Seismic Evidence that the Juan de Fuca Plate is Fracturing from the Bottom Up

The processes that control the end of an oceanic plate's life are not well understood. We have imaged the upper mantle of the Pacific Northwest, where the Juan de Fuca plate subducts obliquely beneath North America, using a combination of onshore and offshore seismic data. The images show a hole in the subducting Juan de Fuca slab below 150km underneath central and northern Oregon. This hole is spatially coincident with both a weak region within the subducting slab inferred from magnetic anomalies, and with volcanism on the overriding North American plate. We suggest that the hole may be a tear in the slab that is propagating updip. Our interpretation is consistent with southern edge of the subducting slab inferred from microseismicity, with deformation internal to the southernmost Juan de Fuca plate, and with spreading rates in the Gorda Ridge. We propose that in the final stages of an oceanic plate's life, deformation of the plate on the surface can be driven by deeper dynamics, and that this bottom-up fragmentation may be the first stage of the accretion process.