AVERAGE EROSION RATE OF THE COLORADO PLATEAU ESTIMATED FROM THERMOCHRONOLOGIC DATA

Over the last two decades the Colorado Plateau has been the object of many thermochronologic studies to determine its geologic history, which is not fully resolved yet. Here we present an estimate of an average long-term erosion rate of the Colorado Plateau from the Late Jurassic to the Early Miocene of ~30m/m.y. This estimate is based on a cross-calibration of previously published apatite fission-track, apatite, zircon and titanite (U-Th)/He data and new zircon fission-track ages from the same samples of the Gold Butte block, which is located on the western rim of the Colorado Plateau in SE Nevada. The estimate is also based on a 10ºC surface temperature, derived from paleobotanical information for the Colorado Plateau. The Gold Butte block is a ~18km thick tilted crustal section of Precambrian metamorphic and igneous basement rocks, similar to those exposed in the Grand Canyon. The Precambrian basement is unconformably overlain by Paleozoic sedimentary rocks. All samples were collected west of the unconformity between the Precambrian basement and the Paleozoic sedimentary cover units, with increasing paleodepth. The various thermochronological data of each sample show a characteristic age/depth profile with a shallow upper part, related to slow erosional cooling, and a steeper lower part, caused by rapid cooling, induced by tectonic exhumation. The shallow parts of the age/depth profiles of each thermochronometer are too thick to be interpreted as static PRZs/PAZs. Therefore we regard them as moving PRZs/PAZs, associated with slow erosional exhumation. For the given erosion rate we estimate the removal of about 4km of overlaying strata in the Gold Butte block area from the late Jurassic until the onset of Miocene Basin and Range extension and deposition of the Horse Spring Formation. Due to the abundance of regional unconformities in the Colorado Plateau it is not possible to point to a specific location, where this whole section is still preserved, but parts of it are exposed north of the Gold Butte block. These results are comparable with other thermorchronologic data from surrounding areas.