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 Hercyn ian basement of the Central Pyrenees. A deep-seated level (infrastruct ure) 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 st eep foliations. Since the recognition of this structural zonation, dif ferent interpretations have been proposed. Disagreements concern timin g of deformation and tectonic regime considered to be prevalent of the structural development of each tectonic level. The proposed interpret ation regards the progressive development and differentiation of both structural domains and considers the following factors to be significa nt for the structural arrangement of the Pyrenean segment of the Hercy nian belt: an increase with depth of the horizontal shear component of deformation related to tangential and transcurrent tectonics, a marke d inhomogeneity of deformation and penetrability of related foliations , and the gradual evolution from an earlier compressive crustal shorte ning event to a late transpressive event responsible for crustal short ening and trend-parallel stretching.