Faults that displace Quaternary units can be observed in scarps in Trans-Pecos Texas and are restricted to two north-trending zones in contrast to late Tertiary faults that cover the region and strike north, northwest, and west. The western zone of Quaternary faults, near El Paso, is usually included as the southern part of the Rio Grande rift. The eastern zone of Quaternary faults extends for 300 km from southern New Mexico along the Salt Basin graben through Van Horn, Texas; its probable extensions and subparallel associates extend southward to Presidio, Texas. This belt of faults is parallel to the Rio Grande rift zone and should be considered a southeast extension of that zone.
These fault zones die out southward into the edge of the Chihuahua tectonic belt, a region underlain by a thick Mesozoic carbonate and clastic section that in turn rests on a thick layer of evaporites. The evaporite zone may mask Cenozoic normal faulting and thus may define a zone of no data rather than a southern limit of basin-and-range or Rio Grande graben tectonics.
All Quaternary and most Tertiary faults trend parallel to pre-existing structures. The map pattern of Quaternary scarps suggest a maximum extension oriented about S80°W; however, first-motion studies of the 1931 Valentine, Texas earthquake show a maximum elongation direction of S50° to 55° W. This difference may be unique to this one earthquake or may be due to the pre-existing lines of weakness that control the location of presently active faults.