Lateral collapses and active sedimentary processes on the northwestern flank of Stromboli volcano, Italy

The marine volcaniclastic system extending to 3 km water depth on the northwest flank of Stromboli volcano has been surveyed using seismo-acoustic profiling, multibeam bathymetry, deep-towed long-range side-scan sonar imaging and box-core sampling. Deposits and sea-floor topography record repeated flank collapses that have occurred during the last 13 ka and created the present Sciara del Fuoco scar. An extensive (170 km 2) debris-avalanche deposit characterized by subdued hummocks (megablocks) is buried by turbidites formed from gravity currents that originated in Sciara del Fuoco. The currents were mainly deflected along the margin of the avalanche deposit, but also spilt over onto it. The turbidites reflect topographic flow stripping and their thinning towards slightly steeper slopes indicates that some material that settled there drained further downslope as secondary higher concentration flows. The estimated volume of the entire fan-shaped bulge amounts to 12 ± 2.5 km 3, about 3.5 km 3 of which is attributed to the post-collapse sedimentation. The remaining 8.5 ± 2.5 km 3 is in the order of the volume computed for the material removed by the multiple sector collapses that affected the NW Stromboli flank. Two units are identified in the distal part of the bulge, through the recognition of their fronts, and these indicate a layered structure for successive debris-avalanche deposits. Granulometric and particle-type characterization of seabed samples shows clast population evolution with sedimentary transport distance, from the main entrance point of debris in Sciara del Fuoco, to the confluence with the Stromboli Canyon, and into the Marsili Basin some 45 km from the island. Changes in particle populations are due to selective sedimentation and also mixing with a wide range of terrigenous detritus brought by currents that descended the main Stromboli Canyon from the continental shelf. Abundant fine sand and silt is produced at the volcano and occurrence of mainly coarse-grained material along the channel to the Marsili Basin indicates that a considerable volume of volcaniclastic material from Stromboli lies in the sedimentary succession accumulating on the distal basin floor, where a deep-sea fan is growing.