TECTONIC APPLICATIONS OF MAGNETIC-SUSCEPTIBILITY AND ITS ANISOTROPY

Anisotropy of low field magnetic susceptibility (AMS) is a versatile p etrofabric tool. For magnetite, AMS primarily defines grain-shape anis otropy; for other minerals, AMS expresses crystallographic control on magnetic properties. Thus, we may infer the orientation-distribution o f a dominant mineral from the AMS of a rock. AMS principal directions can record current directions from sediment, flow-directions from magm a, finite-strain directions from tectonized rocks and stress-direction s from low-strain, low-temperature, neotectonic environments. AMS meas urements may reveal some aspects of the strain-path, where carefully s elected. For example, we may compare different parts of a heterogeneou sly strained domain, different minerals in a homogeneously strained si te, AMS with schistosity/mineral lineation, and AMS with remanence-ani sotropy. Such measurements isolate the orientation-distributions of di fferent minerals, adding a temporal scale to the kinematic sequence. N ormally, we can interpret the principal directions of AMS distribution s as a physically significant direction, such as a current direction, magmatic flow or finite-strain axis. However, calibrating the AMS elli psoid shape against the magnitude of the controlling physical process is very difficult. Primarily, this is because the shape of the AMS ell ipsoid combines contributions from several minerals whose individual A MS ellipsoids are of different shape. Thus, small variations in the pr oportions of minerals change the shape of the rock's AMS ellipsoid, ev en if the alignment process were of constant intensity. In deformed ro cks, AMS is more strain-sensitive than calcite twinning or the alignme nt of calcite or quartz c-axes. Not all AMS fabrics relate to crystall ographic or grain alignment. First, displacement fabrics generate AMS where an isotropic matrix of high susceptibility displaces unevenly sp aced objects of low susceptibility and suitable scales. Second, AMS lo cation fabrics occur where sub-isometric magnetite grains are close en ough, in certain directions, for their demagnetizing fields to interac t. This accounts for the AMS of many magnetite-dominated signals where there is no aligned magnetite. Third, the AMS of single-domain magnet ite is inverse to shape so that such grains may oppose the AMS contrib ution of parallel minerals. Finally, transitional sedimentary-tectonic or magmatic-tectonic fabrics yield smeared, temporal sequences of AMS principal directions that cannot be immediately attributed to a singl e alignment process. These transitional AMS ellipsoids mix primary and secondary AMS components, making it difficult to characterize either component. However, such fabric combinations may permit us to recogniz e the sense of shearing in flow processes.