Manta-Ray : a study of the relationship between fluids and seismicity in the Lesser Antilles

The Lesser Antilles subduction zone is one of very few regions where old oceanic crust formed at slow spreading rates is being subducted. Crust accreted at slow spreading ridges 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. The water stored in these serpentinites is released in the subduction process and then migrates upwards, towards the seafloor, where it forms fluid escape features, such as mud-volcanoes and pockmarks. Seismic reflection profiles from offshore Antigua and Barbuda image high amplitude seismic reflectors extending from the top of the downgoing crust roughly 15 km down into the mantle. They possibly originate from low-angle detachments related to exhumation of mantle material at the slow spreading Mid-Atlantic Ridge. As serpentinite rheology differs from that of basalt and gabbro, and because the amount of fluids from dewatering serpentinite is significantly higher than from mafic crust, the rheological properties of the plate interface and the margin are likely to influence slip behavior.

During the Manta-Ray cruise in 2022, bathymetric and seismic data were acquired in the Lesser Antilles region with the objective to study the influence of subduction of this ultramafic basement on the tectonic deformation, fluid circulation and seismogenesis. At the accretionary prism fluid extrusion sites have been identified in the bathymetric data and their structures are finely imaged by high-resolution seismic data. Mud-volcano structures are linked to deeper faults and individual mud flows are imaged with a high enough a resolution to be described. In the region of the previously imaged deep reflectors, a 3D seismic experiment was conducted during which 23 deep sounding seismic profiles were acquired, crossing 75 ocean-bottom seismometers and using a 5000 cu-inch airgun array and a 6 km long seismic streamer. The extension of the reflectors east of the trench was proven and initial interpretation of the seismic data clearly shows the existence of several fault families dipping in different directions. In this area also numerous fluid extrusion features were identified which might have formed during accretion at the Mid-Atlantic ridge.

During the LAVAS project, starting in 2025, we will further study the relationship between the hydration the downgoing plate and seismicity in the Lesser Antilles. In the scope of the project passive seismic data will be acquired using a sailing vessel along the different fluid extrusion sites to record signals from fluids leaving the seafloor. A submarine glider survey along these sites will help to identify possible methane accumulation in the water column. Satellite images will be used to identify natural hydrocarbons leaking from the seafloor and arriving at the sea surface. In an associated pedagogic project seismometers will be installed in schools on the islands of Guadeloupe and Saint Martin.