Palaeomagnetism

The Mediterranean region represents a complex mosaic of continental, microcontinental and ophiolitic terranes, whose overall evolution has been controlled by relative movements between the African and Eurasian plates. Palaeomagnetic studies have played an important part in deciphering the sequence of tectonic events in this region. The 33 papers presented here span the full width of the Mediterranean basin and present results from Permian to Quaternary rocks.

Palaeomagnetism and archaeomagnetism are fascinating specialized studies because they are applicable to such a wide range of problems in geology, archaeology and geophysics. They can also be undertaken cheaply, when compared with most other geophysical techniques, and, at first sight, simply. In fact, real comprehension of the magnetic processes that have occurred in rocks and other types of material over several thousands or many millions of years is still extremely difficult to assess and measure. On this basis, this book cannot explain all such features, nor can it attempt to cover all the actual and potential applications of the method. All that can be attempted is to give an impression of the ways in which such techniques can be used in a wide variety of fields, and how these techniques are usually applied. The magnetization of rocks is, in fact, one of the earliest of the true sciences, but we are still not in a position to answer many of the problems posed. Consequently some of the examples given of applications are, essentially, state-of-the-art comments, rather than being a review as such. The changing position of the geomagnetic poles with time is still not adequately defined, for example, and some of the more recent conventional views are given, although the emphasis is placed on more subjective, probably more controversial, evaluations.

Palaeomagnetism is a technique used to understand complex deformation patterns in fold-and-thrust belts; it can be used to characterize the distribution, magnitude and timing of vertical axis rotations, an elusive variable using other methods. A combination of palaeomagnetic and structural geology analyses has helped to unravel the geometry and kinematics of fold-and-thrust belts around the world and of different geological ages for more than 50 years. This volume comprises three sections: the first shows thorough overviews of western Mediterranean arcs and the western Carpathians; the second depicts several examples from the Andes, the Alps, Anatolia, Pyrenees, Iberian Ranges and the Atlas; and the third shows the latest research on the use of palaeomagnetism to understand fold-and-thrust belts in 3D and 4D in a more quantitative way and it also includes some methodological proposals to avoid common errors. In the papers of the first two sections, the combination of palaeomagnetic analyses with structural data, AMS or magnetostratigraphic analyses demonstrate the usefulness of palaeomagnetism in deciphering complex deformation patterns in fold-and-thrust belts.

During the last 30 years the study of the magnetic properties of rocks and minerals has substantially contributed to several fields of science. Perhaps the best known and most significant advances have resulted from the study of palaeomagnetism, which led to quantitative confirmation of continental drift and polar wandering through interpretation of the direction of remanent magnetism observed in rocks of different ages from different continents. Palaeomagnetism has also, through observations of reversals of magnetiz ation, ancient secular variation and ancient field intensities provided data relevant to the origin of the geomagnetic field, and other investigations have contributed significantly to large-scale and local geological studies, the dating of archaeological events and artefacts and more recently to lunar and meteoritic studies. Rock and mineral magnetism has proved to be an interesting study in its own right through the complex magnetic properties and interactions observed in the iron-titanium oxide and iron sulphide minerals, as well as contributing to our understanding of remanent magnetism and magnetization processes in rocks. Simultaneous with the development of these studies has been the develop ment of instruments and techniques for the wide range of investigations involved.

Methods in Paleomagnetism covers the proceedings of the NATO Advanced Study Institute on Paleomagnetic Methods, held in the Physics Department of the University of Newcastle upon Tyne on April 1-10, 1964. The book focuses on apparatus and techniques used in paleomagnetism and rock magnetism. The selection first offers information on sampling techniques in the field and measurement of natural remanent magnetization. Discussions focus on ballistic and spinner magnetometers; paleomagnetic sampling with a portable coring drill; portable apparatus for collecting small oriented cores; and portable field-sampling equipment. The book also takes a look at procedures to test the stability of magnetization, as well as physical properties of demagnetization; thermal demagnetization by the continuous method; and apparatus for thermal demagnetization by the progressive method. The text ponders on measurement of isotropic and anisotropic susceptibility and magnetic measurements in applied fields. Topics include preliminary account of a refined technique for magnetic susceptibility anisotropy measurement of rocks; errors in anisotropy measurements with the torsion balance; and measurement of the anisotropy of the susceptibility with an astatic magnetometer. The selection is a valuable reference for readers interested in paleomagnetism.

This volume presents lectures given at the NATO Advanced Study Institute held 11-22 April 1988 at Newcastle upon Tyne, England. The aim of the Institute was to improve the interaction between workers in observational geomagnetism (using historical data) and archaeo- and palaeo-magnetism (using the remanent magnetization of man-made artefacts and of natural sediments and rocks) and those trying to interpret the data in terms of mechanisms inside or outside the Earth, particularly those developing dynamo theories of the field. The material discussed ranged from magnetic bacteria swimming round a circle in a few seconds, the effect of El Nino, through secular variation with time scales of tens to thousands of years and the'mechanics of individual field reversals and excursions (aborted reversals?) to possible modulation of average reversal frequency on the hundred million year time scale. Many members of the Physics Department helped with the organization, and we are most grateful to them, and in particular to Anne Codling for her very many contributions. We also gratefully acknowledge the painstaking work of Aileen Dryburgh and Lynn Whiteford in so carefully typing the manuscript.