Widespread Hydration of the Back Arc and the Link to Variable Hydration of the Incoming Plate in the Lesser Antilles From Rayleigh Wave Imaging

Abstract

Subduction zone dynamics are important for a better understanding of natural hazards, plate tectonics, and the evolution of the planet. Despite this, the factors dictating the location and style of volcanism are not well-known. Here we present Rayleigh Wave imaging of the Lesser Antilles subduction zone using ocean bottom and land seismic data collected as part of the VoiLA experiment. This region is an important global endmember that represents a slow (<19 mm/yr) convergence rate of old (80-120 Ma), Atlantic lithosphere formed at a slow spreading ridge. We image the fast slab, the fast-overriding plate and the slow mantle wedge across the entire arc. We find slow velocity anomalies (~4.1 km/s) in the mantle wedge directly beneath the arc with local minima beneath Dominica/Martinique, Montserrat and the Grenadines. We observe that slow velocities in the wedge extend 200 km into the back arc west of Martinique. The slowest mantle wedge velocity anomaly is more muted than several global wedges, likely reflecting the lower temperatures and less partial melt predicted for the Antilles. Subducted fracture zones and plate boundaries are a potential source of hydration, since they are located nearby the anomalies, although not directly beneath them. To match our observations, geodynamic models with a broadly hydrated mantle wedge are required, which can be achieved via deep hydration of the slab, and fluid release further into the back arc. In addition, 3-D flow and melt migration or ponding are required to explain the shape and location of our anomalies. KeypointsWe image a fast slab and upper plate with slow anomalies found in the wedge beneath the central part of the arc and the backarc Low velocity wedge anomalies are associated with projected fracture zones and tectonic plate boundaries on the downgoing slab Accepted ArticleThis article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

Slab to back-arc to arc: fluid and melt pathways through the mantle wedge beneath the Lesser Antilles

“…We compare the imaged seismic attenuation structure with published seismic velocity models from local earthquake tomography (V P and V P /V S ) (43,51) along with V S from teleseismic Rayleigh waves (45) and ambient noise (46). Then, we interpret the most likely thermal structure, volatile and melt content, and their pathways, by using experimental and numerical geodynamic predictions as a guide (25,43,45,52,53).…”

“…We compare the imaged seismic attenuation structure with published seismic velocity models from local earthquake tomography (V P and V P /V S ) (43,51) along with V S from teleseismic Rayleigh waves (45) and ambient noise (46). Then, we interpret the most likely thermal structure, volatile and melt content, and their pathways, by using experimental and numerical geodynamic predictions as a guide (25,43,45,52,53). We make these interpretations within the context of the two slab dehydration pulses inferred from high V P /V S (>1.8) anomalies in local tomography (43), indicating serpentinised slab crust and mantle at 60-80 km and >120 km depth, respectively.…”

“…Serpentine-derived fluids identified using boron isotopes (19) imply unusually high degrees of mantle hydration along the Marathon and Mercurius FZs (Figure 1), representing the boundary between the Proto-Caribbean and Equatorial Atlantic oceanic domains (44). Tomographic imaging and receiver functions (45)(46)(47)(48) show along-arc variations in S-wave velocity (V S ), with the slowest segment of upper plate mantle and mantle wedge lying beneath Dominica and extending 100 km into the back-arc.…”

“…Crucial unanswered questions remain about the LAA. Notably, why are low V S anomalies in the back-arc mantle wedge are offset from FZs, and why there is no high V P /V S in the sub-arc mantle wedge (43,45) as seen beneath Pacific arcs? To address these questions, this study investigates the locations and mechanisms of flux melting in the mantle wedge and the resultant melt pathways beneath the LAA.…”

2‐D Analytical P‐to‐S and S‐to‐P Scattered Wave Finite Frequency Kernels

“…These kernels assume a homogeneous Earth, and therefore do not include the effects of structural heterogeneity like the previously described approaches. Although the kernels could easily be extended to 3‐D, we investigate the 2‐D case here, as a first step and for the sake of simplicity and also given that many tectonic environments, for instance rifts (e.g., Armitage et al., 2015; Chambers et al., 2021, 2019; Lavayssiere et al., 2018; Rychert et al., 2012), ridges (e.g., Harmon et al., 2020, Harmon, Wang et al., 2021, 2018; Agius et al., 2021, 2018; Eakin et al., 2018; Rychert et al., 2021, 2018; Rychert et al., 2020; Saikia et al., 2021b, 2021a; Wang et al., 2020), or subduction zone trenches (e.g., Harmon et al., 2013, Harmon, Rychert et al., 2021, 2008; Chichester et al., 2020; Cooper et al., 2020; Harmon & Rychert, 2015; Rychert et al., 2008; Schlaphorst et al., 2021; Syracuse et al., 2008) are often characterized by at least some structures that are expected to be relatively 2‐D. The idea is to determine the utility of using the simplest and computationally efficient way to implement a kernel and also recover the magnitude of velocity discontinuities.…”