Tertiary basins and Alpine tectonics in the Cantabrian Mountains (NW Spain)

Two sub-aerial major Tertiary basins developed during the Alpine orogeny in the Cantabrian Mountains: the Oviedo Basin and the Duero Basin. The Oviedo Basin, now deeply eroded, is located in front of a thrust developed by the tectonic inversion of a Late Jurassic–Early Cretaceous rift-related normal fault. The northern margin of the Duero Basin, with a well-preserved succession up to 2500 m thick, occurs in front of a large basement-cored uplift developed as a result of thrusting over a long ramp connected to a midcrustal detachment. The movement along this thrust carried the Oviedo Basin southwards in a piggyback manner. Both basins show syntectonic unconformities ahead of the mountain front, associated with syndepositional thrust-related folds. Syntectonic alluvial fans developed along the mountain front. These are more extensive in the Duero Basin (with radii up to 25 km) than in the Oviedo Basin (with radii up to 5 km). In both basins, the succession was overall prograding and changes vertically from polymictic to siliceous conglomerates. This lithological variation is attributed to a climatic change towards more humid conditions. The later siliceous episodes were essentially post-tectonic in both basins. An erosion–deposition mass balance is calculated for the Duero Basin.