The early Eocene (52–44 Ma) was a time of tectonic reorganization and widespread magmatism in Washington, Oregon, and British Columbia (west coast of the United States and Canada) that culminated with establishment of the Cascade arc. Details of this tectonic transition remain enigmatic, and diverse scenarios involving ridge-trench interaction, slab breakoff, and/or plume magmatism have been proposed. This study focuses on the ca. 48 Ma Basalt of Summit Creek, a ∼1500-m-thick sequence of subaerial lavas that erupted during the interval of time between accretion of the Siletzia terrane to the west and inception of the Cascade arc. The sequence dominantly consists of moderately evolved tholeiitic basalts (9.3–3.1 wt% MgO; Mg# = 0.66–0.29) and scarce rhyolites that erupted in the forearc but are chemically and isotopically similar to oceanic basalts of the Crescent Formation, found ∼100 km to the west as part of the Siletzia terrane. Basalt of Summit Creek lavas lack subduction signatures (e.g., high field strength element depletions) and require a mechanism whereby asthenospheric melts were able to reach the surface having undergone little or no chemical interaction with fluids derived from the subducting slab or metasomatized mantle. We suggest that the accretion of Siletzia led to breakoff of the Farallon slab, and that Basalt of Summit Creek lavas formed by decompression melting of mantle that upwelled through the rupture. Slab breakoff typically produces a short-lived linear magmatic belt; the Basalt of Summit Creek appears to be part of such a belt, previously unrecognized, that parallels the Siletzia boundary and formed between 50 and 48 Ma.

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