Age of Carving the Westernmost Grand Canyon: Conflicts and Potential Resolutions that Reconcile Geologic and Thermochronologic Data

Abstract:

Conflicting models for the timing of carving Grand Canyon, especially the westernmost Grand Canyon, involve either a 70 Ma segment of Grand Canyon or a segment that is < 5-6 Ma. Geologic data such as the late Miocene-Pliocene Muddy Creek constraint, the north-derived Paleogene Hindu fanglomerate, and the 19 Ma Separation Point basalt on the south rim favor the young model. However, thermochronologic data do not resolve the controversy. (U-Th)/He (AHe) data combined with 4He/3He modeling of a sample near Separation Canyon suggest river-level rocks cooled from ~100 to 30ºC at 70 Ma. Alternatively, apatite fission track (AFT) combined with AHe data show variable cooling paths in different locations, with some samples cooling steadily since 70 Ma and others remaining at ~40-80ºC until 5-6 Ma. Either model could be compatible with geologic data that show Laramide (90-70 Ma) cooling resulted from northward stripping of the Hualapai Plateau. Variable cooling sample-to-sample is also geologically plausible. This study area offers an important test-bed for interpreting the sensitivity of thermochronologic data in areas of slow cooling at relatively shallow (~ 1 km) burial depths. Recent 4He/3He modeling of Separation Canyon samples suggests a period of 90-70 Ma Laramide cooling, 70-10 Ma post-Laramide residence at ~40-60 ºC, and cooling to surface temperatures at 5-6 Ma. New HeFTy modeling of all available data was done considering the Precambrian age and cooling histories of the apatite, surface residence in the Cambrian and Devonian, and burial to 40-140 ºC during the Laramide. Our models indicate that most grains underwent substantial pre-Laramide radiation damage, and that peak Laramide burial and associated temperatures may not have been high enough to completely reset the AHe age and anneal lattice damage. Our overall conclusion is that published thermochronologic constraints are not yet able to fully resolve the “old” versus “young” canyon models because most AHe ages are old (> 50 Ma), the Precambrian apatite crystals have variable, often high eU values and complex radiation damage due to insufficient annealing as a result of thin cover following the Laramide. Additional AFT, AHe, and 4He/3He modeling on the same samples is underway and will likely define better cooling paths for this complex region.