- Email: [email protected]
Upper mantle structure and dynamics beneath Southeast China
Physics of the Earth and Planetary Interiors 182 (2010) 161–169
Contents lists available at ScienceDirect
Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi
Upper mantle structure and dynamics beneath Southeast China Zhouchuan Huang a,b,∗ , Liangshu Wang a,∗∗ , Dapeng Zhao b,∗∗ , Mingjie Xu a , Ning Mi a , Dayong Yu a , Hua Li a , Cheng Li a a b
School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China Department of Geophysics, Tohoku University, Sendai 980-8578, Japan
a r t i c l e
i n f o
Article history: Received 9 October 2009 Received in revised form 6 July 2010 Accepted 26 July 2010 Keywords: Teleseismic tomography Southeast China Late Mesozoic igneous rocks Upwelling mantle flow Subducted Eurasian plate Deep earthquakes
a b s t r a c t We applied teleseismic tomography to 5671 relative travel-time residuals from 257 teleseismic events recorded by 69 seismic stations to determine the 3D P-wave velocity structure of the upper mantle under Southeast (SE) China. Our results show prominent low P-wave velocity (low-Vp) anomalies under SE China which may reflect the remnant magma chambers and channels of the Late Mesozoic igneous rocks, which may be reheated by the upwelling mantle flow from the lower mantle driven by the deep subduction in East Asia during the Cenozoic. High-Vp anomalies are revealed in the upper mantle to the east of Taiwan, which represent the subducted Eurasian plate. Our result also suggests the break-off of the subducted Eurasian plate caused by its interaction with the Philippine Sea plate under Central and North Taiwan. The slab break-off may have created a mantle window through which the asthenospheric flow arises, causing the high heat flow and rapid uplift in the Taiwan orogen. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Southeast (SE) China is located at the southeastern margin of the Eurasian plate which is strongly interacting with the Philippine Sea (PHS) plate near Taiwan. The PHS plate moves toward the northwest at a rate of ∼82 mm/year (Seno et al., 1993; Yu et al., 1997), resulting in the Taiwan orogen which is one of the youngest and most active orogens in the world (Sibuet and Hsu, 2004). The structure and tectonics around Taiwan have been of interest to scientists all over the world. Many researchers have investigated the detailed bathymetry, seismicity, and earthquake source parameters and studied the distribution of morphological features, seismogenic structures and the state of strain and stress in and around Taiwan (e.g., Kao et al., 1998, 2000; Chen et al., 2009; Wu et al., 2009). Global and local seismic tomography revealed the structural heterogeneities in the crust and upper mantle and provided clear evidence for the subduction of the Eurasian and PHS plates (e.g., Bijwaard et al., 1998; Zhao, 2004; Wang et al., 2006; Wu et al., 2007; Cheng, 2009; Li et al., 2009). Several tectonic models have been proposed to illustrate the plate interactions around Taiwan based on seismicity, submarine observations, tomographic images
∗ Corresponding author at: School of Earth Sciences and Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, China. Tel.: +86 25 83593561; fax: +86 25 83686016. ∗∗ Corresponding authors. E-mail addresses: [email protected] (Z. Huang), [email protected] (L. Wang), [email protected] (D. Zhao).
and GPS measurements (e.g., Teng et al., 2000; Lallemand et al., 2001; Malavieille et al., 2002; Sibuet and Hsu, 2004). To the northwest of Taiwan, the Late Mesozoic igneous rocks exist widely in the Southeast of Mainland China, covering ∼39% of the entire area (Zhou et al., 2006). The nature and origin of the igneous rocks have attracted much attention (e.g., Jahn et al., 1990; Lapierre et al., 1997; Chen and Jahn, 1998). Zhou and Li (2000) and Zhou et al. (2006) summarized the distribution of the igneous rocks in SE China based on the geological, geochemical and isotopic studies. They suggested that the igneous rocks are closely related to the subduction of the paleo-Pacific plate in the Late Mesozoic, and the driving forces are the extension-induced deep crustal melting and the underplating of basaltic magmas. Their results were obtained from the analysis of rock samples from the surface and some xenoliths from the lower crust and uppermost mantle. So far there have been few studies on the structure and dynamics of the crust and mantle under the study region, which hampers our understanding of the origin of the Late Mesozoic igneous rocks in SE China. The upper mantle structure under SE China is important for us to understand the deformation within the Eurasian plate and mantle dynamics associated with the interaction between the Eurasian and PHS plates. Furthermore, the India-Asia collision and the subduction of the Pacific plate controlled the tectonic activities of East Asia in the Cenozoic. It is very interesting and important to clarify how SE China has responded to the tectonic processes thousands of kilometers away from the plate boundaries. In this study we use teleseismic tomography to determine the first high-resolution local tomography of the upper mantle under SE China. Our results provide clear evidence for the deep origin of the
0031-9201/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2010.07.010
162
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
Fig. 1. (a) Tectonic background of the study area (dashed square). The bold arrow shows the motion direction of the Philippines Sea plate relative to the Eurasian plate (Yu et al., 1997). (b) The 69 seismic stations used in this study are shown in different symbols, and the observation periods are shown at the upper-right corner. White lines show major faults in SE China, while the dashed lines denote the province boundaries. F1: the Shaoxing-Jiangshan Fault which is the boundary between the Yangtze and SE China blocks; F2: the Zhenghe-Dapu Fault; F3: the Changle-Nan’ao Fault. FJ: Fujian province; JX: Jiangxi province.
Fig. 2. (a) Epicentral locations of the 257 teleseismic events (solid circles) used in this study. The square denotes the present study area. The three circles show the epicentral distances of 30◦ , 60◦ and 90◦ , respectively. (b) Seismograms of a typical teleseismic event recorded by some seismic stations. The station names and the corresponding epicentral distances are shown on the left side of the seismograms. The vertical lines show the P-wave arrivals we picked manually. The event location (star) and the corresponding stations (triangles) and ray paths are shown on the inset map.
Late Mesozoic igneous rocks in SE China and the subducted Eurasian plate under Taiwan. The present study also provides important information for understanding the mantle dynamics of SE China in response to the Cenozoic collision and subduction in East Asia.
collected 5671 P-wave arrival times generated by 257 teleseismic events recorded by the 69 stations. The seismic rays of the collected data crisscross well in both the horizontal and vertical directions down to 700 km depth (Fig. 3). We used the tomographic method of Zhao et al. (1994, 2006) to invert the relative travel-time residuals for the 3D velocity structure under the study region. Theoretical travel times were calculated by using the iasp91 Earth model (Kennett and Engdahl, 1991). Travel-time residuals were obtained by subtracting the theoretical travel times and origin time from the observed arrival times, and relative residuals were calculated for each event by subtracting its corresponding mean residual from the raw residuals (Zhao et al., 1994). Distribution of average relative residuals at each of the stations is shown in Fig. 4, which reflects the lateral heterogeneity under the study area. The residuals vary significantly with the azimuth, suggesting the existence of significant high P-wave velocity (high-Vp) materials under both Taiwan and Mainland China, and low-Vp materials under the Chinese coastal areas and Taiwan Strait. A 3D grid was set up in the study area. Velocity perturbations from the 1D iasp91 Earth model at the grid nodes were taken as unknown parameters. The velocity perturbation at any point in the model was computed by interpolating the velocity perturbations at the eight grid nodes surrounding that point. A 3D ray tracing
2. Data and methods We used arrival-time data from teleseismic events recorded by 69 seismic stations of three arrays deployed in SE China and Taiwan (Fig. 1). The first array consists of 26 portable stations deployed by Nanjing University in Fujian and Jiangxi provinces (NJU-FJ and NJUJX); the second array consists of 36 permanent stations of the Fujian Seismic Network (FJ-NET); and the third array includes 7 stations of Broadband Array in Taiwan for Seismology (BATS). These stations were generally equipped with broad-band three-component seismographs with the recording times of 8–16 months (Fig. 1), while some stations of the FJ-NET are equipped with short-period seismometers. We selected earthquakes with epicentral distances between 26◦ and 90◦ (Fig. 2a), which have magnitudes ≥5.5 and were recorded by more than 5 stations simultaneously. We picked manually the P-wave arrival times from the original seismograms (Fig. 2b), and the picking accuracy is estimated to be 0.1–0.2 s. As a result, we
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
163
Fig. 3. Distribution of the 5671 ray paths from the 257 teleseismic events used in this study in the plan view (a) and in the (b) north-south and (c) east-west vertical cross-sections. The squares in (a) denote the seismic stations used.
