Seismic image of the subducted trailing fragments of the Farallon plate

The Farallon plate was an enormous oceanic plate located west of the Americas during the Cenozoic and Mesozoic eras. This plate has now been almost completely subducted beneath the American plates. In the Northern Hemisphere, the Farallon plate broke up to form a number of independent smaller plates1–3 when its western edge approached the North American plate. Here we present a tomographic image of the subducted trailing fragments of the Farallon plate in the upper mantle beneath the western margin of North America. The relatively cold, subducted fragments appear as an intricate region of high seismic S-wave velocity. Comparison of the structure of this high-velocity region with tectonic plate reconstructions and volcanic records enables us to identify individual fragments of the subducted Farallon plate and thus reconstruct qualitatively the kinematic evolution of the Farallon slab in the upper mantle beneath North America.