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Collisional tectonics between the Eurasian and Philippine Sea plates from tomography evidences in Southeast China
Tectonophysics 606 (2013) 14–23
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Collisional tectonics between the Eurasian and Philippine Sea plates from tomography evidences in Southeast China Hong-Wei Zheng a, Rui Gao a,⁎, Ting-Dong Li b, c, Qiu-Sheng Li a, Ri-Zheng He a a b c
Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Chinese Academy of Geological Sciences, Beijing 100037, China Consulting Research Center, Ministry of Land and Resources, Beijing 100035, China
a r t i c l e
i n f o
Article history: Received 3 September 2012 Received in revised form 21 February 2013 Accepted 14 March 2013 Available online 22 March 2013 Keywords: Teleseismic tomography Southeast China Subducted Eurasian plate Subducted Philippine Sea slab
a b s t r a c t The upper mantle structure of Southeast China is important for us to understand the deformation and mantle dynamics process associated with the interaction between the Eurasian plate and Philippine Sea (PHS) slab. We determined a detailed three-dimensional P-wave velocity (Vp) structure of the crust and upper mantle down to 400 km depth beneath Southeast China by applying teleseismic tomography to 6869 high-quality P-wave arrival times. The data were collected very carefully from the original seismograms of 635 teleseismic events recorded by 65 broadband stations deployed in Southeast China. Our images show that the high-Vp PHS slab subducts toward the north along the Ryukyu trench at the latitude of about 24°N and extends down to 350 km depth and even more. High-Vp anomalies are imaged in the upper mantle under central and southern Taiwan, which represent the subducted Eurasian plate. Break-off Eurasian plate at a big angle subducting eastward is revealed under central Taiwan at depths from the upper mantle to 400 km. While continuous Eurasian plate under South Taiwan is mainly imaged from the Moho down to 400 km depth, a torn mantle window within the Eurasian continent beneath central and northern Taiwan created by the northward motion of the Philippine Sea plate is the upwelling path of the asthenosphere. The tomographic images also show the low-Vp anomalies spread widely under the coastal areas of Mainland China and Taiwan Strait. The structure of the crust and upper mantle suggests that the mountain building process in the central part of Taiwan is mainly attributed to the subduction–collision tectonics at the boundary between the Eurasian continental lithosphere and the subducting oceanic lithosphere of the PHS slab. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The southeastern margin of China is located at the juncture zone of the Eurasian plate and the Philippine Sea (PHS) slab, with unusual tectonic features (Fig. 1). Particularly, it is famous for the intensive magmatic activities, recording the multiphasic assembly and breakup processes of plates (Cheng, 1994). Taiwan Island is the only zone located in the youngest trench–arc–basin system in China and it is also the most active collision orogen between continental plate and oceanic slab in the world. Therefore, it is the best field laboratory for studying the collision and subduction processes between oceanic slab and continental plate. The upper mantle structure under the southeastern margin of China not only reveals the complex tectonic evolution, but also records and displays the ongoing plentiful geological phenomena during the collision between oceanic plate and continental plate. About the collision and subduction processes of the Eurasian continental plate and the PHS slab, previous researchers have relatively unanimous viewpoint on the collision and subduction mechanisms ⁎ Corresponding author. Tel.: +86 10 68999730; fax: +86 10 68997803. E-mail address: [email protected] (R. Gao). 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.03.018
in the northern and southern areas of Taiwan Island. In the northern area of Taiwan Island, the PHS slab is subducting under the Eurasia plate along the Ryukyu trench (Ai et al., 2007; Lallemand et al., 2001; Mcintosh et al., 2005; Wu et al., 2009a, 2009b). In the southern area of Taiwan Island, the Eurasia continental plate is subducting under the PHS slab (Kim et al., 2005; Lallemand et al., 2001; Mcintosh et al., 2005; Sibuet and Hsu, 2004; Wang et al., 2006; Wu et al., 2007; Zhang et al., 2008). However, there have been three different points of view on the collision mechanism in central Taiwan. The first point is that there is an eastern-dipping Eurasian continental plate under central Taiwan (Chen et al., 2004; Huang et al., 2010; Sibuet and Hsu, 2004). The Eurasian plate crust is involved in the collision orogen, and its mantle continues to subduct. Wang et al. (2006, 2009) proposed that the Eurasian plate is subducting eastward under the PHS slab in central Taiwan, with depth up to 300 km. The second viewpoint supports the presence of thickened crust and the absence of subducting mantle (Rau and Wu, 1995; Wu et al., 1997; Mcintosh et al., 2005; Zhang et al., 2008). In central Taiwan, the continental crustal layer is thickened to form ‘root’ under the Central Range when the Eurasia continental plate collides with the PHS slab. The third viewpoint is that the PHS slab is westward subducting under the Eurasia continental plate (Ai et al.,
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Fig. 1. Tectonic background in SE China.
