Upper crustal structure of the southern Rio Grande rift: A composite record of rift and pre-rift tectonics

A P-wave seismic velocity model derived from analysis of data from a seismic refraction/reflection survey provides the first regional-scale view of the subsurface structure of the upper crust of the southern Rio Grande rift. The seismic survey was conducted along a 205-km-long east-west transect that crosses a portion of the Basin and Range province and the Rio Grande rift in southernmost New Mexico and west Texas. In the upper few kilometers of the crust, the model shows a series of low- and high-velocity zones that correlate with the mid-Tertiary to Holocene Basin and Range structure at the surface. Typically, basins are 10-20 km wide and similar to 1-3 km deep. Beneath the mid-Tertiary to Holocene structures are velocity anomalies interpreted to be the result of tectonic activity of Paleozoic to early Tertiary age. The geometry of high-velocity zones at 3-10 km depth in the eastern half of the model correlates well with Laramide block uplifts mapped at the surface and suggests that the thrusts that bound these blocks may sole into a regional detachment at 10-15 km depth. In the western half of the model, a low-velocity zone that reaches depths as great as 11 km has geometry highly suggestive of a deep basin. This previously unrecognized feature may represent a combination of great thicknesses of the Upper Jurassic to Lower Cretaceous rocks of the Chihuahua Trough and Bisbee Basin stacked above rocks of the older Paleozoic Pedregosa Basin. Alternatively, portions of this region may have been thickened by thrust stacking during the Laramide orogeny.