technique was used to compute travel times and ray paths (Zhao et al., 1992). The large and sparse system of observation equations that relate the observed relative residuals to the unknown velocity parameters was resolved by using a conjugate-gradient algorithm LSQR (Paige and Saunder, 1982) with damping and smoothing regularizations (Zhao et al., 2006). The station elevations were taken into account in the 3D ray tracing and inversion. 3. Analysis and results Teleseismic tomography cannot determine the 3D crustal structure well because the teleseismic rays arrive at stations nearly vertically and so they do not crisscross near the surface. Hence it is necessary to correct the teleseismic relative residuals for the heterogeneous crustal structure. In this work we made the crustal correction using the model CRUST2.0 (Bassin et al., 2000) which is specified on a 2◦ × 2◦ grid for the lateral velocity variations of the crust and Moho topography. We also made detailed analyses of the trade-off between the data variance reduction and the model norm and selected the final 3D velocity model based on the result of the trade-off analysis (Fig. 5a). Figs. 6 and 7 show our final tomographic model. The root-meansquare (RMS) value of the travel-time residuals is significantly reduced after the inversion (Fig. 5b). The 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 and Taiwan Strait, being consistent with the distribution of the relative residuals (Fig. 4). To confirm the main features of the tomographic results, we conducted detailed resolution analyses. The checkerboard resolu-
tion test (Zhao et al., 1992) 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 space, the image of which is straightforward and easy to remember. Therefore, by just examining the inverted images of the checkerboard, one can easily understand where the resolution is good and where it is poor. 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 ∼100 km in the horizontal direction and 50–100 km in depth (Fig. 8). The test results show that the checkerboard pattern is recovered well at depths greater than 200 km under most of the study area, while at shallower depths it is only well recovered right beneath the seismic network. The restoring resolution test (Zhao et al., 1992) is another way of synthetic test. In this test the obtained tomographic image is used to construct the input model (Fig. 9a–e). The velocity perturbations at the grid nodes with those ≥+1.5% and ≤−1.5% are changed to +3% and −3%, respectively, while those between −1.5% and +1.5% are changed to 0%. The inverted results (Fig. 9a –e ) show that the velocity anomalies in our images are well recovered. Through these synthetic tests, we believe that the main features of the tomographic results (Figs. 6 and 7) are reliably resolved by our data set and inversion. 4. Discussion The most prominent feature in the tomographic images is the extensive low-Vp anomalies under SE China (Figs. 6 and 7). This feature is consistent with the thinner lithosphere (An and Shi, 2006) and higher heat flow (Hu and Wang, 2000; Hu et al., 2000) in this
164
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
Fig. 4. Distribution of the average relative travel-time residuals at each of the seismic stations for the teleseismic events in each quadrant (a–d) and for all the events (e). Crosses and dots denote the early and delayed arrivals, respectively. The scale for the residuals is shown beside (e).
region. Two low-Vp anomalies are imaged in the upper mantle: one is located under the inland area (Fig. 7c), the other more prominent one is located under the coastal area (Figs. 6 and 7). A low-Vp anomaly is visible in the mantle transition zone and it is connected with the low-Vp anomalies in the upper mantle. Fig. 10 shows an east-west vertical cross-section of P-wave tomography along a profile passing through SE China determined by Huang and Zhao (2006), which shows that low-Vp anomalies extend widely under SE China from the surface down to the lower mantle, similar to that under the Philippine Sea. Around the low-Vp anomalies are high-Vp zones which represent the subducting Pacific plate under Mariana, the Burma plate under Tibet and Tengchong, and the PHS plate under Taiwan. These high-Vp anomalies in the lower mantle
suggest that the subducted slabs have penetrated the mantle transition zone and entered the lower mantle (Zhao, 2004). The low-Vp anomalies under SE China and the Philippine Sea may represent the mantle upwelling flow driven by the deep slab subduction. Thus the question: why does the mantle flow arise under SE China but is it bounded by the Yangtze Block to the northwest (Fig. 7)? Our tomographic results reveal prominent low-Vp anomalies in the upper mantle under the Yangtze Block which may represent the root of the stable block with widespread Archean basement (Figs. 6 and 7) (Yang et al., 1986; Chen and Jahn, 1998; Zheng et al., 2006). In contrast, the Late Mesozoic igneous rocks widely spread in SE China (Zhou and Li, 2000; Zhou et al., 2006). Magma chambers and channels are thus required in the upper man-
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
165
Fig. 5. (a) Trade-off curve for the variance of velocity perturbations and root-mean-square (RMS) travel-time residual for the tomographic inversions with different values of the damping parameter. The numbers beside the dots denote different damping values from 1.0 to 100.0. The thick arrow represents the optimal damping value used in this study. (b) Distribution of the relative travel-time residuals of our teleseismic data before (gray columns) and after (thick lines) the tomographic inversion.
Fig. 6. Plan views of P-wave tomography obtained in this study. The layer depth is shown below each map. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown at the bottom. The gray bold lines and the black thin lines denote major faults and province boundaries in Mainland China, while the red triangles show the active volcanoes.
166
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
Fig. 7. Vertical cross-sections of P-wave tomography along the five profiles shown on the inset map. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown below (e). The three long-dashed lines in each panel denote the Moho, 410- and 660-km discontinuities. The bold bars at the top of each cross-section denote the land areas, while the reverse triangles indicate the trench locations (Bird, 2003). White dots and red triangles denote the earthquakes (Engdahl et al., 1998) and active volcanoes within 30-km width along each cross-section. The short-dashed lines show the inferred outline of the subducted slab. EUR: Eurasian plate; PHS: Philippine Sea plate.
Fig. 8. Results of a checkerboard resolution test. Open and solid circles denote fast and slow velocities, respectively. The velocity perturbation scale is shown at the bottom. The layer depth is shown below each map.
