New Mexico Geological Society Annual Spring Meeting — Abstracts

Forty-nine apatite fission-track (AFT) and two zircon fission-track ages were collected during a reconnaissance study of south-central New Mexico in order to investigate the cooling and tectonic history of uplifts associated with the southern Rio Grande rift. Mack et al. (1994) propose that the southern rift has been affected by four episodes of extension beginning at about 35 Ma. The main phases of faulting started in the early Oligocene, the late Oligocene, the middle Miocene, and the latest Miocene to early Pliocene, with each phase disrupting earlier rift basins, and in some cases, reversing the dip of the early rift half-grabens found in the vicinity of the southern Caballo Mountains. The timing of denudation derived from AFT data in the Caballo, Mud Springs, San Diego, and Dona Ana Mountains are consistent with the episodes of uplift and erosion preserved in the Oligocene to Miocene Hayner Ranch and Rincon Valley Formations in the southern Caballo Mountains.

Each mountain block studied in the southern rift has a unique history. AFT ages in the Proterozoic rocks on the east side of the San Andres Mountains record cooling of this mountain block at 21 to 22 Ma in response to the phase of extension that began in the late Oligocene. Younger AFT ages of 7 to 8 Ma related to the middle Miocene episode of extension are exposed on the upthrown side of highangle faults cutting the Proterozoic rocks. AFT ages from the eastern Organ Mountains are 10 to 17 Ma and the mean track lengths are long, while the ages on the west side are 20 to 29 Ma and the mean track lengths are shorter. A westward-tilted partial annealing zone for apatite that formed during protracted cooling of the Organ batholith in the late Oligocene to early Miocene is preserved in this mountain block. Rapid denudation (200 to 400 m/m.y.) and tilting of the range occurred in the middle Miocene. The base of an apatite partial annealing zone that formed during burial of southern New Mexico in the Mesozoic is preserved in the Sacramento Mountains. The earliest phase of extension is recorded in the AFT data from Proterozoic rocks exposed at the base of the Sacramento escarpnlent. Evidence for significant denudation related to Laramide deformation, or late Oligocene and middle Miocene extension is not observed in the AFT data from the Sacramento Mountains. AFT data derived from high elevation samples in Oligocene to Miocene intrusions (Capitan, Sierra Blanca, and Black Range) record rapid cooling of these shallowly emplaced plutons.

The AFT data from the northern and southern Rio Grande rift are similar in many respects. The trend of young AFT ages and greater denudation on footwall blocks adjacent to the master faults I controlling the geometry of the half-grabens that was observed in northern New Mexico is also found in southern New Mexico. Horst blocks in the rift that are bordered by normal faults on two or three sides invariably have young (< 12 Ma) AFT ages and high (>10°C/Ma) cooling rates. In the places where Mesozoic AFT ages are preserved, the estimated cooling rates are low (<2°C/Ma) in both the northern and southern rift. The primary difference in the cooling histories of mountain ranges in the northern and southern rift is the paucity of AFT ages that are older than 30 Ma in the southern rift. Part of this trend may be a function of sampling bias.

Zircon FT and K-Ar data are used to examine the distribution of late Mesozoic to Eocene volcanism in southern New Mexico. A previolisly unrecognized Cretaceous intrusion was identified southeast of Hillsboro using zircon FT dating. The porphyry from this intrusion was the source of volcanic clasts in the late Cretaceous to early Tertiary McRae Formation located in the northeastern Caballo Mountains. In addition, a zircon FT age of 49.6 ± 3.8 Ma for the rhyolite sill capping Salinas Peak at the north end of the San Andres Mountains, and K-Ar ages for the Orogrande, Cuchillo, and Tres Hermanos stocks are used to constrain the northern limit of 40 Ma volcanism in south-central New Mexico.