Flank collapse, sediment failure and flow-transition: the multi-stage deposition of a volcanic sector collapse offshore Montserrat, Lesser Antilles

Volcanic sector collapses generated some of the most voluminous mass transport deposits on Earth and triggered devastating tsunamis with numerous casualties. The associated sector collapse deposits occur around many volcanic islands all over the world. The shelf around the volcanic island of Montserrat (Lesser Antilles) and the adjacent Montserrat-Bouillante-Graben host more than ten surficial or buried landslide deposits with most of them classified as volcanic debris avalanche deposits by previous studies. The most intensively studied deposit (Deposit 2) is associated with a landslide that occurred at ~ 130 ka and comprises a volume of 10 km³, including remnants of the volcanic flank and secondarily mobilized seafloor sediments. Here, we present new 2D and 3D seismic data as well as MeBo drill core data from Deposit 2 that reveal multi-phase deposition including an initial blocky volcanic debris avalanche followed by secondary seafloor failure and a late- erosive event. Late-stage erosion is evidenced by a channel-like incision on the hummocky surface of Deposit 2 about 15 km from the source region. Erosional incisions into the top of sector collapse deposit have also been reported from Ritter Island, Papua New Guinea – the only other volcanic landslide deposit that was studied at similarly high resolution. This may imply that late stage erosive turbidites are a common process during volcanic sector collapse. This requires geological and oceanographic processes that can create high flow velocities close to the source of the collapse area leading to a late down-slope acceleration of sediments that were suspended in the water column.