QUANTIFYING THE EVOLUTION OF GRAND CANYON TOPOGRAPHY WITH THERMOCHRONOLOGY AND NUMERICAL MODELING

This study revisits a long-standing debate regarding the Cenozoic erosion history of the proto-Colorado River and Grand Canyon, Arizona. When and how the upper Colorado River established its course across Laramide-age uplifts has been difficult to ascertain due to a lack of early Cenozoic drainage system indicators. End-member scenarios predict the presence (i.e. antecedence) or lack (i.e. superposition) of a Laramide-age paleocanyon. We present new low-temperature thermochronometer data and 3D thermo-kinematic erosion and age prediction modeling to quantify the topographic history of the eastern Grand Canyon. Apatite (U-Th)/He data was collected from two vertical transects along the North and South Kaibab trails. The numerical modeling predicts 3D and transient thermal field and thermochronometer cooling ages for sample locations. Model inputs include user defined erosion rates and histories, thermophysical rock properties, and basal crust and surface temperatures. Predicted and observed cooling ages were compared to test between the end-member models for canyon evolution. AHe ages from the two vertical transects range between 60 and 35 Ma and provide new constraints on the exhumation history of Mesozoic overburden from the Kaibab Uplift during the middle Cenozoic. Over 100 model simulations were used to explore a range of input parameters for the two end-member topographic evolution scenarios and the results are as follows: (1) Laramide-age canyon models produce predicted ages within the 2σ age uncertainty of observed ages and indicate an erosion rate of ~0.3mm/yr that abruptly changes to 0.03mm/yr at 60Ma. Further changes in erosion rates throughout the Neogene are unnecessary to provide the best fit. (2) Results for middle Cenozoic erosion without the presence of significant paleo-relief produces predicted ages that fall within 1σ of observed ages and indicates a late Cretaceous to late Eocene erosion rate of 0.07mm/yr, followed by an erosional hiatus during the late Oligocene to middle Miocene. Penetration of the Kaibab surface by ~30Ma is required to fit lower elevation samples and indicates a change to a more incisive erosional style. Both best-fit results suggest the presence of a paleo-canyon on the Kaibab Uplift along the axis of the modern Grand Canyon that predates Miocene drainage reversal.