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
167
Fig. 9. Results of a restoring resolution test (see text for details). Locations of the profiles are shown in Fig. 7. White and black colors denote low and high velocities, respectively. The velocity perturbation scale is shown at the bottom. Other labels are the same as those in Fig. 7.
tle to cause the related magmatism. The chambers and channels, even after 100 Ma, are good passages for the rising mantle flow that is driven by the subduction in the Cenozoic. Thus the mantle flow can reheat the chambers and channels and reduce the seismic velocity there. From this viewpoint, the low-Vp anomalies under SE China may also represent the deep origin of the Late Mesozoic igneous rocks. The low-Vp anomalies under the coastal area are more prominent than that under the inland area. The contrast may result from the much stronger (more volcanic rocks) and younger (142–67 Ma compared with 180–142 Ma) magmatism along the coastal area in the Late Yanshanian than that in the inland area in the Early Yanshanian (Zhou and Li, 2000; Zhou et al., 2006). A significant high-Vp zone is imaged in the uppermost mantle along the coastal area (Fig. 6), which was also revealed by Sn-wave tomography (Pei et al., 2007). Although extensive rising mantle
flow occurred under SE China in the Cenozoic, the Cenozoic volcanism with extreme high heat flow took place in only a few limited sites along the active faults (Liu et al., 1995; Hu et al., 2000; Fedorov and Koloskov, 2005). At the same time, low heat flow and low temperature around the Moho are also found in some sites in the coastal area (Hu and Wang, 2000; Hu et al., 2000). These results suggest that the crust and uppermost mantle under SE China are not significantly affected by the Cenozoic rising mantle flow as mentioned above. Since the Late Mesozoic igneous rocks exist widely along the coastal area with extensive volcanism, we interpret that the highVp anomalies are caused by the cooled igneous rocks in the uppermost mantle. Nishimoto et al. (2008) made a similar interpretation for the high-Vp anomalies in the lower crust under Northeast Japan. High-Vp anomalies are revealed in the upper mantle to the east of Taiwan, which vary significantly from north to south (Fig. 7).
168
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
Fig. 10. An east-west vertical cross-section of P-wave tomography along a profile passing through Southeast China (Huang and Zhao, 2006). Location of the profile is shown on the inset map. The surface and seafloor topography along the profile is shown on the top. White dots show the earthquakes that occurred within a 50-km width from the profile. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown at the bottom. The dashed lines denote the 410- and 660-km discontinuities.
The high-Vp anomalies under North and Central Taiwan extend from the upper mantle down to the mantle transition zone (AA , BB , CC ), while those under South Taiwan are mainly located in the upper mantle (DD , EE ). Considering the distribution of deep and intermediate-depth earthquakes (Engdahl et al., 1998; Wu et al., 2008), we interpret the high-Vp bodies under North and South Taiwan as the subducted PHS plate and the Eurasian plate, respectively. Under South Taiwan, the subducted Eurasian plate looks continuous from the Moho down to 400 km depth, and deep earthquakes occur in the slab down to 200 km depth (Fig. 7d and e). The subducted Eurasian slab has passed through the 410-km discontinuity under Central Taiwan (Fig. 7b and c) and may have reached to the mantle transition zone under the Ryukyu Trench beneath Northeast Taiwan (Fig. 7a) (Teng et al., 2000). Receiver-function analyses show that the mantle transition zone is thicker with a shallower 410-km discontinuity to the east of Taiwan (Ai et al., 2007). The result suggests that the Eurasian plate has subducted into the mantle transition zone, or at least down to the 410-km discontinuity, being consistent with our present results. Our tomographic images also suggest the break-off of the subducted Eurasian plate under Central and North Taiwan (Fig. 7b), which has been proposed by the previous studies (e.g., Chai, 1972; Davies and von Blankenburg, 1995; Teng et al., 2000; Chemenda et al., 2001; Lallemand et al., 2001; Malavieille et al., 2002; Sibuet and Hsu, 2004). The slab break-off plays an important role in the flipping of the subduction polarity (Teng et al., 2000). To the south of Taiwan, the oceanic part of the Eurasian plate (South China Sea (SCS) slab) is subducting under the PHS plate from the Manila Trench (Kao et al., 2000). The subducted SCS slab dragged the adjacent continental
lithosphere to subduct under Central and North Taiwan (Malavieille et al., 2002; Sibuet and Hsu, 2004). As the PHS plate moved northwestward, strong interactions occurred between the PHS plate and the subducted continental slab and caused the detachment of the slab (Lallemand et al., 2001; Sibuet and Hsu, 2004). As mentioned above, the low-Vp anomalies in the upper mantle may have existed since the Late Mesozoic, which must have been heating the above Eurasian plate all the time and so weakened the plate, which made the slab easier to break-off. The slab break-off may have created a mantle window through which the hot mantle material or asthenospheric flow arose into the crust, causing high heat flow and rapid uplift in the Taiwan orogen (Lee and Cheng, 1986; Teng et al., 2000). 5. Conclusions We determined a detailed 3D P-wave velocity structure of the upper mantle under SE China by applying teleseismic tomography to a large number of high-quality data recorded by many portable and permanent seismic stations. Our tomographic images show strong structural heterogeneities under the study region. Prominent low-Vp anomalies are revealed in the upper mantle under SE China, which may be closely related to the widespread Late Mesozoic magmatism in the region. In the Cenozoic, active subductions of the Pacific, Burma and PHS plates took place in East Asia. The subducted plates have reached to the lower mantle, and subsequently drove the lower mantle material to arise into the upper mantle through the Mesozoic magma chambers and channels. The subducted Eurasian plate under Taiwan is clearly imaged as highVp anomalies with significant lateral variations from south to north
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
in the upper mantle. The Eurasian slab is continuous under South Taiwan, but it is broken-off under Central and North Taiwan. The break-off of the subducted slab created a mantle window through which the asthenospheric flow arises, causing the high heat flow and rapid uplift in the Taiwan orogen. Acknowledgements We thank the Seismological Bureau of Fujian province, the Institute of Earth Science of Academic Sinica, Taiwan, and the IRIS Data Management Center for providing the waveform data used in this study. Dr. G. Jiang kindly helped us at the data processing stage. Profs. G. Helffrich, C. Chiarabba and an anonymous referee provided constructive review comments, which improved the manuscript. Discussions with Prof. Q. Wang, X. Zhou, J. Qiu and Dr. Z. He were very helpful. We thank H. Zhu, Z. Li and others for their help during the field seismic experiments in Fujian and Jiangxi provinces. This work was supported by grants (40634021, DD0963 and YPH08043) from the National Natural Science Foundation of China, the Scientific Research Foundation of Graduate School of Nanjing University, and a grant (Kiban-A 17204037) to D. Zhao from Japan Society for the Promotion of Science (JSPS). Most of the figures were made by using Generic Mapping Tools (Wessel and Smith, 1998). References Ai, Y., Chen, Y., Zeng, F., Hong, X., Ye, W., 2007. The crust and upper mantle structure beneath southeastern China. Earth Planet. Sci. Lett. 260, 549–563. An, M., Shi, Y., 2006. Lithospheric thickness of the Chinese continent. Phys. Earth Planet. Inter. 159, 257–266. Bassin, C., Laske, G., Masters, G., 2000. The current limits of resolution for surface wave tomography in North America. EOS Trans. AGU 81, F897. Bijwaard, H., Spakman, W., Engdahl, R., 1998. Closing the gap between regional and global travel time tomography. J. Geophys. Res. 103, 30055–30078. Bird, P., 2003. An updated digital model of plate boundaries. Geochem. Geophys. Geosyst. 4, 1027. Chai, B., 1972. Structure and tectonic evolution of Taiwan. Am. J. Sci. 272, 389–422. Chemenda, A., Yang, R., Stephan, J., Konstantinovskaya, E., Ivanov, G., 2001. New results from physical modeling of arc-continent collision in Taiwan: evolutionary model. Tectonophysics 333, 159–178. Chen, J., Jahn, B., 1998. Crustal evolution of southeastern China, Nd and Sr isotopic evidence. Tectonophysics 284, 101–133. Chen, R., Kao, H., Liang, W., Shin, T., Tsai, Y., Huang, B., 2009. Three-dimensional patterns of seismic deformation in the Taiwan region with special implication from the 1999 Chi-Chi earthquake sequence. Tectonophysics 466, 140–151. Cheng, W., 2009. Tomographic imaging of the convergent zone in Eastern Taiwan—a subducting forearc sliver revealed? Tectonophysics 466, 170–183. Davies, J., von Blankenburg, F., 1995. Slab break-off: a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens. Earth Planet. Sci. Lett. 129, 85–102. Engdahl, E., van der Hilst, R., Buland, R., 1998. Global teleseismic earthquake location with improved travel times and procedure for depth determination. Bull. Seismol. Soc. Am. 88, 722–743. Fedorov, P., Koloskov, A., 2005. Cenozoic volcanism of Southeast Asia. Petrology 13, 352–380. Hu, S., Wang, J., 2000. Heat flow, deep temperature and thermal structure across the orogenic belts in Southeast China. J. Geodyn. 30, 461–473. Hu, S., He, L., Wang, J., 2000. Heat flow in the continental area of China: a new data set. Earth Planet. Sci. Lett. 179, 407–419. Huang, J., Zhao, D., 2006. High-resolution mantle tomography of China and surrounding regions. J. Geophys. Res. 111, B09305. Jahn, B., Zhou, X., Li, J., 1990. Formation and tectonic evolution of the SEChina and Taiwan: isotopic and geological constraints. Tectonophysics 183, 177–189.
