New Mexico Geological Society Annual Spring Meeting — Abstracts

Conflicting models for the timing of carving Grand Canyon, especially the controversial westernmost Grand Canyon, involve either an “old” 70 Ma segment of Grand Canyon versus a “young” < 5-6 Ma segment. 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 are in conflict between two studies. (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 during the Laramide orogeny 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 geologically plausible, although perhaps unlikely, such that 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 the Separation Canyon samples suggests a period of 90-70 Ma Laramide cooling, 70-10 Ma post-Laramide residence at ~40-60 ºC, then another period of cooling to surface temperatures at 5-6 Ma. We also summarize all available thermochronologic data from river-level Precambrian basement rocks of westernmost Grand Canyon. New HeFTy modeling of all samples was done considering the Precambrian age and cooling histories of the grains, cooling to near surface temperatures in the Cambrian and Devonian, and then burial by 1-3 km of Phanerozoic strata by 90 Ma (hence 40-140ºC from 90-100 Ma). 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 system and anneal out lattice damage. Only some samples record a second 5-6 Ma cooling pulse. Our overall conclusion is that published thermochronologic constraints were not yet able to fully resolve the “old” versus “young” canyon models because 1) most AHe ages are old (> 50 Ma), 2) the Precambrian apatite crystals have variable, often high effective uranium (eU) values and complex radiation damage due to long term burial and insufficient annealing as a result of thin cover (~ 1 km) 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. Once cooling paths are better established, geologic interpretations will need to consider: 1) a still unresolved combination of northward stripping of the Phanerozoic section that left less than 1 km of strata above basement rock, 2) fault throw on the Hurricane and other Laramide faults, 3) irregular cliff retreat of the ~1 km high Permian escarpment, and 4) the carving of paleocanyons.