During the summer of 1987, magnetic anomaly data were collected by surface ship as part of an extensive SeaMARC II investigation of the East Pacific Rise (EPR) from 13¿N to the Orozco transform. The survey extended to either side of the rise axis onto seafloor at least 1.8 million years (m.y.) in age, enabling the recent evolution of the structural and kinematic framework of the plate boundary to be studied in detail. North of 13¿50'N there has been a major perturbation in the evolution of the plate boundary. Swaths of lineaments that trend oblique to EPR-parallel topography form a north pointing, V-shaped discordant zone on the Pacific and Cocos plates that is broadly symmetric about the EPR axis. On the Pacific plate a zone of discordant morphology 130 km long and between 6 and 14 km wide with a structural grain that is highly oblique to the present-day spreading direction is observed on seafloor 0.9--1.8 m.y. in age. A similar but more subtle feature of the same age is also present on the Cocos plate. These zones of discordant lineaments can be correlated with changes in the magnetic lineation pattern. On the Pacific plate the disturbed zone lies between anomalies J and 2, creating greater than normal distance between the anomalies. On the Cocos plate the disturbed zone is characterized by a distinct, high-amplitude, northwestward trending magnetic anomaly. The observed structural grain and the changes in the magnetic anomaly patterns associated with the disturbed zones are very similar to those observed at propagating ridges.

Based on the magnetic anomalies, a propagation rate of 10.8 cm/yr in a N10¿W direction is estimated for the past 1.8 m.y. A detailed examination of the structures developed within the disturbed zone on the Pacific plate indicates that the rift propagation in this area can best be explained by the model of Wilson (1990) which involves cyclic rift failure with inward curvature of both rift tips. Plate adjustment to the propagation event is ongoing. There is a pronounced change in the morphology of the rise axis with distance from the propagation event. Close to and for ~100 km behind the propagator tip, the EPR crest is not well developed and is characterized by a series of low (relief of 100--200 m) ridges and troughs with a poorly defined neovolcanic zone. Further away (>100 km) from the propagator tip the rise crest is a single, linear, horst-shaped ridge with a well-developed narrow axial graben at the axis of the ridge. Regional morphologic and magnetic data suggest that this cycle of rift propagation may have begun approximately 2.5 million years before present (m.y.B.P.) near the eastern ridge-transform intersection of the paleo-O'Gorman transform and is continuing today at the Orozco transform. ¿ American Geophysical Union 1992