LOW-TEMPERATURE THERMOCHRONOLOGIC CONSTRAINTS ON THE TERTIARY COOLING AND UNROOFING HISTORY OF THE SOUTHERN SANGRE DE CRISTO RANGE, NEW MEXICO

Understanding the timing of unroofing in the southern Rocky Mountains-Rio Grande Rift is key to differentiating between hypotheses proposed to explain high elevations in the region today. Low-temperature thermochronology in the southern Sangre de Cristo Range places constraints on the unroofing history of part of this region from Laramide to late Tertiary time. The southern Sangres separate the Española Basin of the Rio Grande Rift from the High Plains of northern New Mexico. The range is bisected by the Pecos-Picuris fault and has experienced different cooling histories in its eastern and western blocks. In the western Santa Fe Range, previously published AFT vertical transect data show an age-elevation relationship and vary from 74 to 55 Ma. New AHe dates for a subset of these samples are the same age or younger than corresponding AFT dates, and are similarly correlated with elevation. Both systems indicate cooling and rapid unroofing through the shallow crust during the early Tertiary, with the AHe system extending this history to lower temperatures. In addition, the preservation of these AHe dates limits the amount of mid-Tertiary subsidence and burial experienced by the Santa Fe Range during early rifting and basin formation.

The Pecos River valley is located in the center of the range near the Pecos-Picuris fault and preserves previously published mid-Tertiary AFT dates ranging from 44 to 28 Ma that are not correlated with elevation. New AHe dates for a subset of these samples show dispersion correlated with eU concentration, suggesting the presence of a preserved He partial retention zone that was established between the end of the Laramide orogeny and the beginning of late Tertiary exhumation. The late Tertiary episode is preserved in the uplift east of the Pecos-Picuris fault, where AFT dates from 34 to 12 Ma are correlated with elevation in some drainages. AHe dates also extend this history to lower temperatures, and with AFT dates indicate rapid cooling as recently as 14-12 Ma for samples at the lowest elevations. Thus, low-temperature thermochronometry records phases of unroofing associated with contractional and extensional deformation in early and late Tertiary times. Additional work may reveal the more precise timing and magnitude of a cryptic mid-Tertiary cooling signal in this region.