Tectonically induced bending of incoming plates at subduction zones can result in normal faulting in the upper ocean crust. Seismic surveys and numerical models indicate enhanced permeability and fluid circulation when this occurs. Yet, direct geological evidence of such effects on the seafloor is lacking. Here we report Human Occupied Vehicle (HOV) based observations of the existence of fluid discharge features on the seafloor of the incoming plate of the Mariana subduction zone. These features include fluid discharge points and associated pockmarks, which are striking, and occur in abundance in several depth related fields. The existence of Galatheid crabs, a typical seep related organism, also indicates fluid discharge from the seafloor. Alteration of the coexisting basaltic ocean crust is extensive, with iddingsite-rich muds within and overlapping the apparent fluid discharge zones. Our findings are significant in that they suggest that structural deformation of the incoming plate could substantially influence chemical exchange between the upper ocean crust and seawater in a new way. We further suggest that these fluid discharge points may represent previously unknown niches for H 2-based chemolithotrophic life and microbial ecosystems at deep trenches. Observations reported here contrast both chemically and physically with serpentine mud volcano formation associated with the shallower Mariana forearc region.

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SUMMARY The post-spreading magmatic activities in the northeastern South China Sea (SCS) margin are very strong, evidenced by widely distributed high-velocity lower crust (HVLC) and numerous volcanoes. However, there are large contrasts in magmatic activities and crustal structure between the Southern Depression (TSD) of the Tainan Basin and the volcanic continental slope area further south. We analyse their crustal P-wave velocity structures based on a newly acquired wide-angle ocean bottom seismic data set. The Cenozoic strata below the TSD, a Cenozoic failed rift, are relatively thick (∼3–4.5 km) with velocities from 1.6 to 3.6–3.9 km s–1, whereas the Mesozoic strata are relatively thin (∼1–2.5 km) with velocities from 4.3 to 4.6–5.2 km s–1. In the TSD, magmatic activities are relatively weak and the crust is severely thinned (∼4 km). The crust is 9–15 km thick below the volcanic continental slope area, which shows extensive volcanism. We identified HVLC below the failed rift of the TSD (Zone 1) and attributed it to mantle serpentinization, whereas the imaged HVLC below the volcanic continental slope (Zone 3) and HVLC adjacent to the failed rift of the TSD (Zone 2) are due to post-spreading magmatic underplating/intrusions. At the model distance ∼90 km, lateral transition from magmatic underplating/intrusions to mantle serpentinization occurred abruptly. We concur that post-spreading cooling and thermal contraction in the nearby SCS oceanic lithosphere can trigger decompressive melting and deformation in the thinned continental slope zone. Our study shows that, in addition to mantle serpentinization in the continent–ocean transition (COT) zone, mantle can also be serpentinized below the rift during early-stage rifting. Weak syn-rifting magmatism and mantle serpentinization below the failed rift support that the northeastern SCS has a magma-poor margin.

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