2007). Ai et al. (2007) thought that this subduction model can be used to explain the phenomena that Taiwan Island moves to the NW direction with velocity of 8 cm/year (Yu et al., 1997). As a whole, knowing whether there is a subducted lithosphere beneath Taiwan is important for our understanding of the regional tectonic evolution. In this study, we used a large amount of teleseismic events recorded by stations in Taiwan and the Fujian provinces to determine the highresolution tomography of the crustal and uppermost mantle structures under the southeastern margin of China. Combining the data from Taiwan and Fujian provinces gives a better coverage of the deep structure because their ray-paths cross each other. Furthermore, the detailed crustal and uppermost mantle structures in the two regions can provide more robust evidence for interaction between the Eurasian plate and PHS slab. 2. Tectonic setting Southeast China is located at the southeastern margin of the Eurasian plate which is strongly interacting with the PHS slab near Taiwan. Most of Taiwan is under a northwest–southeast compression with a convergence rate of about 8 cm/year (Yu et al., 1997). Taiwan Island is formed
over the past 4 Ma with a high rate of crustal deformation and a strong seismic activity (Suppe, 1984). There have been different tectonic views on the formation of Taiwan. For instance, the model of arc–continent collision (Angelier et al., 1986; Chai, 1972; Ho, 1986; Lallemand et al., 2001; Lee et al., 2006; Lin, 2002; Suppe, 1981; Teng, 1990) supports the presence of a slab, whereas that of arc–arc collision (Hsu and Sibuet, 1995) favors its absence. Whatever models, all authors agree that the uplift of Taiwan results from the collision of the Luzon arc with the Eurasian plate. The oblique collision between the Luzon Arc and the Eurasian plate is still propagating southwest, which causes the migration of the deformation front and foreland basin westward and southward, resulting in a mature overfilled basin in the north and an immature one in the south (Suppe, 1984; Zhou et al., 2003). To the east of Taiwan, the PHS slab subducts northwestward beneath the Eurasian lithosphere from the Ryukyu Trench and is overriding the South China Sea floor of the Eurasian plate beneath South Taiwan (Wang et al., 2009). 3. Data and method In this study, we utilized seismic waveforms recorded by portable broadband seismic stations in Fujian Province (FJB) and three local
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networks, Fujian Seismic Network (FJN), Broadband Array in Taiwan for Seismology (BATS), and New China Digital Seismology Network (NCDSN) (Fig. 2). FJB was deployed in 2008 by the Institute of Geology, Chinese Academy of Geological Sciences. It consists of 20 portable broadband seismic stations within the Fujian Province. The data spans the time interval from August 2008 to April 2009. FJN was deployed in 1999 by the Earthquake Administration of Fujian Province. It consists of 9 broadband and 20 short period stations within the Fujian Province. All of them have operated continuously from June 1999 to now. The data from FJN used in this study spans the time interval from January 2000 to October 2005. BATS consisted of 15 broadband stations (13 on Taiwan and 2 on Mainland China offshore). Some stations in BATS operated with three components from the mid 1990s to the present, and recorded continuously at 40 samples per second (Kao et al., 1998). The data from BATS used in this study spans the time interval from January 1995 to November 2005. The last data set was acquired from NCDSN. In southern China, only one broadband station WZH is located near the Fujian Province. The data spans the time interval from October 2000 to August 2003. Tremendous efforts were made to process very carefully the huge amount of data sets. The arrival time data for the same events but
appeared in the different data sets were merged together, and the repeated and inconsistent data were removed. The data were collected very carefully from the original seismograms of 635 teleseismic events (Fig. 3) recorded by 65 broadband stations deployed under Southeast China. These teleseismic events were in the distance range between 30° and 90° with magnitude greater than 5.0 and focal depths greater than 10 km for the tomographic inversion. All of the events were recorded by at least five stations simultaneously. As a result, we picked manually 6869 P-wave arrival times from the original seismograms (Fig. 4). The picking accuracy of the arrival times is estimated to be 0.1–0.2 s. We used the tomographic method of Zhao et al. (1994) to determine the 3-D P-wave velocity structure beneath the study area. In this study we mainly used the relative travel time residuals of teleseismic P-waves in tomographic inversion. This method allows 3-D velocity variations everywhere in the model and is adaptable to a velocity structure which contains complex-shaped velocity discontinuities. The discontinuities represent known geological boundaries, like the Moho discontinuity and/or a subducting slab boundary, etc. A 3-D grid net is set up in the modeling space. The velocity perturbations at the grid nodes from a 1-D starting velocity model are taken to be unknown parameters. The velocity perturbation at any point in the model is calculated by linearly interpolating the velocity perturbations at the eight grid
Fig. 2. The 65 seismic stations used in this study. Different symbols denote seismic stations deployed in different observation projects as shown in the lower-left corner. Black lines show major faults in SE China.
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Fig. 3. Distribution of teleseismic events used in this study. The red dots show the teleseismic events. The yellow square is the present study area. The concentric circles show the epicentral distance of 30°, 60° and 90°, respectively.
nodes surrounding that point. A 3-D ray-tracing technique (Zhao et al., 1992) is used to calculate travel times and ray paths accurately and rapidly. Station elevations are taken into account in the ray tracing. A conjugate–gradient algorithm (Paige and Saunders, 1982) with a damping and smoothing regularization is used to invert the large and sparse system of observation equations. For details of the method, see Zhao et al. (1992, 1994). Using the 1-D iasp91 Earth model (Kennett and Engdahl, 1991), we calculated theoretical P-wave travel times (Tcal), and corrected them for the Earth's ellipticity (Dziewonski and Gilbert, 1976). A travel-time residual (tij) from the j-th event to the i-th station can be expressed as: obs
t ij ¼ T
cal
−T :
ð1Þ
In order to minimize the effects of hypocenter mislocation, origin times and velocity anomalies outside of the study area, we calculated relative travel-time residuals (rij) from Eq. (1), as follows 1 nj t ij : r ij ¼ t ij − ∑i¼1 nj
ð2Þ
Here nj is the number of stations that recorded the waveform data of the j-th event.
Three-dimensional grid nodes were set up in the modeling space (Fig. 5). In this work, we performed many such tests by adopting different grid spacings and found that the optimal grid spacing for the tomographic inversion of our data set is 1° × 1° in central region and 2° in the edge portions of the study region in the horizontal direction and 25–50 km in depth. The optimum damping factor made an adequate balance between the total reduction of travel time residuals and the roughness of the model. Considering the balance between the reduction of the RMS travel-time residual and the smoothness of the velocity model obtained, we chose 20.0 as the best value of damping parameter for the present data set and model parameterization after many experiments (Fig. 6).
4. Checkerboard resolution tests To confirm the main features of the tomographic results and ascertain the adequacy of ray coverage and reliability of obtained images, we conducted detailed resolution analyses. The checkerboard resolution test (Humphreys and Clayton, 1988; Zhao et al., 1992, 1994) is a well-used synthetic test. Positive and negative velocity perturbations (3%) are assigned to the 3D grid nodes that are arranged in the modeling
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Fig. 4. Distribution of seismic ray paths in the model space.
Fig. 5. 3-D configuration of the grid nodes in the model. Crosses show the grid node location.
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Fig. 6. Trade-off curve for the root-mean-square (RMS) travel-time residuals versus RMS of P-wave velocity perturbations (in %) at the grid nodes (see text for details). Numbers beside the diamond symbols denote the damping parameters adopted for the tomographic inversions. The red dot denotes the optimal damping parameter for the final tomographic model.
