Major problems regarding the magnetization of oceanic crustal layer 2

Abstract

The recognition that the linear magnetic anomaly patterns of the ocean basins can be interpreted in terms of geomagnetic field reversal history and the success in anomaly modeling using models consisting of sequences of normally and reversely dipole-magnetized rectangular prisms, have led to a number of estimates for the thickness of a magnetized layer capable of accounting for the observed anomaly patterns. However, the picture emerging is one of directional complexity, and a major problem for the International Phase of Ocean Drilling (IPOD) will be to explain how magnetically complex oceanic basement often gives rise to a relatively simple, two-dimensional anomaly pattern. Features of this real complexity that are discussed are the unexpectedly frequent occurrence of reversely magnetized basalts within the median valley of the mid-Atlantic ridge, the occurrence of reversals in vertical sections through layer 2, the occurrence of long sections through layer 2 basalts that are uniformly magnetized in directions well away from expected dipole field inclinations, and the effects of significant presence of intrusives within layer 2 lava flow sequences. A critical evaluation of usefulness of magnetic studies of oceanic layer 2 is given in which it is demonstrated that some conventional targets of magnetic studies such as absolute plate motion are unlikely to be achieved, while novel features such as contributions to our knowledge of how the layer is formed and later tectonically and chemically altered are readily apparent.