Significance of AMS in multilayer systems in fold-and-thrust belts. A case study from the Eocene turbidites in the southern Pyrenees (Spain)



The anisotropy of magnetic susceptibility (AMS), and other magnetic fabric techniques have been tested in a multilayer system of Eocene turbidites from the Southern Pyrenees. The identified magnetic fabrics can be grouped into (1) fabrics related to layer parallel shortening (LPS) acquired in pre-folding conditions, (2) fabrics related to cleavage development and (3) fabrics related to shear parallel to the thrust movement and intermediate cases. AMS depends on (i) the paramagnetic contribution to the susceptibility and (ii) on the position of sites in the fold-thrust belt. The study of sites in the southern Pyrenees shows that the processes underwent by rocks and recorded by the different magnetic fabrics are independent from their lithology. The most paramagnetic samples show fabrics related to LPS and cleavage, whereas shear parallel to the thrust movement is observed in the most ferromagnetic ones. Measurement of other magnetic fabrics shows the presence of more than one magnetic fabric at site scale with independence of the AMS ellipsoid aspect ratio. The intersection lineation fabrics are interpreted as composite magnetic fabrics, developing a progression between LPS, intersection fabrics (from magnetic foliation parallel to bedding, to magnetic foliation parallel to cleavage) and magnetic axes switching from K1 parallel to the intersection lineation, to K1 parallel to the dip direction of cleavage or amplification direction. This change can be inferred from the transition from the outer (farther from the Axial Zone) to inner domains of the studied zone, consistent with the intensity of cleavage at outcrop scale. Copyright © 2010 John Wiley & Sons, Ltd.