169
Kao, H., Shen, S., Ma, K., 1998. Transition from oblique subduction to collision: earthquakes in the southernmost Ryukyu arc-Taiwan region. J. Geophys. Res. 103, 7211–7229. Kao, H., Huang, G., Liu, C., 2000. Transition from oblique subduction to collision in the northern Luzon arc-Taiwan region: constraints from bathymetry and seismic observations. J. Geophys. Res. 105, 3059–3080. Kennett, B., Engdahl, E., 1991. Travel times for global earthquake location and phase identification. Geophys. J. Int. 105, 429–465. Lallemand, S., Font, Y., Bijwaard, H., Kao, H., 2001. New insights on 3-D plates interaction near Taiwan from tomography and tectonic implications. Tectonophysics 335, 229–253. Lapierre, H., Jahn, B., Charvet, J., Yu, Y., 1997. Mesozoic felsic arc magmatism and continental olivine tholeiites in Zhejiang Province and relationship with the tectonic activity in southeastern China. Tectonophysics 274, 321–338. Lee, C., Cheng, W., 1986. Preliminary heat-flow measurements in Taiwan. In: Horn, M.K. (Ed.), Transactions of Forth Circum-Pacific Energy and Mineral Resources Conference. Circum-Pacific Energy and Mineral Resources, Tulsa. Li, Z., Xu, Y., Hao, T., Xu, Y., 2009. Vp and Vp/Vs structures in the crust and upper mantle of the Taiwan region, China. Sci. China Ser. D 52, 975–983. Liu, R., Xie, G., Zhou, X., Chen, W., Fan, Q., 1995. Tectonic environments of Cenozoic volcanic rocks in China and characteristics of the source regions in the mantle. Chinese J. Geochem. 14, 289–302. Malavieille, J., Lallemand, S., Dominguez, S., Dechamps, A., Lu, C., Liu, C., Schnurle, P., 2002. Arc-Continent collision in Taiwan: new marine observations and tectonic evolution. Geol. Soc. Am. Special Paper 358, 189–213. Nishimoto, S., Ishikawa, M., Arima, M., Yoshida, T., Nakajima, J., 2008. Simultaneous high P–T measurements of ultrasonic compressional and shear wave velocities in Ichino-megata mafic xenoliths: their bearing on seismic velocity perturbation in lower crust of northeast Japan arc. J. Geophys. Res. 113, B12212. Paige, C., Saunder, M., 1982. LSQR: an algorithm for sparse linear equations and sparse least squares. ACM Trans. Math. Softw. 8, 43–71. Pei, S., Zhao, J., Sun, Y., Xu, Z., Wang, S., 2007. Upper mantle seismic velocities and anisotropy in China determined through Pn and Sn tomography. J. Geophys. Res. 112, B05312. Seno, T., Stein, S., Gripp, A., 1993. A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geological data. J. Geophys. Res. 89, 17941– 17948. Sibuet, J., Hsu, S., 2004. How was Taiwan created? Tectonophysics 379, 159–181. Teng, L., Lee, C., Tsai, Y., Hsiao, L., 2000. Slab break-off as a mechanism for flipping of subduction polarity in Taiwan. Geology 28, 155–158. Wang, Z., Zhao, D., Wang, J., Kao, H., 2006. Tomographic evidence for the Eurasian lithosphere subducting beneath south Taiwan. Geophys. Res. Lett. 33, L18306. Wessel, P., Smith, W., 1998. New improved version of the Generic Mapping Tools released. EOS Trans. AGU 79, 579. Wu, W., Hsu, S., Lo, C., Chen, H., Ma, K., 2009. Plate convergence at the westernmost Philippine Sea Plate. Tectonophysics 466, 162–169. Wu, Y., Chang, C., Zhao, L., Shyu, B., Chen, Y., Sieh, K., Avouac, J., 2007. Seismic tomography of Taiwan: improved constraints from a dense network of strong motion stations. J. Geophys. Res. 112, B08312. Wu, Y., Chang, C., Zhao, L., Teng, T., Nakamura, M., 2008. A comprehensive relocation of earthquakes in Taiwan from 1991 to 2005. Bull. Seismol. Soc. Am. 98, 1471–1481. Yang, Z., Cheng, Y., Wang, H., 1986. The Geology of China. Oxford Monograph Geology Geophysics. Clarendon Press, Oxford, 303 pp. Yu, S., Chen, H., Kuo, L., 1997. Velocity field of GPS stations in the Taiwan area. Tectonophysics 274, 41–59. Zhao, D., 2004. Global tomographic images of mantle plumes and subducting slabs: insight into deep Earth dynamics. Phys. Earth Planet. Inter. 146, 3–34. Zhao, D., Hasegawa, A., Horiuchi, S., 1992. Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res. 97, 19909–19928. Zhao, D., Hasegawa, A., Kanamori, H., 1994. Deep structure of Japan subduction zone as derived from local, regional, and teleseismic events. J. Geophys. Res. 99, 22313–22329. Zhao, D., Lei, J., Inoue, T., Yamada, A., Gao, S., 2006. Deep structure and origin of the Baikal rift zone. Earth Planet. Sci. Lett. 243, 681–691. Zheng, J., Griffin, W., O’Reilly, S., Zhang, M., Pearson, N., Pan, Y., 2006. Widespread Archean basement beneath the Yangtze craton. Geology 34, 417–420. Zhou, X., Li, W., 2000. Origin of Late Mesozoic igneous rocks in Southeastern China: implications for lithosphere subduction and underplating of mafic magmas. Tectonophysics 326, 269–287. Zhou, X., Sun, T., Shen, W., Shu, L., Niu, Y., 2006. Petrogenesis of Mesozoic granitoids and volcanic rocks in South China: a response to tectonic evolution. Episodes 29, 26–33.
