Gravity and flexure models of the San Luis, Albuquerque, and Tularosa basins in the Rio Grande rift, New Mexico, and southern Colorado

Abstract

—The Rio Grande rift is a Cenozoic continental rift zone that trends south from central Colorado through New Mexico and merges with a broader extensional zone in west Texas and Chihuahua, Mexico. The net upper crustal extension increases southward along the rift, accompanied by a southward-widening of the rift from a narrow zone encompassing a single basin in the north to a wide zone of multiple, laterally adjacent basins to the south. In this study we model the isostatic responses of the lithosphere to extension along the Rio Grande rift, and compare and contrast this response in the San Luis and Albuquerque basins in the narrow part of rift and the Tularosa basin in the wider region of the rift. Our flexural isostatic models are based on a joint-inversion of gravity and deflection data along the footwalls of major basin-bounding faults. The main difficulty in constraining isostatic response to extension in the Rio Grande rift is a lack of constraints on footwall deflection. Our approach is to use topography as a proxy for flexural deflection, and then to compare the estimated end-loads required for plate flexure in the footwall with the end-loads estimated from hanging wall and basin geometry. Using this approach we determine the extent to which flexural isostasy could have generated elevated rift-flanks, and how this process might change along strike. Our results indicate that flexural responses to extension are consistent with rock and surface uplift in the footwalls of the Albuquerque and the Tularosa basins in the central and southern Rio Grande rift. In the San Luis basin, however, although a flexural curve may adequately fit footwall topography, the plate-end loads required for flexure are inconsistent with those inferred from the hanging wall and basin geometries. Additionally, flexure-based estimates of crustal extension agree with geologic estimates in the Albuquerque and Tularosa basins, but not in the San Luis basin. We infer therefore, that rift-flank uplift due to flexure is likely only in the central and southern parts of the Rio Grande rift; to the north, we infer an additional (perhaps pre-rift) contribution to rift-flank topography.