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Boltaña kinematics Mochales, 2011 The Western South-Pyrenean Zone includes the Jaca-Pamplona Basin, bounded by the Pamplona fault (to the W) and the South Pyrenean Central Unit (SPCU, to the E). From north to south several units can be distinguished: Internal Sierras (in contact with the Axial Zone, with materials from Upper Cretaceous to Lower Eocene); Flysch unit or Jaca-Pamplona turbiditic Basin (monotonous thin strata of sandstone/marls belonging to Eocene turbiditic facies with interlayered resedimented carbonates ); Val Ancha or Canal de Berdún (correspond to the Arguis-Pamplona marls of Bartonian age); Guarga Synclinorium (Late Eocene and Oligocene continental rocks deformed by the underlying Guarga thrust); External Sierras (hanging wall of the Frontal South-Pyrenean sole thrust, where interference between Eocene deformation with N-S structures and Oligocene deformation with Pyrenean trend appears). Figure 5. Structural sketch of the Pyrenees (from Barnolas and Pujalte, 2004; Tejero and Fernández-Gianotti, 2004). Main structures: CFS: South Pyrenean Frontal Thrust, CFN: North Pyrenean Frontal Thrust, CPP: Petites Pyrénées Thrust, Lk: Lakora Thrust, SE: External Sierras, Ga: Gavarnie Thrust, ZN: Nogueres Zone, Bo: Bòixols Thrust, Mo: Montsec Thrust, SM: Marginal Sierras, Sg: Segre oblique ramps, Cg: Cinca oblique ramps, Pe: Pedraforca sheet, CV: Vallfogona Thrust. The Central South-Pyrenean Zone is comprised between the Segre river structural alignment and the western limit of the SPCU, that is the Cinca river structural alignment (Figure 5). From north to south, Séguret (1972) defined three units that were displaced southwards: the Nogueres Zone, The Gavarnie-Monte Perdido Unit and the South Pyrenean Central Unit (SPCU). The Nogueres Zone is linked to the Axial Zone and presents a succession of allochthonous Variscan units composed by a cover of Upper Carboniferous, Permian and Triassic rocks, tilted toward the south (Séguret, 1972; 17
Boltaña kinematics Mochales, 2011 Saura, 2004; Saura and Teixell, 2006). They are called the têtes plongeantes of the Nogueras zone (Figure 5). The Gavarnie-Monte Perdido Unit comprises paleozoic rocks underlying Cretaceous rocks. Southwards it presents a Cretaceous-Eocene cover detached from the basement at the level of the Upper Triassic. The minor Monte Perdido sheet (equivalent to the Internal Sierras in the western sector) is higher and included into the detached series (Figure 5). The SPCU presents several allochthonous imbricate thrust sheets linked to south verging folds in piggyback sequence. This unit was transported southwards during Eocene times (Puigdefàbregas et al., 1991) producing the partitioning of the southern margin of the Pyrenees. This unit is of special relevance in our study, therefore it will be described in detail in section 2.5. 2. 3. Crustal-scale structure of the Pyrenees Over the years, several interpretations for the crustal structure of the Pyrenees have been adopted. From the 60s, the generalized allochthonous approach included thick-skinned and thinskinned models. The thick-skinned model considers the North-Pyrenean fault as the main structure and plane of symmetry of a fan-like structure defined by vertical thrusts in depth (Mattauer, 1968, Séguret, 1972; Choukroune, 1976). The Axial Zone (south of the North-Pyrenean fault) would be the consequence of the thickening of the central part of the Pyrenees, dominated by the Variscan basement. The thin-skinned model considers the Pyrenees as an asymmetric range with thrust sheets predominantly displaced southwards (Hossack et al., 1984; Cámara and Klimowitz, 1986; Muñoz, 1986, Teixel, 1998; among others). The north-verging sheets are considered back-thrusts and the North-Pyrenean fault (inherited from Early-Late Cretaceous sinistral movement of the Iberian plate) would be cut in depth by north-verging thrusts formed in Eocene-Miocene times. According to this model, the Axial Zone would be a tectonic window of an antiformal stack whose thrust sheets moved southwards. The asymmetry of the orogen would be related to crustal subduction of the Iberian plate underneath the European one. Both models consider geophysical studies as seismic profiles and magnetotelluric and gravimetric surveys (ECORS team, 1988; Torné et al., 1989; Pous et al., 1995; Soriau & Granet, 1995; Casas et al., 1997; Vacher & Soriau, 2001, among others). From these studies, thickening of the crust under the Axial Zone, an interruption of the Mohorovicic discontinuity below the North-Pyrenean fault and a reduced crustal thickness of the North-Pyrenean Zone with regards to the South-Pyrenean Zone (Daignières et al., 1982; Choukroune et al., 1989) can be reasonably well established. Programs as ECORS-Pyrénées and ECORS-Arzacq revealed that the Paleozoic basement is uplifted by thrusts in the Axial Zone and a fan-like thrust pattern in the crust. The Lower crust and the Moho discontinuity (Choukroune et al., 1989; Daignières et al., 1994), placed 33 km below the surface of the South-Pyrenean Front and 55-60 km below the Axial Zone, dip toward the north, deepening under the European Lower crust (where the Moho is 28-30 km deep). Considering the seismic profiles and other geophysical data, several balanced sections have been performed (Figure 6), integrating surface structures and subsurface data. Thrusting throughout the crust and upper mantle generate the thickening of the Pyrenean crust. From ECORS-Pyrénées section, a total shortening of 100 km (Roure et al., 1989) and 150-165 km (Muñoz, 1992; Beaumont et 18
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