Fifty years after plate tectonic theory was developed, the defining mechanism of the plate is
still widely debated. The relatively short, simple history of young ocean lithosphere makes it
an ideal place to determine the property that defines a plate, yet the remoteness and
harshness of the seafloor have made precise imaging challenging. We use S-to-P receiver
functions to image discontinuities beneath newly formed lithosphere at the Juan de Fuca
and Gorda Ridges. We image a strong negative discontinuity at the base of the plate …