Oligocene Laccoliths on the Colorado Plateau: A Key to Understanding Cenozoic Rock Cooling and Exhumation

Abstract:

The Colorado Plateau is an exceptional region of high elevation, extensive canyonlands, and minor crustal deformation in the North American Cordillera. Numerous low-temperature thermochronology studies have attempted to constrain the timing and rate(s) at which this enigmatic Cenozoic landscape developed. However, such studies have struggled to robustly interpret the thermal histories of the Plateau’s rocks because most contain detrital crystals with diverse time-temperature histories that are partially reset, which confounds efforts to see clear thermochronologic signals of Cenozoic erosion. To overcome this challenge, we targeted sample collection from places on the Plateau where rock thermal histories were reset by Oligocene magmatism: the thermal aureoles of laccolith-cored Henry, La Sal, and Abajo mountains. There, shallow plutons (i.e., laccoliths) heated the late Paleozoic and Mesozoic sedimentary rocks ca. 26 Ma, locally resetting apatite fission track and (U-Th)/He ages so they record subsequent Miocene-Quaternary cooling. Sandstone samples from each mountain range yield single-crystal apatite He ages that vary with effective U concentration (eU). In these key samples, minimum He ages are ~5 Ma with eU <10 ppm, and maximum ages are ~25-20 Ma with eU >60 ppm. We model and interpret these age-eU patterns together with (1) stratigraphic constraints, (2) the timing of magmatic heating constrained by zircon U/Pb laccolith geochronology, and (3) the extent of resetting temperatures >100 ˚C by apatite fission track analysis. Results require prolonged sample residence in the apatite He partial retention zone (40-60 ˚C) from 25 to <10 Ma; best fit time-temperature solutions show onset of rapid cooling to surface temperatures in the Plio-Pleistocene. We interpret this as an erosional event that removed ~1.5 km at time-averaged erosion rates between 0.3 and >1 km/Myr in parts of the Henry and Abajo mountains. The Plio-Pleistocene was a time of change both regionally and globally, as Colorado River base level dropped with its final integration through the Grand Canyon ca. 5.3 Ma and global climate cooling possibly enhanced erosion rates on the Plateau’s high-relief mountain ranges. Ongoing work evaluates the relationship between the erosion of the laccolith-cored ranges and regional denudation.