The Role of Crustal Accretion Variations in Determining Slab Hydration at an Atlantic Subduction Zone

We present a 2D P‐wave velocity model from the outer rise region of the Lesser Antilles island arc, the first wide‐angle seismic study of outer rise processes at an Atlantic subduction zone. The survey consists of 46 OBS receivers over a 174 km profile with velocities resolved to 15 km below top basement. The final velocity model, produced through tomographic inversion, shows a clear decrease in the velocity of the lower crust and upper mantle of the incoming plate as it approaches the trench. We attribute this drop to outer rise bend‐related hydration, similar to Pacific cases, but superimposed on spatial variations in hydration generated at the slow‐spreading ridge axis. In thin, tectonically controlled crust formed under magma‐poor spreading conditions the superposition of these sources of hydration results in compressional velocities as low as 6.5 km s−1 beneath the PmP reflector. In contrast, segments of crust interpreted as having formed under magma‐rich conditions show velocity reductions and inferred hydrous alteration more like that observed in the Pacific. Hence, variations in the style of crustal accretion, which is observed on 50–100 km length scales both along and across isochrons, is a primary control over the distribution of water within the slab at Atlantic subduction systems. This heterogeneous pattern of water storage within the slab is likely further complicated by along strike variations in outer rise bending, subducting fracture zones and deformation at segment ends and may have important implications for our understanding of long‐term patterns of hazard at Atlantic subduction systems. Plain Language Summary Subduction zones are locations where one of the Earth's tectonic plates is dragged below the other into the mantle. These are locations of significant seismic and volcanic hazard. Water present in the down‐going plate plays an important role in determining the distribution of these hazards. In well‐studied Pacific subduction zones much of this water enters the down‐going plate as it is deformed just prior to subduction in a region known as the outer rise. This hydration can be estimated using seismic techniques which measure the time energy takes to travel through the plate, with altered “wet” rocks shown to be anonymously slow. As the structure of the crust in the Pacific is relatively uniform this hydration is evenly distributed. At Atlantic subduction zones such as the Lesser Antilles the structure of the subducting plate is mor