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space, the image of which is straightforward and easy to remember. To make a checkerboard, we assigned positive and negative 3% velocity perturbations to the 3-D grid nodes adjacent to each other that are arranged in the model space (Fig. 5) and then calculated the travel times for this model. Note that, with the images determined by the synthetic inversion, one can easily understand whether the resolution is good or not. Fig. 7 shows the checkerboard test results at different depths. The checkerboard test results for the P-wave structures show good resolution beneath the Mainland China and the Taiwan Island from depths of 10 to 400 km where the patterns of the checkerboard test are recovered by numerous ray paths recorded by several stations. Fig. 4 shows 3-D ray-path distributions of the teleseismic data at depths of 0–400 km exhibiting good coverage of the ray paths at all depths under the Mainland China and Taiwan Island, which leads to a good resolution. The checkerboard test results also show good resolution under Taiwan Strait at depths of 60–400 km because of the utilization of the arrival times recorded by the stations in the Fujian and Taiwan provinces (Fig. 2).
Fig. 7. Results of checkerboard resolution test. The depth of the layer is shown on the top of each map. Solid and open circles denote high and low velocities, respectively. The velocity perturbation scale is shown at the bottom.
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The test results show that the checkerboard pattern is recovered well in the mantle at depths greater than 60 km under most of the study area, while at shallower depths it is only well recovered right beneath the seismic network. 5. Results and discussions Fig. 8 presents the maps at several depths for the results of Vp structures determined in this study. Tectonic lines, major fault zones, and major volcanic center are also shown in the velocity images. The
Fig. 8. P-wave velocity images at every depth slices. The layer depth is shown below each map. Red and blue colors denote low and high velocities, respectively. The velocity perturbation scale is shown at the bottom. The black lines denote the major tectonic boundaries. The red triangle denotes the major volcanos.
tomographic images show the existence of two prominent high-Vp anomalies under Taiwan and Mainland China, while low-Vp anomalies spread widely under the coastal areas of Mainland China and Taiwan Strait. In the depth range of 25 to100 km, significant high-Vp anomalies exist along the eastern coast of Mainland China (Fig. 8), which was also revealed by Sn-wave tomography (Pei et al., 2007) and P-wave tomography (Huang et al., 2010). In most recent work, Huang et al. (2010) combined BATS, FJ-NET and 26 portable stations deployed by Nanjing University in the Fujian and Jiangxi province data to constrain the velocity structure in the region similar to that of this study. Huang et al. (2010) interpreted that the high-Vp anomalies were caused by the cooled igneous rocks in the uppermost mantle. Another obvious feature is the high-Vp in eastern Taiwan in the upper mantle to the east of Taiwan (Fig. 8). The high-Vp anomalies under eastern Taiwan extend from the upper mantle down to 100 km depth. Considering the distribution of deep and intermediate-depth earthquakes (Engdahl et al., 1998; Wu et al., 2008), we interpret that the high-Vp clearly reflects the oceanic lithospheric of the PHS slab. This feature was supported by the previous tomographic studies that the high-Vp anomalies were imaged along the eastern coast of Taiwan (Chou et al., 2009; Huang et al., 2010; Kim et al., 2005; Wang et al., 2009; Wu et al., 2007, 2009a, 2009b). Under the volcanic area north of Taiwan Island, a prominent low-V zone exists from the surface down to 60 km or even deeper (Fig. 8). This result is in good agreement with the recent tomography results (Wu et al., 2007). In the depth range of 10 to 200 km, significant low-Vp anomalies exist under southeastern China (Fig. 8). Such wide extent of the low-Vp zones is consistent with the suggestions about the thinning of the lithosphere under southeastern China (e.g., Kong et al., 1991; Liao et al., 1988; Ren et al., 2002; Xu and Xie, 2005; Zhao and Windley, 1990). The previous geological and geophysical researches illustrated that the Cenozoic igneous rocks (Niutoushan and Mingxi areas, Fujian Province) with mantle xenoliths correspond to low-Vp zones (Dong et al., 1993; Lin, 1992; Zhao et al., 2004). Our tomographic images show that the depth of low-Vp zone under the Niutoushan area and Mingxi area is 60 km and 200 km, respectively (Fig. 8). It is still debated that whether or not a slab of subducted lithosphere exits beneath central Taiwan. Subduction zone seismicity does not provide conclusive answers because no earthquakes occur beyond ~ 200 km depth. Fig. 9 shows four vertical cross sections of velocity images together with the background seismicity and a volcano along profiles A–B, C–D, E–F and G–H. Break-off high-Vp anomalies at a big angle toward the east that are revealed in the upper mantle under central Taiwan extend from about 120 km down to 400 km depth (Fig. 9a), which has been proposed by the previous studies (e.g., Chemenda et al., 2001; Huang et al., 2010; Lallemand et al., 2001; Sibuet and Hsu, 2004; Teng et al., 2000; Wang et al., 2006, 2009). While continuous high-Vp anomalies under South Taiwan are mainly imaged from the Moho down to 400 km depth, and deep earthquakes occur in the slab down to 200 km depth (Fig. 9b–c), which were also revealed by a previous tomographic imaging (Huang et al., 2010; Wang et al., 2006, 2009; Wu et al., 2007). Together with the previous studies, we interpret that these high-Vp zones represent the subducted Eurasian continental lithosphere. Receiver–function analyses suggest that the Eurasian plate has subducted down to the 410 km discontinuity at least (Ai et al., 2007). Most of previous local and regional tomography results support the assumption that an eastern dipping Eurasian plate may exist beneath south and central Taiwan (Chen et al., 2004; Huang et al., 2010; Rau and Wu, 1995; Wang et al., 2006, 2009). The mechanism of the Taiwan uplift is presumed to be produced by the plate pull force due to the Eurasian continent subduction (Sibuet and Hsu, 2004). South China Sea slab, the oceanic part of the Eurasian plate, is subducting under the PHS slab from the Manila Trench at the south of
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Fig. 9. Vertical cross-sections of P-wave tomography along the four profiles shown on the inset map. Red and blue colors denote slow and fast velocities, respectively. White dots and red triangles denote the earthquakes and active volcanoes (Hualian volcano and Niutoushan igneous rocks areas) within 30-km width along each cross-section. The short-dashed lines show the inferred outline of the subducting Eurasian plate and PHS slab. The velocity perturbation scale is shown at the base of the figure.
