Continental arc systems often show evidence of large-scale migration both toward and away from the incoming plate. In oceanic arc systems, however, while slab roll-back and the associated processes of backarc spreading and arc migration toward the incoming plate are commonplace, arc migration away from the incoming plate is rarely observed. We present a new compilation of marine magnetic anomaly and seismic data in order to propose a new tectonic model for the eastern Caribbean region that includes arc migration in both directions. We synthesized new evidence to show two phases of backarc spreading and eastward arc migration toward the incoming Atlantic. A third and final phase of arc migration to the west subdivided the earlier backarc basin on either side of the present-day Lesser Antilles arc. This is the first example of regional multidirectional arc migration in an intra-oceanic setting, and it has implications for along-arc structural and geochemical variations. The back and forth arc migrations were probably due to the constraints imposed by the neighboring American plates on this isolated subduction system, rather than variations in subducting slab buoyancy.

The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interface suggest an unusually wide megathrust seismogenic zone reaching ∼65  km depth. Our results provide an excellent framework for future understanding of regional seismic hazard in eastern Caribbean and the volatile cycling beneath the Lesser Antilles arc.

Oceanic lithosphere carries volatiles, notably water, into the mantle via subduction at convergent plate boundaries. This subducted water exercises a key control on the production of magma, earthquakes, formation of continental crust and mineral resources. However, identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to observed surface expressions has proved challenging 1 . The two Atlantic subduction zones are valuable end members to study this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform 2 . As part of an integrated, multi-disciplinary project in the Lesser Antilles 3 , we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine, i.e. hydrated mantle rather than crust or sediments, is a dominant supply of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over the past ~10 Myr. Dehydration of these fracture zones is consistent with the locations of the highest rates of earthquakes and prominent low shear velocities, as well as time-integrated signals of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data provide the clearest indication to date that the structure and hydration of the downgoing plate are directly connected to the evolution of the arc and its associated hazards.The 750 km-long Lesser Antilles volcanic arc (LAA), located along the eastern margin of the Caribbean Plate, is the result of slow (1-2 cm/year) westward subduction of Atlantic and proto-Caribbean oceanic lithosphere (Fig 1). Water hosted in hydrous phases within the subducting plate will be released as the slab sinks into the mantle and warms up. As the water migrates out of the slab the stress on faults is reduced, causing earthquakes. At the same time, the addition of water to the overlying mantle wedge reduces the solidus temperature which may enhance melting. LAA magma production rates lie at the lower end of the global range, probably due to the low convergence rates, and are very unevenly distributed, being greatest in the centre of the arc (Dominica and Guadeloupe) 4 . The LAA also displays notable along-arc variations in geochemistry, volcanic activity, crustal structure, and seismicity [5][6][7][8] . Subducting plate velocity and age are often held responsible for variations in convergent margin behaviour 9 but are unlikely to have first-order influence on lateral variations within the LAA as neither vary significantly along-strike. Instead, variations in LAA magmatism and seismicity have been proposed to reflect; (i) a combination of a strong north to south increase in sediment input 10 , (ii) subduction of bathymetric ridges below the central arc 11 , which may enhance plate stress and coupling, (iii) and/or subduction of strongly hydrated fracture zones 12 at several locations along arc (Fig. ...

We present a high-resolution 2-D P-wave velocity model from a 225-km-long active seismic profile, collected over~60-75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three nontransform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two-layer velocity structure with a high gradient Layer 2 (~0.9 s −1) above a lower gradient Layer 3 (0.2 s −1). Here, PmP coincides with the 7.5 km s −1 contour, and velocity increases to >7.8 km s −1 within 1 km below. We interpret these segments as magmatically robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between the upper and lower crust and PmP shallower than the 7.5 km s −1 contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. While velocity-depth profiles fit within previous envelopes for slow-spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments, we make the first wide-angle seismic identifications of buried oceanic core complexes in mature (>20 Ma) Atlantic Ocean crust. They have a~20-km-wide "domal" morphology with shallow basement and increased upper crustal velocities. We interpret their midcrustal seismic velocity inversions as alteration and rock-type assemblage contrasts across crustal-scale detachment faults.

The margins of the Caribbean and associated hazards and resources have been shaped by a poorly understood history of subduction. Using new data, we improve teleseismic P-wave imaging of the eastern Caribbean upper mantle and compare identified subducted-plate fragments with trench locations predicted from plate reconstruction. This shows that material at 700–1200 km depth below South America derives from 90–115 Myr old westward subduction, initiated prior to Caribbean Large-Igneous-Province volcanism. At shallower depths, an accumulation of subducted material is attributed to Great Arc of the Caribbean subduction as it evolved over the past 70 Ma. We interpret gaps in these subducted-plate anomalies as: a plate window and tear along the subducted Proto-Caribbean ridge; tearing along subducted fracture zones, and subduction of a volatile-rich boundary between Proto-Caribbean and Atlantic domains. Phases of back-arc spreading and arc jumps correlate with changes in age, and hence buoyancy, of the subducting plate.