Tectonothermal evolution of a foreland fold and thrust belt:: the Cantabrian Zone (Iberian Variscan belt, NW Spain)

Analysis of the conodont colour alteration index and the Kubler index of illite allowed us the characterization of four types of very low- or low- grade metamorphism in the Cantabrian Zone (CZ) and determination of their regional and temporal distribution. These types are: (1) an orogenic Variscan metamorphism present only in restricted areas of the western and northwestern parts of the CZ where epizonal conditions are reached; (2) a burial metamorphism that appears in the basal part of some nappes, where anchizonal conditions are sometimes achieved; the thermal peak preceded emplacement of the nappes; (3) a late-Variscan metamorphism in the southern and south-eastern parts of the CZ; a cleavage, cutting most of the Variscan folds, is associated with this metamorphism, which has been related to an extensional episode; (4) a contact metamorphism and hydrothermal activity associated with minor intrusive bodies. The extension continued after the Variscan deformation giving rise to hydrothermal activity during Permian times.