KEYNOTE: THE HISTORY AND CAUSES OF THE RIFTING, SHEARING AND SPREADING THAT OPENED THE MOUTH OF THE GULF OF CALIFORNIA

The present 145-210 km-wide East Pacific Rise (EPR) in the mouth of the Gulf of California was accreted at an inter-continental spreading center that propagated NE across a Late Cretaceous-Paleocene pluton, as far as Tamayo fracture zone, at 5.2-3.1Ma. But this was just the latest phase in the separation of the granitic crust of the Cabo Block from rocks of similar age and chemistry on and offshore of the Nayarit mainland. Pliocene propagation of the EPR axis was not just through rifted continental crust, but through preexisting oceanic crust accreted at spreading centers (the Maria Magdalena and Los Frailes axes) that developed 6-5Ma from pull-aparts at major intracontinental shear zones, and were propagating southwestward out of the mouth of the Gulf before they were overlapped by the lengthening EPR axis, and rendered extinct. The maximum combined width of oceanic crust from the EPR and the other rises is 250km, but this accreted crust is bordered by broad bands of stretched submerged continental crust on both margins. Most geodynamic models, supported by new geophysical evidence, have BC obliquely torn away from the mainland by the basal drag of underlying oceanic lithosphere, once part of the Guadalupe and Magdalena microplates, that acquired northwestward Pacific motion at ~11.5 Ma. Restoring post-11.5Ma northwestward rifting, shearing and spreading within the mouth of the Gulf only brings the tip of BC adjacent to the Tres Maria Islands region, but strong evidence suggests that the tip was originally conjugate with the Banderas Bay region 150-160km to the south. It may have been dragged north prior to microplate capture, before the main phase of rifting. Subduction of the microplates west of BC ceased several million years before 11.5Ma; magnetically determined motion relative to the Pacific plate shows that after ~14Ma they were moving N to NNW, almost parallel to the coast and to the Tosca-Abreojos strike-slip fault zone.