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We present a new crustal-scale transect of the eastern Basque-Cantabrian Zone, through the Cinco Villas Massif, the Leiza-Aralar Thrust System and the South-Pyrenean Zone. The restoration of this transect to its pre-shortening stage allows us to assess the architecture of the hyperextended domain and the style of the Alpine contractional deformation. During the Cretaceous, extension led to a hyperthinned crust with local mantle unroofing to the base of the eastern Basque-Cantabrian Basin. The mantle unroofing process was driven by a complex system of detachments putting into contact Mesozoic sediments in the hanging wall with mantle rocks in the footwall. At this stage, extensive fluid circulation caused serpentinization of the uppermost mantle body. Furthermore, the thermal anomaly created during the unroofing caused high temperature metamorphism of the overlying sediments of the Leiza detachment system and hydrothermalism in further basins. The Alpine convergence gave rise to the tectonic inversion of the Mesozoic basins. Tectonic structures inherited from the Cretaceous hyperextension played a major role in mountain building. The southward indentation of the European crust forced the northwards subduction of the Iberian crust and the basement-cover decoupling along the Triassic evaporites. Restoration of this section to the end of the extensional period enabled us to estimate a shortening of~90 km., En este trabajo se presenta un nuevo modelo de estructura cortical en el borde oriental de la Zona Vasco-Cantábrica, atravesando el Macizo de Cinco Villas, el sistema de cabalgamientos de Leiza-Aralar y la Zona Surpirenaica. La restauración de esta sección al estadio previo a la convergencia alpina permite establecer la arquitectura del dominio cortical hiperextendido Mesozoico y el estilo de la deformación compresiva posterior. Durante el Mesozoico, la deformación extensional asociada a la apertura del Golfo de Vizcaya dio lugar a la formación de dominios de corteza hiperextendida y a la exhumación parcial del manto bajo la Cuenca Vasco-Cantábrica oriental. El proceso de exhumación del manto se produjo por la acción de un complejo sistema de detachments extensionales que pusieron en contacto la cuenca cretácica en el bloque superior con el manto en su bloque inferior. En este estadio, la intensa circulación de fluidos a lo largo de estos detachments causó la serpentinización de la parte superior del cuerpo mantélico. Además, la anomalía térmica creada en este proceso, generó un metamorfismo de alta temperatura que afectó a los materiales mesozoicos localizados inmediatamente sobre el manto exhumado, así como hidrotermalismo en cuencas más alejadas de la aureola metamórfica. La orogenia Alpina dio lugar a la inversión de las cuencas mesozoicas, y las estructuras tectónicas heredadas de la etapa de hiperextensión cortical jugaron un papel determinante en la configuración de la estructura orogénica. La indentación hacia el sur de la corteza Europea en la corteza Ibérica forzó la subducción de esta última hacia el norte y el desacoplamiento del basamento con respecto a la cobertera sedimentaria mesozoico-terciaria a lo largo del nivel de despegue de las evaporitas del Triásico superior. La restauración de esta sección cortical al final del periodo extensional indica un acortamiento cenozoico de aproximadamente 90 km., We thank the PhD financial support for IDF given by the University of Oviedo-Banco Santander, Government of the Principality of Asturias and the FPU grant of the Spanish Ministry of Education. This is a contribution of the ESF TopoEurope Project PYRTEC (SV-PA-10-03-IP2- PYRTEC), the Consolider-Ingenio 2010 Project TOPOIBERIA (CSD2006-00041) and the Ministry of Economy and Competitiveness Project MISTERIOS (CGL2013-48601-C2-2-R).

The presence of ophicalcites in serpentinized peridotites together with fragments of these rocks in Cretaceous breccias along several North‐Pyrenean basins led to the interpretation of complete mantle exhumation to the seafloor. The westernmost outcrop of peridotites in the Pyrenean‐Cantabrian belt is located in Ziga (eastern Basque‐Cantabrian Basin), associated to a band of HT metamorphism related to the Leiza fault (Marble Unit). We present a petrological and geochemical study of the marbles and Ziga peridotite‐hosted calcite, including standard stable isotope composition and clumped isotope geothermometry. These isotopic techniques allow the determination of different types of formational fluids and crystallization temperatures, and are a useful tool for studying carbonation processes in hyperextended basins. Fieldwork and analytical studies lead us to conclude that during the opening of the Bay of Biscay, mantle rocks were unroofed at the base of the sedimentary pile of the eastern Basque‐Cantabrian Basin. However, the ophicalcite veins were recrystallized from meteoric fluids at low temperatures (≤49°C). The primary carbonate phase may have been formed either during the mid‐Cretaceous unroofing of the mantle or in a post‐exhumation stage. The process of mantle exhumation was accompanied with HT‐LP metamorphism and fluid circulation along major faults that reset the marine isotopic signature in the nearest marbles. For comparison, ophicalcites from Urdach and Lherz (North‐Pyrenean Zone) were included in the clumped isotope study. Results show that they were recrystallized from hot (∼200–230°C), saline fluids, and from meteoric fluids at near ambient temperatures (∼32–42°C), respectively., We thank the PhD financial support for IDF, given by the University of Oviedo‐Banco Santander, Government of the Principality of Asturias and the FPU grant of the Spanish Ministry of Education, Culture and Sport. This is a contribution of the Ministry of Economy and Competitiveness Project MISTERIOS (CGL2013‐48601‐C2‐2‐R), the Consolider‐Ingenio 2010 Project TOPO‐IBERIA (CSD2006‐00041), and the ESF TopoEurope Project PYRTEC (SV‐PA‐10‐03‐IP2‐PYRTEC).

The crustal structure of the transition zone between the Pyrenees and the Cantabrian Mountains was investigated from a set of seismic refraction / wide angle reflection profiles acquired in 1997. An E-W profile of 540 km shows important variations in crustal thickness and velocity distribution from the Cantabrian Mountains to the western Pyrenees. A continuous Alpine crustal root is observed, with Moho depths of 46-48 km, except for the western part of the Basque Cantabrian Basin, Where the Moho is located at about 40 km, Another outstanding feature is the inferred presence of a discontinuous layer of 6.40-6.90 km s. at midcrustal depths beneath the cantabrian mountains and the Eastern Basque -Cantabrian basin. Some geological and geophysical constraints suggest that it can represent a wedge of the lower crust of the Cantabrian Margin identing the Iberian crust durin the almipe compression. This indentation was conditioned and /or affected by some NE-SW structures and this explains its discontinuous presence along the profile. The anomalously low velocity for this lower crustal material could be a consequence of its intense deformation during the collision and crustal indentation., Estas investigaciones se han financiado con cargo a los proyectos CICYT AMB95-0922 y AMB98-1012