We report new fission-track (FT) ages for detrital zircons for 34 sandstone samples and 2 volcanic ash beds from the “Hoh Formation,” exposed along the western side of the Olympic Mountains of western Washington State. The “Hoh Formation” is now formally known as the coastal unit of the Olympic Structural Complex, or Coastal OSC for short. About 35 zircons were dated per sample. The fission-track grain-age (FTGA) distributions are all strongly discordant; grain ages range from 10 to older than 100 Ma. Low vitrinite-reflectance values, short etch times for the zircons, and a broad range of grain ages indicate that the zircon FT ages are unreset and thus preserve information about cooling events in the source region for these sedimentary rocks. Five areas were sampled repeatedly and yield similar FTGA distributions, demonstrating that sampling errors are not a problem. We show that almost all of the samples contain a well-defined young component that was probably derived from a contemporaneous active volcanic source, presumably the adjacent Cascadia arc. Binomial peak-fitting was used to estimate the FT minimum age, which is the age of the youngest concordant fraction of zircon FT grain ages in a FTGA distribution. In most cases, minimum ages are similar to fossil ages where available. This result supports our contention that zircon FT minimum ages from volcaniclastic sandstones commonly can be used as a proxy for depositional age.
Our zircon FT minimum ages indicate that the Coastal OSC is made up mainly of lower Miocene (ca. 24 to 16 Ma) sedimentary rocks. We use these age data, together with other geologic constraints, to reconstruct a tectonic history. Sedimentary rocks of the Coastal OSC were derived from a mixed-source region that included an active volcanic arc and also older units, including Cretaceous metamorphic rocks, probably located in the Omineca crystalline belt in the Canadian Rockies. The upper part of the Clallam Formation, located on the northern side of the Olympic Peninsula, appears to be a remnant of the sedimentary system that fed the Coastal OSC. The sediments that formed the Coastal OSC were initially deposited seaward of the Cascadia trench, at water depths of >2000 m. This debris was deposited seaward of the Cascadia trench, at water depths of >2000 m, and subsequently accreted beneath the frontal 50 to 100 km of the wedge. Owing to continued accretion at the front of the wedge, and erosion of the forearc high in back of the wedge, these lower Miocene sediments were moved rearward within the Cascadia subduction wedge. A simple relationship based on the cross-sectional area of the wedge and a steady accretion flux indicates that it would have taken ∼22 m.y. for the Coastal OSC to reach its present position 140 km landward of the toe of the wedge. This estimate is in good agreement with the unit's early Miocene age.