Large-scale margin collapses along a partly drowned, isolated carbonate platform (Lansdowne Bank, SW Pacific Ocean)

The Lansdowne Bank is a partly drowned, isolated carbonate platform of around 4000 km2 located 300 km west New Caledonia, in the SW Pacific Ocean, water depths 20 to 100 m. multibeam bathymetric data, high resolution seismic reflection profiles and sediment gravity cores have been acquired on bank top adjacent slopes. This dataset reveals an almost continuous 4 wide outer reef rim ca. 50 m depth, surrounding gently deepening inner platform, reaching up depth. bordered by very steep slopes showing numerous erosional morphologies such as canyons, channels gullies. Along with these bypass features, spectacular margin collapses slope failures are evidenced km-wide edge intraslope failure scars, respectively, resulting typical “scalloped” geometry margin. These scars can lead complete collapse impact subsequent development. hectometer kilometer-scale blocks debris shed slope, likely emplaced rock fall/avalanching processes originating from brittle early cemented upper sediments. In turn, triggered un-cemented mud-prone middle lower generate more cohesive, submarine flows that could be at origin erosive within fields. Estimated individual volumes reach 3 km3. Quaternary sea-level lowstands, would led exposure, fracturing karstification, development sea cliff, well rising believed play significant role mass wasting event emplacement, yet “bottom up” upslope propagation retrogressive headward erosion cannot ruled out. terms geomorphic stratigraphic constraints, documented affect terrace 70 depth bank, which, depending its age origin, provide minimum for events. Finally, considering shallow headscarps, material involved slides their vicinity nearby main island numerical simulations tsunamigenic potential performed. They showed able produce meter-scale wave northern coast less than hour.