NEW INSIGHTS ABOUT THE INTERACTION OF THE PACIFIC-FARALLON SPREADING CENTER WITH THE BAJA CALIFORNIA SUR CONTINENTAL MARGIN

The interaction of the Pacific-Farallon spreading center with the North American convergent margin off Baja California supposedly ceased at 12 Ma when plate convergence and seafloor spreading stopped. From 12 to 5 Ma right lateral transform motion occurred along the Tosco-Abreojos fault system (TAFS) between 23°N-28°N until the opening of the Gulf of California.

We propose a new geodynamic evolution based on full bathymetry coverage (23°N-27°N), seismic reflection and magnetic profiles (FAMEX cruise, R/V L'Atalante, 2002). The upper-slope of the margin exhibits active faulting and clearly evidence strike-slip motion along the TAFS. The lower-slope exhibits compressional deformation until 8-7 Ma. Seaward the data unveil a major clockwise rotation of the Pacific-Farallon spreading direction starting at ≈14 Ma that formed a series of short spreading centers that later become extinct at ≈ 8-7 Ma. We suggest that until 8-7 Ma the transcurrent motion between Pacific and North America along Baja California was accommodated by seafloor spreading and oblique convergence along the trench. This change in spreading direction was followed by a concomitant, progressive demise of both Pacific-Farallon seafloor spreading and Farallon-North America subduction that are attributed to the break-off of the Farallon slab. This also resulted in the opening of a trench-parallel slab window beneath Baja California. After 8-7 Ma the transcurrent motion between Pacific and North America along Baja California was accommodated along the TAFS and in the Gulf of California.

Tectonic features along the TAFS are indicative of right-lateral motion between the Baja California Peninsula and the Pacific plate. The TAFS may thus represent the boundary between the Pacific plate and a Baja California Block, bounded to the east by the Gulf of California spreading system. Motion partitioning between the Pacific and North American plates for the last 5 Myr must be reconsidered in the light of this observation.