LONG TERM EXHUMATION OF THE OREGON COAST RANGE USING DETRITAL THERMOCHONOLOGY FROM THE EOCENE TYEE FORMATION, OREGON, USA

New detrital apatite fission track data and thermal modeling from the southern Oregon Coast Range reveal a multi-phase post depositional thermal history of a 2000-meter section of Eocene forearc basin sediments of the Tyee Formation. Six pooled fission track dates range from 37 ± 3 Ma (2s) to 46 ± 4 Ma (2s) and increase upsection at a slope of about 200 meters per million years. All Tyee Formation samples are younger than or contemporaneous with the depositional age of the sampled strata (circa 49 – 46.5 Ma for Tyee Formation), requiring post depositional heating sufficient to anneal fission tracks. We use a Bayesian Monte Carlo Markov chain statistical approach deployed with the software package QTQt to quantify the acceptable thermal histories that could result in the observed pattern of fission track ages and track lengths. Modeling efforts to date include individual runs for each sample and composite vertical transect modeling of all Tyee Formation samples together, utilizing estimated stratigraphic height of each sample based on published mapping and stratigraphic work. Results reveal a transient thermal pulse lasting from shortly after deposition until 39 Ma with heating at 50-80 °C per million years achieving maximum temperatures circa 45 Ma, followed by cooling from 45 to 39 Ma at 10-20 °C per million years. The preferred geothermal gradient at time of maximum temperature is 50 °C per kilometer. We interpret this thermal pulse as a regional signal related to magmatism and extension in the aftermath of Siletzia accretion, referred to as the Tillamook Magmatic Episode of the northern Oregon Coast Range where bimodal magmatism and regional dike swarms are documented. Preferred model results indicate that no major heating occurred following the cessation of this early thermal transient (39 Ma). This new understanding of the thermal evolution of Eocene forearc basin sediments places important constraints on the framework for tectonic evolution of the Oregon coastal block as well as the evolution of hydrocarbon resources in the Paleogene forearc basin.