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

Goran Andjić , John Wakabayashi, Alexey Ulianov, Douwe J.J. van Hinsbergen 

Ocean plate stratigraphy preserved in the Franciscan Complex recorded the Mesozoic plate tectonic evolution of the North American Cordillera and eastern Pacific basin. 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. Moreover, plate reconstruction shows that, to the south, this lithosphere subducted southward at an intra-oceanic 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. We infer that it formed as part of a back-arc basin during the Cretaceous, bounded by eastward-dipping subduction zones on both its western and eastern margins. Northward translation of the western intraoceanic subduction-related terranes, likely driven by oblique Kula plate subduction, may explain why low-latitude intraoceanic arc records are currently only found from British Columbia northwards.