Taiwan (e.g., Ai et al., 2007; Chemenda et al., 2001; Kao et al., 1998, 2000; Lallemand et al., 2001; Mcintosh et al., 2005; Wu et al., 1997). The subducted South China Sea slab drags the Eurasian lithosphere to subduct down to central Taiwan (Malavieille et al., 2002; Sibuet and Hsu, 2004). This is why the continental plate can subduct into the oceanic plate. Kao et al. (2000) considered that to the north of 23°N, collision was clearly the predominant process with broad deformation on Taiwan Island, after systematically studying the detailed bathymetry, seismicity, and source parameters of large-and moderate-sized earthquakes that occurred in the northern Luzon arc-Taiwan region between 1964 and 1996. Similarly, the tectonic stress regime in Taiwan is dominated by compression as the PHS slab collides with the Eurasia continental plate (Wu et al., 2010). The plate collision viewpoint consists of our east–west vertical cross-sections tomography image (Fig. 9a–c) of the developing collision between the subducted Eurasian continental lithosphere and the PHS slab beneath south and central Taiwan. In the G–H profile (Fig. 9d), a north-dipping high-velocity zone indicates the subducting oceanic lithosphere of the PHS slab, showing consistency with the previous studies (e.g., Rau and Wu, 1995; Huang et al.,
1997; Kao and Rau, 1999; Lin, 2000; Annemarie et al., 2003; Wang et al., 2004, 2009; Mcintosh et al., 2005; Wu et al., 2007, 2009a, 2009b; Chou et al., 2009). The PHS slab has subducted toward the north along the Ryukyu trench at the latitude of about 24°N and extends down to 350 km depth and even more. The subducting PHS slab becomes even better illuminated, once we plot the intermediate-depth and deep earthquakes occurring down to about 300 km depth on top of our new tomography results. The tomography image and numerical modeling results (Wu et al., 2009a, 2009b) agree with this opinion. On account of the research areas limited in the south of 25.5°N, Wang et al. (2009) imaged that the subducted depth of the PHS slab is only limited at 200 km. An upper-mantle low-Vp anomaly, ranging from 100 to 400 km in depth, is generally imaged above the subducting PHS slab (Fig. 9d), which might be caused by the fluids released from the dehydration process associated with the PHS slab subduction (Wang et al., 2009). Low-Vp anomalies behind the PHS slab and Eurasian plate (Fig. 9a–d), resulting in the thinning and fracture of the lithosphere under Mainland China and Taiwan Strait, may be caused by the upwelling of the hot asthenospheric materials associated with the plate subduction. Low-Vp
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Acknowledgments We thank Prof. Zhao Dapeng for allowing us to use his 3-D tomographic inversion software in this work and related studies. We also thank the China Earthquake Networks Center for providing the waveform data used in this study. We thank Prof. Sihua Zheng and Prof. Yinshuang Ai for helping us at the data collection stage. We are especially grateful to Prof. Laurent Jolivet, guest editor and two anonymous reviewers for their constructive comments, which improved the manuscript. This work was supported by grants from the Natural Science Foundation of China (40904026; 40830316; 41274095), Open Fund of Geo-detection Laboratory, Ministry of Education of China, China University of Geosciences (GDL0901), the Basic Outlay of Scientific Research Work from the Ministry of Science and Technology, China (J0911; J1204; J1216), and Experiment and Integration of Deep Probe Techniques in China (SinoProbe-02; No. 201011044; No. 201011042). Most of the figures in the paper were made by using the GMT software package distributed by Wessel and Smith (1998). Fig. 10. 3D perspective view of the Eurasian plate and PHS slab interaction in Southeast China based on the tomography (modified from Lallemand et al., 2001). This model accepts the northward subducted PHS slab and the eastward subducted Eurasian plate as well as the previous models, also obviously proposes that Eurasian plate lithosphere is broken off under northern and central Taiwan.
anomalies beneath the Hualian volcano originate from the depths of 400 km in northwestward profile (Fig. 9a), indicating a torn mantle window within the Eurasian continent beneath central and northern Taiwan created by the northward motion of the Philippine Sea plate which is the upwelling path of the asthenosphere. On the basis of the tomographic cross sections above and several previous tomography studies in this region, we present the following 3D perspective view of the Eurasian plate and PHS slab interaction in Southeast China in Fig. 10. Our model shows that the northward moving PHS slab resulted in the collision of PHS slab with Eurasian plate beneath Taiwan. In northern Taiwan, the PHS slab subducts toward the north along the Ryukyu trench beneath the Eurasian plate. In central Taiwan, a torn mantle window in Eurasian lithosphere is caused by subducted PHS slab. Consequently, the image shows that the eastward subducted Eurasian plate is broken off. In southern Taiwan, the Eurasian plate continuously subducted eastward beneath PHS slab. 6. Conclusions In this study, we used a large number of P-wave arrival times recorded by portable and permanent seismic stations to reveal the structure of the crust and upper mantle beneath Southeast China. Our images show that the high-Vp PHS slab subducts toward the north along the Ryukyu trench at the latitude of about 24°N and extends down to 350 km depth and even more. High-Vp anomalies are imaged in the upper mantle under central and southern Taiwan, which represent the subducted Eurasian plate. Break-off Eurasian plate at a big angle subducting eastward is revealed under central Taiwan at depths from the upper mantle to 400 km. Therefore, our model supports the first viewpoint of the summarized collision mechanism in central Taiwan mentioned in the Introduction section. While continuous Eurasian plate under South Taiwan is mainly imaged from the Moho down to 400 km depth, a torn mantle window within the Eurasian continent beneath central and northern Taiwan created by the northward motion of the Philippine Sea plate is the upwelling path of the asthenosphere. The tomographic images also show that the low-Vp anomalies spread widely under the coastal areas of Mainland China and Taiwan Strait. The structure of the crust and upper mantle suggests that the mountain building process in the central part of Taiwan is mainly attributed to the subduction– collision tectonics at the boundary between the Eurasian continental lithosphere and the subducting oceanic lithosphere of the PHS slab.
