The structural features and evolution of the northern Nazca plate can be explained by the existence of an east-trending spreading center in the eastern Pacific from approximately 40 to 25 m.y. ago, and reorganization of spreading centers about 10 m.y. ago. East-west anomalies from the Oligocene spreading center are found between the Carnegie Ridge and Grijalva scarp to the south. The offset along the scarp is the result of a northeastward-increasing age difference between crust to the north and south. Volcanic extrusions during a period of very slow spreading along this center may have built the ancestral Carnegie-Cocos Ridge. This east-trending spreading center was reactivated during late Miocene time along the present Galapagos rift zone.