Electrical resistivity structure of the lithosphere-asthenosphere boundary beneath the extinct ridge of the South China Sea

Extinct spreading ridges are globally widespread and are crucial to understanding the lifespan of oceanic plates. Yet the nature of the LAB beneath extinct ridges remains enigmatic. In this study, we investigate the LAB structure beneath the SCS basin, where a ~700-km-long extinct ridge system stopped spreading at ~15 Ma. A 120 km long marine magnetotelluric (MT) transect perpendicular to the extinct mid-ocean ridge in the southwest sub-basin of SCS was carried out at September 2021. The electrical resistivity model reveals a relatively low-resistivity layer at depths of 50-80 km, potentially corresponding to 0.1%-0.9% partial melts. This low-resistivity layer is heterogeneous and absent directly beneath the extinct ridge axis. This observation supports a model in which melts are efficiently extracted beneath the ridge axis, leaving the central region depleted, while partial melts are retained in the surrounding areas on either flank. Additionally, residual melts at shallower depths have likely solidified due to plate cooling, while deeper melts indicate the depth of the LAB. These findings propose a new mechanism for the emplacement of long-lived partial melts at the LAB and suggest that a discontinuous melt-rich layer may commonly occur near extinct spreading ridges globally.