APATITE-ZIRCON U-TH/HE AND FISSION-TRACK DATING OF THE OKANOGAN DOME EXHUMATION (WA, USA)

The North American Cordillera experienced accretion, collision, and subsequent extension during Late Mesozoic-Early Cenozoic, which resulted in exhumation of metamorphic core complexes (MCC) in the hinterland of the orogen. Many aspects regarding the formation of metamorphic core complexes remain unresolved, including the relative role of tectonic forces and surface processes on denudation histories that control mass and heat transfer within the crust, and production/reorganization of topography at the surface. The Okanogan Dome (WA, USA) is the southernmost member of north‐south trending gneiss and migmatite domes that are exposed in the Shuswap MCC, the largest in the North American Cordillera, where coeval Paleocene-Eocene crystallization of migmatites, upper crustal extension, and exhumation has been documented. Here, we present new results from multiple low-temperature chronometers (zircon and apatite U-Th/He and apatite fission track) from the Okanogan Dome in order to have a complete thermal record of the timing and rates of the late-stage exhumation and cooling history of the footwall units. We sampled various lithologies along a transect from core of the dome to the bounding Okanogan Detachment zone. Zircon He (ZHe) ages are 37-51 Ma, whereas apatite He (AHe) ages vary between 32-51 Ma in the same sample suites. ZHe ages are consistently older than AHe ages and both thermochometers show a slight decrease in age towards the detachment zone. Apatite fission track (AFT) ages range between 40-70 Ma, but mostly concentrate c. 40-50 Ma. Double and triple dates from individual samples, combined with existing higher temperature thermochronometers, reveal cooling rates as high as c. 80 °C/ Myr between 51-41 Ma and a decrease to <20 °C/Ma between 40-32 Ma. Our dataset suggests that, even though spatial and temporal variations exist, overall cooling and exhumation to shallow crustal levels took place in Eocene-Oligocene, following rapid cooling associated with Eocene extension.