The structural history of the Vall Ferrera area, the transition zone between the Aston Massif and the Salat-Pallaresa anticlinorium (Central Pyrenees, France, Spain)

Cambro-Ordovician clastic sediments varying in composition from microconglomerates to slates occur. A more precise stratigraphic assignment is impossible, because of the lack of fossils and traceable stratigraphic horizons. The Cambro-Ordovician rocks have undergone four Hercynian deformation phases. Each phase was accompanied by the formation of cleavages (slaty, fracture or crenulation cleavage). In the Vail Ferrera area these four deformation phases are well developed. Their relative age relationships were known from other parts of the Central Pyrenees and could be established in many outcrops. The first deformation, accompanied by a regional synkinematic metamorphism and caused by a N-S compression, divided the orogene into (1) the infrastructure (e.g., the Aston massif) with medium to high grade metamorphism and a flat cleavage plane, and into (2) the suprastructure (e.g., the Salat Pallaresa anticlinorium), with low grade metamorphism and a steep cleavage plane. The second phase produced N-S trending folds and was caused by an E-W compression. The third phase made a conjugate fold system in a NW-SE and a NE-SW direction; the asymmetry proves that an E-W compression caused these folds. The fourth phase was produced by a renewed N-S compression and a vertical E-W cleavage was formed. Finally blockfaulting occurred by which the Mérens fault, which seperates the Aston massif from the Hospitalet massif, was formed. A literature study, concerned with the different types of deformation, shows that the direction of the extension can be parallel or perpendicular to the fold axis with transitions in between. Such a ”deformation pattern” is found in the Vail Ferrera area. The relationship between this pattern and the metamorphism is given.