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

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks-in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas-provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt

Updip rupture of the 2004 Sumatra earthquake extended by thick indurated sediments

During subduction, weak, unlithified sediments are scraped off the down-going plate and accumulate near the subduction trench axis. The weak nature of the sediments usually impedes the propagation of fault rupture during an earthquake. However, measurements of slip during the 2004 Sumatra–Andaman Mw 9.2 earthquake show that fault rupture propagatedupdip, extending unusually close to the subduction trench, in the southern part of the rupture area. Here we present seismic reflection images of the southern part of the 2004 Sumatra–Andaman earthquake rupture area. We show that sedimentary strata, greater than 4?km in thickness, form coherent blocks that have been thrust onto the continental margin during subduction. The blocks form a 130-km-wide plateau overlying the seismogenic zone and the plate boundary megathrust lies near to the base of the sediments. The sediments consist of the Nicobar and Bengal Fan turbidites and exhibit strong internal cohesion. We suggest that dewatering and lithification of the sediments during burial made them unusually competent and strong, thus enabling rupture during the 2004 earthquake to propagate beneath the plateau, close to the Sunda Trench. Extending fault rupture so close to the trench, and thus further seaward, may have enhanced the tsunami hazard by displacing a greater thickness of water