Hydration in the Crust and Upper Mantle Near the Extinct Spreading Ridge in the Eastern Sub‐Basin, South China Sea

Abstract During the final stages of seafloor spreading in the East Sub‐basin of the South China Sea, spreading transitioned from slow to ultraslow before cessation. However, post‐spreading volcanism has obscured the original fabric along or near the extinct spreading ridge (ESR). We present a detailed V P / V S model along a profile perpendicular to the ESR, constructed using independent tomography of P ‐ and S ‐wave data. This model provides critical constraints on the crustal lithology and tectonic evolution of the ESR. Our results show significant thinning of the crust (∼3.3–4.3 km) near the ESR. Within the thin crustal regions, anomalously elevated V P / V S ratios (>1.9) may indicate tectonic uplift of serpentinites. These observations indicate a substantial reduction in magma supply during the terminal phases of spreading, resulting in a near absence of lower crust. During this period, tectonic extension dominated, and crustal fracturing facilitated seawater infiltration into the upper mantle, promoting serpentinite formation.