No abstract

Gravity data, the geometry of the Wadati‐Benioff zone beneath western Mexico, and the seafloor morphology of the Rivera‐Cocos plate boundary west of the Middle America trench suggest that the subducted part of this boundary lies directly beneath and is oriented parallel to the Southern Colima rift. Thus, the Southern Colima rift likely formed in response to divergence between the subducting Rivera and Cocos plates due to direct coupling between these two plates and the overriding North American plate. In contrast, the subducted plate boundary lies east of and oblique to the Northern Colima and Central Colima grabens. East of the Central Colima graben a low density zone overlies the boundary and underlies surface exposures of Cretaceous granitoids and associated thermal springs and shallow focus earthquakes; characteristics that are explained by thermal convection induced in the upper mantle by divergence between the subducted Rivera and Cocos plates. These characteristics along with the adjacent locations of the low density upper mantle and the Central Colima graben are consistent with crustal extension produced by the uniform‐sense normal simple shear mechanism.

The interaction of the Pacific-Farallon spreading centers with the North American convergent margin off Baja California, Mexico, supposedly ceased at 12 Ma, when plate convergence and seafloor spreading stopped. We propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 23°N to 27°N (Famex cruise of the R/V L'Atalante, April 2002). The data unveil a major clockwise rotation of the Pacific-Farallon spreading direction, starting ca. 14 Ma, that formed a series of short spreading centers that became extinct ca. 8–7 Ma. We suggest that the transcurrent motion between the 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

No abstract