A new interpretation of the structural and metamorphic history of the western Aston Massif (Central Pyrenees, France)

Abstract

Detailed structural analysis has yielded new data on the structural and metamorphic evolution of the western Aston Massif and on the nature of the transition zone between the supra- and infrastructure in the central Pyrenees. The low-grade suprastructure is characterized by a steep axial-plane cleavage, whereas a flat-lying schistosity predominates in the high-grade infrastructure. Structural cross sections through the transition zone indicate that the steep cleavage is a first (F1) structure which becomes progressively deformed towards the higher grade rocks by second and third generations of structures (F2 and F3). The third deformation in particular is characterized by a flat-lying axial-plane schistosity. Two dome-shaped metamorphic isograd patterns occur in the area. These isograd patterns match with a structural dome which largely results from F2 deformation. Third generation flat-lying structures, while being moderately developed in the transition zone, predominate in the central part of the dome, suggesting thermally induced vertical shortening. Two younger generations of broadly post-metamorphic steep folds (F4 and F5) only slightly affected the major structure. The relationship of deformation and metamorphism is different for the two metamorphic thermal highs. In areas where these two thermal highs intersect, two phases of metamorphic mineral growth can be demonstrated, indicating the complex nature of the Pyrenean metamorphism in such localities.