Magnetic fabrics studies (AMS) are a useful tool in order to describe the distribution of deformation in orogenic areas where conventional techniques are difficult to apply, especially due to the lack of proper strain markers. In the present study, AMS and structural analysis are used to define the distribution of deformation in the Central Axial and Nogueres Zones, an area of strong structural changes (i) in the geometry of the antiformal stack defining the Pyrenean Axial Zone and (ii) the distribution of Alpine cleavage. The studied rocks are Lower-Middle Triassic red beds that crop out in three different stacked thrust units (Bielsa, Nogueres and Orri). Primary sedimentary fabrics are preserved in the uppermost thrust units (Nogueres Zone), but a high percentage of the sampling sites shows an overprint of Alpine compression on magnetic fabrics, with the magnetic lineation mostly parallel to the tectonic grain defined by compressional structures and the magnetic foliation showing different orientations between the poles to cleavage and bedding. The development of compressional fabrics strongly depends on the structural position of the sites, and two deformation gradients can be inferred: the southern margin of the Axial Zone (Orri and Bielsa units) shows strong internal deformation, increasing towards the North; farther north (in the restored cross-section), deformation is in general terms lower, but increases towards the basal thrust of the Nogueres Zone. The heterogeneous distribution of Alpine internal deformation indicates a preferred development of cleavage in the Variscan basement and overlying units of the southern margin of the Axial Zone (mainly in the Orri unit), that could be partly controlled by the tectonic load resulting from the stacking of thrust sheets. © 2013 Elsevier Ltd.

The authors thank Cristina Garcia and Teresa Román for their help during the field and laboratory work and Manuel Tricas (SAI Preparación de Rocas, Universidad de Zaragoza) for the preparation of thin sections. This work is part of the research projects CGL2009-08969 and CGL2006-05817 (Spanish Ministry of Education) and is included in the objectives of the pre-doctoral grant AP-2009 – 0554 (Esther Izquierdo, Programa de formacion de profesorado universitario, Spanish Ministry of Education). Funding from Instituto de Estudios Altoaragoneses are also acknowledged. The authors acknowledge the careful and constructive revisions from E. Saura and an anonymous reviewer.

Peer Reviewed