LARAMIDE DEFORMATION IN THE FRA CRISTOBAL RANGE, SOUTHERN NEW MEXICO

The Fra Cristobal Range is a fault-block uplift of the Basin and Range that adjoins the eastern edge of the Rio Grande rift north of Truth or Consequences in south-central New Mexico. Exposed in the range is part of a belt of east-verging Laramide faults and folds extending more than 200 km along the east side of the Rio Grande. We recently completed geologic mapping of the entire range, following up on previous mapping efforts that are largely unpublished. In classic Laramide foreland style, reverse faults in granitic Precambrian basement pass upward to partly overturned fault-propagation and fault-bend folds in Paleozoic and Mesozoic sedimentary cover. Small to medium-scale detached thrusts have developed locally east of the major basement faults. Some Laramide reverse faults were reactivated by Cenozoic normal faulting associated with the Rio Grande rift, creating unusual structural geometry.

In the Fra Cristobals, we find no evidence for the large-scale (20 to 30 km) right-lateral wrench faulting postulated by some previous authors. The main Laramide reverse fault is not even continuous; in places the Precambrian is folded but not faulted. Rather than being linear like documented wrench faults, the main fault makes sharp bends without the extensional or compressional duplexes that would be mandated by strike slip. Exposed fault surfaces yield indicators of primarily dip slip. En echelon folds and faults characteristic of documented wrench faults do not exist. A postulated wrench fault in Massacre Gap in the southern part of the range is not a fault, but a diapiric anticline confined to thick evaporites in the lower Permian Yeso Group.