Pn tomography of South China Sea, Taiwan Island, Philippine archipelago, and adjacent regions

Abstract

The South China Sea (SCS) and its surrounding areas are geologically highly heterogeneous from the interactions of multiple plates in Southeast Asia (Eurasian plate, Indian‐Australian plate, Philippine Sea plate, and Pacific plate). To understand the tectonics at depth, here we combined bulletin and handpicked data to conduct Pn tomography of the region. The results show distinct features that are correlated with the complex geology at surface, suggesting a lithosphere‐scale tectonics of the region. Low Pn velocities are found along a belt of the western Pacific transpressional system from the Okinawa Trough and eastern East China Sea, across central and eastern Taiwan orogeny, to the island arcs of the Luzon Strait and the entire Philippine Islands, as well as under the Palawan Island and part of the continental margin north of the Pearl River Basin. High velocities are found under Ryukyu subduction zone, part of the Philippine subduction zone, part of the Eurasian subduction beneath the southwestern Taiwan, and the continent‐ocean boundary between the south China and the SCS basin. The Taiwan Strait, the Mainland SE coast, and the main SCS basin sea are relatively uniform with average Pn values. Crustal thicknesses show large variations in the study region but also coherency with tectonic elements. The Pn pattern in Taiwan shows linear trends of surface geology and suggests strongly lithosphere‐scale deformation of the young Taiwan orogenic belt marked by the deformation boundary under the Western Foothill and the Western Coastal Plain at depth, and the crustal thickness shows a complex pattern from the transpressional collision. Our observations are consistent with rifting and extension in the northern margin of the SCS but are not consistent with mantle upwelling as a mechanism for the opening and the subsequent closing of the SCS. The Philippine island arc is affected by volcanisms from both the Asian and Philippine Sea subductions in the south but mainly from the Asian subduction in the north and under the Luzon Strait.

Pn Anisotropic Tomography of Hainan Island and Surrounding Areas: New Insights Into the Hainan Mantle Plume

“…Our new high‐resolution tomographic model clearly illustrates a close correlation between the low‐V anomalies and the distribution of volcanic rocks in the northern Hainan Island, southern Leizhou Peninsula, and Weizhou Island in the study areas (Figure 7). Furthermore, the low‐V anomaly around the PRMB extends to the Leiqiong areas along the southeastern coast of the South China Block (Figure 7), in good agreement with the previous tomographic results (e.g., Z. Huang, Zhao, & Wang, 2015; X. Li et al., 2017; Mi et al., 2021; Xia et al., 2016). Although Mi et al.…”

“…Huang, 2014;Lei, Zhao, Steinberger, et al, 2009;X. Li et al, 2008X. Li et al, , 2017Mi et al, 2021;Tian et al, 2021;Xia et al, 2016;X.…”

“…From Figure 4b, we can obtain $${t}_{0}$$ = 6.31 s and $$\overline{{V}_{m}}=1/\text{slope}$$ = 7.99 km/s. We determine $${\overline{V}}_{c}$$ = 6.30 km/s, using the CRUST1.0 velocity model (Laske et al., 2013) and previous studies (X. Li et al., 2017; S. Li & Mooney, 1998; Liang et al., 2004). Thus, using Equation 2, we can obtain $$\overline{H}$$ = 32.3 km, which is adopted in the initial model.…”

“…The Leiqiong area is the largest Quaternary volcanic province in the South China Block, which could be associated with the upwelling of the Hainan plume. Many researchers have investigated the formation mechanism and morphological characteristics of the Hainan plume using geochemical data, receiver functions, and seismic tomography (e.g., J. Huang, 2014; Lebedev & Nolet, 2003; Lei, Zhao, Steinberger, et al., 2009; X. Li et al., 2017; Liang et al., 2004; Mi et al., 2021; Montelli et al., 2006; Montelli, Nolet, Dahlen, et al., 2004; X. Wang et al., 2013; X. Yang et al., 2021). An increasing number of tomographic results have shown that the Hainan plume originates from the lower mantle (e.g., Z. Huang, Zhao, & Wang, 2015; Lebedev & Nolet, 2003; Lei et al., 2013; Lei & Zhao, 2006; Xia et al., 2016; D. Zhao et al., 2021).…”

“…Although there have been many Pn tomographic models in Mainland China including the Hainan Island (e.g., Hearn et al., 2004; Liang et al., 2004; Pei et al., 2007; S. Wang et al., 2013; Zhou & Lei, 2016), their resolution is still insufficient to define the Hainan plume structure in the uppermost mantle. Recently, several Pn tomographic studies focused on the South China block and the northern SCS, but the raypath coverage in those studies is still not dense enough to resolve the detailed velocity and anisotropy structure of the Hainan plume (e.g., X. Li et al., 2017; Mi et al., 2021; Y. Xu et al., 2007). Over the period of May 2018 and September 2020, the National Institute of Natural Hazards (NINH), Ministry of Emergency Management of China (MEMC) deployed HAVESArray (Hainan Active Volcano Experiment of Seismology Array) portable seismic stations in the Hainan Island, Leizhou Peninsula and Guangdong province with a station interval of ∼20–30 km (Figure 2).…”