OLIGOCENE ROCK COOLING OF THE NORTH-CENTRAL COLORADO PLATEAU REGION: EROSION OR A VARIABLE GEOTHERMAL GRADIENT?

Apatite thermochronology studies of the Colorado Plateau broadly document three periods of rock cooling during the Cenozoic: Paleogene, middle Cenozoic (Oligocene), and latest Miocene to Recent. Whereas early and late Cenozoic rock cooling clearly documents significant erosional responses to Laramide shortening and the final integration of the Colorado River, respectively, we argue that regional Oligocene cooling of the north-central Colorado Plateau cannot be unambiguously attributed to rock exhumation. Here, we present new apatite (U-Th-Sm)/He results from Paleozoic-Mesozoic sandstones sampled along the Colorado River corridor between Lees Ferry, Arizona and Hite, Utah far (>20 km) from the Oligocene Henry, La Sal, and Abajo intrusive complexes. Though most of these results suffer from age variability largely inappropriate for detailed thermal modeling, Oligocene cooling ages are abundant. Several key samples from the Triassic and Permian rocks along the main stem of the Colorado River have interpretable positive-slope apatite He age-eU patterns that document Oligocene and late Miocene-Recent cooling, with ages as young as ~5 Ma at eU < 10 ppm and as old as ~20 Ma at eU > 50 ppm. Using a simple 1D thermal model and the RDAAM in HeFTy, we demonstrate that the Oligocene cooling of these samples can be attributed to either (1) ~1 km of erosion between ca. 25 and 15 Ma, as previous workers suggested, or (2) relaxation of a geothermal gradient transiently doubled by local mid-crustal magmatism associated with the vigorous magmatic flare-up that swept through the southwestern USA ca. 40-25 Ma. We find the latter a compelling driver of this rock cooling, for it offers a simple explanation for its regional distribution; does not require an Oligocene river system capable of transporting significant volumes of sediment out of the region; and is consistent with the Oligocene tectonics of the southwestern USA. Because these data cannot definitively distinguish between erosional and magmatic cooling in this case, we suggest that Oligocene rock cooling documented by thermochronology in the north-central Colorado Plateau region be referred to as the Oligocene Cooling Event and not be solely attributed to rock exhumation without additional evidence.