Formation of Magnetic Anomaly Pattern of Mid-Atlantic Ridge

Abstract

Surveys of the crest of the Mid-Atlantic Ridge and similar portions of the other mid-ocean ridges characteristically reveal a pattern of magnetic anomalies which have an elongation of about 4:1 parallel with the median line of the ridge. The linear pattern can be roughly simulated by a model in which steep-sided blocks within the oceanic crust between the sea floor and the depth of the Curie point isotherm have alternately normal and reversed directions of remanent magnetization. Vine & Matthews (1963) suggested that this structure might be due to the continued injection of basaltic feeder dykes along the median line of the ridge, each dyke shouldering aside its predecessors and being magnetized in the direction of the Earth's magnetic field. As the direction of the Earth's field reverses periodically this mechanism can account in a crude way for the formation of the model. A survey of the crest of the Mid-Atlantic Ridge at 45°N (Loncarevic, Mason & Matthews 1966) showed a less regular pattern of blocks suggesting that the mechanism of dyke injection is less regular than that utilized in the original theory. It seems plausible that dyke injections do not always occur precisely along the median line but are scattered about it. An analogue model experiment involving marbles, and a computer program written for titan are used to investigate the consequence of such scatter. First results indicate that the great majority of dyke injections must occur within the width of the median valley; the standard deviation of the distribution of dykes (assumed normal) must be less than about 5 km (half the width of the median valley).