Geological and geophysical investigation of the Rivera-Cocos plate boundary : implications for plate fragmentation

Typescript (photocopy).An analysis of the geological and geophysical character of the Rivera-Cocos plate boundary indicates that it consists of three distinct morphotectonic zones. The easternmost zone, delineated by the El Gordo Graben, is a zone of lithospheric extension. The westernmost zone, delineated by a regional bathymetric high, is a structurally complex zone formed as a result of rift propagation and convergence between the Rivera and Cocos plates. The central zone, delineated by normal oceanic depths and continuous seafloor spreading produced magnetic lineations, is a zone of undisturbed lithosphere. The nature and spatial relationships of these zones are best explained if the present day Rivera-Cocos Euler pole lies within the central morphotectonic zone, a location which is a permissible solution to plate motion results. These results, in conjunction with the morphotectonic features present in the eastern Pacific and western Mexico, suggest that the fragmentation of the proto-Cocos Plate into the Rivera and Cocos plates was initiated around 5.0 Ma due to the approach of the East Pacific Rise towards the northern terminus of the Middle America Trench. The reorientation of plate driving and resistive forces associated with the attempted subduction of young lithosphere produced changes in the motion of the proto-Cocos Plate. These changes resulted in the formation of a southwestward propagating rift along the Rivera-Cocos plate boundary, the present day manifestation of which is the El Gordo Graben. Free-air gravity data suggests that this rift may still be propagating towards the East Pacific Rise; when it reaches the East Pacific Rise, the separation of the Rivera and Cocos plates will be complete. Also the change in motion of the subducting proto-Cocos Plate fragmented the overriding North American Plate producing the Jalisco Block. Morphologic relationships and plate motion results also suggest that a new fragmentation event, the breakup of the Cocos Plate along the Orozco Fracture Zone and further deformation within western Mexico, is occurring due to the approach of the northern terminus of the Pacific-Cocos Spreading Center towards the Middle America Trench