Plate‐Plume Interactions in Southeast Asia: New Insight From Sn‐Wave Tomography

Abstract A novel shear‐wave velocity (Vs) model of the uppermost mantle beneath Southeast Asia is derived from a substantial amount of high‐quality Sn‐wave arrival‐times from the ISC‐EHB Bulletin. Our model reveals striking lateral heterogeneities closely linked to surface tectonics. The average Sn‐wave velocity in the region is 4.55 km/s. The trenches and continental collision zone exhibit significant high‐velocity (high‐Vs) anomalies of up to +0.40 km/s, indicating the presence of cold and dense lithospheric materials. Low‐velocity (low‐Vs) anomalies (as low as −0.40 km/s) are observed beneath volcanic arc areas, suggesting the existence of a hot, wet and partially melting mantle wedge. Furthermore, low‐Vs anomalies (−0.26 km/s) appear beneath eastern Tibet and its adjacent areas, possibly indicating eastward material flow from Tibet that has been blocked by the Yangtze block and divided into two branches, with the southward flow likely encountering the Khorat Plateau and bifurcating again. Integrating with previous Pn anisotropy and shear‐wave splitting measurements, we suggest that the flow around the Indochina block may also be contributed jointly by both westward mantle convection in the big mantle wedge and from the Hainan plume. Such a geodynamic model could explain the widespread distribution of late Cenozoic basalts and the formation of the Southeast Asian basalt province. A prominent ∼1,550‐km‐long, northeast‐southwest oriented low‐Vs belt is clearly imaged, extending from northern Hainan and reflecting a residual track of the Hainan plume. Combined with the subduction history, mantle plume activity, and large earthquakes in Southeast Asia, our findings shed new light on the regional geodynamics and plate‐plume interactions.