The Cascade arc (western North America) is the world’s youngest continental arc, and because the down-going Juan de Fuca plate is young, it is also the hottest end member among subduction zones worldwide. We present evidence that the arc initiated <5 m.y. after accretion of the Siletzia oceanic terrane terminated the earlier subduction system and caused the northern portion of the Farallon slab to break off. Cascade magmatism began ca. 46 Ma with a new trench outboard of Siletzia, a reconfiguration commonly attributed to a seaward jump of the subduction zone. However, the presence of young buoyant oceanic lithosphere that would have resisted being forced into the mantle and the very rapid reestablishment of arc magmatism are hard to reconcile with initiation of a new subduction zone by this process. We propose an alternative mechanism in which the arc was reestablished as the intact southern portion of Farallon slab migrated northward from California (United States), converting a transform margin to a convergent one. This model utilizes plate reconstructions, petrology, mantle tomography, and geochronology to explain how subduction was initiated in a setting where the slab was young and hot and why the earliest Cascade magmatism occurred toward the middle rather than an end of the arc.

This content is PDF only. Please click on the PDF icon to access.