Magnetic fabrics in the Western Central-Pyrenees: an overview

Abstract

In the Western Central-Pyrenees numerous investigations during the past years have yielded an exceptional high density of localities (more than 700 sites) where the AMS and rock magnetic properties have been determined. This unique AMS dataset helps in understanding the orogenic evolution of the Pyrenees and its foreland basins. Processes related to AMS development in different structural units permit to identify: (i) an early recording of strain related to the transtensional or transpressional stages during the Early Triassic and Early Cretaceous, (ii) a moderate shortening related to the fold-and-thrust belt and foreland basin evolution during the Cretaceous–Tertiary orogenic stage, and (iii) a relatively strong deformation that produced cleavage related to the emplacement of basement thrusts in the Axial Zone during the Middle–Late Eocene. The main achievement of this contribution is the integration in cross-section and map-view of magnetic fabrics (orientation and parameters) in the western Central-Pyrenees, which allow constraining the processes affecting magnetic fabrics at the orogen scale. AMS results indicate: (i) a dominant bedding-controlled foliation, very sensitive to subtle layer parallel shortening (LPS) processes, especially in the foreland basins; (ii) magnetic lineations parallel to fold axes or strike of thrusts outside the cleavage front that can be deflected by vertical-axis rotations in particular areas; and (iii) an overall decrease of the definition of AMS fabrics (magnetic lineation or the anisotropy of the ellipsoid) associated with incipient cleavage and intersection lineation fabrics in the internal zones of the orogen. Therefore, AMS is a sensitive indicator to delineate the strain patterns at the orogen-scale, in particular, in those areas undergoing a modest degree of deformation.