Elongated structural and metamorphic domes are a common feature in the cores of orogenic belts. They are characterized by alignment of their long axes parallel to the trend of the orogenic belt and gneissic or migmatitic cores mantled by deformed metasedimentary rocks.
Different levels of exposure of gneiss domes are observed in the Bossòst and Aston-Hospitalet domes of the central and eastern Pyrenees. The exposed core of the Bossòst dome comprises undeformed late Variscan leucogranite, with older - as yet undated - foliated tonalite observed only in the central part of the core. The Aston-Hospitalet dome displays deeper structural levels with complete exposure of its orthogneiss core, whose pre-Variscan protolith age is currently under investigation. The mantling metasedimentary cover is mainly exposed at the eastern and western margins of the Aston-Hospitalet dome, while it covers the Bossòst dome more completely. Both dome structures are actually a pair of half domes, as they are cut by easterly trending steep fault and mylonite zones, the Bossòst and the Merens fault, respectively, that are of latest Variscan or Alpine age and that resulted in relative uplift of the northern half domes.
The main structural fabrics of both domes are similar. The major schistosity forms girdles around shallowly plunging, easterly trending fold axes. Conspicuous mineral lineations shallowly plunge to the SE–ESE or NW–WNW. The orthogneiss cores of the Aston-Hospitalet half domes show the same orientation of gneissic schistosity and mineral lineation, suggesting coeval deformation of the metasedimentary cover and the orthogneiss during the main Variscan deformation phase. In the southern Bossòst dome mineral lineations are associated with a flat-lying extensional shear zone that facilitated uplift of the underlying higher metamorphic core. A similarly oriented extensional shear zone has been reported previously from the eastern Hospitalet dome. Metamorphic studies in both domes show that the development of the main schistosity occurred at pressures around 5 kbar, while later metamorphism associated with emplacement of late Variscan granitoids near or partially into the orthogneiss core occurred at much shallower depths (6 km).
The previously emplaced, and during the main Variscan deformation foliated, orthogneisses acted as strain partitioners during later N–S directed compression. Enhanced by beginning uplift of the more rigid orthogneiss cores and pluton emplacement, the roofing metasedimentary cover rocks experienced orogen-parallel extension.
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