Sources of magnetic anomalies on the Mid-Atlantic Ridge

Abstract

STRONGLY magnetised basalts occur at the basement surface in ridge crest areas (see, for example, refs 1, 2) and various models for the magnetic sources have been devised. A uniformly magnetised layer, extending downwards from the basement surface for 400–500 m is consistent with most of the geophysical information, although some older estimates and the low magnetisation of some sets of rocks³ require a layer thickness of as much as 2.5 km. During Leg 37 of the Deep Sea Drilling Project⁴ five basement holes, all deeper than 100 m, were drilled from DV Glomar Challenger along a seafloor spreading flow line from the median valley of the Mid-Atlantic Ridge in the project FAMOUS survey area into the American Plate (Fig. 1). Basement penetration ranged from 108 m to 582 m and totalled 1,453 m with 243 m recovery. Here we report the results of a shipboard palaeomagnetic reconnaissance of 295 vertically oriented basement samples.