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Collisional tectonics between the Eurasian and Philippine Sea plates from tomography evidences in Southeast China Hong-Wei Zheng a, Rui Gao a,⁎, Ting-Dong Li b, c, Qiu-Sheng Li a, Ri-Zheng He a a b c
Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China Chinese Academy of Geological Sciences, Beijing 100037, China Consulting Research Center, Ministry of Land and Resources, Beijing 100035, China
a r t i c l e
i n f o
Article history: Received 3 September 2012 Received in revised form 21 February 2013 Accepted 14 March 2013 Available online 22 March 2013 Keywords: Teleseismic tomography Southeast China Subducted Eurasian plate Subducted Philippine Sea slab
a b s t r a c t The upper mantle structure of Southeast China is important for us to understand the deformation and mantle dynamics process associated with the interaction between the Eurasian plate and Philippine Sea (PHS) slab. We determined a detailed three-dimensional P-wave velocity (Vp) structure of the crust and upper mantle down to 400 km depth beneath Southeast China by applying teleseismic tomography to 6869 high-quality P-wave arrival times. The data were collected very carefully from the original seismograms of 635 teleseismic events recorded by 65 broadband stations deployed in Southeast China. Our images show that the high-Vp PHS slab subducts toward the north along the Ryukyu trench at the latitude of about 24°N and extends down to 350 km depth and even more. High-Vp anomalies are imaged in the upper mantle under central and southern Taiwan, which represent the subducted Eurasian plate. Break-off Eurasian plate at a big angle subducting eastward is revealed under central Taiwan at depths from the upper mantle to 400 km. While continuous Eurasian plate under South Taiwan is mainly imaged from the Moho down to 400 km depth, a torn mantle window within the Eurasian continent beneath central and northern Taiwan created by the northward motion of the Philippine Sea plate is the upwelling path of the asthenosphere. The tomographic images also show the low-Vp anomalies spread widely under the coastal areas of Mainland China and Taiwan Strait. The structure of the crust and upper mantle suggests that the mountain building process in the central part of Taiwan is mainly attributed to the subduction–collision tectonics at the boundary between the Eurasian continental lithosphere and the subducting oceanic lithosphere of the PHS slab. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The southeastern margin of China is located at the juncture zone of the Eurasian plate and the Philippine Sea (PHS) slab, with unusual tectonic features (Fig. 1). Particularly, it is famous for the intensive magmatic activities, recording the multiphasic assembly and breakup processes of plates (Cheng, 1994). Taiwan Island is the only zone located in the youngest trench–arc–basin system in China and it is also the most active collision orogen between continental plate and oceanic slab in the world. Therefore, it is the best field laboratory for studying the collision and subduction processes between oceanic slab and continental plate. The upper mantle structure under the southeastern margin of China not only reveals the complex tectonic evolution, but also records and displays the ongoing plentiful geological phenomena during the collision between oceanic plate and continental plate. About the collision and subduction processes of the Eurasian continental plate and the PHS slab, previous researchers have relatively unanimous viewpoint on the collision and subduction mechanisms ⁎ Corresponding author. Tel.: +86 10 68999730; fax: +86 10 68997803. E-mail address: [email protected] (R. Gao). 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.03.018
in the northern and southern areas of Taiwan Island. In the northern area of Taiwan Island, the PHS slab is subducting under the Eurasia plate along the Ryukyu trench (Ai et al., 2007; Lallemand et al., 2001; Mcintosh et al., 2005; Wu et al., 2009a, 2009b). In the southern area of Taiwan Island, the Eurasia continental plate is subducting under the PHS slab (Kim et al., 2005; Lallemand et al., 2001; Mcintosh et al., 2005; Sibuet and Hsu, 2004; Wang et al., 2006; Wu et al., 2007; Zhang et al., 2008). However, there have been three different points of view on the collision mechanism in central Taiwan. The first point is that there is an eastern-dipping Eurasian continental plate under central Taiwan (Chen et al., 2004; Huang et al., 2010; Sibuet and Hsu, 2004). The Eurasian plate crust is involved in the collision orogen, and its mantle continues to subduct. Wang et al. (2006, 2009) proposed that the Eurasian plate is subducting eastward under the PHS slab in central Taiwan, with depth up to 300 km. The second viewpoint supports the presence of thickened crust and the absence of subducting mantle (Rau and Wu, 1995; Wu et al., 1997; Mcintosh et al., 2005; Zhang et al., 2008). In central Taiwan, the continental crustal layer is thickened to form ‘root’ under the Central Range when the Eurasia continental plate collides with the PHS slab. The third viewpoint is that the PHS slab is westward subducting under the Eurasia continental plate (Ai et al.,
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Fig. 1. Tectonic background in SE China.
