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Insights from the P Wave Travel Time Tomography in the Upper Mantle Beneath the Central Philippines

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Insights from the P Wave Travel Time Tomography in the Upper Mantle Beneath the Central Philippines remote sensing Article Insights from the P Wave Travel Time Tomography in the Upper Mantle Beneath the Central Philippines Huiyan Shi 1 , Tonglin Li 1,*, Rui Sun 2, Gongbo Zhang 3, Rongzhe Zhang 1 and Xinze Kang 1 1 College of Geo-Exploration Science and Technology, Jilin University, No.938 Xi Min Zhu Street, Changchun 130026, China; [email protected] (H.S.); [email protected] (R.Z.); [email protected] (X.K.) 2 CNOOC Research Institute Co., Ltd., Beijing 100028, China; [email protected] 3 State Key Laboratory of Geodesy and Earth’s Dynamics, Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan 430071, China; [email protected] * Correspondence: [email protected] Abstract: In this paper, we present a high resolution 3-D tomographic model of the upper mantle obtained from a large number of teleseismic travel time data from the ISC in the central Philippines. There are 2921 teleseismic events and 32,224 useful relative travel time residuals picked to compute the velocity structure in the upper mantle, which was recorded by 87 receivers and satisfied the requirements of teleseismic tomography. Crustal correction was conducted to these data before inversion. The fast-marching method (FMM) and a subspace method were adopted in the forward step and inversion step, respectively. The present tomographic model clearly images steeply subduct- ing high velocity anomalies along the Manila trench in the South China Sea (SCS), which reveals a gradual changing of the subduction angle and a gradual shallowing of the subduction depth from the north to the south. It is speculated that the change in its subduction depth and angle indicates Citation: Shi, H.; Li, T.; Sun, R.; the cessation of the SCS spreading from the north to the south, which also implies that the northern Zhang, G.; Zhang, R.; Kang, X. part of the SCS opened earlier than the southern part. Subduction of the Philippine Sea (PS) plate Insights from the P Wave Travel Time ◦ ◦ Tomography in the Upper Mantle is exhibited between 14 N and 9 N, with its subduction direction changing from westward to ◦ ◦ ◦ Beneath the Central Philippines. eastward near 13 N. In the range of 11 N–9 N, the subduction of the Sulu Sea (SS) lies on the Remote Sens. 2021, 13, 2449. https:// west side of PS plate. It is notable that obvious high velocity anomalies are imaged in the mantle ◦ ◦ doi.org/10.3390/rs13132449 transition zone (MTZ) between 14 N and 9 N, which are identified as the proto-SCS (PSCS) slabs and paleo-Pacific (PP) plate. It extends the location of the paleo-suture of PSCS-PP eastward from Academic Editor: José Borneo to the Philippines, which should be considered in studying the mechanism of the SCS and Fernando Borges the tectonic evolution in SE Asia. Received: 5 June 2021 Keywords: seismic tomography; upper mantle; slab tear; Philippine subduction; South China Sea; Accepted: 19 June 2021 proto-South China Sea Published: 23 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- iations. The Philippines are located in the collision and convergence region of the Eurasian plate, the PS plate and the Indo-Australian plate. The interaction of the three plates has created a complex tectonic environment in the area. The Philippines are surrounded by the bidirectional subduction of the PS plate and the SCS (Figure1). The Philippine subduction zone plays a critical role in the tectonic evolution of the SE Asia. The kinetic energy Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. generated by the strong convergence of the Eurasian and PS plates [1] is regulated and This article is an open access article absorbed by the bidirectional subduction system in this region [2]. distributed under the terms and The mechanism of the SCS opening dynamics and tectonic evolution are still unre- conditions of the Creative Commons solved issues. Many kinematic models have been put forward to explain the opening Attribution (CC BY) license (https:// process of the SCS [3–9]. However, there is no widely accepted model of the SCS spreading. creativecommons.org/licenses/by/ One of the major controversies in establishing the SCS opening model is which of the 4.0/). eastern and southwestern basins of the SCS opened first. The Philippines lies on the eastern Remote Sens. 2021, 13, 2449. https://doi.org/10.3390/rs13132449 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, x FOR PEER REVIEW 2 of 17 Remote Sens. 2021, 13, 2449 2 of 17 eastern and southwestern basins of the SCS opened first. The Philippines lies on the east- boundaryern boundary of the of SCS. the SCS. The westwardThe westward subduction subduction of the of PS the plate PS beneathplate beneath the central the central Philip- pinePhilippine limited limited the eastward the eastward spreading spreading of the of SCS, the SCS, and theandSCS the SCS plate plate subducted subducted eastward east- beneathward beneath the Philippines the Philippines [10]. Previous[10]. Previous tomographic tomographic results results revealed revealed the the slab slab tear tear of theof SCSthe SCS slab slab [11,12 [11,12].]