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International audience; The Nile deep-sea fan is the largest Plio-Quaternary depocentre in the Mediterranean Sea, extending over an area of >150,000 km2 within which rapid deposition drives syn-sedimentary collapse tectonics, as well as widespread seafloor venting of mixed thermogenic and biogenic gases. It is thus an ideal setting for the near-seabed accumulation of gas hydrates, which are stable below water depths of about 1100 m in the warm waters of the eastern Mediterranean. Gas hydrates have yet to be sampled on the Nile fan, although their presence has been suggested from semi-published industry seismic and well log data. Here we integrate available industry data with a regional grid of academic geophysical data (seismic and multibeam) acquired by Géoazur, in order to identify a BSR on the central Nile fan, invert it to geothermal gradients, and examine its relationship to fluid venting. The BSR is observed on several intersecting seismic profiles of varying frequency content and offset, as a discontinuous reflection of negative polarity that lies 220-330 ms below seafloor, deepening downslope in water depths of 2000-2500 m. The BSR occurs within a relatively small area (2500 km2), among a larger system of slope-parallel extensional faults. Inversion of BSR depth to temperature gradients was performed using a velocity-depth function for deep-sea sediments, a phase boundary for methane hydrate in equilibrium with seawater of 3.86% salinity (Mediterranean average), and water temperatures from MEDATLAS. The results indicate spatially varying gradients of 27-42˚C/km, up to twice background values reported in the geothermally cool eastern Mediterranean offshore. ‘Shallow’ BSRs yielding elevated thermal gradients within the Nile fan can be explained in terms of advective heat transfer by upward fluid flux. Upward fluid migration is supported by evidence of widespread seafloor fluid venting, but seafloor data indicate few or no vents in the area of the BSR. In contrast, in an area with many gas vents over 50 km from the BSR, subsurface gas hydrate accumulations are indicated by resistivity log data at two well-sites. If gas and gas hydrates are present across wider areas, why is the BSR so restricted? We hypothesise that these observations can be explained in terms of spatially varying fluid flux, such that away from the BSR a greater flux of fluids rich in dissolved gas leads to rapid gas hydrate formation and the growth of gas chimneys. We intend to test this hypothesis during a forthcoming Franco-Brazilian campaign, focused on understanding the dynamics of the fluid vents. Acknowledgement : this work has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 656821 (SEAGAS project).

International audience; One of the objectives of the European project SEAHELLARC (SEismic and tsunami risk Assessment and mitigation scenarios in the western HELLenic ARC) was to identify the key elements controlling the geological structure of the Peloponnese continental margin in order to better understand the distribution of earthquakes in the area. Vintage multichannel seismic lines were reprocessed, interpreted and integrated with detailed morpho-bathymetric data and high-resolution single channel seismic data. According to the interpretation, most of the upper continental margin has been created within the former Alpine Gavrovo, Ionian and Apulian thrust units, including their Miocene to Quaternary cover and their Triassic salt-bearing formations. The Ionian thrust complex seems to have been continuously overthrusting the Apulian Ridge, itself characterised by strongly fractured acoustic basement features. Ductile and mobile Triassic evaporitic sequences are present in most of the major Ionian zone morpho-structures. The westward directed thrusts of the Ionian zone reached their final positions in Lower Pliocene and were followed by uplift of the ridges that is continuing in the present time. Previous compressional structures are being reactivated as normal faults with associated diapirism in the Triassic evaporites. NNE-SSW shear displacements related to trans-extensional motion may have activated "pull apart" basins with ENE-WSW normal faulting at their heads. This NNE-SSW dextral strikeslip motion agrees with the present anticlockwise rotation of the Peloponnese. The focal mechanism and the NNE-SSW distribution of aftershocks of a major earthquake in June 2008 along the Andravida fault define a major lineament that can be followed past the Katakolo Peninsula to the diapiric morpho-structures defining the eastern boundary of the Strophades basin.

International audience; Funded by the EEC Sixth Framework Program, the SEAHELLARC project was aimed to evaluate, and better understand the causes, of the various natural geohazards (chiefly earthquakes and tsunamis), which frequently affect the western Peloponnese area and particularly its coastal domain; this region is one of the most seismically active of Greece and therefore of the Mediterranean Sea. Based on a set of new geophysical data, such as detailed swath bathymetry and high-resolution sub-bottom Chirp, we have distinguished and studied four contrasted domains along this area of the Peloponnese active continental margin underlined by intense crustal seismicity and marked by very contrasted and often sharp continental slopes; from east to west these are: (1) an area including the continental shelf and the upper slope; there sedimentary overload and destabilizations, syn-sedimentary faults, mass transport deposits and active sedimentary by-pass mechanisms are the main risk factors. (2) The middle to lower continental slopes, is mainly expressed by two, N-S trending, fault-related, depressions, where active deformations, well recorded by actual tilting of the sedimentary blanket, occur. (3) West of this deep structural depressions exists a poorly sedimented ridge area (from which merges the small Strophades Islands) also showing N-S and E-W trending lineaments resulting in a dense network of fractures and scarps and leading too a particularly complex sub-marine morphology; this area, together with the westernmost deep domain (4), which bounds the continental margin, clearly records the effects of significant active tectonic. Our studies of the shallow and recent sedimentary cover of the continental margin off western Peloponnese, confirm that this active margin segment is an area where geohazards can be expected. In addition to fault ruptures, generated at depth by the specific tectonic framework, sedimentary collapses, particularly along the shelf break nearby Cape Katakolo, may trigger significant local tsunamis, which may in turn induce strong damages all along the nearby coasts up to the town of Pylos.

International audience