Tectono-sedimentary characteristics and subduction initiation in the middle Kyushu-Palau Ridge and adjacent basins: A comprehensive study of multichannel seismic reflection profiles

The Kyushu Palau Ridge and the basins on the both sides are ideal areas for understanding the formation and evolution of marginal seas and the initiation mechanism of plate subduction. We studied the tectonic sedimentary characteristics and crustal structure of the Kyushu-Palau Ridge (KPR) and adjacent basins using newly collected deep reflection seismic data combined with the borehole data, and discussed the origin and subduction initiation mechanism of the West Philippines Sea. Seismic profiles and drill cores revealed that two sets of sediment caprocks are developed from different sources in the Western Philippines Basin. The thickness of the lower set is very different, and most of them are volcanic materials that derived from island arc volcanos of KPR, showing continuous thickening towards KPR, and volcanoclastic rock skirts are developed near the foot of KPR. The thickness of the upper set is relatively stable, being composed of deep-water fine-grained sediments. The crust thickness of the western Philippines Basin and Parece Vela Basin is 6-8km, which is similar to the global average oceanic crust thickness. The Moho surface of the Western Philippines Basin is in a wide-gentle fold shape, and fluctuates synchronously with the ocean crust basement beneath sediments. The Western Philippines Basin might be located in the northern margin of Australia in the southern hemisphere in the Mesozoic, and was gradually developed on a continental margin arc (including some continental fragments) due to inter-arc extension and submarine spreading in the Paleogene. Seismic profiles and drilling in the Western Philippines Basin have revealed tectonic compression events during the Eocene. The subduction along the paleo-IBM (Izu-Bonin-Mariana) might be induced by the remote effect of the Indo-Asian collision. The subduction process was accompanied by lateral propagation and persistent compressive stress field, which consequently resulted in the island arc broke up in about 30Ma. This study has certain theoretical significance for studying the plate tectonic evolution and subduction initiation mechanism in Southeast Asia.