Hinterland-foreland zonation of the Variscan orogen in the Central Pyrenees: comparison with the northern part of the Iberian Variscan Massif

A new sequence of Variscan deformations is proposed for the Palaeozoic rocks of the Central Pyrenees. The non-metamorphic units include south-directed thrust systems and related folds with a poorly developed cleavage. In the metamorphic units north-verging, recumbent to inclined folds (D1), associated with a subhorizontal to south dipping cleavage, are refolded by south-verging, upright to inclined folds (D2), with a subvertical to north-dipping axial plane cleavage, and offset by south-directed thrusts approximately coeval with D2. The structural evolution of these units suggests a subdivision of the Variscan Central Pyrenees into two different regions consistent with the zones known for a long time in the core of the Ibero-Armorican or Asturian arc (northern part of the Iberian Variscan Massif). The structure of the Pyrenean non-metamorphic units has foreland affinities and is comparable to that of the Cantabrian Zone, whereas the deformation observed in the Pyrenean metamorphic units is characteristic of the hinterland and is consistent with the features of the West Asturian–Leonese Zone or Central–Iberian Zone. Since the Pyrenean non-metamorphic units are located southwards of the metamorphic ones and the Variscan thrusts are south-directed, we tentatively correlate the Variscan Pyrenees with the northern branch of the Ibero-Armorican or Asturian arc. The Pyrenees are an ESE–WNW-trending, Alpine cordillera that resulted from collision between the Iberian and Euro-Asian plates from Late Cretaceous–Early Miocene. The collision induced tectonic inversion of previous Permian–Mesozoic extensional basins and incorporated Palaeozoic basement rocks in some Alpine thrust sheets. The resulting cordillera is an asymmetrical, doubleverging belt consisting of large outcrops of Mesozoic and Cenozoic rocks deformed by mainly northdirected structures in the north part of the belt and involved in principally south-directed structures in the southern part of the cordillera, separated by a large strip of Palaeozoic rocks in the core of the Pyrenees (Seguret 1972, amongst others). The Pyrenees have been traditionally divided into the eastern, central and western Pyrenees, but the Alpine age structures mapped at surface, subsurface and offshore, enabled the identification of the western prolongation of the Pyrenean Chain along the onshore and offshore part of the north Atlantic margin of the Iberian Peninsula from the BasqueCantabrian Basin to the Cantabrian Mountains (Alonso et al. 1996, amongst others). The first systematic studies of the Palaeozoic rocks of the Central Pyrenees were conducted during the sixties and seventies by geologists of Leiden University, The Netherlands (see Zwart 1979, and references therein) (Fig. 1). These studies provided a large number of geological maps and cross sections, and a great deal of stratigraphic, palaeontological, petrological and structural data. Since then, various aspects of the structures of the Palaeozoic rocks of the central Pyrenees and their origin have been a matter of debate between geologists from different schools. One of the most controversial aspects is the succession of deformation events proposed. Although most authors accept that the Variscan orogeny gave rise to a polyphase deformation and that some of the structures recognized in the Palaeozoic rocks were formed during the superimposed Alpine orogeny, the described features and order of the different deformation phases identified are extremely variable (e.g. Zwart 1960, 1964, 1968, 1979, 1981, 1986; Seguret & Proust 1968a, b; Matte 1969; Clin et al. 1970; Muller & Roger 1977; Soula 1982; Soula et al. 1986; Vissers 1992). Another major topic of debate concerns the occurrence of two structural domains, called ‘infrastructure’ and ‘suprastructure’, distinguished by their metamorphic grade and by the attitude of the main cleavage (Fig. 2). Most authors agree that the main cleavage has a flat-lying attitude and it is usually associated From: Poblet, J. & Lisle, R. J. (eds) Kinematic Evolution and Structural Styles of Fold-and-Thrust Belts. Geological Society, London, Special Publications, 349, 169–184. DOI: 10.1144/SP349.9 0305-8719/11/$15.00 # The Geological Society of London 2011. with high temperature metamorphism in the ‘infrastructure’, whereas the cleavage is subvertical and was generated in low grade metamorphic conditions in the ‘suprastructure’, though the relative age and main structural features of these two structural domains has been extensively discussed (e.g. Zwart 1960, 1964, 1968, 1979, 1981, 1986; Seguret & Proust 1968a, b; Matte 1969; Clin et al. 1970; Van den Eeckhout & Zwart 1988; Vissers 1992). The last controversial matter is related to the contractional and/or extensional geodynamic settings and their relative age invoked to explain the origin of the Palaeozoic structures, sedimentation, metamorphism and magmatism in the Pyrenees (e.g. Wickham & Oxburgh 1985, 1986; Soula et al. 1986; Matte & Mattauer 1987; Van den Eeckhout & Zwart 1988; Pouget et al. 1989; Pouget 1991; Vissers 1992, 1993; Poblet & Casas 1993a). In many of these studies, the Palaeozoic rocks of the Pyrenees were treated as an almost isolated massif unrelated to the Variscan orogenic system that extends all across Europe. Thus, the structural evolution, the structural domains and/or the geodynamic settings of the Variscan rocks of the Pyrennes proposed in many publications does not fit those observed in neighbouring portions of the Variscan orogenic belt, and, this lack of correspondence was hardly addressed. This aspect, together with the complex structure of the Pyrenean Palaeozoic basement resulting from the superposition of the Variscan deformation, a Mesozoic extensional event and a Tertiary contractional episode, led some authors to consider the Palaeozoic rocks of the Pyrenees as a geological anomaly, and therefore ignored it or did not attempt to correlate it with the rest of the Variscan belt in their reconstructions of the geology during Palaeozoic times (Matte 1995, 1998, 2001 amongst many others). Here, we propose a new sequence of Variscan deformations that has resulted from geological mapping and structural analyses for more than two decades in various regions along the central Pyrenees formed by different types of rocks of various ages and different metamorphic grades. This new structural evolution leads us to establish a zonation of this portion of the Variscan belt in two different regions, originally located in internal and external structural positions of the orogenic belt, and supplies us data to discuss the similarities and differences between some features of the Palaeozoic Pyrenean rocks and those observed in the Palaeozoic rocks that crop out in the northwest Iberian Peninsula, one of the most well known regions of the Variscan belt adjacent to the Pyrenees.