The deep seismic reflection MARCONI-3 profile: Role of extensional Mesozoic structure during the Pyrenean contractional deformation at the eastern part of the Bay of Biscay

Abstract

The 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 Eocene–Miocene that constitutes the North-Pyrenean contractional front.

Geometric relationships and thickness variations depict that this overall structure results from the following.

• Mesozoic extensional stage which includes a Late Jurassic (?)–Late Aptian syn-rift stage in which the Parentis Basin formed; and an Albian–early Late Cretaceous post-rift stage in which diapirs of Triassic evaporites grew close to this major fault. • Compressive deformational stage coeval to the Pyrenean orogeny which led to (1) the development of a latest Cretaceous up to Middle Miocene foreland basin; and (2) from Late Eocene, the formation of a basement-involving thrust wedge in the innermost foreland basin, the squeezing of the diapiric structures formed previously in the Parentis Basin and, later (Middle Miocene), the partial inversion of some pre-existent faults located in the southern Parentis Basin margin.

This geodynamic evolution together with the structure of the area evidences that the extensional structure resulting from the opening of the Bay of Biscay played an important role both in the location of the North-Pyrenean front and in the North-Pyrenean foreland contractional deformation features. Specially, the lack of significant inversion structures in the Parentis Basin, despite that it belongs to a severely thinned crustal area before the Alpine compression, denotes that the Mesozoic Landes High acted as an important buffer for the propagation of the Pyrenean contractional deformation to the north. This deformational buffer was active until Early Miocene and probably vanished afterwards during the last stages of Pyrenean orogen development when some basement faults reactivated in the Parentis Basin