Structural geology, petrofabrics and magnetic fabrics (AMS, AARM, AIRM)

Anisotropy of magnetic susceptibility (AMS) was recognized as a feature of minerals in 1899, and petrofabric-compatible AMS fabrics were reported from 1942–1958. Shortly thereafter, cleavage and mineral lineation were associated with the principal axes of the AMS ellipsoid. AMS is describable by a magnitude ellipsoid, somewhat similar in concept to the finite strain ellipsoid, with principal susceptibilities (κMAX, κINT, κMIN) as its axes and their average value being the mean susceptibility (κ). Orientations of the AMS axes usually have a reasonably straightforward structural significance but their magnitudes are more difficult to interpret, being the result of mineral abundances and different mineral-AMS. The strain ellipsoid is dimensionless (i.e., of unit-volume) and readily compared from one outcrop to another but the AMS ellipsoid represents the anisotropy of a physical property. Thus, (κ) determines the relative importance of AMS for different specimens, or compared outcrops, or component AMS subfabrics. AMS provides a petrofabric tool, unlike any other, averaging and sampling the orientation-distribution of all minerals and all subfabrics in a specimen. Sophisticated laboratory techniques may isolate the AMS contributions of certain minerals from one another, and of certain subfabrics (e.g. depositional from tectonic). However, suitable data processing of the basic AMS measurements (κMAX, κINT, κ MIN magnitudes and orientations, and the mean susceptibility, κ) may provide the same information. Thus, AMS provides the structural geologist with a unique tool that may isolate the orientations of subfabrics of different origins (sedimentary, tectonic, tectonic overprints etc.).