Abstract Critical gravity and magnetic data suggest the presence of a continuous zigzag exhumed mantle body inside the attenuated crust of the north Iberia continental margin. We propose that this body greatly conditioned the structural domains of the Cantabrian–Pyrenean fold-and-thrust belt during their evolution from hyperextension in Early Cretaceous times to shortening and inversion during the Cenozoic. This may be seen as a new line for cross-section construction and balancing, because previous cross-sections do not incorporate comparable volumes of exhumed mantle. Five structural cross-sections, constrained by the results of 3D gravity inversion, feed our discussion of the complexities of the doubly vergent Pyrenean orogen in view of the inversion of a precursor hyperextended rifted margin. In all sections, crustal rocks underthrust the lithospheric mantle in the hyperextended region, supporting that the near-surface exhumed mantle lithosphere acts as a more rigid buttress, allowing weaker continental material to be expelled outwards and upwards by thrusting during the Alpine collision; thus giving rise to two uplifted crustal triangular zones at the boundaries with the exhumed mantle. Contractional slip is localized in lithospheric-scale thrusts, which in turn reactivate parts of the extensional system. The NE–SW transfer zones that offset the rift therefore behave as compartmental faults during the orogenic phase. The amount of shortening increases from 34 km in the Cantabrian Cordillera, where the Basque–Cantabrian Basin partially preserves its original extensional geometry, to 135 km in the nappe stack of the central Pyrenees.

Abstract The Ayoluengo Field, commonly cited as the only onshore oil field in Spain, was discovered in June 1964 by Amospain, a joint venture of Chevron, Texaco and CAMPSA, by then the state-owned company. Ayoluengo is located about 300 km north of Madrid in the southern part of the Basque–Cantabrian Basin, a geological region where natural oil seeps, tar sands and asphalt have been recognized in outcrops since the early twentieth century. Now, 50 years after the first oil in 1967, the field has a cumulated production of 17 million barrels of oil. The 50-year production concession expired at the end of January 2017 and the field is now closed, awaiting a bidding process for a new concession to be awarded. The Ayoluengo Field consists of a NE–SW-orientated salt-cored anticline, related to Triassic salt movements. The field is divided into two large structural blocks by a normal fault. Oil and gas production comes from a series of thin lenticular fluvio-lacustrine sandstone packages of Late Jurassic–Early Cretaceous age. More than 50 separate oil and gas sandstone beds have been identified by drilling. This multilayer reservoir, together with the structural component, means that Ayoluengo is considered to be a grouping of hundreds of small oil and gas fields. After years of intense exploration activity, the Ayoluengo Field still, surprisingly, remains a unique oil discovery and is the only onshore commercial oil field in Spain and also the only one in the entire Iberian Peninsula. This geological singularity has resulted in recurrent discussions between petroleum geologists because it is difficult to explain why a petroleum system is working uniquely at this particular spot and nowhere else within such a vast territory.