Magnetic fabric: methods and applications — an introduction
F. Martín-Hernández, C. M. Lüneburg, C. Aubourg, M. Jackson, 2004. "Magnetic fabric: methods and applications — an introduction", Magnetic Fabric: Methods and Applications, F. Martín-Hernández, C. M. Lüneburg, C. Aubourg, M. Jackson
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Fifty years have now passed since Graham (1954) published his seminal paper advocating the use of anisotropy of magnetic susceptibility (AMS) as a rapid and sensitive petrofabric tool. During these five decades, Graham’s ‘under-exploited’ method has become standard, and AMS and related techniques are now routinely applied to characterizing fabrics in a wide variety of geological materials (e.g. the GEOREF database lists over 500 journal publications with ‘magnetic anisotropy’ as keywords).
Magnetic anisotropy works as a petrofabric tool because individual grains of most minerals are magnetically anisotropic, i.e. easier to magnetize in certain orientations, which are governed primarily by crystallography and/or grain shape. Magnetic anisotropy at the bulk rock scale results from the preferred crystallographic orientation (PCO) and/or preferred dimensional orientation (PDO) of anisotropic mineral grains. AMS can also result from magnetostatic interactions among closely spaced, strongly magnetic grains that are heterogeneously distributed in a matrix of more weakly magnetic minerals. In either case, magnetic anisotropy is directly related to some aspects of rock fabric, and thus it provides a quick, simple and effective characterization tool, even though the relationship between magnetic fabric and petrofabric is quite complex in detail.
The present collection of papers originated, in part, at a special session on magnetic fabrics at the Joint Assembly of the EGS-AGU-EUG (April 2003) in Nice, which highlighted recent methodological advances, theoretical and experimental studies, and characterization of flow and deformation fabrics in rocks and sediments. A similar session at the AGU Fall Meeting (December 2003) in San
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Fabric is a ubiquitous and significant feature of geological materials. The processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains. Petrofabrics thus provide essential keys to understanding the history of geological materials. Magnetic anisotropy is directly related to petrofabric, and has become one of the most rapid, sensitive and widely used tools for its characterization. The relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition, concentration and grain size of mineral grains. Ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved.
The papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations, mineral fabric development, technical advances and rock-magnetic properties.