Contents lists available at ScienceDirect
Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi
Upper mantle structure and dynamics beneath Southeast China Zhouchuan Huang a,b,∗ , Liangshu Wang a,∗∗ , Dapeng Zhao b,∗∗ , Mingjie Xu a , Ning Mi a , Dayong Yu a , Hua Li a , Cheng Li a a b
School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China Department of Geophysics, Tohoku University, Sendai 980-8578, Japan
a r t i c l e
i n f o
Article history: Received 9 October 2009 Received in revised form 6 July 2010 Accepted 26 July 2010 Keywords: Teleseismic tomography Southeast China Late Mesozoic igneous rocks Upwelling mantle flow Subducted Eurasian plate Deep earthquakes
a b s t r a c t We applied teleseismic tomography to 5671 relative travel-time residuals from 257 teleseismic events recorded by 69 seismic stations to determine the 3D P-wave velocity structure of the upper mantle under Southeast (SE) China. Our results show prominent low P-wave velocity (low-Vp) anomalies under SE China which may reflect the remnant magma chambers and channels of the Late Mesozoic igneous rocks, which may be reheated by the upwelling mantle flow from the lower mantle driven by the deep subduction in East Asia during the Cenozoic. High-Vp anomalies are revealed in the upper mantle to the east of Taiwan, which represent the subducted Eurasian plate. Our result also suggests the break-off of the subducted Eurasian plate caused by its interaction with the Philippine Sea plate under Central and North Taiwan. The slab break-off may have created a mantle window through which the asthenospheric flow arises, causing the high heat flow and rapid uplift in the Taiwan orogen. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Southeast (SE) China is located at the southeastern margin of the Eurasian plate which is strongly interacting with the Philippine Sea (PHS) plate near Taiwan. The PHS plate moves toward the northwest at a rate of ∼82 mm/year (Seno et al., 1993; Yu et al., 1997), resulting in the Taiwan orogen which is one of the youngest and most active orogens in the world (Sibuet and Hsu, 2004). The structure and tectonics around Taiwan have been of interest to scientists all over the world. Many researchers have investigated the detailed bathymetry, seismicity, and earthquake source parameters and studied the distribution of morphological features, seismogenic structures and the state of strain and stress in and around Taiwan (e.g., Kao et al., 1998, 2000; Chen et al., 2009; Wu et al., 2009). Global and local seismic tomography revealed the structural heterogeneities in the crust and upper mantle and provided clear evidence for the subduction of the Eurasian and PHS plates (e.g., Bijwaard et al., 1998; Zhao, 2004; Wang et al., 2006; Wu et al., 2007; Cheng, 2009; Li et al., 2009). Several tectonic models have been proposed to illustrate the plate interactions around Taiwan based on seismicity, submarine observations, tomographic images
∗ Corresponding author at: School of Earth Sciences and Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, China. Tel.: +86 25 83593561; fax: +86 25 83686016. ∗∗ Corresponding authors. E-mail addresses: [email protected] (Z. Huang), [email protected] (L. Wang), [email protected] (D. Zhao).
and GPS measurements (e.g., Teng et al., 2000; Lallemand et al., 2001; Malavieille et al., 2002; Sibuet and Hsu, 2004). To the northwest of Taiwan, the Late Mesozoic igneous rocks exist widely in the Southeast of Mainland China, covering ∼39% of the entire area (Zhou et al., 2006). The nature and origin of the igneous rocks have attracted much attention (e.g., Jahn et al., 1990; Lapierre et al., 1997; Chen and Jahn, 1998). Zhou and Li (2000) and Zhou et al. (2006) summarized the distribution of the igneous rocks in SE China based on the geological, geochemical and isotopic studies. They suggested that the igneous rocks are closely related to the subduction of the paleo-Pacific plate in the Late Mesozoic, and the driving forces are the extension-induced deep crustal melting and the underplating of basaltic magmas. Their results were obtained from the analysis of rock samples from the surface and some xenoliths from the lower crust and uppermost mantle. So far there have been few studies on the structure and dynamics of the crust and mantle under the study region, which hampers our understanding of the origin of the Late Mesozoic igneous rocks in SE China. The upper mantle structure under SE China is important for us to understand the deformation within the Eurasian plate and mantle dynamics associated with the interaction between the Eurasian and PHS plates. Furthermore, the India-Asia collision and the subduction of the Pacific plate controlled the tectonic activities of East Asia in the Cenozoic. It is very interesting and important to clarify how SE China has responded to the tectonic processes thousands of kilometers away from the plate boundaries. In this study we use teleseismic tomography to determine the first high-resolution local tomography of the upper mantle under SE China. Our results provide clear evidence for the deep origin of the
0031-9201/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2010.07.010
162
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
Fig. 1. (a) Tectonic background of the study area (dashed square). The bold arrow shows the motion direction of the Philippines Sea plate relative to the Eurasian plate (Yu et al., 1997). (b) The 69 seismic stations used in this study are shown in different symbols, and the observation periods are shown at the upper-right corner. White lines show major faults in SE China, while the dashed lines denote the province boundaries. F1: the Shaoxing-Jiangshan Fault which is the boundary between the Yangtze and SE China blocks; F2: the Zhenghe-Dapu Fault; F3: the Changle-Nan’ao Fault. FJ: Fujian province; JX: Jiangxi province.
Fig. 2. (a) Epicentral locations of the 257 teleseismic events (solid circles) used in this study. The square denotes the present study area. The three circles show the epicentral distances of 30◦ , 60◦ and 90◦ , respectively. (b) Seismograms of a typical teleseismic event recorded by some seismic stations. The station names and the corresponding epicentral distances are shown on the left side of the seismograms. The vertical lines show the P-wave arrivals we picked manually. The event location (star) and the corresponding stations (triangles) and ray paths are shown on the inset map.
Late Mesozoic igneous rocks in SE China and the subducted Eurasian plate under Taiwan. The present study also provides important information for understanding the mantle dynamics of SE China in response to the Cenozoic collision and subduction in East Asia.
collected 5671 P-wave arrival times generated by 257 teleseismic events recorded by the 69 stations. The seismic rays of the collected data crisscross well in both the horizontal and vertical directions down to 700 km depth (Fig. 3). We used the tomographic method of Zhao et al. (1994, 2006) to invert the relative travel-time residuals for the 3D velocity structure under the study region. Theoretical travel times were calculated by using the iasp91 Earth model (Kennett and Engdahl, 1991). Travel-time residuals were obtained by subtracting the theoretical travel times and origin time from the observed arrival times, and relative residuals were calculated for each event by subtracting its corresponding mean residual from the raw residuals (Zhao et al., 1994). Distribution of average relative residuals at each of the stations is shown in Fig. 4, which reflects the lateral heterogeneity under the study area. The residuals vary significantly with the azimuth, suggesting the existence of significant high P-wave velocity (high-Vp) materials under both Taiwan and Mainland China, and low-Vp materials under the Chinese coastal areas and Taiwan Strait. A 3D grid was set up in the study area. Velocity perturbations from the 1D iasp91 Earth model at the grid nodes were taken as unknown parameters. The velocity perturbation at any point in the model was computed by interpolating the velocity perturbations at the eight grid nodes surrounding that point. A 3D ray tracing
2. Data and methods We used arrival-time data from teleseismic events recorded by 69 seismic stations of three arrays deployed in SE China and Taiwan (Fig. 1). The first array consists of 26 portable stations deployed by Nanjing University in Fujian and Jiangxi provinces (NJU-FJ and NJUJX); the second array consists of 36 permanent stations of the Fujian Seismic Network (FJ-NET); and the third array includes 7 stations of Broadband Array in Taiwan for Seismology (BATS). These stations were generally equipped with broad-band three-component seismographs with the recording times of 8–16 months (Fig. 1), while some stations of the FJ-NET are equipped with short-period seismometers. We selected earthquakes with epicentral distances between 26◦ and 90◦ (Fig. 2a), which have magnitudes ≥5.5 and were recorded by more than 5 stations simultaneously. We picked manually the P-wave arrival times from the original seismograms (Fig. 2b), and the picking accuracy is estimated to be 0.1–0.2 s. As a result, we
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
163
Fig. 3. Distribution of the 5671 ray paths from the 257 teleseismic events used in this study in the plan view (a) and in the (b) north-south and (c) east-west vertical cross-sections. The squares in (a) denote the seismic stations used.