2007). Ai et al. (2007) thought that this subduction model can be used to explain the phenomena that Taiwan Island moves to the NW direction with velocity of 8 cm/year (Yu et al., 1997). As a whole, knowing whether there is a subducted lithosphere beneath Taiwan is important for our understanding of the regional tectonic evolution. In this study, we used a large amount of teleseismic events recorded by stations in Taiwan and the Fujian provinces to determine the highresolution tomography of the crustal and uppermost mantle structures under the southeastern margin of China. Combining the data from Taiwan and Fujian provinces gives a better coverage of the deep structure because their ray-paths cross each other. Furthermore, the detailed crustal and uppermost mantle structures in the two regions can provide more robust evidence for interaction between the Eurasian plate and PHS slab. 2. Tectonic setting Southeast China is located at the southeastern margin of the Eurasian plate which is strongly interacting with the PHS slab near Taiwan. Most of Taiwan is under a northwest–southeast compression with a convergence rate of about 8 cm/year (Yu et al., 1997). Taiwan Island is formed
over the past 4 Ma with a high rate of crustal deformation and a strong seismic activity (Suppe, 1984). There have been different tectonic views on the formation of Taiwan. For instance, the model of arc–continent collision (Angelier et al., 1986; Chai, 1972; Ho, 1986; Lallemand et al., 2001; Lee et al., 2006; Lin, 2002; Suppe, 1981; Teng, 1990) supports the presence of a slab, whereas that of arc–arc collision (Hsu and Sibuet, 1995) favors its absence. Whatever models, all authors agree that the uplift of Taiwan results from the collision of the Luzon arc with the Eurasian plate. The oblique collision between the Luzon Arc and the Eurasian plate is still propagating southwest, which causes the migration of the deformation front and foreland basin westward and southward, resulting in a mature overfilled basin in the north and an immature one in the south (Suppe, 1984; Zhou et al., 2003). To the east of Taiwan, the PHS slab subducts northwestward beneath the Eurasian lithosphere from the Ryukyu Trench and is overriding the South China Sea floor of the Eurasian plate beneath South Taiwan (Wang et al., 2009). 3. Data and method In this study, we utilized seismic waveforms recorded by portable broadband seismic stations in Fujian Province (FJB) and three local
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networks, Fujian Seismic Network (FJN), Broadband Array in Taiwan for Seismology (BATS), and New China Digital Seismology Network (NCDSN) (Fig. 2). FJB was deployed in 2008 by the Institute of Geology, Chinese Academy of Geological Sciences. It consists of 20 portable broadband seismic stations within the Fujian Province. The data spans the time interval from August 2008 to April 2009. FJN was deployed in 1999 by the Earthquake Administration of Fujian Province. It consists of 9 broadband and 20 short period stations within the Fujian Province. All of them have operated continuously from June 1999 to now. The data from FJN used in this study spans the time interval from January 2000 to October 2005. BATS consisted of 15 broadband stations (13 on Taiwan and 2 on Mainland China offshore). Some stations in BATS operated with three components from the mid 1990s to the present, and recorded continuously at 40 samples per second (Kao et al., 1998). The data from BATS used in this study spans the time interval from January 1995 to November 2005. The last data set was acquired from NCDSN. In southern China, only one broadband station WZH is located near the Fujian Province. The data spans the time interval from October 2000 to August 2003. Tremendous efforts were made to process very carefully the huge amount of data sets. The arrival time data for the same events but
appeared in the different data sets were merged together, and the repeated and inconsistent data were removed. The data were collected very carefully from the original seismograms of 635 teleseismic events (Fig. 3) recorded by 65 broadband stations deployed under Southeast China. These teleseismic events were in the distance range between 30° and 90° with magnitude greater than 5.0 and focal depths greater than 10 km for the tomographic inversion. All of the events were recorded by at least five stations simultaneously. As a result, we picked manually 6869 P-wave arrival times from the original seismograms (Fig. 4). The picking accuracy of the arrival times is estimated to be 0.1–0.2 s. We used the tomographic method of Zhao et al. (1994) to determine the 3-D P-wave velocity structure beneath the study area. In this study we mainly used the relative travel time residuals of teleseismic P-waves in tomographic inversion. This method allows 3-D velocity variations everywhere in the model and is adaptable to a velocity structure which contains complex-shaped velocity discontinuities. The discontinuities represent known geological boundaries, like the Moho discontinuity and/or a subducting slab boundary, etc. A 3-D grid net is set up in the modeling space. The velocity perturbations at the grid nodes from a 1-D starting velocity model are taken to be unknown parameters. The velocity perturbation at any point in the model is calculated by linearly interpolating the velocity perturbations at the eight grid
Fig. 2. The 65 seismic stations used in this study. Different symbols denote seismic stations deployed in different observation projects as shown in the lower-left corner. Black lines show major faults in SE China.
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Fig. 3. Distribution of teleseismic events used in this study. The red dots show the teleseismic events. The yellow square is the present study area. The concentric circles show the epicentral distance of 30°, 60° and 90°, respectively.
nodes surrounding that point. A 3-D ray-tracing technique (Zhao et al., 1992) is used to calculate travel times and ray paths accurately and rapidly. Station elevations are taken into account in the ray tracing. A conjugate–gradient algorithm (Paige and Saunders, 1982) with a damping and smoothing regularization is used to invert the large and sparse system of observation equations. For details of the method, see Zhao et al. (1992, 1994). Using the 1-D iasp91 Earth model (Kennett and Engdahl, 1991), we calculated theoretical P-wave travel times (Tcal), and corrected them for the Earth's ellipticity (Dziewonski and Gilbert, 1976). A travel-time residual (tij) from the j-th event to the i-th station can be expressed as: obs
t ij ¼ T
cal
−T :
ð1Þ
In order to minimize the effects of hypocenter mislocation, origin times and velocity anomalies outside of the study area, we calculated relative travel-time residuals (rij) from Eq. (1), as follows 1 nj t ij : r ij ¼ t ij − ∑i¼1 nj
ð2Þ
Here nj is the number of stations that recorded the waveform data of the j-th event.
Three-dimensional grid nodes were set up in the modeling space (Fig. 5). In this work, we performed many such tests by adopting different grid spacings and found that the optimal grid spacing for the tomographic inversion of our data set is 1° × 1° in central region and 2° in the edge portions of the study region in the horizontal direction and 25–50 km in depth. The optimum damping factor made an adequate balance between the total reduction of travel time residuals and the roughness of the model. Considering the balance between the reduction of the RMS travel-time residual and the smoothness of the velocity model obtained, we chose 20.0 as the best value of damping parameter for the present data set and model parameterization after many experiments (Fig. 6).
4. Checkerboard resolution tests To confirm the main features of the tomographic results and ascertain the adequacy of ray coverage and reliability of obtained images, we conducted detailed resolution analyses. The checkerboard resolution test (Humphreys and Clayton, 1988; Zhao et al., 1992, 1994) is a well-used synthetic test. Positive and negative velocity perturbations (3%) are assigned to the 3D grid nodes that are arranged in the modeling
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Fig. 4. Distribution of seismic ray paths in the model space.
Fig. 5. 3-D configuration of the grid nodes in the model. Crosses show the grid node location.
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Fig. 6. Trade-off curve for the root-mean-square (RMS) travel-time residuals versus RMS of P-wave velocity perturbations (in %) at the grid nodes (see text for details). Numbers beside the diamond symbols denote the damping parameters adopted for the tomographic inversions. The red dot denotes the optimal damping parameter for the final tomographic model.
