ELEVATED CENOZOIC GEOTHERMAL GRADIENTS AND LATER POST-6 MA INCISION OF THE UNCOMPAHGRE PLATEAU AND UNAWEEP CANYON (WESTERN COLORADO) REVEALED BY LOW TEMPERATURE THERMOCHRONOLOGY

We present new apatite and zircon fission track (AFT and ZFT) and (U-Th)/He (AHe and ZHe) thermochronologic data from the Unaweep Canyon area of the Uncompahgre Plateau to constrain the Mesozoic and Cenozoic thermal and erosion history of this enigmatic canyon. Samples were collected from Precambrian basement rocks at the base of the canyon, from the canyon rim immediately below the Precambrian/Mesozoic nonconformity, and from Cutler Formation sandstones of the Paradox basin at the western mouth of the canyon. The AFT age from below the nonconformity at 2650 m is 25 Ma, with mean track length of 14.85 µm. These data require that this sample was heated to >120°C at around 25 Ma, then rapid cooled to <30°C within a few million years, then slowly cooled until the present. New mapping and other estimates of former Mesozoic overburden above this sample (1600-1900 m) imply an increase in the geothermal gradient to at least 65°C/km during the late Oligocene that is best explained by nearby La Sal Mountains magmatism of this age. Unreset Pennsylvanian-Permian Ancestral Rocky Mountain ZFT ages reveal this sample was not heated to above about 180°C since this time. AFT ages from the base of the canyon at 1450-1700 m are 20-25 Ma, but with significantly shorter mean track lengths of 13.3 to 13.8 µm. Inverse HeFTy time-temperature modeling of these data show these samples cooled from about 70°C to 40°C following late Oligocene heating, and then cooled rapidly to the surface after about 6 Ma, constraining the earliest time of onset of late Cenozoic Dolores River and Unaweep Canyon incision and/or paleocanyon exhumation. The difference in estimated paleotemperature of samples at the base and rim of Unaweep canyon at 6 Ma is about 30°C. Thus the elevation difference of these samples (1000 m) reveals the geothermal gradient had returned to a modern day value of 30°C/km by the late Miocene. Younger AHe ages from the base of the canyon (ca. 18 Ma) are consistent with the thermal histories derived from the AFT data, while ZHe ages from the same samples show radiation damage dependent partial resetting, supporting a significant heating event sometime since 32 Ma.