Debris avalanche deposits offshore St. Vincent (West Indies): Impact of flank-collapse events on the morphological evolution of the island

Major scarp features have been previously identified on Soufrière volcano (St. Vincent, Lesser Antilles) and recently interpreted as flank-collapse structures although no associated debris avalanche deposits were recognized. New offshore high-resolution bathymetry and geophysical data have been collected during the Caraval cruise (N/O L'Atalante, February–March 2002) offshore of St. Vincent. Recent analysis of these data leads us to identify and characterize a debris avalanche deposit on the western flank of Soufrière volcano, which extends down to the Grenada Basin. The origin of the submarine deposit by submarine slump or flank-collapse event is discussed but data analysis tends support the occurrence of a flank-collapse event during the evolution of Soufrière volcano. The collapse volume has been estimated to about 9 km3 and the age of the event is less than 50,000 years (using marine geophysical constraints). The origin of the flank-collapse can be related to local causes (weakening of the flank by intense hydrothermal alteration, magmatic injections, increase of the load by accumulation of volcanic products) and/or to regional causes (asymmetry of the arc, sea level variations induced by climate change that may have an effect on the stability of the volcano). Taking into account this event, we proposed a model for the evolution of Soufrière volcano including a new estimation of the magma production rate. The estimation indicates that Soufrière volcano is one of the most active volcanoes of the arc. A future flank-collapse event could have catastrophic consequences for the entire area due to the emplacement of voluminous, potentially tsunamigenic, debris avalanches.