. However, However, there there were were few studiesfew studies on the on relationship the relationship between between subduction sub- andduction spreading and spreading of the SCS of the plate. SCS Therefore, plate. Therefor researche, research on deep on structuredeep structure of the of Philippine the Phil- subductionippine subduction zone will zone possibly will possibly contribute contribute to study to thestud openingy the opening of the SCS.of the SCS. Figure 1. Tectonic framework of the central Philippine and surrounding regions. The topography Figure 1. Tectonic framework of the central Philippine and surrounding regions. The topography data are provided by GSHHG. The saw-toothed lines indicate the trench axes. The dashed lines dataindicate are providedthe collision by zone. GSHHG. The solid The saw-toothedlines indicate lines active indicate faults. The the trenchred triangles axes. Theindicate dashed the vol- lines indicatecanoes, which the collision are obtained zone. from The solidNCEI lines Volcano indicate Location active Database, faults. TheNOAA red National triangles Centers indicate for the volcanoes,Environmental which Information. are obtained from NCEI Volcano Location Database, NOAA National Centers for Environmental Information. Previous researchers have carried many studies to understand the geodynamic pro- cess Previousof the Philippine researchers subduction have carried zone many [13–16]. studies One to of understand the most important the geodynamic controversial process oftopics the Philippinein this region subduction is the PSCS. zone At [13 present,–16]. One there of thehas mostbeen importantan inconclusive controversial debate about topics inwhere this regionare the isPSCS the PSCS.slabs. Some At present, researchers there hasargued been that an the inconclusive PSCS once debate existed about in the where area arebetween the PSCS the southern slabs. Someboundary researchers of the SCS argued and Borneo that the [17,18]. PSCS Tang once et existed al. found in the500 areakm betweensoutheastward the southern high velocity boundary anomalies of the SCSbelow and northern Borneo Borneo, [17,18]. which Tang et were al. foundidentified 500 kmas southeastwardPSCS slabs [7]. Hall high et velocity al. argued anomalies that the below PSCS northernslabs were Borneo, at 800 km which between were East identified Borneo as PSCSand Southern slabs [7]. Philippines Hall et al. argued [19]. A that high-velocity the PSCS slabsanomaly were was at 800 discovered km between at 400 East km–700 Borneo andkm Southerndepth beneath Philippines the central [19]. APhilippine, high-velocity whic anomalyh was interpreted was discovered as the at PSCS 400 km–700 slab that km depthgenerated beneath by southward the central Philippine,subduction which [12]. wasHowever, interpreted some asresearchers the PSCS slabclaimed that generatedthat the by southward subduction [12]. However, some researchers claimed that the PSCS slab subducted northward to the present SCS [17,20]. According to the tomographic results, Remote Sens. 2021, 13, 2449 3 of 17 Shi et al. proposed that PSCS has subducted northward beneath Borneo and inferred that the Paleo-Tethys and the paleo-Pacific (PP) have never been connected by PSCS [21]. Lin et al. proposed a double-side subduction model to interpret the PSCS based on plate reconstruction [22]. To answer the above questions, a high-resolution tomographic model is needed. In this work, we selected 32,224 useful relative travel time residuals recorded by 87 receivers distributed in the central Philippines. Crustal correction was applied to remove the influ- ence on the upper mantle. These data were adopted to image the detailed upper mantle structure in the Philippine subduction zone. Then, we present a 3-D high-resolution to- mographic model in the upper mantle beneath the central Philippines. Our tomographic model clearly images the steeping and tearing of SCS slabs, which provide evidence to define the opening sequence of the SCS. The PS plate and SS slab are also imaged by the present model. The PSCS-PP slabs are also revealed by our tomographic result, which helps to determine the location of paleo-suture of PSCS and PP. The present study has significant implication for the establishment of the SCS opening mechanism and the study of the tectonic evolution of SE Asia. 2. Data and Methods 2.1. Data The range of study area is (8◦ N~18◦ N, 118◦ E~128◦ E). There are 87 receivers dis- tributed in the study area (Figure2a). In this work, we picked travel time data recorded by these receivers from 1960 to 2020, which was primarily derived from International Seismological Center (ISC) [23]. The selection of teleseismic tomography data should satisfy the following conditions: (1) The magnitude of events is greater than 4.5; (2) The epicentral distance is 30◦–90◦, which reduces the influence of deep structures such as lower mantle; (3) Only events received by more than five receivers can be used for inversion calculation.
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