INVESTIGATING THE UPLIFT CHRONOLOGY OF THE OLYMPIC MOUNTAINS WITH SEDIMENTARY PROVENANCE PROXIES

The Olympic Mountains on the west coast of Washington State are the uplifted portion of the Cascadia accretionary wedge, emergence timing of which remains poorly documented. The accretionary wedge comprises lower Eocene basalts, Paleogene and Neogene marine clastic and low-grade metamorphic rocks. Here, we document the provenance of Paleogene and Neogene units from the Olympic peninsula and from the Seattle Basin further east. Our data from both areas show that Eocene and Oligocene sandstones display U-Pb zircon ages and grain petrography in agreement with a direct supply from the Cascade volcanic arc. Samples of Miocene sandstones and modern river sands from the west of the Olympic peninsula display a youngest zircon age population at 17 Ma. These data indicate that this area was still fed by the volcanic arc at that time. Middle Miocene (~11 Ma) sandstones from the eastern part of the Seattle Basin display U-Pb zircon ages still in agreement with supply from the Cascade volcanic arc. By contrast, contemporary fluvial sandstones from the western part of the basin mostly consist of basaltic clasts and reworked Eocene material. This change in sediment source shows that by 11 Ma, the Olympics had already emerged and reached sufficient topographic prominence to support eastward draining rivers that deposited the late Miocene fluvial units of the Seattle Basin. These data allow us to constrain the initial uplift of the Olympic Mountains sometime after 17 Ma but before 11 Ma. The data are in close agreement with previously published low-temperature thermochronology ages that establish the exhumation of the Olympic Mountains beginning at 18 Ma, and show that the Olympic Peninsula became an emerged topographic high in less than 6 million years.