Orogenic core zones are hosts to elongated gneiss domes, which are commonly aligned parallel to the trend of the orogen. Some gneiss domes were rigid bodies when they attained their present position in the structural framework of an orogen, but their origin is still a matter of conjecture. Diapirism, polyphase folding, extensional tectonism, and most recently, transpression along large-scale crustal shear zones are the models that attempt to explain the tectonic evolution of gneiss domes. In order to test existing or develop new models, we present results from field studies and analogue modelling. For our field studies we have selected the Axial zone, the Variscan core of the Pyrenees, along the French-Spain border. Asymmetrically shaped plutons within the Axial Zone suggest that it acted as a large-scale dextral shear zone during the main Variscan deformation phase. The Axial zone is also home to several gneiss domes that are cored by orthogneisses of possible pre-Variscan or earliest Variscan age. Field work has concentrated on two representative gneiss domes, the Aston and the Bossost massifs. The style of deformation of the overlying metasedimentary rocks is strongly influenced by the gneiss domes. Schistosity and mineral lineation is deflected around the domes. Complex geometries, mineral lineations oriented orthogonal to stretching direction, cannot be explained by simple shear flow. Shear sense indicators do not show patterns characteristic of a simple dextral shear zone. Analogue modelling of dome structures are performed with a shear box apparatus that allows homogeneous deformation in general flow regimes. Experiments include deformation of viscoelastic fluid polymer with inserted rigid dome-shaped objects and layers of different viscosity under transpressional stress regime. The resulting geometries of schistosity and lineation are compared to the observed natural orientations, and help constrain the deformational parameters that caused development of the gneiss domes of the Axial Zone of the Pyrenees. Preliminary experiments under transpressional stress resulted in orientation of lineations similar to what is observed in the field.
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