LOW-TEMPERATURE THERMOCHRONOLOGY FROM LACCOLITH AUREOLES CONSTRAINS LATE CENOZOIC EXHUMATION IN THE NORTH-CENTRAL COLORADO PLATEAU

The timing and spatial pattern of erosion in Utah’s iconic canyon country remains relatively unconstrained compared to the region along the Colorado River downstream of Lee’s Ferry, Arizona, including the Grand Canyon, in part because most low-temperature thermochronometers in this region were not sufficiently reset during Mesozoic burial to clearly constrain subsequent exhumation. In the Henry, La Sal, and Abajo mountains, however, shallowly intruded Oligocene plutons (laccoliths) heated late Paleozoic and Mesozoic country rocks, locally resetting apatite fission track and (U-Th)/He ages so they only record Miocene-Quaternary cooling and exhumation. Sandstone samples from within 3 km of laccoliths in each mountain range yield single-grain apatite He dates that vary by a factor of four with the effective uranium concentration (eU). Together with stratigraphic constraints, the timing of laccolith intrusion determined by zircon U/Pb geochronology, and the extent of resetting temperatures >100˚C by apatite fission track analysis, we interpret these He date-eU patterns using the radiation damage and annealing model in HeFTy. All data require prolonged residence in the apatite He partial retention zone (40-60˚C, 2-3 km depth), from 25 Ma until at least 10 Ma. In the Abajo Mountains, apatite grains from the upper Permian Cutler Group have minimum and maximum He dates of 11.4 ± 1.7 Ma (eU = 3 ppm) and 25.7 ± 0.4 Ma (eU = 70 ppm); best-fit solutions suggest onset of rapid cooling to surface temperatures in the Pliocene, perhaps as recently as 2-3 Ma. Apatite date maxima and minima from the Triassic Wingate sandstone in the Henry Mountains are younger (5.1 ± 0.4 Ma at 10 ppm eU and 21.1 ± 0.3 Ma at 86 ppm eU) and require a similar trend. In the La Sal Mountains, date patterns from the Jurassic Morrison Formation require cooling since the mid-Miocene and are consistent with acceleration of cooling in the Pliocene. These results agree with previous interpretations of borehole samples from the Canyonlands area and bedrock samples from Uncompahgre Plateau, and together suggest that the ca. 6 Ma integration of the Colorado River system through the western Grand Canyon increased erosion rates at nearly the same time across the north-central Colorado Plateau. Additionally, Pleistocene erosion could be significant in these mountain ranges.