The magnetic fabric in “undeformed clays”: AMS and neutron texture analyses from the Rif Chain (Morocco)

In this study we analyse the magnetic and mineral fabric of clay sediments that appear homogeneous and not deformed at the outcrop scale. We integrate low-field, high-field and low-temperature AMS analyses with neutron diffraction pole figures analysis as an alternative approach to studying the deformation mechanisms that drive the development of the magnetic lineation in these sediments. The studied sediments are Neogene claystones sampled from the Rif Chain (Morocco). High-field AMS measurements carried out on a large number of samples show that the relative percentage of the ferrimagnetic contribution is not significant (generally < 5%) and paramagnetic contribution prevails (generally > 95%). The principal axes of both components coincide with those of the low-field AMS at room and low temperatures. These results indicate that the magnetic anisotropy of the studied samples is predominantly carried by the paramagnetic phyllosilicates, i.e. chlorite, as deduced by X-ray diffraction. Neutron texture analysis confirms that the different AMS ellipsoids obtained with different magnetic methods reflect the spatial distribution of the basal planes of chlorite, demonstrating the great potential of the AMS to determine the mineral orientation fabric. The combination of these different approaches enables to establish a more accurate qualitative and quantitative correlation between the chlorite fabric and magnetic anisotropy and yields valuable information about the tectonic meaning of the magnetic fabric.