The role of crustal accretion variations in determining slab hydration at an Atlantic subduction zone 

<p>We present a 2D P-wave velocity model from across 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 up 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 interpret this drop as due 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 two sources of hydration results in compressional velocities as low as 6.5 kms^-1 beneath the PmP reflector, with a slow velocity anomaly extending to ~10km below top basement. 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 we observe 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, in contrast with Pacific subduction zones where the fast-intermediate spreading lithosphere being subducted is more structurally uniform. This heterogeneous pattern of water storage within the slab, which is further complicated by along strike variations in outer rise bending, subducting fracture zones and deformation at segment ends may have important implications for our understanding of long-term patterns of hazard at Atlantic subduction systems.</p>