GEOPHYSICAL AND GEOLOGIC MAPPING OF THE NORTHERN TAOS PLATEAU, RIO GRANDE RIFT, NEW MEXICO

Tectonic structures within basins of the northern Rio Grande rift have historically been identified and studied through integration of geologic mapping with geophysical interpretation. Geologic mapping creates constraints that significantly reduce ambiguity and enable robust interpretation of geophysical data. Conversely, geophysical interpretations bring a third dimension (depth) to modeling that is not easily addressed by geologic mapping alone. In active continental rift environments, delineation of the temporal and spatial evolution of structural basins is often obscured by coeval sedimentation. Integration of geologic and geophysical methods is crucial for understanding rift-related basins and volcanic rocks.

The San Luis Basin is the northernmost of the major basins comprising the Rio Grande rift. The Taos Plateau of northern New Mexico is geographically coincident with the southern part of the basin. The plateau’s surface is dominated by Pliocene volcanic deposits of the Taos Plateau volcanic field (TPVF), obscuring faults and other buried structures. Recently acquired aeromagnetic, land gravity, and magnetotelluric datasets and the resulting preliminary interpretations are integrated with geologic mapping, resulting in new interpretations of rift evolution in the TPVF region. The temporal development of the syn-rift volcanic rocks can be studied by an approach that includes both aeromagnetic interpretations of rock magnetic polarities and geochronologic age estimates. This demonstrates that rocks of the TPVF erupted over the course of several magnetic field reversals. Gravity data show that the modern rift occupies a relatively narrow zone along the eastern margin of the TPVF, and that much of the TPVF region is underlain by pre- and early-rift rocks that are structurally high. A separate structure, an apparent narrow graben, lies along the western boundary of the TPVF. Preliminary magnetotelluric results suggest that sediments underlie parts of the TPVF previously interpreted to rest directly on Precambrian basement. Our results, combined with new geologic mapping, geochronologic data and fault interpretations, suggest a significantly more complex basin evolution history than previously recognized.