Edited by F. Martin-Hernandez, C. Luneburg, C. Aubourg and M. Jackson. The Geological Society of London Special Publication No. 238, 2004 ISBN-13: 978-1-86239-170-3 Price ,135.00, ,81.00 other qualifying societies, ,67.50 Fellow’s price Hardcover – 560 p.
1 The history of a rock’s formation and deformation is recorded, in part, by the alignment of its constituent minerals. Petrofabric, the study of the preferred alignment of minerals and rocks, is used as a proxy measurement for paleocurrent directions in sedimentary rocks, emplacement flow directions in igneous rocks, and strain directions in deformed and metamorphosed rocks. These proxies do not necessarily mimic their targets, so a great deal of clever field work, lab work and analysis must be done to determine their applicability.
2 Most techniques used to measure petrofabric (pebble long-axis counts, universal stage optical microscope measurements of oriented thin sections, and x-ray or neutron goniometry) are tedious and time consuming and only account for the orientations of small numbers of grains. In contrast, measurement of anisotropy of magnetic susceptibility (AMS) is a rapid and sensitive petrofabric tool. It takes less than 3 minutes a sample, and integrates the fabric of all the grains in a core specimen.
3 Application of magnetic anisotropy is often complicated because the relationship between magnetic fabric and petrofabric is not direct. In rocks that host ferromagnetic minerals, such as magnetite, the magnetic fabric signature is dominated by those grains. Iron oxides and sulfides are sensitive to the primary, metamorphic and environmental history of the rocks, leading to both the strength but also the complexity of the method. In rocks lacking a ferromagnetic component, the alignment of paramagnetic (mostly silicates) and diamagnetic (mostly carbonates and quartz) minerals is observed. Usually the major axis of the AMS ellipsoid is parallel to crystal lineation, and the minimum axis is perpendicular to foliation. The magnitudes of the ellipsoid axes, however, depend, in a complex way, with the alignment forces such as finite strain magnitude.
4 In 1954, John Graham published "Magnetic Susceptibility Anisotropy, an Unexploited Petrofabric Element". On the golden anniversary of that trail-blazing publication, the Geological Society of London published its Special Publication No. 238 on "Magnetic Fabric: Methods and Applications". This book reveals the development of this research field over the last 5 decades. There are papers on the history of the method, its physical foundations, instrumental aspects, and current examples of how anisotropy results can be applied to many areas in the earth sciences. The papers do not shy away from presenting important limitations and unresolved problems.
5 Based on special sessions in 2003 at the Joint Assembly of the EGS-AGU-EUG in Nice and the AGU Fall Meeting in San Francisco, this volume is more than a collection of unrelated current research papers. Several reviews are featured, including the fine overview by the editors. The centrepiece is Borradaile and Jackson’s 62 page review of "Anisotropy of magnetic susceptibility (AMS): magnetic petrofabrics of deformed rocks", including an extremely useful 3 page glossary. This paper is rigorous but fully readable. It also introduces a clever new polar plot to display fabric data.
6 The book is divided into 5 sections. The first, "Magnetic Fabric Characterization Methods and Mineral Sources" (6 papers), includes reviews on laboratory methods (Potter) and statistical methods (Jezek and Hrouda). The second section, Sedimentary Fabrics, features 6 case studies. Unfortunately, only one of these deals with the important application of depositional flow directions (Matasova and Kazansky’s impressive study on Siberian loess formation including analysis of wind direction). The rest investigate deformation recorded in sedimentary rocks, and thus should be placed in one of the last sections.
7 The next section on Igneous Fabrics is quite humbling for the method. While the study by Petronis et al. concludes that magnetic fabrics are parallel to emplacement directions in a lacolith, the other 3 papers suggest that magnetic fabrics in igneous rocks are, at best, complicated functions of flow direction, mineralogy, and boundary conditions. In contrast, the fourth section on Tectonic Fabrics (6 papers) includes several successful studies for magnetic methods. I would like to highlight the paper by Chadima et al. who compare neutron goniometry to low and high field magnetic fabrics on deformed Bohemian metasediments.
8 The last section, on Complex Fabrics, Superposition and Alteration has 4 papers revealing difficulties in magnetic methods on rocks with rich geological histories. A particularly impressive work (De Wall and Warr) used heat treatment on the rocks to oxidize siderite in order to the more magnetic magnetite to make better measurements. They applied a paleomagnetic tilt test to the magnetic fabrics to establish the relative timing of diagenesis and deformation.
9 The book includes most of the significant members of the magnetic fabric community. Two authors stand out in the collection: František Hrouda with four papers, and Graham Borradaile with three. They have been leaders over the recent decades, and it is good to see them properly represented here. I am sorry not to see a paper by Ken Kodama or his colleagues who have been working on magnetic fabrics and the deflection of magnetic remanence (e.g. inclination shallowing) which has an important application to paleomagnetic studies.
10 This Geological Society Special Publication is a substantial book, much larger than their typical 200 to 400 pages. While it may be too expensive for individuals to purchase, it should be included in all earth science library collections. Researchers in all fields concerned with rock fabric and deformation will find it useful.