A SHALLOW RIFT BASIN SEGMENTED IN SPACE AND TIME: THE SOUTHERN SAN LUIS BASIN, RIO GRANDE RIFT, NORTHERN NEW MEXICO

Interpretation of gravity, magnetotelluric, and aeromagnetic data in conjunction with geologic constraints reveals details of basin geometry, thickness, and spatiotemporal evolution of the southern San Luis Basin, one of the major basins of the northern Rio Grande rift. Spatial variations of low-density basin-fill thickness are estimated primarily using a 3D gravity inversion method that separates the effects of the low-density basin fill from the effects of pre-rift rocks. The results show a significantly narrower and shallower basin, that is more complex in the subsurface, than indicated by previous modeling efforts. Five distinct subbasins are recognized within the southern San Luis Basin. The oldest and shallowest subbasin is the Las Mesitas graben along the northwestern basin margin, formed during the Oligocene transition from Southern Rocky Mountain volcanic field magmatism to rifting. Sediments are estimated to reach a maximum thickness of ~400 m. Other subbasins that likely initially developed during the Miocene are the dominant tectonic features. This includes the Tres Orejas subbasin, which formed in the southwestern portion of the basin by the Embudo fault zone and a hypothesized fault zone along its western margin. This subbasin reaches a maximum thickness of ~2 km. The Sunshine Valley, Questa, and Taos subbasins occupy the eastern part of the southern San Luis Basin. The southern Sangre de Cristo fault zone controlled their development after ~20 Ma. The east-down Gorge fault zone controlled much of the western margins of these eastern subbasins, although the distinction between the Taos and Tres Orejas subbasins is not well defined. Maximum low-density basin-fill thicknesses are estimated to be 1.2 km for the Sunshine Valley subbasin, 800 m for the Questa subbasin, and 1.8 km for the Taos subbasin. Subbasin-forming tectonic activity along the Gorge fault zone and within the Tres Orejas subbasin ceased by the end of the development of the largely Pliocene Taos Plateau volcanic field. Subsequently, rift-related subsidence became more narrowly centered on the eastern margin of the basin, controlled mainly by the linked Embudo and southern Sangre de Cristo fault zones.