THERMOCHRONOMETRIC CONSTRAINTS ON TERTIARY EROSIONAL EXHUMATION AND INCISION OF THE COLORADO PLATEAU – A PROGRESS REPORT

Most models addressing the origin of the Colorado Plateau landscape tend to focus on how and when this broad region, which was at sea level in late Cretaceous time, came to stand at a modern elevation of ~1.9 km with relatively little structural deformation. Unfortunately, the multiple conceptual models that have been proposed to account for the uplift and incision of the Colorado Plateau are difficult to test because few geologic constraints are preserved in the rock record on or around the Plateau. Understanding the relationship of rock uplift and denudation/incision should be crucial for constraining the paleoelevation history and the long-term landscape evolution of the central Colorado Plateau. New and published thermochronological results from southern Utah provide important constraints on the timing and magnitude of post-Laramide Tertiary denudation and incision on the central Colorado Plateau. Apatite (U-Th)/He ages generally correlate with elevation and show remarkable regional consistency. These data were collected at a distance from the major canyons and thus should not record cooling related to river incision, but rather mean exhumation. The data are difficult to evaluate quantitatively due to the limited vertical exposure window (<400m on basement-cored uplifts), but show that the samples resided within a HePRZ for at least 30 Myr, between ~45 and ~10 Ma. The (U-Th)/He ages indicate that, subsequent to post-Laramide reburial, the cental Colorado Plateau underwent uniform, protracted, early to middle Tertiary erosional exhumation, followed by accelerated erosional exhumation <10 Ma. Furthermore, these data limit the maximum magnitude of late Neogene exhumation to ~1.5 –2.8 km; an estimate that is consistent with results of ~1.4 –1.8 km based on published geomorphic reconstructions. The current topographic relief outside major river drainages or volcanic centers restricts vertical sample arrays to <<1 km and thus limits the use of surface sample for thermochronology. To alleviate this problem a large-scale collaborative thermochronometric study is currently being undertaken on core samples from drillholes with the goal to more fully elucidate the Cenozoic exhumation history of the entire Colorado Plateau in a quantitative fashion.