Hercynian structure of the Axial Zone of the Pyrenees: the Aran Valley cross-section (Spain-France)

Abstract

In the Axial Zone of the Pyrenees Palaeozoic rocks crop out with superimposed Hercynian and Alpine deformation. The Hercynian structures present two domains: infrastructure and suprastructure. A complete crosssection of the central part of the Axial Zone of the Pyrenees is presented, in which Hercynian structures of both domains can be observed. In the Aran Valley cross-section, the infrastucture corresponds to the Garona Dome Domain, where the main Hercynian structures are north-vergent recumbent folds, with an associated, mainly flatlying, foliation (S 2). In the Garona Dome Domain, the peak of the medium-to-high grade metamorphism is simultaneous with the main Hercynian structures. The Aran Valley and Noguera Ribagorça areas correspond to the suprastructure. In these domains the main Hercynian structures are upright folds, with an associated, mainly steep-lying, foliation (S 3). In these suprastructural domains, low grade syn-S 3 metamorphism is present. This crosssection relates the structures of the infra- and the suprastructure, suggesting a deformational sequence for the Axial Zone of the Pyrenees. This sequence is deduced from the fact that the north-vergent folds observed in the Garona Dome are earlier than the main structures of the suprastructure. The boundary of the Garona Dome and the suprastructure is a décollement level observed at the base of the Silurian. The deformational sequence proposed here provides some constraints for models of Hercynian evolution and the emplacement of the Alpine structures of the Pyrenees.