Superposition of tectono-thermal episodes in the southern Cantabrian Zone (foreland thrust and fold belt of the Iberian Variscides, NW Spain)

Abstract

Abstract The combined use of the illite “crystallinity” Kübler index (KI) and the conodont colour alteration index (CAI) has revealed the existence of three thermal episodes in an area affected by thin-skin tectonics, close to the internal zones of the Variscan orogen in NW Spain. In the southernmost part of the study area, the first episode gave rise to a regional syntectonic Variscan metamorphism. The associated deformation involves the development of a slaty cleavage, which is mainly recognized in Precambrian rocks. Towards the foreland, the syntectonic metamorphism disappears and only an incipient burial metamorphism, giving rise to anchizonal conditions in the basal part of the thrust units, is observed. Another metamorphic episode occurred close to the Carboniferous-Permian boundary in an extensional tectonic regime. This metamorphism is restricted to the northern part of the study area, where it reached anchizonal or epizonal conditions. It is associated with a subhorizontal or moderately north-dipping cleavage and can be considered as a late-Variscan episode. The last thermal episode occurred during the Permian. It was produced by heat flow due to hydrothermal fluids, whose migration was favoured by faults. The effects of this episode are irregularly distributed, and they are apparent in the unconformable Stephanian rocks in which anchizonal or epizonal conditions were reached. It is interpreted as a post-Variscan episode.