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Magnetic anisotropy and X-ray diffraction study of clay minerals in the decollement horizons of the western Helvetic nappes, SW Switzerland

Institut de Géologie, rue E. Argand 11, CH-2007 Neuchêtel, Switzerland

^* E-mail: pierre.dick{at}geol.unine.ch

(Received 14 December 1999; revised 21 November 2000)

Anisotropy of magnetic susceptibility (AMS) and X-ray diffraction studies on shale decollement horizons in the Western Helvetic nappes were conducted to delineate the interactions between clay particles and deformation during a non-coaxial thrusting deformation. The reorientation of magnetic lineations in the decollement horizons, from a NW–SW thrusting direction to a NE–SW oriented lateral escape, provides the opportunity to study the evolution of magnetic fabrics and clay mineralogy during a two-step deformation history. The AMS of these shales originates from paramagnetic phyllosilicate minerals: illite, phengite and chlorite. Clay mineral studies indicate that magnetic lineations parallel to the thrust directions correspond to the spatial organization of the basal planes of phengites, whereas those parallel to the lateral escape movement are caused by illites. This work indicates that the change in direction of magnetic lineations from orogenic contraction to lateral escape not only records the strain history but also the dissolution and neoformation of paramagnetic minerals.

KEYWORDS: magnetic anisotropy, X-ray diffraction, Helvetic nappes, Switzerland