COMBINED AFT, AHE, 4HE/3HE THERMOCHRONOLOGIC DATA INDICATE CARVING OF WESTERNMOST GRAND CANYON IN THE LAST 6 MA

Conflicting hypotheses for the timing of carving Grand Canyon involve either a 70 Ma (“old”) or < 6 Ma (“young”) Grand Canyon. We evaluate the controversial westernmost segment of Grand Canyon where the geologic data firmly favor the “young” Canyon model but thermochronologic data have been in conflict. This controversy is resolved via the first integration of three methods on the same sample: apatite helium bulk ages (AHe), ⁴He/³He diffusion profiles (⁴He/³He), and apatite fission track ages, lengths, and compositional data (AFT). Thermal histories generated by RDAAM in HeFTy that best predict observed data differ between data sets: AFT data are best predicted by T-t paths that cool from temperatures of 80-140°C during the Laramide and reach surface temperatures (20±10°C) after 20 Ma. AHe data are initially best predicted by T-t paths that cool rapidly from 75-120°C to surface temperatures during the Laramide. New ⁴He/³He data (assuming no or minor zonation) are best predicted by a tightly constrained thermal history that cools from 75°C to 60°C during the Laramide and resides there until 5 Ma, when it cools to surface temperatures. Thus AFT and ⁴He/³He data favor the “young” Canyon model and AHe data favor the “old” Canyon model, a discrepancy that is mimicked in the regional dataset and the overall debate. A resolution of all data is possible by lowering the rmr0 value in RDAAM inverse modeling of AHe data to represent higher He retentivity in these samples than assumed in previous studies. This is justified because Laramide peak burial temperature and duration were likely insufficient to completely anneal radiation damage that accumulated during long term, low-temperature residence since the Proterozoic. Thus, low eU values alone do not accurately reflect low closure temperatures since burial as predicted by RDAAM when burial conditions are not taken into account. We conclude that new AFT, AHe, and ⁴He/³He analysis of the same sample, new combined AFT/AHe modeling from four other locations, re-modeling of all existing data, and geologic data can only be reconciled by a “young” Westernmost Grand Canyon that was carved in the last 5-6 Ma. Westernmost Grand Canyon remains an excellent test bed for refining the limits of thermochronometric techniques and helium diffusion models in areas with extensive radiation damage of apatite.