Implications for hazard assessment and long-term magmatic evolution of the Lesser Antilles arc: Integrated Ocean Drilling Program (IODP) Expedition 340 - Lesser Antilles Volcanism and Landslides

Flank collapse with emplacement of voluminous and potentially tsunamigenic debris avalanches is the key recurrent process controlling the evolution of Lesser Antilles volcanoes. In island arcs where edifices are small, most debris avalanche material is deposited offshore. On land studies preserve only a very limited part of the information necessary to study these major volcanic events of considerable potential hazard. Furthermore, analysis of available piston sediment cores offshore Martinique and Montserrat identified many more explosive eruptions than documented from on-land studies, demonstrating that marine sediments contain a more complete record of volcanic history. However, these shallow penetration piston cores only provide a very short part of the Lesser Antilles volcanic history. IODP Expedition 340 (JOIDES Resolution, March-April 2012) was dedicated to characterizing volcanic and marine sediments at nine primary sites in the backarc region of the Lesser Antilles arc, using both deep coring and downhole logging. We obtained 484 cores that recovered 2384 meters of sediment around Martinique and Montserrat - areas that are representative of the entire eruptive activity across the Lesser Antilles arc. The deepest hole cored reached depths of 436 meters below seafloor. These new data provide a complete record of eruptive activity and volcaniclastic sedimentation of some of the most active volcanic complexes. Initial results show that the Montserrat volcanic sites could be 4 million years old, while those in Martinique go back 400,000 years. Given that land material from Montserrat only dates back 2.6 million years, these findings show the importance of integrating marine and terrestrial studies. In addition, we observed inclined and convoluted banding of hemipelagic sediment combined with deformed tephra or turbiditic layers. Identifying the origin of these deformed units and their relationship with aerial flank-collapse events on land is an important objective for the post-expedition research. The cores will be carefully screened over the next few years to: 1) provide a more accurate reconstruction of the long-term eruptive history of the Lesser Antilles arc focusing on volcano evolution (cycles of construction and destruction) to guide the prognosis of future activity; 2) establish a more precise timing and description of emplacement processes of debris avalanches produced by flank collapse and; 3) improve geohazards evaluation associated with flank-collapse events with implications for tsunami hazards. Results of this project will provide novel and critical insights into the behaviour of volcanic debris avalanches and the role of flank collapses in the evolution of a volcanic arc which displays a remarkable frequency of flank collapses.