New OBS array reveals relic slab distribution in the mantle of Southeast Asia Curved Subduction System

Southeast Asia is located in an important regional geodynamic intersection zone and is surrounded by inward subduction systems on three sides. It is the largest and most complicated convergent subduction system on Earth and is known as the Southeast Asia Curved Subduction System (CSS). The deep circulation and structure of the subducted slabs, especially the relic slabs within the CSS, have been an elusive scientific mystery. In this study, using an ocean bottom seismometer (OBS) array in the South China Sea in 2019-2020 and the long-term inland seismic network, we present a new seismic tomographic deep structure of the relic slabs in the CSS mantle. Both the eastern and western CSS subducting slabs exhibit high angles and pass down through the mantle transition zone (MTZ), while the southern subducting slab not only passes through MTZ but also lies flat in the lower mantle, giving rise to a very unique "dustpan"-shaped deep mantle structure. Above the "dustpan"-shaped subduction slab, a high-velocity layer located in and around the South China Sea is interpreted as a relic slab and it lies within the MTZ and continues to subduct to the north. This relic slab is broken by the low-velocity Hainan plume and may result from paleo-Pacific subduction.