Tomography of the upper mantle and transition zone in Southeast Asia

SUMMARY Southeast Asia, bordered by significant tectonic plates such as the Indo-Australian, Pacific and Philippine Sea Plates, is distinguished by its frequent tectonic activity and complex geological structures, making it one of the most dynamically evolving regions worldwide. In this study, we introduce a novel 3-D P-wave velocity model of the upper mantle and transition zone in Southeast Asia using regional seismic traveltime tomography based on first-arrival data from the International Seismological Center. We employ an adjoint-state tomography method with normal-vectors independence to accurately invert for 3-D velocities using a 1-D reference model. Synthetic tests confirm the reliability of our model in delineating features of the subduction zone and the surrounding region. Our inversion results highlight distinct subducted slabs within the subduction zone and a pervasive low-velocity zone beneath Sundaland, which may be associated with lithospheric thermal weakening. Additionally, a mushroom-shaped low-velocity anomaly attributed to the Hainan mantle plume is identified beneath Hainan Island. The low-velocity anomaly observed beneath the western part of the Java Sea may be attributed to the combined effects of Sunda-Java slab subduction, lower mantle flow through the Sunda Strait and the influence of the Hainan mantle plume. Notably, beneath the Andaman Sea, we observe an east–west elongation of the northern Sumatra slab, potentially linked to the clockwise rotational opening of the Andaman Sea. Additionally, three potential rifts are identified beneath the subducting Sumatra-Java slab: beneath the Toba Volcano, the Sunda Strait and the eastern segment of Java Island. Extensive high-velocity anomalies beneath the Philippine Islands and the South China Sea suggest a double-sided subduction process involving the Proto-South China Sea slab.