Large Scale Three Dimensional P Velocity Structure beneath the Western U

The results of a recent large scale three-dimensional study of P velocity beneath North America are analyzed from the point of view of the search for the fossil Farallon plate in the mantle beneath the western edge of the North American continent. The large scale velocity structure obtained in the western U.S. is compatible with independent finer scale seismic studies and suggests that the Farallon plate can be traced to depths of 300 to 450 km along the entire North-South extent of the area. At greater depth, no clear evidence is found for its presence: it has either not reached it or is no longer distinguishable from the surrounding mantle. On the basis of the 3-D velocity study we also venture to predict that, whereas in the central and southern part of the western U.S. the top of the plate seems to have sunk to depths in excess of 200 km, it should still be present at shallower depths beneath the Northwestern region. This is in agreement with the progressive migration to the North of the Mendocino transform-transform-trench triple junction and its present day position.