High resolution seismic imaging of the Lesser Antilles incoming plate, outer rise and accretionary prism: Implications for the hydration of slow-spreading Atlantic lithosphere

Subduction of chemically bound water in oceanic crust has long been known to be the primary method by which volatiles are reintroduced to the mantle. It has been proposed that the amount of water stored in the slab should vary globally, depending on factors including the age, spreading rate and degree of alteration. This variable hydration can in turn exert a strong control on subduction zone processes including intraslab seismicity and arc volcanism.

Here we present the results of a new, high-resolution, 2D seismic profile, shot by the R.R.S. James Cook in 2017, across the outer-rise and accretionary prism of the Lesser Antilles arc in the region of the Tiburon Basin. The Atlantic Ocean setting presents the opportunity to test our understanding of incoming plate hydration through comparison with better studied Pacific Ocean examples. Wide-angle P-wave arrivals along the 174km profile, modelled in tomo2d, resolve clear velocity perturbations in the incoming Atlantic lithosphere. Comparison with a coincident multi-channel streamer reflection profile allow us to tie these velocity variations to tectonic features, such as the outer-rise, which is identifiable by recent extensional faulting in the overlying sediments.

As well as being the first such experiment across slow-spreading Atlantic oceanic crust our profile samples significantly older crust (>90Ma) than the majority of existing studies. We identify clear hydration of the lower crust and mantle marked by a significant reduction in lower crustal and mantle velocities from ~30km outboard of the accretionary prism. This result has implications not just for the amount of water being delivered into the Lesser Antilles subduction zone, but for the global subduction water budget as a whole.