Seismic Imaging and Salt Tectonics of the Mediterranean Salt Giant in the Central Algerian Basin

The Mediterranean Salt Giant (MSG) is a thick layer of Messinian evaporites (up to 4 km) that is thought to be deposited during an extreme paleo-environmental event known as the Messinian Salinity Crisis (MSC). After decades of research, there is not yet a consensual model explaining the emplacement and the evolution of the MSG. This is due to the absence of samples of the deep offshore MSG. Past scientific drilling operations were limited to the topmost MSG records because of the risks of intersecting zones of hazardous fluids and overpressure linked to evaporites. The European project SALTGIANT, in which the work of this thesis fits into, is dedicated to understanding the formation of the MSG and its implications for the microbial life, the drilling hazards and the geo-economics of the Mediterranean region and the history of oceanography. In that framework, this thesis aims to improve our seismic images of the offshore MSG and to use the new results to update our understanding of the Messinian salt tectonics. This work is focused on the Algerian basin, in the south-western Mediterranean Sea, where the salt was deposited in an already contractional tectonic setting. I compile, reprocess, and interpret legacy academic seismic data acquired in the central Algerian basin. The re-processing is designed to improve as much as possible the salt and pre-salt structures. It relies on an integrated approach combining geophysics and geological interpretation to iteratively build the velocity model. The new results display a better imaging of salt structures and the seismic facies variations. They shed a new light on the tectono-sedimentary evolution of the central Algerian basin, highlighting the presence of seismic fluid indicators evidencing an active fluid circulation in the basin and its margins. Interpretation of the new seismic sections is done following the most recent nomenclature of the MSC seismic markers. New isochores and thickness maps are produced and compared with the spatial distribution of the salt structures. I interpret contractional salt tectonic structures, such as buckle folds, squeezed diapirs and related salt sheets as evidence of regional thick-skinned shortening episodes. I suggest that extensional stage of the salt system (where the deformation is driven by gravity loading) was short-lived, and that many salt structures were driven by contractional tectonic loading during the Plio-Quaternary. I demonstrate that the initial shortening-related salt deformation in the late Messinian was focussed along the Algerian margin and later shifted outward toward the Balearic margin in the Plio-Quaternary. The shifting of the deformation front is interpreted to be a result of the thickening and strengthening of the overburden. The second peak of deformation may have reactivated faults along the Emile-Baudot escarpment with thick-skinned deformation. I also observe a variation in the intensity of the salt deformation along the margin from SW to NE, which I associate to variable tectonic loading applied along the Algerian margin or the pre-shortening distribution of salt. Fluid indicators are imaged within the Plio-Quaternary of the Algerian basin. They could be thermogenic or biogenic gas sourced from the Messinian Upper Unit, or from the pre-salt, migrating through a hydro-fractured salt. The new results also evidence numerous volcanic structures within the Formentera basin. The distribution of this volcanic edifice could affect fluid circulation, resulting in small-wavelength surface HF anomalies observed locally.