MULTICHRONOMETER STUDY TO CONSTRAIN THE CRETACEOUS TO MIOCENE THERMAL HISTORY OF THE NORTHERN KLAMATH MOUNTAINS

The Northern Klamath Mountains Province (NKMP) in Northern California and Southern Oregon formed as a series of Paleozoic-Mesozoic accreted terranes and was intruded by Jurassic-Cretaceous plutons. In prior work, we strategically analyzed samples from five plutons in the NKMP using low-temperature apatite (U-Th)/He (AHe, closure temperature ~60 ℃) and zircon (U-Th)/He (ZHe, closure temperature ~180 ℃) thermochronology to reconstruct the region’s Cretaceous to present time-temperature history. This was necessary due to a limited Cenozoic rock record. Sample averaged (n=5) AHe dates are Miocene and range from 18.1 ± 2.5 Ma to 24.8 ± 6.7 Ma and ZHe dates are Cretaceous and range from 85.3 ± 10.7 Ma to 131.5 ± 13.7 Ma, with error reported as standard deviation. A thermal data gap still exists between the Late Cretaceous and early Miocene, requiring additional mineral chronometers and thermal modeling to constrain the NKMP’s response to the Eocene collision of Siletzia, Cretaceous marine sedimentation, and regional tectonics. AHe and ZHe results bracket the timing of Siletzia collision north of the NKMP and permit, but do not require significant reburial or cooling at that time. The data and thermal models are consistent with extensive Cretaceous burial and record a slight uptick in cooling rate during the Miocene. AHe dates from the Southern KMP are also Miocene, which may suggest a regional mechanism for cooling at that time.

In order to fill the thermal data gap between the Late Cretaceous and early Miocene, we re-analyzed our same five samples using apatite fission track (AFT, closure temperature ~110 ℃) thermochronology, which has a closure temperature between AHe and ZHe. New AFT dates will allow us to test different cooling scenarios. Old (Cretaceous) AFT dates would suggest rapid removal of the Cretaceous sediment, Eocene ages could be connected to Siletzia’s collision, or point towards monotonic or episodic cooling depending on track lengths, and young (Miocene) dates would indicate more significant exhumation at that time than previously identified. We will present new AFT results combined with previously presented AHe and ZHe data.