The Hainan plume and the origin of tectonic and magmatic activity in Southeast Asia

Southeast Asia hosts widespread Cenozoic intraplate volcanism that is not related to arc volcanism or to the opening of the South China Sea. In this study, we analyze the Southeast Asia portion of the recently developed full waveform seismic model, FWEA23, presenting its first comprehensive interpretation for this region. We observe slow shear velocity (Vs) anomalies (∼4–5%) extending to ∼660 km depth beneath Hainan island, resembling one or more plume-like upwellings. At depths <220 km, the slow anomaly extends westward to the Sagaing fault, eastward to the subduction zones, and northward to ∼26°N latitude. This lateral spreading explains the similarities in timing and geochemical signature between the Cenozoic intraplate volcanism and the Hainan volcano. We observe that the asthenospheric mantle (100–220 km) beneath Southeast Asia is ∼1.4% slower than the global average shear velocity of oceanic asthenosphere, implying that the mantle beneath Southeast Asia is warmer than the global adiabat. We also detect high Vs anomalies (up to ∼3%) in the mantle transition zone, resembling lithospheric slab fragments which trap heat and may have facilitated plume initiation through the slab gaps. Additionally, we present evidence from radial anisotropy (> + 3%), which reveals strong horizontal mantle flow (<200 km) away from the Hainan plume. This is consistent with lateral plume-head spreading, and associated lithospheric thinning across Southeast Asia. Our results provide new constraints on mantle dynamics of Southeast Asia by (i) highlighting the super-adiabatic character of the asthenosphere, (ii) a slab-induced mechanism for Hainan plume generation and (iii) the genesis of the intraplate volcanism.