Magnetic fabric and petrographic investigation of hematite-bearing sandstones within ramp-related folds: examples from the South Atlas Front (Morocco)

An investigation of the anisotropy of magnetic susceptibility (AMS) of hematite-bearing sandstones has been carried out along two fault-propagation folds located at the front of the High Atlas Belt (Goulmima area, Morocco). It reveals two types of unusual magnetic fabrics. The first one is always located in the forelimb of the anticlines and is characterised by an oblique magnetic foliation with respect to bedding. The relationship between bedding and magnetic foliation indicates a sense of shear, which is consistent with the one expected from kinematic modelling. It is proposed that such an oblique magnetic foliation is the result of a weak strain that develops during folding. The other striking feature of the AMS results is that 40% of the magnetic lineations are parallel to the regional shortening direction. From structural and petrographical data, it is suggested that this magnetic lineation corresponds to the intersection lineation between two planar elements that are, respectively, the bedding and iron oxides mineralised veins. The latter result from iron-rich fluid circulations through a fracturing network.