Seismic interpretation of the MARCONI deep seismic survey enables recognition of the upper crustal structure of the eastern part of the Bay of Biscay and the main features of its Alpine geodynamic evolution. The new data denotes that two domains with different Pyrenean and north foreland structures exist in the Bay of Biscay. In the eastern or Basque‐Parentis Domain, the North Pyrenean front is located close to the Spanish coast, and the northern foreland of the Pyrenees is constituted by a continental crust thinned by a north dipping fault that induced the formation of the Early Cretaceous Parentis Basin. In the western or Cantabrian Domain, the North Pyrenean front is shifted to the north and deforms a narrower and deeper foreland basin which lies on the top of a transitional crust formed from the exhumation of lithospheric mantle along a south dipping extensional low‐angle fault during the Early Cretaceous. The transition between these two domains corresponds to a soft transfer zone linking the shifted North Pyrenean fronts and a north‐ to WNW‐directed thrust that places the continental crust of the Landes Plateau over the transitional crust of the Bay of Biscay abyssal plain. Comparison between this structure and regional data enables characterization of the extensional rift system developed between Iberia and Eurasia during the Late Jurassic and Cretaceous and recognizes that this rift system controlled not only the location and features of the Pyrenean thrust sheets but also the overall structure of this orogen.

a b s t r a c tThe new MARCONI-3 deep seismic profile allows recognition of the upper crustal structure of the eastern part of the Bay of Biscay and the main features of its Alpine geodynamic evolution. It denotes that the easternmost part of the Bay of Biscay consists of a thick wedge of uppermost Cretaceous to Cenozoic synorogenic sediments lying unconformably on the top of a thinned continental crust with the Mesozoic Parentis Basin to the north and the coeval Landes High to the south. The Parentis Basin appears as a major half-graben bounded southwards by a north-dipping planar fault. It is filled by a thick sequence of Jurassic-Upper Cretaceous carbonates affected by salt domes and squeezed diapirs made up of Triassic evaporites and mudstones. These salt tectonic structures also affect the overlying uppermost Cretaceous to Lower Miocene synorogenic deposits which are folded upon these structures. The Landes High includes a thin pre-Upper Cretaceous cover tilted to the south. In the Basque shelf, it is deformed by a basement-involving thrust wedge emplaced during the Late EoceneMiocene that constitutes the North-Pyrenean contractional front. Geometric relationships and thickness variations depict that this overall structure results from the following.

Salt is mechanically weaker than other sedimentary rocks in rift basins. It commonly acts as a strain localizer, and decouples supra- and sub-salt deformation. In the rift basins discussed in this paper, sub-salt faults commonly form wide and deep ramp synclines controlled by the thickness and strength of the overlying salt section, as well as by the shapes of the extensional faults, and the magnitudes and slip rates along the faults. Upon inversion of these rift basins, the inherited extensional architectures, and particularly the continuity of the salt section, significantly controls the later contractional deformation.This paper utilizes scaled sandbox models to analyse the interplay between sub-salt structures and supra-salt units during both extension and inversion. Series 1 experiments involved baseline models run using isotropic sand packs for simple and ramp-flat listric faults, as well as for simple planar and kinked planar faults. Series 2 experiments involved the same fault geometries but also included a pre-extension polymer layer to simulate salt in the stratigraphy. In these experiments, the polymer layer decoupled the extensional and contractional strains, and inhibited the upwards propagation of sub-polymer faults. In all Series 2 experiments, the extension produced a synclinal hanging-wall basin above the polymer layer as a result of polymer migration during the deformation. During inversion, the supra-polymer synclinal basin was uplifted, folded and detached above the polymer layer. Changes in thickness of the polymer layer during the inversion produced primary welds and these permitted the sub-polymer deformation to propagate upwards into the supra-salt layers.The experimental results are compared with examples from the Parentis Basin (Bay of Biscay), the Broad Fourteens Basin (southern North Sea), the Feda Graben (central North Sea) and the Cameros Basin (Iberian Range, Spain).

The Late Jurassic–Cretaceous Parentis Basin (Eastern Bay of Biscay) illustrates a complex geological interplay between crustal tectonics and salt tectonics. Salt structures are mainly near the edges of the basin, where Jurassic–Lower Cretaceous overburden is thinner than in the basin centre and allowed salt anticlines and diapirs to form. Salt diapirs and walls began to rise reactively during the Late Jurassic as the North Atlantic Ocean and the Bay of Biscay opened. Some salt-cored drape folds formed above basement faults from the Upper Jurassic to Albian. During Albian–Late Cretaceous times, passive salt diapirs rose in chains of massive salt walls. Many salt diapirs stopped growing in the Mid-Cretaceous when their source layer depleted. During the Pyrenean orogeny (Late Cretaceous–Cenozoic), the basin was mildly shortened. Salt structures absorbed almost all the shortening and were rejuvenated to form squeezed diapirs, salt glaciers and probably subvertical welds, some of which were later reactivated as reverse faults. No new diapirs formed during the Pyrenean compression, and salt tectonics ended with the close of the Pyrenean orogeny in the Middle Miocene. Using reprocessed industrial seismic surveys, we document how salt tectonics affected the structural evolution of this offshore basin largely unknown to the international audience.

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The northwest Mediterranean Basin includes a thick Messinian salt sequence composed of three evaporitic units. From these, the intermediate unit, which is dominantly composed of halite, acted as a gravitational detachment favoring the downslope failure of the overlying sediments in a thin-skinned deformation regime. As a result, the structure of the margin is characterized by an upper extensional domain with basinward-dipping listric normal faults and a lower contractional domain that accommodates upslope extension by folding, salt inflation, or diapir squeezing. Lower to middle Miocene volcanic seamounts (presalt reliefs) located at the upper extensional domain locally disrupted the evaporitic units and produced salt flow perturbations. They acted as passive buttresses during the gravitational failure modifying the structural zonation of the margin. Using an experimental approach (sandbox models), we analyze the role played by seamounts during the kinematic evolution of passive margins and how they alter salt flow and suprasalt deformation during gravitational gliding. The experiments found that the seamounts locally interrupt the structural zonation of the margin because they hindered downdip salt flow during early deformation. Seamounts initially compartmentalize the margin architecture, resulting in the development of two gravitational subsystems with two extensional/contractional pairs that are subsequently reconnected when the accumulation of salt analog upslope of the relief is enough to overthrust it. From this point onward, the cover is passively translated downslope as a regional system. The changes in the viscous layer flow velocity related to the dip differences between the flanks and edges of the seamount determine the kinematic evolution of this system. Our experiments also provide geometric constraints to consider during interpretation of these structures, which are commonly poorly imaged in seismic data.

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