Seismic imaging of the Lesser Antilles subduction zone using S-to-P receiver functions

Abstract:

Constraining the depths and pathways of volatiles and melt within the mantle wedge is important for a better understanding of subduction zone processes. The Lesser Antilles arc presents an important end-member, where oceanic lithosphere generated at the slow spreading Mid-Atlantic Ridge, and so hypothesised to be significantly hydrated in fracture zones and core complexes, is being subducted beneath the Caribbean plate. Here, we calculate S‑to‑P (Sp) receiver functions using land and ocean-bottom data recorded by the Volatile Recycling in the Lesser Antilles (VoiLA) project to image 3-D discontinuity structure of the overriding plate, mantle wedge, and the subducted slab. We illuminate a seismic velocity increase with depth that undulates along-arc at depths that range from 34-43 ± 4 km, shallowest beneath Dominica and Martinique and deepest beneath St. Lucia. Given previous results from geophysics and geochemistry, the shallower discontinuities are likely mid-crustal and related to magma intrusion, whereas deeper discontinuities beneath the other islands represent the Moho. A velocity decrease with depth is imaged beneath the northern arc and back-arc at depths of 76‑79 ± 5 km, which is likely the base of the overriding Caribbean plate. Its visibility beneath the north of the arc is likely related to relatively greater amounts of melt ponding beneath the plate here, and possibly also more melt generation. This is in good agreement with the notion of higher degrees of serpentine breakdown and fluid concentrations in the north from geochemistry. Hints of a negative-positive phase are imaged beneath the arc in the south at depths of 105 ± 5 km and 119 ± 5 km, respectively, which may demarcate the crust of the down-going slab, possibly only visible in the south owing to shallower slab dip in this region. The scale and structure of these discontinuities suggests that greater hydration in the wedge overall towards the north results in broad scale melt ponding beneath the upper plate, and discrete serpentine sources in the slab likely preferentially contribute additional volatiles, magmatism, and volcanism and cause greater crustal thicknesses at certain volcanic locations.