technique was used to compute travel times and ray paths (Zhao et al., 1992). The large and sparse system of observation equations that relate the observed relative residuals to the unknown velocity parameters was resolved by using a conjugate-gradient algorithm LSQR (Paige and Saunder, 1982) with damping and smoothing regularizations (Zhao et al., 2006). The station elevations were taken into account in the 3D ray tracing and inversion. 3. Analysis and results Teleseismic tomography cannot determine the 3D crustal structure well because the teleseismic rays arrive at stations nearly vertically and so they do not crisscross near the surface. Hence it is necessary to correct the teleseismic relative residuals for the heterogeneous crustal structure. In this work we made the crustal correction using the model CRUST2.0 (Bassin et al., 2000) which is specified on a 2◦ × 2◦ grid for the lateral velocity variations of the crust and Moho topography. We also made detailed analyses of the trade-off between the data variance reduction and the model norm and selected the final 3D velocity model based on the result of the trade-off analysis (Fig. 5a). Figs. 6 and 7 show our final tomographic model. The root-meansquare (RMS) value of the travel-time residuals is significantly reduced after the inversion (Fig. 5b). The 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 and Taiwan Strait, being consistent with the distribution of the relative residuals (Fig. 4). To confirm the main features of the tomographic results, we conducted detailed resolution analyses. The checkerboard resolu-
tion test (Zhao et al., 1992) 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 space, the image of which is straightforward and easy to remember. Therefore, by just examining the inverted images of the checkerboard, one can easily understand where the resolution is good and where it is poor. 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 ∼100 km in the horizontal direction and 50–100 km in depth (Fig. 8). The test results show that the checkerboard pattern is recovered well at depths greater than 200 km under most of the study area, while at shallower depths it is only well recovered right beneath the seismic network. The restoring resolution test (Zhao et al., 1992) is another way of synthetic test. In this test the obtained tomographic image is used to construct the input model (Fig. 9a–e). The velocity perturbations at the grid nodes with those ≥+1.5% and ≤−1.5% are changed to +3% and −3%, respectively, while those between −1.5% and +1.5% are changed to 0%. The inverted results (Fig. 9a –e ) show that the velocity anomalies in our images are well recovered. Through these synthetic tests, we believe that the main features of the tomographic results (Figs. 6 and 7) are reliably resolved by our data set and inversion. 4. Discussion The most prominent feature in the tomographic images is the extensive low-Vp anomalies under SE China (Figs. 6 and 7). This feature is consistent with the thinner lithosphere (An and Shi, 2006) and higher heat flow (Hu and Wang, 2000; Hu et al., 2000) in this
164
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
Fig. 4. Distribution of the average relative travel-time residuals at each of the seismic stations for the teleseismic events in each quadrant (a–d) and for all the events (e). Crosses and dots denote the early and delayed arrivals, respectively. The scale for the residuals is shown beside (e).
region. Two low-Vp anomalies are imaged in the upper mantle: one is located under the inland area (Fig. 7c), the other more prominent one is located under the coastal area (Figs. 6 and 7). A low-Vp anomaly is visible in the mantle transition zone and it is connected with the low-Vp anomalies in the upper mantle. Fig. 10 shows an east-west vertical cross-section of P-wave tomography along a profile passing through SE China determined by Huang and Zhao (2006), which shows that low-Vp anomalies extend widely under SE China from the surface down to the lower mantle, similar to that under the Philippine Sea. Around the low-Vp anomalies are high-Vp zones which represent the subducting Pacific plate under Mariana, the Burma plate under Tibet and Tengchong, and the PHS plate under Taiwan. These high-Vp anomalies in the lower mantle
suggest that the subducted slabs have penetrated the mantle transition zone and entered the lower mantle (Zhao, 2004). The low-Vp anomalies under SE China and the Philippine Sea may represent the mantle upwelling flow driven by the deep slab subduction. Thus the question: why does the mantle flow arise under SE China but is it bounded by the Yangtze Block to the northwest (Fig. 7)? Our tomographic results reveal prominent low-Vp anomalies in the upper mantle under the Yangtze Block which may represent the root of the stable block with widespread Archean basement (Figs. 6 and 7) (Yang et al., 1986; Chen and Jahn, 1998; Zheng et al., 2006). In contrast, the Late Mesozoic igneous rocks widely spread in SE China (Zhou and Li, 2000; Zhou et al., 2006). Magma chambers and channels are thus required in the upper man-
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
165
Fig. 5. (a) Trade-off curve for the variance of velocity perturbations and root-mean-square (RMS) travel-time residual for the tomographic inversions with different values of the damping parameter. The numbers beside the dots denote different damping values from 1.0 to 100.0. The thick arrow represents the optimal damping value used in this study. (b) Distribution of the relative travel-time residuals of our teleseismic data before (gray columns) and after (thick lines) the tomographic inversion.
Fig. 6. Plan views of P-wave tomography obtained in this study. The layer depth is shown below each map. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown at the bottom. The gray bold lines and the black thin lines denote major faults and province boundaries in Mainland China, while the red triangles show the active volcanoes.
166
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
Fig. 7. Vertical cross-sections of P-wave tomography along the five profiles shown on the inset map. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown below (e). The three long-dashed lines in each panel denote the Moho, 410- and 660-km discontinuities. The bold bars at the top of each cross-section denote the land areas, while the reverse triangles indicate the trench locations (Bird, 2003). White dots and red triangles denote the earthquakes (Engdahl et al., 1998) and active volcanoes within 30-km width along each cross-section. The short-dashed lines show the inferred outline of the subducted slab. EUR: Eurasian plate; PHS: Philippine Sea plate.
Fig. 8. Results of a checkerboard resolution test. Open and solid circles denote fast and slow velocities, respectively. The velocity perturbation scale is shown at the bottom. The layer depth is shown below each map.
