Water and carbon are transferred from the ocean to the mantle in a process that 22 alters mantle peridotite to create serpentinite and supports diverse 23 ecosystems 1 . Serpentinised mantle rocks are found beneath the seafloor at slow-to 24 ultraslow-spreading mid-ocean ridges 1 and are thought to be present at about half 25 the world's rifted margins 2,3 . Serpentinite is also inferred to exist in the downgoing 26
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. ...
Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent‐ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide‐angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0–7.0 km s−1 velocity contours, corresponding to peridotite serpentinization of 60–30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8–3.5 km, with an average velocity gradient of 1.0 s−1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.