TECTONIC LEVELS IN THE PALEOZOIC BASEMENT OF THE PYRENEES - A REVIEW AND A NEW INTERPRETATION

Two different structural domains, developed during the main Hercynian deformation events, were recognized by de Sitter & Zwart in the Hercynian basement of the Central Pyrenees. A deep-seated level (infrastructure) made of medium to high-grade metasediments and orthogneisses and dome-shaped gently dipping foliations, contrasts with a shallow level (suprastructure) of low to very-low grade metasediments with rather steep foliations. Since the recognition of this structural zonation, different interpretations have been proposed. Disagreements concern timing of deformation and tectonic regime considered to be prevalent of the structural development of each tectonic level. The proposed interpretation regards the progressive development and differentiation of both structural domains and considers the following factors to be significant for the structural arrangement of the Pyrenean segment of the Hercynian belt: an increase with depth of the horizontal shear component of deformation related to tangential and transcurrent tectonics, a marked inhomogeneity of deformation and penetrability of related foliations, and the gradual evolution from an earlier compressive crustal shortening event to a late transpressive event responsible for crustal shortening and trend-parallel stretching.