The basement is traversed by major east-west-striking, late Variscan fracture zones that were reactivated during the alpine orogeny cycle. In the eastern part of the Axial Zone, the fractures are generally upright, a good example being the mylonitic Merens Fault at Pic del Port Vell near Mérens-les-Vals. In the western part, the fractures are more gently dipping to the north and behave as en echelon thrusts arranged in a northwest-southeasterly fashion; along these fractures, the basement of the Axial Zone overthrusts Mesozoic sedimentary units to the south. Good examples are the en echelon thrusts at Eaux Chaudes, Gavarnie and Bénasque—Las Nogueras (referring to the upper reaches of the rivers Noguera Ribagorzana and Noguera Pallaresa). Concomitant with the thrusts, a schistosity developed that affected the basement as well as the sedimentary cover implying an alpine origin. All these fractures account for an overall compression of the Axial Zone by 20% which translates as roughly 10 to 20 km of crustal shortening. As a result, the Axial Zone was squeezed into a south-directed antiformal stack.