Int J Earth Sci (Geol Rundsch) (2003) 92:316–337 DOI 10.1007/s00531-003-0334-y ORIGINAL PAPER Pierre Souquet · Jean-Jacques DelvolvØ · StØphane Brusset Identification of an underfilled foreland basin system in the Upper Devonian of the Central Pyrenees: implications for the Hercynian orogeny Received: 6 May 2001 / Accepted: 18 March 2003 / Published online: 17 June 2003  Springer-Verlag 2003 Abstract A foreland basin succession has been identified in the Frasnian of the Central Pyrenees. This succession comprises a carbonate-dominated transgressive system which recorded the cratonward migration of the foreland basin subsidence, and siliciclastic depocenters which recorded the progression of the thrust-fold deformation. The foreland basin system has always been maintained in deep-marine environments, i.e., at an underfilled deposi- tional state. It was associated with a thrust wedge which descended toward a deep-marine hinterland, i.e., with a type of orogenic wedge usually related to subduction zones. The Frasnian foreland basin system differs from the one known in the Carboniferous which evolved to overfilled depositional state and was associated with a thrust wedge rising toward a mountainous hinterland. Consequently, the Hercynian orogeny in the Pyrenees seems to result first, from a Frasnian thrusting controlled by a subduction zone located north of the Pyrenees, and second, from a Carboniferous thrusting controlled by the surrection of a frontal thrust belt in the Pyrenees. The association of underfilled foreland basin systems and hinterland-dipping thrust wedges, as exemplified in the Frasnian of the Pyrenees, can be interpreted as illustrative of the initial stages of thrust-wedge growth in deep- marine settings. Keywords Foreland basin · Foreland basin stratigraphy · Tectonically controlled sedimentation · Hercynian orogeny · Devonian · Aran valley · Pyrenees · France · Spain Introduction The Paleozoic basins of the Pyrenees are difficult to reconstruct because they have been involved in the Hercynian and Alpine deformations. Nevertheless, an attempt at reconstruction has recently been carried out for the Carboniferous of the Central Pyrenees (DelvolvØ et al. 1998). It resulted in the identification of a foreland basin system analogous to the classical model of DeCelles and Giles (1996). This basin was controlled by the propaga- tion of a forward thrust sequence. The aim of this paper is to analyze the Upper Devonian in the same Central Pyrenean domain which records another foreland basin system, but with significant differences in structural and sedimentary features. The study is focused on the head of the Garonne valley which is termed Aran valley in Spanish territory (Fig. 1). It deals with Devonian sediments which form a syncline (Aran syncline) in the footwall of a major regional thrust (Bossost thrust; Fig. 1). These Devonian sediments have been postulated as extensional basin deposits. A sedimentologic revision leads us to identify foreland deposits in their upper part, which provides a further alternative interpretation for the Aran syncline evolution. An enlargement of the investi- gations along a cross section of the entire chain leads us to question the current models concerning the Hercynian orogeny in the Pyrenees. Moreover, this paper provides an illustrative example of interrelations between nascent thrust wedges and foreland basin systems in deep-marine environments. Regional tectonic framework Main geological features of the Central Pyrenees The Pyrenean chain stretches over the Cretaceous– Cenozoic subduction zone separating the Iberian and European continental lithospheres (Figs. 1, 2; ECORS Team 1990). The Hercynian basement predominates in the Axial Zone where it constitutes a southvergent P. Souquet · J.-J. DelvolvØ · S. Brusset ( ) ) Laboratoire MØcanismes de Transfert en GØologie (UMR 5563 CNRS), UniversitØ Paul-Sabatier–Toulouse III, 38 rue des Trente-Six Ponts, 31062 Toulouse Cedex 04, France e-mail: stephane.brusset@cict.fr Tel.: +33-5-61558340 Fax: +33-5-61558250