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
167
Fig. 9. Results of a restoring resolution test (see text for details). Locations of the profiles are shown in Fig. 7. White and black colors denote low and high velocities, respectively. The velocity perturbation scale is shown at the bottom. Other labels are the same as those in Fig. 7.
tle to cause the related magmatism. The chambers and channels, even after 100 Ma, are good passages for the rising mantle flow that is driven by the subduction in the Cenozoic. Thus the mantle flow can reheat the chambers and channels and reduce the seismic velocity there. From this viewpoint, the low-Vp anomalies under SE China may also represent the deep origin of the Late Mesozoic igneous rocks. The low-Vp anomalies under the coastal area are more prominent than that under the inland area. The contrast may result from the much stronger (more volcanic rocks) and younger (142–67 Ma compared with 180–142 Ma) magmatism along the coastal area in the Late Yanshanian than that in the inland area in the Early Yanshanian (Zhou and Li, 2000; Zhou et al., 2006). A significant high-Vp zone is imaged in the uppermost mantle along the coastal area (Fig. 6), which was also revealed by Sn-wave tomography (Pei et al., 2007). Although extensive rising mantle
flow occurred under SE China in the Cenozoic, the Cenozoic volcanism with extreme high heat flow took place in only a few limited sites along the active faults (Liu et al., 1995; Hu et al., 2000; Fedorov and Koloskov, 2005). At the same time, low heat flow and low temperature around the Moho are also found in some sites in the coastal area (Hu and Wang, 2000; Hu et al., 2000). These results suggest that the crust and uppermost mantle under SE China are not significantly affected by the Cenozoic rising mantle flow as mentioned above. Since the Late Mesozoic igneous rocks exist widely along the coastal area with extensive volcanism, we interpret that the highVp anomalies are caused by the cooled igneous rocks in the uppermost mantle. Nishimoto et al. (2008) made a similar interpretation for the high-Vp anomalies in the lower crust under Northeast Japan. High-Vp anomalies are revealed in the upper mantle to the east of Taiwan, which vary significantly from north to south (Fig. 7).
168
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
Fig. 10. An east-west vertical cross-section of P-wave tomography along a profile passing through Southeast China (Huang and Zhao, 2006). Location of the profile is shown on the inset map. The surface and seafloor topography along the profile is shown on the top. White dots show the earthquakes that occurred within a 50-km width from the profile. Red and blue colors denote slow and fast velocities, respectively. The velocity perturbation scale is shown at the bottom. The dashed lines denote the 410- and 660-km discontinuities.
The high-Vp anomalies under North and Central Taiwan extend from the upper mantle down to the mantle transition zone (AA , BB , CC ), while those under South Taiwan are mainly located in the upper mantle (DD , EE ). Considering the distribution of deep and intermediate-depth earthquakes (Engdahl et al., 1998; Wu et al., 2008), we interpret the high-Vp bodies under North and South Taiwan as the subducted PHS plate and the Eurasian plate, respectively. Under South Taiwan, the subducted Eurasian plate looks continuous from the Moho down to 400 km depth, and deep earthquakes occur in the slab down to 200 km depth (Fig. 7d and e). The subducted Eurasian slab has passed through the 410-km discontinuity under Central Taiwan (Fig. 7b and c) and may have reached to the mantle transition zone under the Ryukyu Trench beneath Northeast Taiwan (Fig. 7a) (Teng et al., 2000). Receiver-function analyses show that the mantle transition zone is thicker with a shallower 410-km discontinuity to the east of Taiwan (Ai et al., 2007). The result suggests that the Eurasian plate has subducted into the mantle transition zone, or at least down to the 410-km discontinuity, being consistent with our present results. Our tomographic images also suggest the break-off of the subducted Eurasian plate under Central and North Taiwan (Fig. 7b), which has been proposed by the previous studies (e.g., Chai, 1972; Davies and von Blankenburg, 1995; Teng et al., 2000; Chemenda et al., 2001; Lallemand et al., 2001; Malavieille et al., 2002; Sibuet and Hsu, 2004). The slab break-off plays an important role in the flipping of the subduction polarity (Teng et al., 2000). To the south of Taiwan, the oceanic part of the Eurasian plate (South China Sea (SCS) slab) is subducting under the PHS plate from the Manila Trench (Kao et al., 2000). The subducted SCS slab dragged the adjacent continental
lithosphere to subduct under Central and North Taiwan (Malavieille et al., 2002; Sibuet and Hsu, 2004). As the PHS plate moved northwestward, strong interactions occurred between the PHS plate and the subducted continental slab and caused the detachment of the slab (Lallemand et al., 2001; Sibuet and Hsu, 2004). As mentioned above, the low-Vp anomalies in the upper mantle may have existed since the Late Mesozoic, which must have been heating the above Eurasian plate all the time and so weakened the plate, which made the slab easier to break-off. The slab break-off may have created a mantle window through which the hot mantle material or asthenospheric flow arose into the crust, causing high heat flow and rapid uplift in the Taiwan orogen (Lee and Cheng, 1986; Teng et al., 2000). 5. Conclusions We determined a detailed 3D P-wave velocity structure of the upper mantle under SE China by applying teleseismic tomography to a large number of high-quality data recorded by many portable and permanent seismic stations. Our tomographic images show strong structural heterogeneities under the study region. Prominent low-Vp anomalies are revealed in the upper mantle under SE China, which may be closely related to the widespread Late Mesozoic magmatism in the region. In the Cenozoic, active subductions of the Pacific, Burma and PHS plates took place in East Asia. The subducted plates have reached to the lower mantle, and subsequently drove the lower mantle material to arise into the upper mantle through the Mesozoic magma chambers and channels. The subducted Eurasian plate under Taiwan is clearly imaged as highVp anomalies with significant lateral variations from south to north
Z. Huang et al. / Physics of the Earth and Planetary Interiors 182 (2010) 161–169
in the upper mantle. The Eurasian slab is continuous under South Taiwan, but it is broken-off under Central and North Taiwan. The break-off of the subducted slab created a mantle window through which the asthenospheric flow arises, causing the high heat flow and rapid uplift in the Taiwan orogen. Acknowledgements We thank the Seismological Bureau of Fujian province, the Institute of Earth Science of Academic Sinica, Taiwan, and the IRIS Data Management Center for providing the waveform data used in this study. Dr. G. Jiang kindly helped us at the data processing stage. Profs. G. Helffrich, C. Chiarabba and an anonymous referee provided constructive review comments, which improved the manuscript. Discussions with Prof. Q. Wang, X. Zhou, J. Qiu and Dr. Z. He were very helpful. We thank H. Zhu, Z. Li and others for their help during the field seismic experiments in Fujian and Jiangxi provinces. This work was supported by grants (40634021, DD0963 and YPH08043) from the National Natural Science Foundation of China, the Scientific Research Foundation of Graduate School of Nanjing University, and a grant (Kiban-A 17204037) to D. Zhao from Japan Society for the Promotion of Science (JSPS). Most of the figures were made by using Generic Mapping Tools (Wessel and Smith, 1998). References Ai, Y., Chen, Y., Zeng, F., Hong, X., Ye, W., 2007. The crust and upper mantle structure beneath southeastern China. Earth Planet. Sci. Lett. 260, 549–563. An, M., Shi, Y., 2006. Lithospheric thickness of the Chinese continent. Phys. Earth Planet. Inter. 159, 257–266. Bassin, C., Laske, G., Masters, G., 2000. The current limits of resolution for surface wave tomography in North America. EOS Trans. AGU 81, F897. Bijwaard, H., Spakman, W., Engdahl, R., 1998. Closing the gap between regional and global travel time tomography. J. Geophys. Res. 103, 30055–30078. Bird, P., 2003. An updated digital model of plate boundaries. Geochem. Geophys. Geosyst. 4, 1027. Chai, B., 1972. Structure and tectonic evolution of Taiwan. Am. J. Sci. 272, 389–422. Chemenda, A., Yang, R., Stephan, J., Konstantinovskaya, E., Ivanov, G., 2001. New results from physical modeling of arc-continent collision in Taiwan: evolutionary model. Tectonophysics 333, 159–178. Chen, J., Jahn, B., 1998. Crustal evolution of southeastern China, Nd and Sr isotopic evidence. Tectonophysics 284, 101–133. Chen, R., Kao, H., Liang, W., Shin, T., Tsai, Y., Huang, B., 2009. Three-dimensional patterns of seismic deformation in the Taiwan region with special implication from the 1999 Chi-Chi earthquake sequence. Tectonophysics 466, 140–151. Cheng, W., 2009. Tomographic imaging of the convergent zone in Eastern Taiwan—a subducting forearc sliver revealed? Tectonophysics 466, 170–183. Davies, J., von Blankenburg, F., 1995. Slab break-off: a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens. Earth Planet. Sci. Lett. 129, 85–102. Engdahl, E., van der Hilst, R., Buland, R., 1998. Global teleseismic earthquake location with improved travel times and procedure for depth determination. Bull. Seismol. Soc. Am. 88, 722–743. Fedorov, P., Koloskov, A., 2005. Cenozoic volcanism of Southeast Asia. Petrology 13, 352–380. Hu, S., Wang, J., 2000. Heat flow, deep temperature and thermal structure across the orogenic belts in Southeast China. J. Geodyn. 30, 461–473. Hu, S., He, L., Wang, J., 2000. Heat flow in the continental area of China: a new data set. Earth Planet. Sci. Lett. 179, 407–419. Huang, J., Zhao, D., 2006. High-resolution mantle tomography of China and surrounding regions. J. Geophys. Res. 111, B09305. Jahn, B., Zhou, X., Li, J., 1990. Formation and tectonic evolution of the SEChina and Taiwan: isotopic and geological constraints. Tectonophysics 183, 177–189.
