STRUCTURE AND VOLCANIC GEOLOGY OF THE RIO GRANDE RIFT IN THE SOUTHERN SAN LUIS BASIN, NORTHERN NEW MEXICO – INSIGHTS FROM GEOPHYSICS

The San Luis Basin is the largest of the basins comprising the northern Rio Grande rift; the southern portion underlies mainly Pliocene rocks of the Taos Plateau volcanic field of northern New Mexico. The fault-bound basin includes thick accumulations of Santa Fe Group rift sediments and syn-rift volcanic deposits. Basin geometry, bounding structures, and the nature and distribution of basin fill are studied using ground-based gravity and aeromagnetic 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 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 independently constrain the model using surficial geology, magnetotelluric soundings, and limited well data. The model suggests that Santa Fe Group sediments reach a maximum thickness of about 1.5 km and occur in a 10-15 km wide zone along the eastern and southeastern portions of the basin, bounded on the east by the Sangre de Cristo fault system. 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. Other parts of the basin contain lesser accumulations (not exceeding 1 km) of Santa Fe Group sediments. An exception is a sub basin along the western basin boundary in the northeast Tusas Mountains, where basin depths may exceed 1.5 km. These results show that subsidence related to the Rio Grande rift is narrower and more structurally complex than previously thought. Aeromagnetic anomalies mainly reflect the burial depth and magnetization variations of relatively shallow syn-rift volcanic rocks. Prominent lineaments sourced within volcanic rocks are present along northwest and northeast trends, and may be related to faulting or boundaries within the volcanic section. Large volcanoes have unusual aeromagnetic expressions, with lows thought to be due to low magnetization over their central zones. Integration of gravity and aeromagnetic data with geologic constraints aids in the construction of a 3D view of Rio Grande rift rocks in the southern San Luis Basin.