Overview of anisotropy of magnetic susceptibility fabrics in Alpine cover rocks from the Southern Central Pyrenees

Abstract

Anisotropy of Magnetic susceptibility (AMS) has been traditionally used as an indirect approach for the characterization of mineral preferred orientation and rock petrofabric. In the Southern Central Pyrenees, a robust data set of magnetic fabrics covering a wide range of ages, structural context, and lithologies has been used for an assessment at the orogen scale, of the sensibility of AMS as a record of strain. AMS results reveal : (i) a dominant bedding-controlled foliation, very sensitive to subtle layer parallel shortening (LPS), especially in the Southern foreland basin ; (ii) magnetic lineations parallel to fold axes or strike fthrusts outside the cleavage front that can be deflected by vertical-axis rotations in particular areas ; and (iii) an overall decrease of either magnetic lineation or the anisotropy of the magnetic ellipsoid associated with incipient cleavage and intersection lineation fabrics in the internal zones of the orogen. These results show a magnetic foliation that is strongly controlled by the bedding/lamination arrangement, and a magnetic lineation with an overall distribution parallel to the Pyrenean orogen but more sensitive to record local deformational processes. These results suggest a strong stability of bedding-related magnetic fabrics, and that only intense deformation processes and development of compression-related cleavage can modify the primary, sedimentary fabric. The distribution of AMS both at regional and local scale evidences that the anisotropy degree of the magnetic ellipsoid is more sensitive than the orientation of the principal susceptibility axes toto strain changess. The presented dataset highlights the requirement of evaluating factors such as mineralogy, age deposit, and pressure-temperature conditions during deformation in order to accurately reconstruct the Pyrenean strain history from the AMS results.