Variations in layer 2A thickness and the origin of the central anomaly magnetic high

The seismically determined off‐axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high typically associated with the ridge crest. The positive magnetization contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales (2–3 km) comparable to that of the proposed Layer 2A thickening. We present magnetic remanence data from basalts dredged on and near the East Pacific Rise axis at 12°N which are compatible with a rapid magnetization reduction (∼20 k.y. to decay to 1/e). Together with near bottom magnetic profiles from the ultra‐fast‐spreading East Pacific Rise at 19.5°S, these data suggest that previous estimates of the time constant of alteration inferred from slow‐spreading ridges (0.5 m.y.) may be more than an order of magnitude too high.