Structure of the Rio Grande rift in southern New Mexico and West Texas based on gravity interpretation

Abstract

The Rio Grande rift, which is marked by a positive heat-flow anomaly in southern New Mexico, has been the subject of a gravity study based on 4,500 stations which cover a strip across New Mexico. The area consists of a series of basins and intervening ranges formed during Miocene time. This basin-and-range structure is strongly reflected in the Bouguer gravity anomalies, which range from -125 mgal over unlifts to -190 mgal over basins. Lineaments in trends of gravity anomalies are oblique to the predominant north-south trend of the rift and suggest that, in detail, the crust broke upon fractures oblique to the large-scale north-south trend. Thicknesses of Cenozoic sediments, determined from gravity measurements, range from 2 to 3 km in basins. The gravity effect of sediments is removed by stripping, and a broad +30-mgal gravity anomaly is located over the rift. Regional and residual Bouguer gravity anomaly maps were constructed. The source of the 30-mgal gravity high is interpreted to ba a shallow slab of basalt or a deep upwarp of the mantle that results in crustal attenuation. The low-velocity zone may project up toward the base of the crust under the Basin and Range province. Experiments and the observedmore » fault pattern suggest an extensional origin for the Rio Grande rift fracture system.« less