Résumé
The aim of this study is to unravel the tectono-sedimentary evolution of a hyper-thinned rift, based on the example of the Mauléon Basin, a basin filled by thick synrift deposits. The integrated study combines field data, detailed geological mapping and seismic interpretation. The field study focuses on the Iberian margin of the Mauléon Basin. Seismic interpretation and well calibration along a NS transect of the Mauléon Basin enable imaging the transition with the northern conjugate margin. The synrift records are very different on either side of the basin: the southern margin is composed of a proximal turbiditic s.l. siliciclastic system, whereas the northern margin is characterized by a carbonate system extending from the platform to the basin. We recognize the Mauléon rift as an apparent symmetric hyper-thinned rift, related to a southward dipping Albian detachment and a northward dipping Cenomanian one. Two stages of continental crustal thinning are inferred to explain the development of the Mauléon Basin. First, a Barremian to earliest Albian “ductile pure-shear thinning phase”, responsible for the lower crustal thinning and the formation of a symmetric sag basin. Second, an Albian-Cenomanian simple-shear thinning phase, responsible for the onset of the southward dipping Saint-Palais detachment faulting and for evolution to an asymmetric basin. The Iberian margin appears as an upper plate and the European one as a lower plate during Albian time. At Early Cenomanian time, the basin was affected by structural changes of the margins resulting from shift in detachment direction, interpreted as “flip-flop detachment tectonics".
•The Early Cretaceous Mauléon rift basin evolves from a symmetric sag basin to an asymmetric basin.•Basin asymmetry is recorded by different depositional profiles along the rift conjugate margins.•The symmetric sag basin is linked to lower crustal thinning without any brittle deformation of the upper crust.•The basin asymmetry and the hyper-thinned continental crust result from double verging detachment system.