Mesozoic ocean plate stratigraphy reveals a Franciscan plate separating Farallon and North America

Ocean plate stratigraphy preserved in the Franciscan Complex records the Mesozoic plate tectonic evolution of the North American Cordillera and eastern Pacific basin, yet it has not been widely used to constrain plate tectonic reconstructions of these regions. New and published ocean floor and accretion ages for Jurassic−Cretaceous oceanic crust, derived from detrital zircons and radiolarians, indicate that eastward younging ocean floor existed between the Farallon and North American plates during the Mesozoic. Our new plate reconstruction shows that, to the south, this lithosphere subducted southward at an intraoceanic subduction zone beneath Caribbean lithosphere, which was still part of Farallon during the Early Cretaceous. The east-younging lithosphere was thus not the Farallon plate, but a separate Franciscan plate. Our kinematic model shows that it formed as part of a back-arc basin during the Mesozoic that must have been bounded by eastward-dipping subduction zones on both its western and eastern margins. The paleolatitude and age of the Wrangellia superterrane of the Northern Cordillera fit well with this prediction. A Late Cretaceous plate reorganization and formation of the north-moving Kula plate offer kinematic explanations for why the intraoceanic arc records formed at low latitude are currently found only from British Columbia northward.