Tectono-thermal evolution in a region with thin-skinned tectonics: the western nappes in the Cantabrian Zone (Variscan belt of NW Spain)

The palaeotemperature distribution in the transition from diagenesis to met amorphism in the western nappes of the Cantabrian Zone (Somiedo, La Sobia a nd Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (TC). Structural and stratigraphic control in distribu tion of CAI and IC values is observed. Both CAI and IC value distributions show that anchizonal conditions are reached in the lower part of the Somied o Unit. A disruption of the thermal trend by basal thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous rocks of the Aramo Unit; the IC has mainly anchizon al values, whereas the CAI has diagenetic values. Discrepant IC values are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI indicate diagenetic conditions . The anchimetamorphism predated completion of emplacement of the major nap pes; it probably developed previously and/or during the early stages of mot ion of the units. Temperature probably decreased when the metamorphosed zon es of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of tectonic factors on the metamor phism is greater in the internal parts, where the strain and cleavage are a lways present, than in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role of t he burial on the metamorphism.