Magnetic Anomalies and Plate Tectonics with Applications in the Southern Oceans

Abstract

The development of the continental drift and plate tectonics theories are reviewed historically and present day developments are outlined. A method for navigational, magnetic and bathymetric data reduction and presentation, developed for this work, is described with examples from profiles measured in the south Pacific. Modelling of marine magnetic anomalies is reviewed historically and the use of the current model for the magnetic source is discussed. This is investigated in relation to the use of different parameters and magnetised layer configurations, and failures of the model are outlined. Models for the structure of the oceanic crust are discussed in the light of seimicity data and petrological and geological evidence. The magnetic properties of crustal rocks are reviewed and determinations of the thickness and magnetisation of the magnetised layer are discussed. An alternative model for the magnetic source derived from the preceeding studies is presented. In this model the magnetised layer is defined by the Curie point isotherm and uniform magnetisation with depth is assumed. The sloping boundaries are proposed as an explanation of the observed skewness in profiles. Other alternatives are critically reviewed. Thermal models for mid-ocean ridges and resulting isotherms are examined. The proposed model is tested with several profiles from the Indian and Pacific oceans and a two-layer variation is examined for regions close to ridge axes. The model is shown to be very promising and to meet many of the objections advanced against the model in current use. The development of geomagnetic reversal time scales is reviewed and the time scale used in this work is discussed. An attempt made to produce time sequences from two measured anomaly profiles is described. The failure of this is discussed and movement of the ridge is advanced as an explanation. Methods for deriving poles of rotation and rates of opening are reviewed. A least squares method, using magnetic anomaly data, is described. The extension of this to determine uncertainties is discussed. Pole positions and angles of opening obtained with this model are shown to agree, within derived uncertainties, with other determinations for Indian-Antarctic and Pacific-Antarctic poles of opening. The method has the advantage of producing statistically sound ellipses of confidence. The entire work is reviewed and future avenues of investigation are suggested.