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National audience; This study focuses on breccias, widely developed in the Bas Agly syncline at the LateJurassic/Early Cretaceous boundary. Both their age and formation processes aredebated and remained hypothetical until today.According to our detailed sedimentological study, the breccia localized in the LateJurassic are spatially associated with large-throw normal faults and show a clearsedimentary character. They result from the creation of a steep topography thatbecomes unstable, producing major rockfalls. The studied breccias, are characterizedby poorly sorted polygenic deposits of pebbles to boulders composed of highly angularpluri-millimetric to pluri-metric carbonate clasts. A lateral evolution is observed, withpebble-size clasts found near the normal fault and boulder-size clasts away from thefault. This evolution is related to the rockfall process as the total kinetic energy acquiredby the small clasts during the fall is lower than that acquired by the bigger ones; as aresult, the latter are able to travel farther. These breccias are related to a first stage ofextension during the Late Jurassic and marks the beginning of the Early Cretaceousextension. However, the heterogeneity and complexity of the structure of carbonatebreccias require a detailed characterization of both their petrographic evolution anddiagenetic paleoenvironment in order to confirm our sedimentological model.Based on observations under S.E.M, polarized-light and fluorescence microscopy adetailed inventory of the diagenetic phases has been performed. From these analyses, adiagenetic sequence and a chronology of the cements have been reconstructed, usingprinciples of overlapping, superposition and inclusion. The oxygen and carbon isotopiccompositions of some cements and matrices of these breccias allowed to specify thediagenetic environments.The occurrence of metamorphic minerals overprinting the latest phase of calcitecementation strongly suggests that the breccia deposited prior to the Albo-Cenomanianregional metamorphic phase in contrast to previous stratigraphic attributions (Eoceneage) and support our interpretation of an interstratified sedimentary breccia of LateJurassic age. Thus, it is crucial to know the real age of these rocks. In order to solve thisissue, we initiated a geochronological study, using in-situ U-Pb dating by LA-ICP-MSand focusing on the carbonate matrix of the breccias. This approach has provensuccessful and yielded ages consistent with the proposed depositional environment.

International audience; The ”Landes-Parentis area” (LPA) is located at the transition between the eastern end of the abyssal plain of the Bay of Biscay, the Aquitaine basin to the east and the highly inverted Basque-Cantabrian basin to the south. It includes the Meso-Cenozoic Parentis basin (PB) bordered to the south by a Paleozoic high named Landes High (LH). The purpose of the study is to highlight the deformation of the Landes-Parentis area during extension from the end of Jurassic to the early Cretaceous, and then during inversion from the Upper Cretaceous onward.To understand this evolution, we analyzed ten regional 2D seismic lines through the LPA from the coast to the oceanic crust and from the LH to the Armorican margin. We mapped the main structures and stratigraphic unconformities. Well data from the PB has been used for the age dating of the different unconformities.The maps show that the Mesozoic cover observed in the PB extends to the foot of the con- tinental slope while all the Jurassic and Lower Cretaceous series are absent on the LH. The LPA continental slope is structured with west dipping normal fault. Further east, the contact area between PB and LH is particularly complex. Indeed, the PB presents an asymmetry between a gentle south dipping northern flank and a steep and faulted southern flank in a direct contact with LH. The LPA is also crossed by a NW-SE to NNW-SSE oriented, sinistral accident: the Landes transfer zone (LTZ). This transfer zone seems to continue in the LH by a set of NW-SE normal faults.East of this accident, the southern flank of the BP shows EW-oriented anticlines structures, attributed to salt ripples. The structural style shows normal faults inverted during the Tertiary and ramp anticlines. To the west, the southern flank presents two tilted blocks limited by EW- oriented crustal faults with a north-dipping normal movement. These faults affect the Triassic to Lower Cretaceous sedimentary series. They are sealed by the Upper Cretaceous to Tertiary sedimentary deposits. The structural scheme is mainly extensive.These results show that the NW-SE direction of the LPA, oblique with respect to the main NNE-SSW transfer zones of the Pyrenees and the lateral partitioning of the local deformation are probably strongly influenced by the presence of Landes High that play the role of a rigid block.

GeoMod 2016 conference Montpellier, France | 17–20 October 2016, Analogue modelling has been previously applied to the analysis of the impact of the progradation of a sedimentary system above a ductile layer (McClay et al., 1998). Commonly, diapiric architecture increases in complexity with: (i) lobe progradation, where the radial shape of the prograding system causes a complex network of polygonal or circular depocenters separated by salt ridges (Gaullier and Vendeville, 2005); (ii) basement topography beneath the ductile layer (Ge et al., 1997); and (iii) the confluence of two sedimentary wedges (Guerra and Underhill, 2012). The outstanding results obtained in all these models, however, did not consider ongoing tectonic processes during diapirism. We present 2 models with two phases (extension and post-extension phases) with different timing of progradation of sedimentary wedge in order to analyse how the time relationship between extension and sedimentary progradation control the formation of diapiric structures and their geometries.