Oblique tectonic inversion and basement thrusting in the Cameros Massif (Northern Spain)

We present the structural study of the Cameros Massif (NW Iberian Chain, Spain), taking into account data from surface geology, deep boreholes and seismic reflection profiles. The main structural feature is a single, I50km-long thrust with a 20-30 km horizontal displacement towards the N-NNW. In the hangingwall the up to 5.000m. thick Mesozoic (Jurassic-Cretaceous) cover forms the Cameros Massif and the Variscan basement constitutes the Sierra de la Demanda to the west of the Cameros Massif. The foreland is the western sector of the Ebro Basin filled with continental Tertiary deposits that reach a maximum thickness of 4.000-5.000m. An important detachment level is constituted by the Keuper marls and gypsum. The age of compression ranges from the Oligocene to the lower part of the Upper Miocene. The Tertiary compressive structures fit into a large-scale process of tectonic inversion : a) An extensional stage with a NE-SW extension direction took place during the Upper Jurassic-Lower Cretaceous, giving rise to the development of the strongly subsiding half-graben Cameros Basin, b) The Tertiary N-S compression brought about a crustal shortening and the transport of the Cameros block towards the Ebro Basin. The NW-SE striking main normal faults that controlled the deposition in the Mesozoic basin during the extensional stage were reactivated as oblique ramps during the Tertiary compressive stage.