Palaeomagnetic studies on rock formations in the High Atlas and Anti-Atlas regions of Morocco

Abstract

The first part of this thesis (Chapters I to 8) describes the results of palaeomagnetic studies on samples collected from the High- Atlas and Anti-Atlas regions of Morocco. New palaeomagnetic pole positions are described from formations of Late Precambrian, Lower Cambrian, Devonian, Triassic, Jurassic and Cretaceous ages. The new Moroccan Late Precambrian and Lower Cambrian poles are shownto be more consistent with reliable Lower Palaeozoic poles of the same age from North America than ones from Southeast Africa, when the two continents are plotted on a Bullard-type reconstruction. It is concluded that a small part of Northwest Africa, including Morocco, may originally have been attached to North America, and that the rest of Africa was widely separated from North America at that time. Collision between Africa and Eastern North America probably occurred during the Carboniferous period, resulting in the formation of the Hercynian fold-belts along the present-day North Atlantic margins of the two continents and the welding of extreme Northwest Africa to the rest of the continent. Subsequent separation probably took place along a slightly different line, so that Morocco was left attached to Africa. Radiometric dating results from Moroccan igneous intrusions paralleling the local coastline yielded a mean age of 183 my, i. e. Lower Juraszic. other evidence suggests that the Mesozoic rifting between North America and Northwest Africa began at this time, and it is considered likely that the intrusion of the Moroccan Lower Jurassic igneous bodies was directly related to this event. Palaeomagnetic results from the Moroccan Cretaceous sediments indicate a gradual widening of the North Atlantic Ocean during the course of the Mesozoic. There are no significant differences between mean Triassic, Jurassic and Cretaceous poles from Morocco, and corresponding poles from South-east and Central Africa, and it is concluded that no significant post-Triassic relative tectonic movement has occurrcd between North-west and South-east Africa. The second part of the thesis (Chapters 9 and 10) describes an attempt at the AF demagnetisation of sediments by means of alternating fields of strangths up to 5000 Oe. Experiments were performed to investigate the effects of small asymmetries (up to 5%) in waveform of the applied alternating field, and it was shown that asymmetrics present in 'the natural AC mains supply at the Close House Laboratory at certain times, of the day are sufficiently high to produce appreciable AMM's in rock samples, if no sample rotation system is used during the demao-natisation process