LARAMIDE EXHUMATION HISTORY OF THE ZUNI MOUNTAINS, WEST-CENTRAL NEW MEXICO

The Laramide cooling history of the Zuni Mountains on the southeastern Colorado Plateau is evaluated using apatite fission-track (AFT) thermochronology. AFT ages for Proterozoic to Permian rocks exposed in the southeastern Zuni Mountains are 40 to 70 Ma and the mean track lengths are 13.0 to 14.1 µm, indicative of moderate to rapid cooling rates. The AFT ages increase toward the north and are oldest in the Permian Yeso and Glorieta formations on the northern dip slope of the Zuni antiform. The base of the late Mesozoic apatite partial annealing zone (PAZ) preserved elsewhere on and around the Colorado Plateau (e.g., Grand Canyon, northern Sierra Nacimiento) is not preserved in the southeastern Zuni Mountains. The base of the apatite PAZ, which roughly corresponds to the 110°C paleoisotherm, is present in the crystalline basement rocks of the northern Sierra Nacimiento, but is somewhere above the Permian Glorieta Sandstone in the Zuni Mountains. Thus the Proterozoic basement in the Zuni Mountains was hotter and likely buried more deeply than the basement rocks of the Sierra Nacimiento during late Cretaceous time. Consequently, the Zuni Mountains seem to have experienced greater Laramide exhumation compared to the northern Sierra Nacimiento. This observation is at odds with the modern topographic expression of these two ranges; the Sierra Nacimiento is a pronounced topographic feature and the Zuni Mountains are rather subtle. The difference in modern topographic expression is likely related to proximity to the Rio Grande rift and the integration of drainages along the margins of the southeastern Colorado Plateau. Late Cenozoic volcanism along the Jemez Lineament and Oligocene to Miocene volcanism in the Mogollon-Datil volcanic field to the south appear to have had little influence on the cooling history of surface rocks exposed in the Zuni Mountains.