THE SUBDUCTED RIVERA-COCOS PLATE BOUNDARY - WHERE IS IT, WHAT IS IT, AND WHAT IS ITS RELATIONSHIP TO THE COLIMA RIFT

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 So uthern Colima rife. Thus, the Southern Colima rift likely formed in re sponse to divergence between the subducting Rivera and Cocos plates du e to direct coupling between these two plates and the overriding North American plate. In contrast, the subducted plate boundary lies east o f 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 associate d thermal springs and shallow focus earthquakes; characteristics that are explained by thermal convection induced in the upper mantle by div ergence between the subducted Rivera and Cocos plates. These character istics along with the adjacent locations of the low density upper mant le and the Central Colima graben are consistent with crustal extension produced by the uniform-sense normal simple shear mechanism.