Abstract
Abstract Partial melt at the oceanic lithosphere-asthenosphere boundary is critical for plate motion, yet its origin remains debated. Here we image the electrical resistivity structure across the extinct spreading ridge of the South China Sea within 15–19 million years old oceanic crust using seafloor magnetotelluric data. Our results reveal a relatively low-resistivity layer at depths of 50–80 km beneath 17–19 million years old crust on both flanks of the extinct ridge axis, indicating the presence of 0.1%–1.3% partial melts. The top of this low-resistivity layer is consistent with the depth of calculated 1300 °C isotherm and indepedent observation of seismic shear wave velocity anomaly. The low-resistivity is discontinuous and weak directly beneath the ridge axis, suggesting a ~ 60 km-wide melt-depleted zone. We infer that beneath active ridges, two melt zones coexist: a shallower, focused triangular zone, overlying a deeper, diffuse layer of low-degree melts. Following ridge cessation, melt extraction beneath the axis leaves a heterogeneous, thinner layer that persists over geological timescales. The mechanical stability of this layer suggests that ridge-generated melts may be a widespread and enduring feature, and may account for the partial melts frequently observed at the oceanic lithosphere-asthenosphere boundary.