Fe Group rift sediments and possibly significant amounts of pre-rift sedimentary rocks beneath the volcanic cover. Basin geometry and the nature and distribution of basin fill are studied using gravity, magnetic, and magnetotelluric (MT) methods. Gravity methods are relevant to establishing basin geometry because low-density sediments of the Santa Fe Group overlie relatively moderate- to high-density Precambrian and pre-rift sedimentary and volcanic 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 constrain the model using surface geology, limited well data, and magnetic source depth estimates. The results are compared to several 2D MT models across the area. MT data are useful for distinguishing electrical conductors, such as sediments and sedimentary rocks, from resistors, such as crystalline volcanic and Precambrian rocks.

Significant thicknesses (greater than 1 km) of low density and conductive rocks are imaged along the eastern and western margins of the basin, places where large accumulations of Santa Fe Group are expected. The southern part of the basin may be formed by two separate sub basins along each side of the Rio Grande, and models suggest that the Santa Fe Group is thinner (about 1.5 km) there than previously thought. Preliminary integration of gravity and MT results suggest that there may be thicker pre-rift sedimentary rocks under other portions of the Taos Plateau than previously thought, especially in the area immediately west of Timber and Brushy Mountains. Pre-rift rocks are interpreted to account for significant portions of the gravity lows that have previously been attributed to large thicknesses of Santa Fe Group, particularly near the Questa caldera, Taos, and the Embudo Fault. The southern basin boundary derived from the gravity inversion closely follows the apparent aeromagnetic expression of strike slip faulting near the mapped basin margin and Embudo Fault.