Pn Velocity Structure and Anisotropy of the Uppermost Mantle Beneath Southeast Asia

Abstract

Systematic investigation of the uppermost mantle structure beneath Southeast Asia is of great significance to understand the dynamics of oceanic plate subductions, continental collisions, and material upwelling associated with a mantle plume. To obtain high-resolution images of Pn wave velocity structure and anisotropy of the uppermost mantle beneath Southeast Asia, we collect 182,875 high-quality Pn arrival time data of 8,647 local earthquakes recorded at 860 stations from seismograms of temporary HAVESArray and permanent seismic networks in and around Hainan Island, as well as from the ISC-EHB Bulletin. Our results generally align with previous findings but provide new insights into the tectonic evolution and dynamics of the Indochina block and the Hainan mantle plume. The Andaman-Sumatra-Java-Timor and Philippine-Taiwan-Ryukyu arcs, associated with active volcanoes, exhibit prominent low-velocity (low-V) anomalies, whereas trenches and sea basins show higher velocities. Notably, the northwestern part of the Indochina block displays significant low-V anomalies with an NW-SE fast propagation direction (FPD), which are divided into two branches with dominant N-S and NE-SW FPDs at the northwest margin of the Khorat Plateau. This result suggests that southeastward mantle flow of the Indochina block bifurcates when encountering the Khorat Plateau. In addition, distinct velocity anomalies beneath the South China Sea and the southern Indochina block reflect unique traces of material upwelling from the Hainan mantle plume, contributing to the wide distribution of late Cenozoic basalts in Southeast Asia.