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space, the image of which is straightforward and easy to remember. To make a checkerboard, we assigned positive and negative 3% velocity perturbations to the 3-D grid nodes adjacent to each other that are arranged in the model space (Fig. 5) and then calculated the travel times for this model. Note that, with the images determined by the synthetic inversion, one can easily understand whether the resolution is good or not. Fig. 7 shows the checkerboard test results at different depths. The checkerboard test results for the P-wave structures show good resolution beneath the Mainland China and the Taiwan Island from depths of 10 to 400 km where the patterns of the checkerboard test are recovered by numerous ray paths recorded by several stations. Fig. 4 shows 3-D ray-path distributions of the teleseismic data at depths of 0–400 km exhibiting good coverage of the ray paths at all depths under the Mainland China and Taiwan Island, which leads to a good resolution. The checkerboard test results also show good resolution under Taiwan Strait at depths of 60–400 km because of the utilization of the arrival times recorded by the stations in the Fujian and Taiwan provinces (Fig. 2).
Fig. 7. Results of checkerboard resolution test. The depth of the layer is shown on the top of each map. Solid and open circles denote high and low velocities, respectively. The velocity perturbation scale is shown at the bottom.
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The test results show that the checkerboard pattern is recovered well in the mantle at depths greater than 60 km under most of the study area, while at shallower depths it is only well recovered right beneath the seismic network. 5. Results and discussions Fig. 8 presents the maps at several depths for the results of Vp structures determined in this study. Tectonic lines, major fault zones, and major volcanic center are also shown in the velocity images. The
Fig. 8. P-wave velocity images at every depth slices. The layer depth is shown below each map. Red and blue colors denote low and high velocities, respectively. The velocity perturbation scale is shown at the bottom. The black lines denote the major tectonic boundaries. The red triangle denotes the major volcanos.
tomographic images show the existence of two prominent high-Vp anomalies under Taiwan and Mainland China, while low-Vp anomalies spread widely under the coastal areas of Mainland China and Taiwan Strait. In the depth range of 25 to100 km, significant high-Vp anomalies exist along the eastern coast of Mainland China (Fig. 8), which was also revealed by Sn-wave tomography (Pei et al., 2007) and P-wave tomography (Huang et al., 2010). In most recent work, Huang et al. (2010) combined BATS, FJ-NET and 26 portable stations deployed by Nanjing University in the Fujian and Jiangxi province data to constrain the velocity structure in the region similar to that of this study. Huang et al. (2010) interpreted that the high-Vp anomalies were caused by the cooled igneous rocks in the uppermost mantle. Another obvious feature is the high-Vp in eastern Taiwan in the upper mantle to the east of Taiwan (Fig. 8). The high-Vp anomalies under eastern Taiwan extend from the upper mantle down to 100 km depth. Considering the distribution of deep and intermediate-depth earthquakes (Engdahl et al., 1998; Wu et al., 2008), we interpret that the high-Vp clearly reflects the oceanic lithospheric of the PHS slab. This feature was supported by the previous tomographic studies that the high-Vp anomalies were imaged along the eastern coast of Taiwan (Chou et al., 2009; Huang et al., 2010; Kim et al., 2005; Wang et al., 2009; Wu et al., 2007, 2009a, 2009b). Under the volcanic area north of Taiwan Island, a prominent low-V zone exists from the surface down to 60 km or even deeper (Fig. 8). This result is in good agreement with the recent tomography results (Wu et al., 2007). In the depth range of 10 to 200 km, significant low-Vp anomalies exist under southeastern China (Fig. 8). Such wide extent of the low-Vp zones is consistent with the suggestions about the thinning of the lithosphere under southeastern China (e.g., Kong et al., 1991; Liao et al., 1988; Ren et al., 2002; Xu and Xie, 2005; Zhao and Windley, 1990). The previous geological and geophysical researches illustrated that the Cenozoic igneous rocks (Niutoushan and Mingxi areas, Fujian Province) with mantle xenoliths correspond to low-Vp zones (Dong et al., 1993; Lin, 1992; Zhao et al., 2004). Our tomographic images show that the depth of low-Vp zone under the Niutoushan area and Mingxi area is 60 km and 200 km, respectively (Fig. 8). It is still debated that whether or not a slab of subducted lithosphere exits beneath central Taiwan. Subduction zone seismicity does not provide conclusive answers because no earthquakes occur beyond ~ 200 km depth. Fig. 9 shows four vertical cross sections of velocity images together with the background seismicity and a volcano along profiles A–B, C–D, E–F and G–H. Break-off high-Vp anomalies at a big angle toward the east that are revealed in the upper mantle under central Taiwan extend from about 120 km down to 400 km depth (Fig. 9a), which has been proposed by the previous studies (e.g., Chemenda et al., 2001; Huang et al., 2010; Lallemand et al., 2001; Sibuet and Hsu, 2004; Teng et al., 2000; Wang et al., 2006, 2009). While continuous high-Vp anomalies under South Taiwan are mainly imaged from the Moho down to 400 km depth, and deep earthquakes occur in the slab down to 200 km depth (Fig. 9b–c), which were also revealed by a previous tomographic imaging (Huang et al., 2010; Wang et al., 2006, 2009; Wu et al., 2007). Together with the previous studies, we interpret that these high-Vp zones represent the subducted Eurasian continental lithosphere. Receiver–function analyses suggest that the Eurasian plate has subducted down to the 410 km discontinuity at least (Ai et al., 2007). Most of previous local and regional tomography results support the assumption that an eastern dipping Eurasian plate may exist beneath south and central Taiwan (Chen et al., 2004; Huang et al., 2010; Rau and Wu, 1995; Wang et al., 2006, 2009). The mechanism of the Taiwan uplift is presumed to be produced by the plate pull force due to the Eurasian continent subduction (Sibuet and Hsu, 2004). South China Sea slab, the oceanic part of the Eurasian plate, is subducting under the PHS slab from the Manila Trench at the south of
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Fig. 9. Vertical cross-sections of P-wave tomography along the four profiles shown on the inset map. Red and blue colors denote slow and fast velocities, respectively. White dots and red triangles denote the earthquakes and active volcanoes (Hualian volcano and Niutoushan igneous rocks areas) within 30-km width along each cross-section. The short-dashed lines show the inferred outline of the subducting Eurasian plate and PHS slab. The velocity perturbation scale is shown at the base of the figure.
