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

Anisotropy of magnetic susceptibility (AMS) and X-ray diffraction studies o n 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 lineati ons in the decollement horizons, from a NW-SW thrusting direction to a NE-S W oriented lateral escape, provides the opportunity to study the evolution of magnetic fabrics and clay mineralogy during a two-step deformation histo ry. The AMS of these shales originates from paramagnetic phyllosilicate min erals: illite, phengite and chlorite. Clay mineral studies indicate that ma gnetic lineations parallel to the thrust directions correspond to the spati al organization of the basal planes of phengites, whereas those parallel to the lateral escape movement are caused by illites. This work indicates tha t the change in direction of magnetic lineations from orogenic contraction to lateral escape not only records the strain history but also the dissolut ion and neoformation of paramagnetic minerals.