SEISMIC IMAGE OF THE SUBDUCTED TRAILING FRAGMENTS OF THE FARALLON PLATE

The Farallon plate was an enormous oceanic plate located west of the A mericas during the Cenozoic and Mesozoic eras. This plate has now been almost completely subducted beneath the American plates. In the North ern Hemisphere, the Farallon plate broke up to form a number of indepe ndent smaller plates(1-3) when its western edge approached the North A merican plate. Here we present a tomographic image of the subducted tr ailing fragments of the Farallon plate in the upper mantle beneath the western margin of North America. The relatively cold, subducted fragm ents appear as an intricate region of high seismic S-wave velocity, Co mparison of the structure of this high-velocity region with tectonic p late reconstructions and volcanic records enables us to identify indiv idual fragments of the subducted Farallon plate and thus reconstruct q ualitatively the kinematic evolution of the Farallon slab in the upper mantle beneath North America.