Taiwan (e.g., Ai et al., 2007; Chemenda et al., 2001; Kao et al., 1998, 2000; Lallemand et al., 2001; Mcintosh et al., 2005; Wu et al., 1997). The subducted South China Sea slab drags the Eurasian lithosphere to subduct down to central Taiwan (Malavieille et al., 2002; Sibuet and Hsu, 2004). This is why the continental plate can subduct into the oceanic plate. Kao et al. (2000) considered that to the north of 23°N, collision was clearly the predominant process with broad deformation on Taiwan Island, after systematically studying the detailed bathymetry, seismicity, and source parameters of large-and moderate-sized earthquakes that occurred in the northern Luzon arc-Taiwan region between 1964 and 1996. Similarly, the tectonic stress regime in Taiwan is dominated by compression as the PHS slab collides with the Eurasia continental plate (Wu et al., 2010). The plate collision viewpoint consists of our east–west vertical cross-sections tomography image (Fig. 9a–c) of the developing collision between the subducted Eurasian continental lithosphere and the PHS slab beneath south and central Taiwan. In the G–H profile (Fig. 9d), a north-dipping high-velocity zone indicates the subducting oceanic lithosphere of the PHS slab, showing consistency with the previous studies (e.g., Rau and Wu, 1995; Huang et al.,
1997; Kao and Rau, 1999; Lin, 2000; Annemarie et al., 2003; Wang et al., 2004, 2009; Mcintosh et al., 2005; Wu et al., 2007, 2009a, 2009b; Chou et al., 2009). The PHS slab has subducted toward the north along the Ryukyu trench at the latitude of about 24°N and extends down to 350 km depth and even more. The subducting PHS slab becomes even better illuminated, once we plot the intermediate-depth and deep earthquakes occurring down to about 300 km depth on top of our new tomography results. The tomography image and numerical modeling results (Wu et al., 2009a, 2009b) agree with this opinion. On account of the research areas limited in the south of 25.5°N, Wang et al. (2009) imaged that the subducted depth of the PHS slab is only limited at 200 km. An upper-mantle low-Vp anomaly, ranging from 100 to 400 km in depth, is generally imaged above the subducting PHS slab (Fig. 9d), which might be caused by the fluids released from the dehydration process associated with the PHS slab subduction (Wang et al., 2009). Low-Vp anomalies behind the PHS slab and Eurasian plate (Fig. 9a–d), resulting in the thinning and fracture of the lithosphere under Mainland China and Taiwan Strait, may be caused by the upwelling of the hot asthenospheric materials associated with the plate subduction. Low-Vp
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Acknowledgments We thank Prof. Zhao Dapeng for allowing us to use his 3-D tomographic inversion software in this work and related studies. We also thank the China Earthquake Networks Center for providing the waveform data used in this study. We thank Prof. Sihua Zheng and Prof. Yinshuang Ai for helping us at the data collection stage. We are especially grateful to Prof. Laurent Jolivet, guest editor and two anonymous reviewers for their constructive comments, which improved the manuscript. This work was supported by grants from the Natural Science Foundation of China (40904026; 40830316; 41274095), Open Fund of Geo-detection Laboratory, Ministry of Education of China, China University of Geosciences (GDL0901), the Basic Outlay of Scientific Research Work from the Ministry of Science and Technology, China (J0911; J1204; J1216), and Experiment and Integration of Deep Probe Techniques in China (SinoProbe-02; No. 201011044; No. 201011042). Most of the figures in the paper were made by using the GMT software package distributed by Wessel and Smith (1998). Fig. 10. 3D perspective view of the Eurasian plate and PHS slab interaction in Southeast China based on the tomography (modified from Lallemand et al., 2001). This model accepts the northward subducted PHS slab and the eastward subducted Eurasian plate as well as the previous models, also obviously proposes that Eurasian plate lithosphere is broken off under northern and central Taiwan.
anomalies beneath the Hualian volcano originate from the depths of 400 km in northwestward profile (Fig. 9a), indicating a torn mantle window within the Eurasian continent beneath central and northern Taiwan created by the northward motion of the Philippine Sea plate which is the upwelling path of the asthenosphere. On the basis of the tomographic cross sections above and several previous tomography studies in this region, we present the following 3D perspective view of the Eurasian plate and PHS slab interaction in Southeast China in Fig. 10. Our model shows that the northward moving PHS slab resulted in the collision of PHS slab with Eurasian plate beneath Taiwan. In northern Taiwan, the PHS slab subducts toward the north along the Ryukyu trench beneath the Eurasian plate. In central Taiwan, a torn mantle window in Eurasian lithosphere is caused by subducted PHS slab. Consequently, the image shows that the eastward subducted Eurasian plate is broken off. In southern Taiwan, the Eurasian plate continuously subducted eastward beneath PHS slab. 6. Conclusions In this study, we used a large number of P-wave arrival times recorded by portable and permanent seismic stations to reveal the structure of the crust and upper mantle beneath Southeast China. Our images show that the high-Vp PHS slab subducts toward the north along the Ryukyu trench at the latitude of about 24°N and extends down to 350 km depth and even more. High-Vp anomalies are imaged in the upper mantle under central and southern Taiwan, which represent the subducted Eurasian plate. Break-off Eurasian plate at a big angle subducting eastward is revealed under central Taiwan at depths from the upper mantle to 400 km. Therefore, our model supports the first viewpoint of the summarized collision mechanism in central Taiwan mentioned in the Introduction section. While continuous Eurasian plate under South Taiwan is mainly imaged from the Moho down to 400 km depth, a torn mantle window within the Eurasian continent beneath central and northern Taiwan created by the northward motion of the Philippine Sea plate is the upwelling path of the asthenosphere. The tomographic images also show that the low-Vp anomalies spread widely under the coastal areas of Mainland China and Taiwan Strait. The structure of the crust and upper mantle suggests that the mountain building process in the central part of Taiwan is mainly attributed to the subduction– collision tectonics at the boundary between the Eurasian continental lithosphere and the subducting oceanic lithosphere of the PHS slab.
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