The main Variscan deformation event in the Pyrenees: new data from the structural study of the Bielsa granite
Introduction
The study of the Variscan structure of the Pyrenees faces the difficulty that two orogenic cycles are superimposed. The Alpine thrusting and folding are responsible for the presently observed overall structure of the range (Déramond et al., 1985, Williams, 1985), and within the Palaeozoic basement it is difficult to ascertain whether the observed thrusts are reactivated Variscan detachments or newly-formed Alpine structures.
Granite bodies can be very useful as kinematic markers, in recording events that are strictly related to their emplacement, through their magmatic foliation and lineation patterns. Progressive deformation during crystallization is well recorded by C–S structures, and by microstructural signatures of high- and subsequent low-temperature solid-state deformation. When deformed in the solid-state, well-defined criteria of shear sense can be derived. However, these structural records correspond to a short time interval when compared with the whole tectonic history of the orogenic belt. And since granites are very hard to pervasively-imprint after their final crystallization, most frequently, in this case they are imprinted only by brittle deformation (joints and faults). Therefore, structural studies within and around granite massifs allow the constraint of the deformational stages in orogens.
In spite of the local overprints of the Variscan structures by the Alpine ones, several structural studies on Variscan granite plutons of the Pyrenees have demonstrated that the main Variscan tectonic phase, called D2 (Zwart, 1986), was transpressional (Leblanc et al., 1996, Evans et al., 1998, Gleizes et al., 1998a, Gleizes et al., 1998b, Gleizes et al., 2001). Among these plutons, the Bielsa granite has not been studied until now from the structural point of view. In order to characterise both the Variscan structures in the Bielsa granite and the relative importance of later Alpine deformation, a detailed structural study was carried out. The interest of this study lies in: (1) the location of the Bielsa granite near the southern border of the Axial Zone, allowing complete characterization of the Variscan deformation in this area, south of the large shear band affecting other Pyrenean granites (Gleizes et al., 1997); (2) the elevation differences between the studied sites (more than 1500 m) that allows to check the consistency of the structural study at different structural levels, in both the inner and outer parts of the pluton; and (3) owing to the outcrop conditions, the Bielsa granite helps in constraining the age and geometrical relationships of shear bands distributed throughout the Pyrenees (Soula et al., 1986c). Finally, this study of the Bielsa pluton allows the testing of the transpressional model proposed by Gleizes et al. (1998a).
This study focuses on the structure of the Variscan Bielsa granite that is located in the Axial Zone of the Central Pyrenees (Fig. 1). We intend to establish the relative contribution of the Variscan regional deformation processes to the intrusion of the Bielsa granite. We used field and microstructural data as well as the anisotropy of magnetic susceptibility (AMS) to characterise its internal structure and we relate it to the structural evolution of the wall rocks. AMS reliability was supported by a rock-magnetic study.
Section snippets
Geological setting
The Axial Zone of the Pyrenees belongs to the European Variscan Orogen (Fig. 1). The main phase of the Variscan orogeny in the Pyrenees is characterised by a crustal shortening event marked by large-scale south-verging thrusting and subsequent widespread regional penetrative deformation (Soula et al., 1986a), the so-called D2 phase. The U–Pb ages published in the last decade for the Pyrenean granites indicate that the Hercynian plutonism of the Pyrenees is essentially Carboniferous in age, and
Field and microstructural data
From a structural point of view, the Bielsa granite is rather heterogeneous, although in many outcrops no mineral preferred orientation can be observed with the naked eye. In some areas, the main deformational structures are a magmatic foliation and lineation, defined by the preferred orientation of feldspar phenocrysts and biotite crystals (Fig. 3a). In other areas, solid-state deformation structures, S–C structures and shear bands (Fig. 4), with biotite concentrated on C planes and a strong
AMS analysis
Samples for this study were collected from 60 stations distributed throughout the Bielsa granite (Table 1, Fig. 2), with a total of 409 standard specimens analysed. During sampling, the structural complexity resulting from post-emplacement folding involving the Triassic units was taken into account. Sampling sites were located next to the two limbs of folds to determine as much as possible the effect of rotation of granite blocks around horizontal axes. In order to prevent the occurrence of
Inferred geometry of the Bielsa granite
Constraining the geometry of the pluton is a first step to infer its mode of emplacement. Although gravimetry is usually the key to determine the 3-D shape of igneous bodies, in this case the Bielsa granite is a part of a Tertiary thrust sheet and, according to our macrostructural interpretation (Fig. 2), it is probably cut by a shallow-dipping thrust (Bielsa thrust) belonging to the Axial Zone antiformal stack (Muñoz, 1992, Martı́nez-Peña and Casas-Sainz, 2003). Therefore, it is not possible
Conclusions
This original structural study of the Bielsa granite reveals the existence of dextral shear bands of both Variscan and post-Triassic ages. The age of some bands is demonstrated to be Variscan where they are sealed by Triassic materials (southern border of the pluton). In places where the shear bands rework Triassic sediments, as frequently observed in the eastern part of the pluton, their post-Triassic activity is also demonstrated. However these shear-bands, which cannot be distinguished from
Acknowledgements
Earlier drafts of this manuscript were greatly improved by helpful comments from J.M. Tubı́a and G. Gleizes. The authors thank J.L. Bouchez and K. Benn for instructive reviews of the paper. We also thank the University of the Pais Vasco for allowing us to make free use of the KLY-2 susceptibility meter, M. Tricas for the realisation of thin sections, E. Guerrero for technical assistance with the SQUID magnetometer and A. Puzo and M. Gracia for their help with collection of samples. This work
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