We report the first low-temperature thermochronologic data from the Oregon Coast Range. This includes apatite and zircon fission-track data from fifteen samples of the Umpqua Group and Dothan Formation collected across the Wildlife Safari Fault near Roseburg, Oregon. This structure marks the boundary between the Siletzia terrane and the Klamath Mountains Province, where collision and accretion at 51–49 Ma produced a fold-and-thrust belt. Our sampling was designed to test whether fission track thermochronometry records structurally controlled exhumation across this fault during Siletzia accretion. The data fail to support this hypothesis. Instead, apatite fission track ages define a single thermally reset population at 45.3 ± 1.1 Ma that is uniform across all sampled stratigraphic and structural positions. Unimodal, moderately shortened track lengths (12.5–14.1 μm) record protracted cooling through the apatite partial annealing zone. Zircon fission track data show a time-continuous partial annealing pattern with youngest grain ages of 49–43 Ma, indicating temperatures reached at least the lower zircon partial annealing zone. These data record a regionally pervasive mid-Eocene thermal event that we interpret as syn-collisional heating followed by protracted cooling.
Klisura et al. (Fri,) studied this question.