WIDE-ANGLE SEISMIC CONSTRAINTS ON THE EVOLUTION OF THE DEEP SAN-ANDREAS PLATE BOUNDARY BY MENDOCINO TRIPLE JUNCTION MIGRATION

Recent,wide-angle seismic observations that constrain the existence an d structure of a mafic layer in the lower crust place strong constrain ts on the evolution of the San Andreas plate boundary system in northe rn and central California. Northward migration of the Mendocino Triple Junction and the subducted Juan de Fuca lithospheric slab creates a g ap under the continent in the new strike-slip system. This gap must be filled by either asthenospheric upwelling or a northward migrating sl ab attached to the Pacific plate. Both processes emplace a mafic layer , either magmatic underplating or oceanic crust, beneath the Californi a Coast Ranges. A slab of oceanic lithosphere attached to the Pacific plate is inconsistent with the seismic observation that the strike-sli p faults cut through the mafic layer to the mantle, detaching the laye r from the Pacific plate. The layer could only be attached to the Paci fic plate if large vertical offsets and other complex structures obser ved beneath several strike-slip faults are original oceanic structures that are not caused by the faults. Otherwise, if oceanic slabs exist beneath California, they do not migrate north to fill the growing slab gap. The extreme heat pulse created by asthenospheric upwelling is in consistent with several constraints from the seismic data, including a shallower depth to the slab gap than is predicted by heat flow models , seismic velocity and structure that are inconsistent with melting or metamorphism of the overlying silicic crust, and a high seismic veloc ity in the upper mantle. Yet either the Pacific slab model or the asth enospheric upwelling model must be correct. While the mafic material i n the lower crust could have been emplaced prior to triple junction mi gration, the deeper slab gap must still be filled. A preexisting mafic layer does not reduce the inconsistencies of the Pacific slab model. Such material could, however, compensate for the decrease in mafic mag ma that would be produced if asthenospheric upwelling occurred at lowe r temperature. These low temperatures, however, may be inconsistent wi th asthenospheric rheology.