The Olympic Peninsula is the uplifted portion of the Cascadia accretionary wedge and forms the core of a 200 km scale oroclinal bend on the west coast of Washington State. The accretionary wedge started forming 45 million years ago following the accretion of the Siletzia igneous province along the Cascadia subduction margin. Low-temperature thermochronology studies have shown that the core of the peninsula has been continuously exhumed for the last 14 million years. The earlier onset of oroclinal bending, uplift, and emergence remains poorly documented. Here, we explore the Cenozoic drainage history of the Cascadia forearc and accretionary wedge to reconstruct the deformation history of the Olympic Peninsula. We use detrital zircon provenance and grain petrography data from modern rivers draining the Cascades, the Cascadia forearc and accretionary wedge, as well as from Eocene to late middle Miocene sedimentary units from the same areas. We first show a clear difference of sedimentary provenance between sedimentary units in the accretionary wedge, with older units reflecting mélange and imbricated strata that began as part of Siletzia, and younger units reflecting trench-fill material sourced from the Cascades and accreted to the wedge. We show that the accretionary wedge was directly fed from the Cascade arc until at least 16.5±0.5 Ma, providing a maximum age for the emergence of the Olympic Peninsula. Fluvial deposits in the Cascadia forearc basin dated at 13.3±1.3 Ma display zircon age spectra and sedimentary grain petrography features typical of recycled accretionary wedge material. Although these deposits may also reflect local input, middle Miocene exhumation rates suggest the Olympic Peninsula was an active sediment source. Our results bracket the timing of emergence of the Olympic Peninsula to a narrow window in the late middle Miocene. We suggest that the initial onset of accretionary wedge deformation and oroclinal bending predates this by at least 10 million years, in the upper Oligocene, and is marked by flexural subsidence and high sedimentation rates recorded in strata of the Seattle Basin. Our results support a composite history for the development of the Cascadia accretionary wedge rather than models predicting a gradual and steady build-up.

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