Anisotropy of Magnetic Susceptibility

Abstract

Anisotropy of magnetic susceptibility is an important technique which depicts preferred orientation of magnetic minerals in a rock or unconsolidated sediments. Hence the property is used for study of primary structures and rock fabric. The technique is non-destructive and can be used in nearly all types of rocks because it does not need a rock to contain specific strain markers like deformed fossils, reduction spots, ooids, etc. The method has an advantage as it can determine weak deformation even where lineation and foliation have not developed. In rocks with well developed tectonic fabrics, the principal magnetic susceptibility directions are closely related to orientation of structural features (e.g. fold, fault, foliation, lineation). Different types of AMS fabrics are described. Differences between magnetic and petrofabric strains are highlighted. Importance of sampling in a region of superimposed deformation is described. It is emphasized that objectives of the study should be formulated prior to selection of sample sites. Hrouda diagram is described for understanding the roles of simple and pure shear deformations in a region of simultaneous development of folding and thrusting. The technique has been successfully employed to ascertain the displacement patterns along some of the prominent Lower Himalayan thrusts.