Factors affecting the record of strain fabrics at the anisotropy of magnetic susceptibility: West-Central South-Pyrenean cleavage domain (Southern Pyrenees; NE Spain)

Abstract

Anisotropy of magnetic susceptibility (AMS) is widely used to determine the petrofabric of sedimentary rocks. The identification of magnetic lineation parallel to the stretching direction or normal to the shortening direction is usual in weakly deformed environments. When deformation increase, AMS fabrics progress to magnetic foliation parallel to cleavage and magnetic lineation parallel to the expected amplification direction. In order to analyze the factors influencing the AMS fabrics in an area with a well developed cleavage, four sections across the cleavage Southern Pyrenean domain have been analyzed. The coeval development of cleavage, folding and thrusting and the relatively constant shortening direction during Pyrenean evolution produce an alignment of structures parallel to the tectonic grain of the mountain belt. Factors affecting the AMS cleavage record are related to the presence of ferrimagnetic fabrics linked to the shear parallel to the thrust movement, the influence of previous strain magnetic fabrics affecting both paramagnetic and ferrimagnetic particles, the degree of competition between bedding-related and cleavage-related fabrics and the inhomogeneous deformation linked to local structures. These factors resulted in some cases in deviation of results from the expected cleavage-related magnetic fabrics in a sector with clear observation of cleavage at outcrop scale. The obtained data show that in central zones from the Pyrenees, the expected geometrical relations between strain and AMS are not observed and that a low sensitivity to cleavage can be apparently identified. However, a cleavage imprint in the magnetic parameters can be identified.