Paper No. 45-10
Presentation Time: 4:50 PM
THERMOCHRONOLOGIC DATA FROM THE MANIOBRA FORMATION AND HAYFIELD MOUNTAINS, MANIOBRA VALLEY, CALIFORNIA
New apatite fission track (AFT) thermochronology from the Maniobra Valley, California provide insights into the thermal history, thickness, and regional significance of the Eocene Maniobra Formation, Maniobra Valley, California. We report data from 6 samples totaling 183 individual apatite grains. The samples are taken from a diversity of geologic units, including 3 detrital sedimentary samples from different stratigraphic levels of Eocene Maniobra Formation and 3 samples from the Hayfield Mountains granitic basement--one directly beneath the basal Maniobra nonconormity, one superjacent to this nonconformity in a locally derived silicified paleosol, and a third bedrock sample taken adjacent to a fault that offsets the Maniobra Formation. With the exception of the fault-adjacent sample, all of the data cluster tightly around an AFT age of 38.5 Ma +/- 3.0 Ma (2σ SD). Best estimates for the depositional age of the Maniobra Formation, from marine microfossils, is circa 53-46 Ma. The substantially older depositional age for the Maniobra Formation compared to both the detrital and bedrock AFT age requires thermal resetting of the AFT system that must include 1) post-depositional heating of the samples above the AFT closure temperature (circa 110° C) and 2) subsequent cooling by circa 38.5 Ma. The bedrock sample taken adjacent to the fault that cuts the Maniobra section is significantly younger than all the other AFT samples (25.2 Ma +3.6/-4.2 Ma), likely related to structurally controlled thermal activity during deposition of the Miocene Diligencia Formation. We integrate individual grain data, track length measurements, and stratigraphic heights for individual samples into a thermal history model to constrain the timing and magnitude of the post-depositional heating and cooling, as well as the likely total thickness of the Eocene Maniobra section.