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. ¿ American Geophysical Union 1995