SILETZIA, A LARGE IGNEOUS PROVINCE IN THE OREGON AND WASHINGTON COAST RANGE – CORRELATION WITH THE 2012 GEOMAGNETIC POLARITY TIMESCALE AND IMPLICATIONS FOR A LONG-LIVED YELLOWSTONE HOT SPOT

Siletzia is a Paleocene and Eocene Large Igneous Province in coastal Oregon, Washington, and southern Vancouver Island that was accreted to North America in the early Eocene. New U/Pb magmatic ages from interbedded tuffs, detrital zircon ages from sedimentary interbeds, and ⁴⁰Ar/³⁹Ar ages from the oceanic basalts are correlated with global nannoplankton zones and magnetic polarity using the 2012 Gradstein et al timescale. They show that Siletzia was rapidly erupted between 56-49 Ma, during northeast Pacific basin plate reorganization. The great crustal thickness of Siletzia, its stratigraphic history, and isotopic signatures similar to early Columbia River basalts indicate offshore formation, probably as an oceanic plateau. Accretion was completed between 51 and 49.5 Ma in Oregon, based on CP11 zone coccoliths in marine strata overlying the continental onlap at Roseburg, OR. Following accretion, a regional MORB sill complex (48-45 Ma) was intruded at the top of Siletzia, possibly representing asthenospheric upwelling during initiation of the new Cascadia subduction zone. About 8 m.y. after accretion, renewed tholeiitic and alkalic magmatism and emplacement of regional dike swarms occurred in the forearc during margin-parallel extension. Peak eruption of the Tillamook Volcanics occurred at 41.5 Ma (Chron 19r), within coccolith subzone CP14a. The precise age control was used to constrain a plate motion model for Siletzia in GPlates, with a long-lived hot spot at the present coordinates of Yellowstone. A northeast-striking Kula-Farallon ridge intersects the Yellowstone hot spot (YHS), which provides a nearshore source for Siletzia on the Farallon plate at 56-49 Ma. The Eocene basalt basement of the Yakutat terrane, now in Alaska, may have formed on the adjacent Kula plate and accreted north of Vancouver Island at about the same time. Following accretion of Siletzia, North America overrode the YHS at about 42 Ma, producing the voluminous basaltic to alkalic magmatism and margin-parallel extension of the Tillamook episode in the forearc. Clockwise rotation of western Oregon about a pole in the backarc has moved the Tillamook center northward about 250 km from the North American hot spot track. In this model, the impact of the YHS on the evolution of the Cordillera has been profound, wide-ranging, and long lasting.