169
Kao, H., Shen, S., Ma, K., 1998. Transition from oblique subduction to collision: earthquakes in the southernmost Ryukyu arc-Taiwan region. J. Geophys. Res. 103, 7211–7229. Kao, H., Huang, G., Liu, C., 2000. Transition from oblique subduction to collision in the northern Luzon arc-Taiwan region: constraints from bathymetry and seismic observations. J. Geophys. Res. 105, 3059–3080. Kennett, B., Engdahl, E., 1991. Travel times for global earthquake location and phase identification. Geophys. J. Int. 105, 429–465. Lallemand, S., Font, Y., Bijwaard, H., Kao, H., 2001. New insights on 3-D plates interaction near Taiwan from tomography and tectonic implications. Tectonophysics 335, 229–253. Lapierre, H., Jahn, B., Charvet, J., Yu, Y., 1997. Mesozoic felsic arc magmatism and continental olivine tholeiites in Zhejiang Province and relationship with the tectonic activity in southeastern China. Tectonophysics 274, 321–338. Lee, C., Cheng, W., 1986. Preliminary heat-flow measurements in Taiwan. In: Horn, M.K. (Ed.), Transactions of Forth Circum-Pacific Energy and Mineral Resources Conference. Circum-Pacific Energy and Mineral Resources, Tulsa. Li, Z., Xu, Y., Hao, T., Xu, Y., 2009. Vp and Vp/Vs structures in the crust and upper mantle of the Taiwan region, China. Sci. China Ser. D 52, 975–983. Liu, R., Xie, G., Zhou, X., Chen, W., Fan, Q., 1995. Tectonic environments of Cenozoic volcanic rocks in China and characteristics of the source regions in the mantle. Chinese J. Geochem. 14, 289–302. Malavieille, J., Lallemand, S., Dominguez, S., Dechamps, A., Lu, C., Liu, C., Schnurle, P., 2002. Arc-Continent collision in Taiwan: new marine observations and tectonic evolution. Geol. Soc. Am. Special Paper 358, 189–213. Nishimoto, S., Ishikawa, M., Arima, M., Yoshida, T., Nakajima, J., 2008. Simultaneous high P–T measurements of ultrasonic compressional and shear wave velocities in Ichino-megata mafic xenoliths: their bearing on seismic velocity perturbation in lower crust of northeast Japan arc. J. Geophys. Res. 113, B12212. Paige, C., Saunder, M., 1982. LSQR: an algorithm for sparse linear equations and sparse least squares. ACM Trans. Math. Softw. 8, 43–71. Pei, S., Zhao, J., Sun, Y., Xu, Z., Wang, S., 2007. Upper mantle seismic velocities and anisotropy in China determined through Pn and Sn tomography. J. Geophys. Res. 112, B05312. Seno, T., Stein, S., Gripp, A., 1993. A model for the motion of the Philippine Sea plate consistent with NUVEL-1 and geological data. J. Geophys. Res. 89, 17941– 17948. Sibuet, J., Hsu, S., 2004. How was Taiwan created? Tectonophysics 379, 159–181. Teng, L., Lee, C., Tsai, Y., Hsiao, L., 2000. Slab break-off as a mechanism for flipping of subduction polarity in Taiwan. Geology 28, 155–158. Wang, Z., Zhao, D., Wang, J., Kao, H., 2006. Tomographic evidence for the Eurasian lithosphere subducting beneath south Taiwan. Geophys. Res. Lett. 33, L18306. Wessel, P., Smith, W., 1998. New improved version of the Generic Mapping Tools released. EOS Trans. AGU 79, 579. Wu, W., Hsu, S., Lo, C., Chen, H., Ma, K., 2009. Plate convergence at the westernmost Philippine Sea Plate. Tectonophysics 466, 162–169. Wu, Y., Chang, C., Zhao, L., Shyu, B., Chen, Y., Sieh, K., Avouac, J., 2007. Seismic tomography of Taiwan: improved constraints from a dense network of strong motion stations. J. Geophys. Res. 112, B08312. Wu, Y., Chang, C., Zhao, L., Teng, T., Nakamura, M., 2008. A comprehensive relocation of earthquakes in Taiwan from 1991 to 2005. Bull. Seismol. Soc. Am. 98, 1471–1481. Yang, Z., Cheng, Y., Wang, H., 1986. The Geology of China. Oxford Monograph Geology Geophysics. Clarendon Press, Oxford, 303 pp. Yu, S., Chen, H., Kuo, L., 1997. Velocity field of GPS stations in the Taiwan area. Tectonophysics 274, 41–59. Zhao, D., 2004. Global tomographic images of mantle plumes and subducting slabs: insight into deep Earth dynamics. Phys. Earth Planet. Inter. 146, 3–34. Zhao, D., Hasegawa, A., Horiuchi, S., 1992. Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res. 97, 19909–19928. Zhao, D., Hasegawa, A., Kanamori, H., 1994. Deep structure of Japan subduction zone as derived from local, regional, and teleseismic events. J. Geophys. Res. 99, 22313–22329. Zhao, D., Lei, J., Inoue, T., Yamada, A., Gao, S., 2006. Deep structure and origin of the Baikal rift zone. Earth Planet. Sci. Lett. 243, 681–691. Zheng, J., Griffin, W., O’Reilly, S., Zhang, M., Pearson, N., Pan, Y., 2006. Widespread Archean basement beneath the Yangtze craton. Geology 34, 417–420. Zhou, X., Li, W., 2000. Origin of Late Mesozoic igneous rocks in Southeastern China: implications for lithosphere subduction and underplating of mafic magmas. Tectonophysics 326, 269–287. Zhou, X., Sun, T., Shen, W., Shu, L., Niu, Y., 2006. Petrogenesis of Mesozoic granitoids and volcanic rocks in South China: a response to tectonic evolution. Episodes 29, 26–33.