RIDGE-TRENCH COLLISION ALONG BAJA CALIFORNIA SUR CONTINENTAL MARGIN : PLATE TECTONIC EVOLUTION SINCE 15 MA

The interaction of the Pacific-Farallon spreading centers with the North American convergent margin off Baja California is reexamined on the basis of new evidence from a marine survey (R/V l'Atalante 2002) and from sampling of inland volcanoes. Swath-bathymetry data off Baja California Sur reveal a series of fossil segments from the Pacific-Farallon spreading ridge and confirm that the spreading ridge did not subduct. The oceanic fabric, south of the Shirley Fracture zone, unveils a drastic change in plate motion after chron 5B (~15 Ma), with a 45 to 90 degrees rotation of the spreading centers; magnetic anomaly data show that seafloor spreading rapidly slowed down and ceased completely between 12 and 8 Ma. This event coincides with the onset of right-lateral motion along the Tosco-Abreojos fault system, which runs along the western margin of Baja California. This fault appears to have been active from 8 Ma to Present, despite the opening of the Gulf of California at 5-3.6 Ma. This observation suggests a long-lasting motion-partitioning between the Pacific plate, a Baja California block and the North American plate, respectively. Inland, petrologic and geochemical evidence for melting of a young downgoing oceanic lithosphere is found along a 600km-long belt of volcanoes parallel to the Baja California margin. Their Late Miocene lavas, which range in age from 13 to 7 Ma and include tholeiitic and alkali basalts as well as adakites and magnesian andesites, are indicative of the presence of a slab-window beneath Baja California, even though, at these latitudes, the Pacific-Farallon spreading ridge did not subduct. In this paper, we attempt to reconcile these new constraints within the geodynamic framework proposed by Wilson et al. (2005) for the tectonic evolution of the North American margin.