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Geological Society, London, Special Publications; 2004; v. 238; p. 77-107;
DOI: 10.1144/GSL.SP.2004.238.01.08
© 2004 Geological Society of London

Sedimentary Fabrics

The anisotropy of magnetic susceptibility (AMS) in low-grade, cleaved pelitic rocks: influence of cleavage/bedding angle and type and relative orientation of magnetic carriers

Timothy N. Debacker1, Philippe Robion2 & Manuel Sintubin3

1 Structural Geology & Tectonics Group, Katholieke Universiteit Leuven, Redingenstraat 16, B-3000 Leuven, Belgium timothy.debacker{at}geo.kuleuven.ac.be
2 CNRS-UMR 7072 ‘Laboratoire de Tectonique’, Département des Sciences de la Terre et de l’Environnement, Université de Cergy-Pontoise, Avenue du Parc-le Campus- Bat I, 95031 Cergy Pontoise, France philippe.robion{at}geol.u-cergy.fr
3 Structural Geology & Tectonics Group, Katholieke Universiteit Leuven, Redingenstraat 16, B-3000 Leuven, Belgium manuel.sintubin{at}geo.kuleuven.ac.be

Cambrian and Silurian, low-grade, pelitic rocks of the single-phase deformed Brabant Massif consistently have a maximum magnetic susceptibility axis (K1) parallel to the cleavage/bedding intersection. In contrast, the minimum susceptibility axis (K3) either coincides with the bedding pole, with the cleavage pole or occupies an intermediate position. Anisotropy of anhysteretic remanence (AARM) and X-ray pole figure goniometry allow the distinguishing of the orientation distributions of the ferromagnetic and paramagnetic (white mica and chlorite) carriers, respectively. Mismatches between K3 and the poles to the macroscopic fabric elements (i.e. bedding and cleavage) are attributed to different orientations of the different magnetic (s.l.) carriers. A strong relationship exists between the cleavage/bedding angle and the shape parameter: low, respectively high angles leading to oblate, respectively prolate susceptibility ellipsoids. However, differences are observed between the Cambrian and Silurian samples in terms of the shape parameter and the behaviour of the degree of anisotropy with changing cleavage/bedding angle. This is tentatively attributed to differences in relative orientation and mineralogy of the magnetic (s.l.) carriers. These results demonstrate the influence of the relative orientation of the different carriers on AMS and suggest that, although being a petrofabric tool, AMS cannot be used as a strain gauge in the case of composite magnetic fabrics.