3D travel-time tomography of the incoming plate at the Northern Lesser Antilles subduction trench

Seismogenesis in subduction zones is profoundly shaped by the nature of the down-going crust and its lithosphere, their inherent structures, and hydration state. However, studying how these factors influence local seismicity remains challenging, particularly in regions with complex crustal and mantle structures. Simultaneously, the subduction of heterogeneous crust presents an opportunity to study the impact of variable physical properties on seismicity in close proximity. Thus, characterising the nature of such crust and the upper mantle immediately seaward of the deep-sea trench, at the smallest scale possible, may help in understanding spatial variations of seismogenic asperities, the variability of fluid input in the margin, and the overall heterogeneity of the subduction zone.

The Lesser Antilles are one of the few places on Earth where such processes can be studied in a natural laboratory setting. Oceanic crust accreted at the slow-spreading Mid-Atlantic Ridge (MAR) is permeated by first- and second-order, closely spaced fracture zones, which delineate crustal segments in which accretion alternates between being tectonically and magmatically dominated. The crust is further characterised by prominent faulting behaviour, showcasing detachment faulting near the spreading centre, normal faulting consistent with the spreading fabric, and faulting during the flexural bending at the subduction trench. Consequently, upon subduction, the crust is irregularly hydrated and contains variable amounts of reworked mafic crust and correspondingly variable amounts of uplifted mantle.

Here we present the first 3D traveltime tomography combined with controlled source seismic data of Central Atlantic crust seaward of the Northern Lesser Antilles subduction trench. We jointly invert first arrivals and Moho reflections using TOMO3D, which allows for high-resolution imaging of crustal and upper mantle structures at small spatial scales, and the comprehensive visualisation of the Moho. Our study is based on data recorded during the Manta-Ray cruise (R/V L’Atalante - 2022), in which 75 ocean-bottom seismometers were deployed offshore Antigua and Barbuda. Through an instrument spacing of 5 km and 10 km on the central and surrounding profiles, respectively, an unprecedented level of resolution has been achieved. Seismic shots were produced using a 5000 cu inch tuned airgun array, and were additionally recorded by a 6000 m seismic streamer. Previous studies have imaged high-amplitude seismic reflectors in the oceanic crust, which extend up to 15 km below the top of the basement and dip towards the MAR. Investigating their nature and their role in fluid migration is an additional objective of this study.

Our results provide (1) a crucial first step towards a more precise characterisation of the anomalous crust and its influence on Caribbean seismicity and tectonics, (2) a key constraint on the nature and tectonic origin of the deep reflectors, and (3) the foundation for deeper investigations into the particular role of fluids in seismogenesis.