VOLCANIC STRATIGRAPHY AND TECTONIC SETTING OF THE SILETZIA LARGE IGNEOUS PROVINCE IN WASHINGTON AND VANCOUVER ISLAND

Geophysical, geochemical, geochronologic, and stratigraphic observations all suggest that the basalts that underlie western Oregon, western Washington, and southern Vancouver Island form a coherent terrane of Eocene age that is named Siletzia. The total volume of basalt within this terrane is comparable to large igneous provinces emplaced above hot spots. However, the tectonic setting of Siletzia remains debated between an origin as an oceanic plateau or as a marginal rift basin. We present new high-precision U-Pb zircon dates of silicic tuffs and intrusive rocks throughout northern Siletzia, as well as detrital zircon spectra and maximum depositional ages for continentally derived sandstones that have long been considered to form the stratigraphic base of the terrane. Our new dates help clarify the volcanic stratigraphy of northern Siletzia and show that it was emplaced between 53-48 Ma, similar to the duration of magmatism in southern Siletzia. The continentally derived sediments are <45 Ma, distinctly younger than the rest of northern Siletzia, and were likely thrust under the terrane at a later date. Importantly, a younger age for these sediments no longer necessitates that Siletzia formed along a rifted margin and we consider our revised stratigraphy to provide support for Siletzia’s origin as an accreted oceanic plateau. Plate reconstructions place the Yellowstone hotspot off the coast of western Oregon at ca. 50 Ma, which may provide a mechanism for generating the voluminous basalts. Since the eruption of Siletzia overlaps with 51-49 Ma regional shortening in western Washington, we suggest that the plateau was built concurrent with its attempted subduction and that the buoyancy of such young crust may explain why it jammed the subduction zone, rather than subducting easily like other plateaus.