Submarine Slope Failures along the Northern Sicilian Continental Margin (Southern Tyrrhenian Sea) and Possible Implications for Geo-hazard

Mass wasting and downslope movements are common processes that have contributed to shape the northern Sicilian continental margin (southern Tyrrhenian Sea) since the Late Quaternary. Nevertheless, processes controlling their evolution are still partially unknown and a variety of geologic factors can be responsible for their formation. In this work we present an overview of the main mass wasting features (submarine canyons, landslides, debris flows) observed and mapped in different sectors of the northern Sicilian margin. The margin is characterized by a narrow, steep continental shelf (1-2°) and a very irregular and steep (6-8°) upper slope. The main aims of this work are: (1) to outline the morphology of the submarine canyons and of the related mass failure features, (2) to describe the main geological processes that control mass failure and (3) highlight their potential implications for the geo-hazard. The study is based on Multi Beam echosounder, Side Scan Sonar data and seismic reflection profiles, for a depth range of 20-1500 m, and on sediment samples and scattered gravity cores, collected to define the textural and compositional properties of the seafloor. Our study focused on the Castellammare Gulf-San Vito Trough system, the Gulf of Palermo, the Termini-Cefalù offshore and the Ustica offshore. In the Castellammare Gulf two main channel systems converge at the San Vito Canyon. This latter develops northwards with a very sinuous pattern, crossing the San Vito through. In the western margin of the Gulf, where the upper slope is very steep (12-13°) and the continental shelf is very narrow or absent, there are a lot of short, straight, sub-parallel gullies that are only few metres deep. In this sector the gully heads are located very close to the coastline and are associated to retrogressive slides. According to the aforesaid, downward erosive flows appear to be the most common mechanism generating widespread slope failure inside the Castellammare Gulf. In the Palermo Gulf a difference in slope configuration can be observed between western and central-eastern sector. Submarine canyons are confined to the upper slope or indent the shelf-edge and enter the Palermo intraslope basin at a depth of around 1,300 m. The canyons evolved through concurrent top-down turbiditic processes and bottom-up retrogressive mass failures. Most of the mass failure features of the area are related to canyon shaping processes. The western sector of the Palermo Gulf slope displays a highly dissected substrate, showing steep to very steep gradients, that appear to favour a retrogressive evolution of the canyons. Headward erosion processes are the main controlling factor in shaping this sector. NNE trending canyons occurring in the central-eastern sector develop in connection with the Oreto and Eleuterio rivers, breaching the shelf break, and show a linear to sinuous thalweg path. A similar pattern can be recognized in the eastern sector of the Termini-Cefalù offshore. The occurrence of pockmarks and carbonate mounds also suggests the probable role that fluid seeps play in the mass wasting processes of the area. Furthermore, the occurrence of pockmarks and highs that probably consist of authigenic carbonates above faulted and folded strata suggests a local relationship between structural control, fluid escape processes and mass failure. In the Ustica offshore volcanism related features (debris avalanche, gravitational collapse of volcanic edifices) are mainly linked to neotectonic activity and volcanism that tends to build, load and steepen the submarine slopes with time. This paper presents a valuable high-resolution morphologic dataset of the northern Sicily continental margin, which constitutes a reliable base for evaluating the geo-hazard potential related to slope failures in the area, through monitoring of areas where mass movement might be forthcoming and modelling landslides consequences to develop geohazard mitigation strategies.