Neogene graben linked to flexural fold reactivation in the southern border of the Sierra de Cantabria (Alava -Navarra)

In the hanging wall of the Sierra de Cantabria thrust (western sector of the southern Pyrenees) there are two Neogene graben with long axes oriented NE-SW and E-W. They are 10x3 and 15x3 km wide, respectively and filled with Miocene and Pliocene clastic deposits. Their southern margins are controlled by near vertical faults, which are roughly parallel to the strike of the OUgocene and Cretaceous beds, folded during the Pyrenean Tertiary shortening. These faults, formerly interpreted as normal extensional faults, can be explained as the result of reactivation, during the Late Miocene to the Pliocene, of former flexural slip NE-SW and E-W compressional folds linked to thrusting. Structures formed on the uncomformable Neogene cover during this reactivation were conditioned by the thickness and lithological changes between Cretaceous and OUgocene units. The interpretation given to these Neogene graben implies that compressional structures extend at least to the Pliocene in this part of the southern Pyrenean borde