THE "HOH FORMATION" IN THE OLYMPIC SUBDUCTION COMPLEX, WASHINGTON: A WINDOW INTO LATE MIOCENE ACCRETION IN THE CASCADIA SUBDUCTION WEDGE

New detrital zircon fission-track ages from 36 sandstone samples collected in the Olympic Mountains in Washington State demonstrate rocks once assigned to the “Hoh Formation” extend at least 50 km eastward from the Pacific Coast into the interior of the range, as originally suggested by Brandon and Vance (1992). Now known as the Coastal unit of the Olympic subduction complex (the Coastal OSC), these rocks contain abundant first-cycle detrital zircons derived from the Cascade arc. Fission-track grain-age (FTGA) distributions are usually polymodal and strongly discordant. Minimum ages, the age of the youngest concordant fraction in FTGA distributions (i.e. the youngest peak), range from 43 to 14 Ma. Low vitrinite reflectance values (Rm~0.5-1.3%) and broad grain-age distributions (>40 m.y.) indicate grain ages preserve the cooling ages of source rocks. Comparison of minimum ages in 17 samples with fossil ages obtained from comparable stratigraphic positions are concordant within <3 m.y. indicating minimum ages are as valid estimates of depositional age. These results also suggest zircon fission-track minimum ages are reliable, if not outstanding proxies of both depositional age and provenance in sediments derived from contemporaneous volcanic arcs. This result is particularly significant because age-diagnostic fossils are very rare in most of the Olympic subduction complex.

Four samples have minimum ages of 16-14 Ma, suggesting deposition of sediment in the Coastal OSC extended into the middle Miocene. Seven samples are Oligocene, even though confirmed Oligocene fossil localities are absent in the Coastal OSC. Four Eocene samples probably represent exotic blocks mixed into the Coastal OSC by faulting or mass wasting. All minimum ages are significantly younger than published 40Ar/39Ar ages from sandstone blocks within mélange of the Coastal OSC.

We suggest the Coastal OSC records a portion of a long history of accretionary growth of the Cascadia wedge. We also suggest accretion and deformation of the Coastal OSC was not limited to a short interval of time in the late middle Miocene, but more likely involved continuous accretion of rocks during subduction of the Juan de Fuca plate. Deformation of the Coastal OSC has not ended, because these rocks are still being deformed by within-wedge shortening, and by mud diapirism.