Seafloor instabilities and sediment transport on the steep and seismically active Ligurian margin (North-Western Mediterranean)

The Ligurian margin has a steep morphology showing complex inherited tectonic structures, variably incised canyons, and numerous submarine slides. The margin has been frequently affected by earthquakes (four historical earthquakes in 1564, 1644, 1817, 1887 AD), and tsunamis (1564, 1817, 1887, 1979 AD). The high anthropic pressure on this densely populated coastal zone was a further reason to investigate about the triggering mechanisms of sediment failures with two campaigns at sea (Malisar1 and Malisar2). The 2006 dataset consists of EM 300 multibeam bathymetry between Nice and Genova, from about 100 m to 2500 m water depth, 3-5 kHz profiles, and 24- or 72-multichannel seismic profiles. In 2007 followed the acquisition of sediment cores, side scan sonar (SAR) imagery, and very high resolution AUV multibeam bathymetry. The dataset reveals numerous fresh scarps and mass-transport deposits involving several cubic kilometres of sediment. Offshore Nice (France), mass-wasting events mainly affect the upper part of the slope, near to the Var and Paillon river mouths, where sediment delivery is the highest. The most abundant failures are located near the shelf break; they are small-scale (< 100 m wide) and affect the uppermost sediment layers (up to 10 m). Larger-scale failures (up to 400 m wide) are located deeper on the slope and affect deposits over greater thickness (up to 40-50 m). Smaller failures mainly result from the under-consolidation state of slope sediment rapidly accumulated, while the triggering of larger failures probably requires an external forcing such as an earthquake-induced acceleration of the seafloor. Between Nice (France) and Imperia (Italy), failures are several kilometres wide and affect slope deposits over 100 to 300 m. They are located near the base of the slope, between 1300 and 2000 m of water depth. The location of these failures could be related to active faults that remain to be detected precisely. Three impressive scarps are located in the epicentral area of the 1887 AD earthquake. One of them is clearly visible at the seafloor and could be related to the 1887 AD event. The two others are partly buried under hemipelagic deposits and could be related to the recurrent earthquakes in that area. Between Imperia and Genova (Italy), submarine slides are mainly restricted to the canyon flanks. A major scarp, 7 km long and 100 m height, is located in an interfluve area between two major canyons, offshore Savona. The failure exhibits large blocks of well-layered deposits in its upstream part, then a chaotic mass several km long in its downstream part. The absence of draping hemipelagite deposits suggests that failure has been triggered recently. The timing of most of the slides is unknown, but ongoing research is aiming at: a) coupling the distribution of submarine slides with the location of epicentres of historical earthquakes, and b) establishing a chronological framework for the stratigraphic units involved in failure.