The Alpine collision between the Iberian and European plates resulted in a complex crustal structure beneath the northern Iberian Peninsula, as revealed from a new set of seismic refraction/wide‐angle reflection profiles. The study area is characterized by two major E‐W ranges, the Cantabrian Mountains and the Pyrenees, which are relayed to the south by the Iberian Chain. Important variations in crustal thickness and velocity distribution are found in a 560‐km‐long E‐W transect. In contrast to the typical 30‐ to 32‐km‐thick European Variscan crust of the western end of the profile, a continuous Alpine crustal root is evidenced from the Cantabrian Mountains to the central Pyrenees, with a Moho depth of 46–48 km, locally rising to ∼40 km depth in between, beneath the Basque‐Cantabrian Basin. Another outstanding feature is the inferred presence of high velocities of 6.40–6.75 km/s at midcrustal depths, which can be associated with portions of a lower crustal wedge from the northern (European) domain indenting the southern (Iberian) crust during the Alpine stage of compression. This indentation produces the delamination of the Iberian crust, with northward underthrusting of its lower half and the consequent crustal thickening. The indenting wedge has a discontinuous presence along the longitudinal section, as it was controlled and/or affected by N‐S to NE‐SW structures. Further evidence of the northward subduction of the Iberian plate is provided by another profile toward the Aquitaine Basin, while a N‐S profile across the Iberian Chain reveals an Alpine midcrustal thickening beneath this belt with Moho depths of ∼42 km.

The complex crustal structure of the boundary zone between the Iberian and European plates, from the western Pyrenees to the Cantabrian Mountains, is probed by three‐dimensional (3‐D) gravity and magnetic modeling constrained by deep seismic profiles. The 3‐D modeling results support the presence of a continuous Iberian crustal root and suggest that the geometry of the orogenic belt that formed in latest Cretaceous‐Tertiary times along the northern margin of Iberia was conditioned by oblique structures separating areas characterized by different tectonic styles. In the western Pyrenees and the Basque‐Cantabrian basin (a thick Mesozoic basin presently incorporated to the Pyrenean‐Cantabrian belt), the relatively narrow thinning of the crust inherited from the Mesozoic rifting stage conditioned a structural style in which portions of the southward indenting European lower crust are interpreted to be back thrusted toward the north and uplifted to shallow depths, promoting the appearance of significant potential field anomalies. In the Basque‐Cantabrian basin, the strongest aeromagnetic anomaly of the whole Iberian Mainland is superimposed on the eastern part of a well‐defined positive gravity anomaly, similar in amplitude and wavelength to those located along the North Pyrenean Zone. These observations suggest that the eastern part of the dense, lower crustal causative body is strongly magnetized and may correspond to a gabbroic cumulate originated in the axis of the ancient Mesozoic rift. To the west, Tertiary compression affected the North Iberian (Cantabrian) passive continental margin, whose geometry and inherited structures conditioned the formation of a double crustal delamination and the uplift of the Cantabrian Mountains.

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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 (498C). 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 (200-2308C), saline fluids, and from meteoric fluids at near ambient temperatures (32-428C), respectively.

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