CONSTRAINTS ON AN ANCIENT GRAND CANYON AND THE TOPOGRAPHIC EVOLUTION OF THE SOUTHWESTERN COLORADO PLATEAU FROM THERMOCHRONOMETRY

The Grand Canyon is a >1500 m deep and > 440 km long gorge carved into the southwestern Colorado Plateau that comprises one of the best-recognized landscapes on Earth. The antiquity of the Grand Canyon is a particularly contentious aspect of the canyon’s origin. The prevailing view of the last few decades is that the canyon was carved after 6 Ma, when diagnostic Colorado River detritus first appeared in Grand Wash Trough where the river now exits the plateau (e.g., Karlstrom et al., 2007). There is consensus that these deposits record when the Colorado River was integrated, but the young canyon model’s fundamental assumption is that river integration was coeval with canyon carving. This relationship, however, is not required by geologic observations: the canyon could be older with later exploitation by the modern river.

The notion of an ancient Grand Canyon has emerged repeatedly in the 150 yr since its discovery, and is supported by recent thermochronologic results. Our 70-90 Ma apatite (U-Th)/He dates (AHe) and ⁴He/³He data from the western Grand Canyon record substantial 70-80 Ma cooling, provide no evidence for strong post-6 Ma cooling predicted by the young canyon model, and thus supports substantial excavation of the western canyon by 70 Ma (Flowers and Farley, 2012). Recent publication of additional western canyon AHe data that are 70-85 Ma (Lee et al., 2013) now yield a total of 29 reproducible analyses from six samples and two independent labs, thus further strengthening the case for an ancient canyon (Flowers and Farley, 2013). Uncontroversial evidence for a paleocanyon system of which an ancient Grand Canyon would logically have been part is provided by the Milkweed, Hindu, and Peach Springs paleocanyons that are widely accepted to be Late Cretaceous-early Tertiary in age, have >1200 m of relief, and intersect the Grand Canyon (e.g., Young, 2001). The more youthful appearance of the modern Grand Canyon may have been generated by post-6 Ma canyon deepening and widening after Colorado River integration. Additional regional AHe data constrain the wider-scale patterns of southwestern plateau landscape evolution.