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

Interpretation of gravity, magnetotelluric (MT), and aeromagnetic data coupled with geologic constraints within the southern San Luis basin reveals a shallower and narrower basin with a more complex structural history than indicated by previous modeling efforts. Spatial variations of low-density basin fill thickness are estimated primarily using a 3D gravity inversion method that improves on previous modeling efforts by separating the effects of the low-density basin fill from the effects of pre-rift rocks. Five distinct subbasins are recognized within the broader southern San Luis Basin. The oldest and shallowest (~400 m-deep), north-trending Las Mesitas graben formed along the northwestern basin margin coeval to the transition between Oligocene volcanism and the onset of rifting. The ~2 km-deep Tres Orejas subbasin formed in the southwestern portion of the basin, coeval to development of the southern-bounding Embudo fault zone and a hypothesized fault zone along its western margin. The Sunshine Valley-Costilla Plain (~1.2 km deep), Questa (~0.8 km deep), and Taos (~1.8 km deep) subbasins occupy the eastern part of the southern San Luis Basin and are bound on the east and west by the west-down, active southern Sangre de Cristo fault zone and east-dipping Gorge fault zone, respectively. These were the dominant structures controlling post-20 Ma subbasin development. Tectonic activity within all subbasins primarily culminated with emplacement of the Plio-early Pleistocene Taos Plateau volcanic field, and rift-related subsidence became more narrowly centered on the eastern margin of the basin, controlled mainly by the linked southern Sangre de Cristo and Embudo fault zones.