Puget Lobe Lecture: Jeff Tepper on the Initiation of the Cascade Arc

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.