The Lesser Antilles subduction zone is one of very few regions where old oceanic crust formed at low spreading rates is being subducted. Crust accreted at slow spreading differs from crust formed at higher rates mainly in its higher content of material originating from the upper mantle, which is mostly hydrated to form serpentinites when arriving at the subduction zone. The water stored in these serpentinites is released once the slab arrives at a depth of 30-60 km and eventually reaches the seafloor where it forms fluid escape features, such as mud-volcanoes and pockmarks. During the previous Antithesis cruises (2013-2016), reflection seismic profiles imaged high amplitude seismic reflections reaching from the top of the downgoing crust and roughly 15 km down into the mantle along a 200-300 km wide region. We propose, that they originate from low-angle detachments related to exhumation of mantle material at the slow spreading Atlantic mid-ocean spreading center.

Cruise Manta-ray aims at studying the influence of this ultramafic oceanic basement onto the tectonic deformation, fluid circulation and seismogenesis, while subducting, during two legs. The first leg consists of a 3D wide-angle and reflection seismic survey in the region where the deep reflectors were imaged in order to characterise the nature of the downgoing basement and quantify its fluid content. During the shorter second Leg a comparison of fluid escape features of two zones, one located above the deep reflectors and one in a region void of them and will allow us to study geochemical differences of fluids returning from dehydration from the subducting crust. The main objectives of the Manta-ray cruise are to constrain the nature and tectonic origin of the deep reflectors, to characterise the influence this anomalous crust has on the seismicity and tectonics of the region and to quantify the impact of the subduction originating from slow spreading on global water cycles.