MAGNETIZATION OF 0-29 MA OCEAN CRUST ON THE MID-ATLANTIC RIDGE, 25-DEGREES-30' TO 27-DEGREES-10'N

A sea-surface magnetic survey over the west flank of the Mid-Atlantic Ridge from 0 to 29 Ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted c rust. We find that magnetization decays rapidly within the first few m illion years, although the filtering effect of water depth on the sea- surface data and the slow spreading rate (<13 km/m.y.) preclude us fro m resolving this decay rate. A distinctly asymmetric, along-axis patte rn of crustal magnetization is rapidly attenuated off-axis, suggesting that magnetization dominated by extrusive lavas on-axis is reduced of f-axis to a background value. Off-axis, we find a statistically signif icant correlation between crustal magnetization and apparent crustal t hickness with thin crust tending to be more positively magnetized than thicker crust, indicative of induced magnetization in thin inside cor ner (IC) crust. In general, we find that off-axis segment ends show an induced magnetization component regardless of polarity and that IC se gment ends tend to have slightly more induced component compared with outside corner (OC) segment ends, possibly due to serpentinized upperm ost mantle at IC ends. We find that remanent magnetization is also red uced at segment ends, but there is no correlation with inside or outsi de corner crust, even though they have very different crustal thicknes ses. This indicates that remanent magnetization off-axis is independen t of crustal thickness, bulk composition, and the presence or absence of extrusives. Remanence reduction at segment ends is thought to be pr imarily due to alteration of lower crust in OC crust and a combination of crustal thinning and alteration in IC crust. From all these observ ations, we infer that the remanent magnetization of extrusive crust is strongly attenuated off-axis, and that magnetization of the lower cru st may be the dominant source for off-axis magnetic anomalies.