6. Rotational kinematics of the Boltaña anticline - Instituto Geológico ...

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Boltaña kinematics Mochales, 2011 al., 2000) has been calculated. On the other hand, 80 km of shortening was deduced from ECORS- Arzacq (Teixell, 1998) in the western sector of the Axial Zone. This disparity is due to diverse interpretations regarding the subcrop of the Mesozoic cover below the South-Pyrenean basement thrusts and the increase of the shortening from E to W (Figure 6, Roure et al., 1989) for instance, or the Iberian continental crust subducted into the mantle (Figure 6b and c, Muñoz, 1992; Berástegui et al., 1993; Teixell, 1998). Teixell (1998) proposed that only the lower crust (mafic and dense) subducted. The collective summary is that the upper-crustal structure is described as a doubledverging orogenic prism where the North and South-Pyrenean Zones converge at depth (Figure 6). Figure 6. Crustal-scale cross section of the Pyrenees based on seismic data combined with gravity. FNP: North- Pyrenean Front; FSP: South-Pyrenean Front. A) ECORS-Pyrénées (Roure et al., 1989). B) ECORS-Pyrénées (Muñoz, 1992; Berástegui et al., 1993). R, O, No: Rialp, Orri and Nogueres sheets. C) ECORS-Arzacq (Teixell, 1998). The Lower crust interpretation is included. G, Ga, Lk: Guarga, Gavarnie and Lakora sheets. From Teixell, 2004; Tejero and Fernández-Gianotti, 2004. 19
Boltaña kinematics Mochales, 2011 2. 4. Evolution of the Pyrenees 2.4.1 Paleogeography The Pyrenean Range originated during the Alpine orogeny in the Western Mediterranean as a consequence of the tectonic inversion of the extensional and transtensional systems developed during Triassic and Cretaceous times. During the Mesozoic period the Pyrenees underwent extensional movements related to the opening of the Atlantic Ocean and the Bay of Biscay. The opening of the Atlantic produced several rifting stages from Late Permian to Early Triassic times (Lucas, 1985; Puigdefàbregas and Souquet, 1986; Ziegler, 1989). However, the South Atlantic opened from Mid- Jurassic to Late Cretaceous and the North Atlantic started at Early Cretaceous (Delfaud, 1969; Peybernès, 1976; Fauré, 1984; Puigdefàbregas and Souquet, 1986). The initial convergence was nucleated in the NW of Spain as early as mid-Cretaceous times (Gibbons and Moreno, 2002) when an accretionary prism resulted of the subduction of the oceanic crust of the Bay of the Biscay beneath Iberia. Nevertheless, wholesale continental collision is considered to start in the eastern Pyrenees, subsequently propagating westwards because of oblique subduction of the Iberian plate below the Eurasian plate (ECORS-Pyrenean team, 1988; Muñoz, 1992; Teixell, 1998; Rosenbaum et al., 2002), and spanned from Late Santonian to Middle Miocene. Subsiding troughs with turbiditic filling were quickly involved in synsedimentary thrusting (Gibbons and Moreno, 2002) in the Iberian side of the orogen. This process enabled a change from generally marine to mostly terrestrial conditions. In this long-lasting convergence, thrusting was directed north and southwards away from the Axial Zone where Variscan basement rocks crop out and towards its two foreland basins: the Aquitaine basin in France (N) and the Ebro basin in Spain (S), forming an asymmetric, double-verging range (Figure 6). During Early Paleogene, the Pyrenean domain was an east-west trending elongate interplate embayment just connected to the west by the Bay of Biscay and surrounded elsewhere by shallow shelf areas (Figure 7a). From Paleocene to Middle-Late Ilerdian (see reference time scale by Gradstein et al., 2004, figure 8) a generalized transgression in a large part of the South Pyrenean Basin (Barnolas and Gil- Peña, 2001) took place. Since then, a south Pyrenean channel with siliciclastic feeding from the east (Puigdefàbregas and Souquet, 1986) developed. Further convergence during Eocene, brought about the formation of the emerging orogenic belt and the splitting of the former single basin into northern and southern foreland basins (Figure 7b). Additional thrusting and uplift during Middle-Late Eocene (Plaziat, 1981) was the cause for retreating marine areas towards the paleo-Bay of Biscay, bounded then by shallow carbonated shelves (Figure 7c). Eventually, the closure of the marine gateway took place during Priabonian times (Costa et al., 2009). After the main convergence stage, during the last Miocene compressional stage, widespread terrestrial conditions with coeval erosion of the original materials (Figure 7d) developed throughout the Pyrenean realm. 20
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