The Ancestral Rocky Mountains (ARM) represent an intraplate deformational event that resulted in a series of Precambrian-cored basement uplifts with adjacent basins that accumulated Pennsylvanian to early Permian strata. Tectonic models for the event are debated largely because of the lack of robust age control in the basin fill. In New Mexico, the ARM event resulted in a series of basins with some of the best biostratigraphic records across the orogenic province. This article utilizes published biostratigraphic and lithostratigraphic data to (1) reconcile the onset of subsidence by first accumulation of Pennsylvanian strata, (2) establish a period of peak subsidence estimated by maximum accumulation rate, (3) correlate estimated peak subsidence with the first appearance of arkose derived from the adjacent denuded Precambrian-cored block, and (4) demarcate synorogenic strata from Permian strata that are postorogenic. Results demonstrate that within New Mexico ARM basins, (1) onset was relatively synchronous, predominantly beginning in early Atokan time; (2) peak subsidence, while potentially younging southward, was relatively coeval in the Middle to Late Pennsylvanian; (3) first occurrence of arkose either predates the period of peak subsidence or is coeval with peak subsidence; and (4) early Permian strata across the study area onlap preexisting faults and folds, and/or form a buttress unconformity with Precambrian basement.

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 ~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.