The Fragmented Death of the Farallon Plate

The processes that accompany the death of an oceanic plate, as a ridge nears a trench, remain enigmatic. How the plate might reorganize, fragment, and eventually be captured by one of the bounding plates are among the unresolved details. We present a tomographic model of the Pacific Northwest from onshore and offshore seismic data that reveals a hole in the subducted Juan de Fuca plate. We suggest that this hole is the result of a tear along a preexisting zone of weakness, is causing volcanism on the North American plate, and is causing deformation in the Juan de Fuca plate offshore. We propose that in the final stages of an oceanic plate's life, deformation on the surface can be driven by deeper dynamics and that the fragmentation and the eventual capture of oceanic plate fragments may be governed by a process that operates from the bottom up. Plain Language Summary A hole in a subducted plate, in the mantle beneath North America, may cause volcanism and earthquakes on the surface of the Earth. Volcanism on the surface of North America appears to have been spatially coincident with a known zone of weakness on the slab for the last similar to 17 million years. We suggest that this hole is caused by tearing along the zone of weakness, a feature that is created when the plate is formed at the ridge. The tearing not only causes volcanism on North America but also causes deformation of the not-yet-subducted sections of the oceanic plate offshore. This tearing may eventually cause the plate to fragment, and what is left of the small pieces of the plate will attach to other plates nearby.