27-3 Cretaceous to Miocene Thermal History of Plutons and Overlying Hornbrook and Dothan Formations in the Northern Klamath Mountains

Session: Evolution of Cordilleran-type orogenic systems

Presenting Author:

Authors:

Abstract:

The Klamath Mountains Province (KMP) lies north of the Sierra Nevada and west of the Cascade Arc, within the North American Cordillera. The KMP is a series of Paleozoic-Mesozoic accreted terranes intruded by Jurassic-Cretaceous plutons. In the northern KMP (NKMP) overlying these plutons, the Hornbrook Formation (Fm) and Dothan Fm display coeval Late Cretaceous depositional ages and stratigraphic relationships that demonstrate they accumulated in a single regional forearc basin. However, their deformation histories differ since deposition. The Valen Lake fault (VLf) is a regional thrust fault that separates undeformed Hornbrook Fm in the east from highly deformed Dothan Fm in the west. The Dothan Fm displays overturned isoclinal folds and prehnite-pumpellyite facies metamorphism, while the Hornbrook Fm is unmetamorphosed and dips gently northeast. In prior work, we analyzed samples from five plutons in the NKMP using low-temperature apatite (U-Th)/He (AHe, closure temperature (Tc) ~60°C) and zircon (U-Th)/He (ZHe, Tc ~180°C) thermochronology. Miocene AHe and Cretaceous ZHe results bracket the timing of Siletzia collision, a large igneous province that accreted to North America in early Eocene time.

In this study, we collected apatite fission track (AFT, Tc ~110°C) pooled ages and track length distributions for five plutons and eight sites from the Dothan and Hornbrook formations to investigate spatial and temporal rock cooling patterns, including the extent of Siletzia collision and its potential to influence burial and exhumation. AFT ages from NKMP plutons yield a Late Cretaceous population averaging 75 Ma (n=3) and an Eocene population averaging 44 Ma (n=2). Our data record different thermal histories for the Dothan and Hornbrook formations. AFT ages from the Dothan Fm, west of the Valen Lake fault, are Oligo-Miocene ranging from 27 to 21 Ma (n=6). East of the VLf in the NKMP, farther from the collision, Hornbrook Fm AFT ages are Late Cretaceous, with the exception of one sample overlying the Ashland pluton (AFT ~26 Ma). Our study reveals a broad pattern surrounding the Valen Lake fault: AFT ages west of the fault are Oligo-Miocene; and AFT ages east of the fault are Eocene to Late Cretaceous. Further analysis and modeling will explore the tectonic controls on exhumation, and test a hypothesis that post-collisional extension was a major driver of Eocene to Miocene rock cooling.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

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