A Matrix Method for Interpreting Oceanic Magnetic Anomalies

Abstract

Summary This paper presents a matrix method for deducing the distribution of magnetization within a specified layer which causes a given magnetic anomaly. It extends the earlier method given by Bott by allowing irregular upper and lower surfaces for the magnetic layer. The method is particularly applicable to interpretation of oceanic magnetic anomalies in terms of a magnetization distribution within Layer 2 of the oceanic crust. It provides a method for applying the Vine-Matthews hypothesis to regions of irregular topography. The method is applied to a profile across the Sheba Ridge, Gulf of Aden. Strip-like magnetic anomalies of several hundred gamma amplitude were first discovered off the California coast (Raff & Mason 1961; Mason & Raff 1961) and are typical of much of the oceanic regions. The strip-like anomalies tend to be parallel to the crest of mid-ocean ridges. The normal interpretation of the anomalies is in terms of the hypothesis of Vine & Matthews (1963) which is itself dependent on the idea of ocean-floor spreading. Following the Vine-Matthews hypothesis, the study of oceanic magnetic anomalies has made outstandingly important contributions to our knowledge of the history of formation of ocean basins in relation to continental drift, and to determining the time-scale of reversals of the Earth’s magnetic field over the last 70 My and more. Vine (1966) and Heirtzler er al. (1968) have used the indirect method of magnetic interpretation to show that typical magnetic profiles of several hundreds of kilometres length can be simulated by a series of two-dimensional rectangular blocks of alternating magnetic polarity which are symmetrical about the ocean ridge crests. These blocks nominally represent Layer 2 of the oceanic crust, which is 2km thick on average: indeed the main source of the anomalies must lie within Layer 2 because Layer 1 consists of effectively non-magnetic sediments and Layer 3 is too deep to explain the anomalies without unacceptably high and irregular variations in magnetization (Bott 1967). A particular sequence of blocks representing the past time-scale of reversals of the geomagnetic field over the last 70 My has been shown by Heirtzler et al. (1968) to give good agreement with observations across the ocean ridge system at widely separated places, provided different relative spreading rates are assumed.