Hydration Processes in the Crust and Upper Mantle of an Extinct Spreading Ridge in the Eastern Sub-Basin, South China Sea

During the final stages of seafloor spreading in the East Sub-basin (ESB) of the South China Sea (SCS), the spreading ridge transitioned from slow to ultraslow before the complete cessation. Post-spreading volcanic activity has obscured the original structures along the extinct spreading ridge (ESR), complicating interpretations of spreading-related tectonics. Using independent tomographic inversion of P- and S-wave data, we constructed a detailed V_P/V_S  model along a profile perpendicular to the ESR, offering critical insights into crustal lithology and tectonic processes of the ESR.

A total of 1749 PSP’ arrivals were obtained, with uncertainties of 0.08 to 0.18 s due to picking error and traveltime correction. To reveal S-wave velocities beneath the sedimentary basement, P-wave velocities were fixed by overdamping the sedimentary layer during Tomo2d modeling, achieving an RMS misfit of 0.148 s and a normalized χ² of 1.37. Vp/Vs ratios were calculated based on the P- and S-wave tomography models, which reveals no distinct layering, predominantly ranging between 1.70 and 1.90. A low V_P/V_S ratio anomaly (<1.70, LRA) is identified in the model distance ~38-82 km, extending to ~1.0 km depth and coinciding with the low-velocity zone at the top of Layer 2. Additional low V_P/V_S anomalies are observed at both ends of the profile, but limited ray coverage and large uncertainties (>0.05) render these poorly resolved structures and excluded from further discussion.

The most striking feature in the V_P/V_S  model are high-ratio anomalies (HRA) in the central portion of the model. One of these HRAs (~1.9-1.99) extends from ~6.0 to ~11 km at depth in the thin oceanic crustal domain, spanning Layer 2 and Layer 3. In the mantle beneath this HRA, a high V_P/V_S zone (>1.8) coincides with the low S-wave velocity anomaly. Within these significantly thinned crustal regions, the abnormally high V_P/V_S ratios suggest the intrusion of serpentinites into the oceanic crust. These observations indicate a substantial reduction in magma supply during the terminal phases of spreading, resulting in rugged basement morphology, an unusually thin crust, and the near absence of lower crust. During this period, tectonic extension dominated seafloor spreading and the crustal fracturing facilitated seawater infiltration into the upper mantle, promoting serpentinite formation and intrusion.