Super-adiabatic asthenosphere in southeast Asia - its tectonic and magmatic implications

Southeast Asia is surrounded by active subduction zones and has a complex tectonic history. It hosts the Indo China block in the west and the South China Sea in the east. Cenozoic intraplate volcanic activity widely spans the Indo China block and the South China sea. These volcanics are not related to any arc volcanism or the opening of the South China Sea during the Miocene. Here, we present an interpretation of a new high resolution seismic tomography model FWEA23, in southeast Asia. We observe extensive slow shear velocity (Vs) anomalies in FWEA23 across most of southeast Asia, extending from the surface down to ~660 km, resembling one or more plumes. We observe the evidence for a strong upwelling beneath Hainan island and the Leizhou peninsula and a weaker one beneath eastern Vietnam, Thailand and Laos. These upwellings spread laterally in all directions beneath southeast Asia at shallow depths (less than ~220 km). We address the tectonic implications of the plume head at shallow depths as well as the deeper origins of the Hainan plume. At depths < 220 km, the slow anomaly extends westward to the Sagaing fault, eastward to the active subduction zones, and northward to ~26°N latitude. We also observe that the asthenospheric mantle (100 - 220 km) beneath southeast Asia is slower than the global average shear velocity of oceanic asthenosphere, implying that the mantle beneath Southeast Asia is warmer than the global adiabat. Additionally, our model indicates a shallow Lithosphere-Asthenosphere boundary (LAB) in the region. We infer that the lateral spreading of the plume at shallow depths is thermally eroding the base of the lithospheric mantle. Also, this lateral spreading could explain the consistency in timing and geochemical features between the Cenozoic intraplate volcanism and the Hainan volcano. We also observe flat, isolated high Vs anomalies in the mantle transition zone, that we interpret as the remnants of subducted slabs. We observe low Vs anomalies in the gaps between the high Vs anomalies. We suggest that these stagnant slabs create a thermal boundary layer on top of the lower mantle by trapping the heat beneath. The built-up heat increases the temperature of the ambient mantle and triggers thermal upwellings that ascend through the slab-gaps.