SPREADING-RATE-DEPENDENT MAGNETIZATION OF THE OCEANIC LITHOSPHERE INFERRED FROM THE ANOMALOUS SKEWNESS OF MARINE MAGNETIC-ANOMALIES

The anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km Myr(-1). Previous magnetization models of the oceanic lithosphere fail to explain this observation. We propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate. Anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization, and the ratio of layer 3 over layer 2A saturation magnetizations varies linearly from about 0.4 at a spreading rate of 10 km Myr(-1) to about 0 at 55 km Myr(-1). For spreading rates higher than 55 km Myr(-1), layer 3 and uppermost mantle magnetizations are negligible. Recent advances concerning accretionary processes at mid-ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation, alteration of layer 2A, and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate.