Applicability of magnetic fabrics in rocks associated with the emplacement of salt structures (the Bicorb-Quesa and Navarre´s salt walls, Prebetics, SE Spain)

Abstract

This work assesses the applicability of the analysis of the anisotropy of magnetic susceptibility (AMS) to rocks associated with salt structures. Magnetic fabrics from 16 sites sampled on Upper Triassic clays (Keuper facies) interbedded within the salt layers of the Bicorb-Quesa and Navarre´s salt walls (Prebetic Zone, SE Spain) are mostly characterized by oblate magnetic ellipsoids with their magnetic foliation parallel to the bedding and magnetic lineation contained in the bedding plane but without a constant relationship with dip direction. To evaluate the quality and significance of the low-field AMS, several experiments were carried out to characterize the magnetic mineralogy of representative samples and enable separation of the contributions from paramagnetic and ferromagnetic (s.l.) minerals. The comparison between the orientation of the observed magnetic lineation and that of the structural elements characterizing the internal geometry of the studied salt structures (antiforms, minor folds, faults and shear zones) and flanks indicates that the magnetic fabric is controlled by deformation processes affecting the salt walls during their growth and/or evolution. Samples were also analyzed from sixteen sites from Miocene rocks outcropping in several syn-diapiric Miocene half-grabens related to the salt walls studied. Their magnetic fabric is interpreted to reflect the dominant stretching direction at the syn-diapiric half grabens during and/or shortly after deposition. Our results indicate that caution is required in interpreting magnetic lineations related to salt flow mechanics from rocks associated with salt structures.