Mathematical simulations of anisotropy of magnetic susceptibility on composite fabrics

The anisotropy of magnetic susceptibility (AMS) is an integral measure of the preferred orientation of all minerals present in a rock. When the AMS is carried by paramagnetic minerals alone, the principal directions of the susceptibility ellipsoid should reflect the crystallographic orientation of the minerals. The relationship between the AMS and deformation depends on several factors, which control the development of lattice-preferred orientation (LPO) in a rock. A mathematical model is presented that simulates the magnetic susceptibility ellipsoid for samples composed of more than one mineral phase. Measurements of the AMS are compared with fabric-based anisotropy models for black slates from the Navia-Alto Sil slate belt in northern Spain. The AMS is carried by paramagnetic minerals, as has been confirmed by high-field torque magnetometry, low-temperature AMS and magnetization curves. The LPO of mica and chlorite has been determined by X-ray texture goniometry. Pole figures and orientation ellipsoids show the changes in the degree of alignment of the phyllosilicates. The models have been tested in samples displaying three types of pole figures: a high-intensity point maximum, a medium-intensity elliptical maximum, and a girdle-shaped distribution. At one site displaying kink bands the LPO shows variations between these types at the outcrop scale. The case studies illustrate the success in modeling different LPO types. Copyright 2005 by the American Geophysical Union.