CONSEQUENCES OF TRIPLE JUNCTIONS FOR THE GEOLOGY OF THE BAJA CALIFORNIA PENINSULA

The Baja California peninsula (Mexico), currently on the Pacific plate, experienced a variety of effects due to the proximity of ridges, cessation of subduction and passage of triple junctions along its western margin. The major triple junction of interest is the Rivera Triple Junction (RTJ), which is now SE of the southern end of Baja California, and involves the Pacific, North America, and Rivera plates at the northern end of the Middle America Trench. The ancestral RTJ formed about 28 Ma as the subducting Farallon plate broke into pieces and lost continuity between its northern and southern subduction zones. This triple junction migrated southward with the northern end of the southern Farallon subduction system. Because the Baja California Peninsula was attached to the North America plate until the late Miocene, it lay in the upper plate of the subduction zone as the RTJ migrated southward and progressively shut off subduction west of the Baja California peninsula. The RTJ migrated southward discontinuously by a series of ridge deaths involving microplates. As a segment of spreading ridge between the Pacific plate and a microplate ceased to spread, the microplate would become attached to the Pacific plate, and its subduction relative to North America would cease and change to Pacific-North America motion. Thus, the RTJ moved southward even though the Pacific plate was moving northward with respect to North America. At the latitude of central and southern Baja California, fossil spreading centers are preserved offshore, and no ridge segment was subducted. The effects of the passage of this triple junction are: 1) cessation of the subduction-related volcanic arc earlier (ca. 17 Ma) in the north than in the south; (2) large amounts of extension west of Baja California in the California Continental Borderland; (3) fossil spreading centers west of the southern peninsula; (4) post-subduction volcanism attributed to slab gaps. The RTJ traveled southward along the west side of Baja California without leaving a record on the upper plate. This may have been because it did not remain in the same place west of the Baja California peninsula for any length of time. The passage of the northern triple junction (Mendocino) can be closely tied to migration of volcanism due to slab window effects; but the slab window effects are more controversial in the Baja California sector.