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Dr. Jeff Tepper The Cascade volcanic chain, the world’s youngest continental arc, was “born” shortly after accretion of the Siletzia oceanic terrane ~50 My ago. That collision, which led to formation of the Olympics, terminated the earlier subduction system and caused a portion of the subducting Farallon slab to break off. When Cascade magmatism began less than 5 My later, there was a new trench located outboard of Siletzia. In this talk I will present a new model, based on petrology, geochronology, plate motion reconstructions, and mantle tomography, that explains how subduction was initiated so quickly and in a setting where the slab was young and hot and in theory too buoyant to subduct.