New Mexico Geological Society Annual Spring Meeting — Abstracts

Low temperature thermochronology, which includes fission-track and (U-Th)/He analysis of the mineral apatite, is a powerful tool for constraining the cooling history of rocks between 40 and 110°C, corresponding to crustal depths <4 km. Most of the low temperature thermochronology data for New Mexico are apatite fission-track (AFT) age and track length results from elevation traverses through mountain ranges and from drillholes on the High Plains and in the San Juan Basin. A few (U-Th)/He dates on apatite (AHe) have been determined for the Sandia Mountains and the High Plains.

The combined data sets record at least four pulses of cooling related to tectonically driven exhumation, changes in heat flow, and river drainage integration. The oldest episode of cooling is related to 50 to 75 Ma Laramide deformation and is recorded by AFT data from the Santa Fe Range, northern Sierra Nacimiento, Zuni Mountains, and Los Pinos Mountains. Middle Cenozoic cooling that began 25 to 30 Ma related to regional scale changes in mantle density and heat flow in the vicinity of the San Juan and Mogollon-Datil volcanic fields are recorded by AFT and AHe data on the High Plains. Miocene (<25 Ma) cooling associated with uplift and erosion of rift flank uplifts on the margins of the Rio Grande rift is observed in many mountain ranges, including the Sandia, Sierra Ladron, Caballo, Mud Spring, San Andres, and Sacramento mountains and the Black Range.

AFT and AHe data from the San Juan Basin indicate that the integration of the San Juan River with the Colorado River system was important in controlling the latter stages of cooling of this significant oilproducing province.

At times, unexpected low-temperature thermochronology results can lead to surprising discoveries. For example, Oligocene AFT cooling ages in the Pecos River valley of the Sangre de Cristo Mountains have been useful in mapping out a previously unrecognized, largely unexhumed Oligocene pluton. In addition, Miocene cooling ages from clasts in the Eocene Baca Formation were key in defining the extent of a Miocene fossil hydrothermal system east of Socorro.