A one‐million‐year history of the 11°45′N East Pacific Rise discontinuity

Abstract

Bathymetric, magnetic and side scan sonar data from a June 1987 SeaMARC II cruise aboard the R/V Moana Wave have been combined with additional data in an area 140 km by 154 km centered on the overlapping spreading center pair (OSC) located at 11°45′N on the East Pacific Rise (EPR). The purpose of this study is to correlate and interpret these data sets in order to determine the recent (<1 m.y.) history of this OSC. The data sets include gridded bathymetry and magnetic field anomalies, a side scan sonar mosaic, and a fault map. The bathymetry and anomaly grids were used as inputs in a three‐dimensional Fourier inversion resulting in a crustal rock magnetization distribution solution. The most prominent feature of all five maps is a roughly V‐shaped anomalous zone centered on the OSC. The zone is characterized by a positive depth anomaly, a negative magnetic field anomaly within the central (positive) anomaly and a zone of high magnetization in the inversion solution. A hummock y texture and sparse, curved faults mark the zone in the side scan mosaic and fault map. The shape of this zone indicates a short period of northward migration of the discontinuity 0.72–0.6 Ma at approximately 75 mm/yr followed by slow (less than 30 mm/yr) and then fast (140–200 mm/yr) southward migration for the past 0.6 m.y.. The anomalous zone is not well defined beyond the edge of the central anomaly which is straight and continuous, indicating that if this discontinuity of the EPR existed 0.72 Ma, its offset was negligible. The present‐day axis has an orientation of 349°, the edge of the central anomaly has an orientation of 353°, and Jaramillo‐age crust to the east of the EPR has an orientation of 353°–355°. This variation in magnetic linealion directions suggests a 4° to 6°‐counter‐clockwise change in the direction of spreading in the past 1 m.y. which seems to have been accommodated by asymmetric spreading. Variations in mean fault trend with age are consistent with a counter‐clockwise change in the direction of spreading for the same time period.