New insights in the structural and metamorphic history of the western Lys-Caillauas massif (Central Pyrenees, France)

Abstract

We distinguish four deformation phases of major importance in the Cambro-Ordovician metasediments of the western Lys-Caillauas massif. During D1 a steep, regionally dominant, foliation developed. This foliation was folded during D2 into a large overturned isoclinal antiform. The third deformation phase D3 resulted in complex rotations of porphyroblasts, crenulation of the S1 foliation and development of a gently dipping crenulation cleavage S3. At a deep structural level, S1 was completely transposed into the plane of S3, which forms a dome-type D3 structure. D1,D2 and D3 are of Variscan age. D4 included Alpine thrusting along the Gavarnie thrust and accompanying folding and faulting. The Alpine folding led to a considerable amount of ductile shortening in the footwall rocks of the thrust. The Cambro-Ordovician rocks show a Variscan plurifacial metamorphism during D3. Peak values were 600-660°C and 2.5-3.5kbar. Two metamorphic domes of slightly different age with respect to D3 are distinguished. One of these metamorphic domes coincides with the D3 structural dome. The Alpine D4 occurred under low grade metamorphic conditions. In comparison with current notions on the Lys-Caillauas massif the data presented a) reveal a more complex deformation history; b) fit the development of some controversial large scale structures within the deformation history; c) give a more complete picture of the metamorphic evolution of the rocks and d) show that Alpine folding was more widespread than previously thought.