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SH wave structure of the crust and upper mantle in southeastern margin of the Tibetan Plateau from teleseismic Love wave tomography
Physics of the Earth and Planetary Interiors 279 (2018) 15–20
Contents lists available at ScienceDirect
Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi
SH wave structure of the crust and upper mantle in southeastern margin of the Tibetan Plateau from teleseismic Love wave tomography
T
⁎
Yuanyuan V. Fua, , Ruizhi Jiab, Fengqin Hanc, Anguo Chena a
Institute of Earthquake Forecasting, China Earthquake Administration, Beijing, China Peking University, Beijing, China c Hebei University of Engineering, Handan, Hebei, China b
A B S T R A C T
The deep structure of southeastern Tibet is important for determining lateral plateau expansion mechanisms, such as movement of rigid crustal blocks along large strike-slip faults, continuous deformation or the eastward crustal channel flow. We invert for 3-D isotropic SH wave velocity model of the crust and upper mantle to the depth of 110 km from Love wave phase velocity data using a best fitting average model as the starting model. The 3-D SH velocity model presented here is the first SH wave velocity structure in the study area. In the model, the Tibetan Plateau is characterized by prominent slow SH wave velocity with channel-like geometry along strike-slip faults in the upper crust and as broad zones in the lower crust, indicating block-like and distributed deformation at different depth. Positive radial anisotropy (VSH > VSV) is suggested by a high SH wave and low SV wave anomaly at the depths of 70–110 km beneath the northern Indochina block. This positive radial anisotropy could result from the horizontal alignment of anisotropic minerals caused by lithospheric extensional deformation due to the slab rollback of the Australian plate beneath the Sumatra trench.
1. Introduction Regional investigation of the lithospheric structure in southeastern margin of the Tibetan Plateau dates back over 25 yr (e.g. Liu et al., 1989), since it is a unique place to study the interaction between the collision of the Burma microplate and Eurasian plate and the eastward or southeastward extrusion of material from the central and eastern part of the plateau (Fig. 1). Refraction studies have been conducted across some large strike-slip faults (e.g. Kan and Lin, 1986; Xiong et al., 1993). Receiver functions have provided valuable constraints on crustal thickness, which decreases gradually from the Tibetan Plateau to the northern part of the Indochina block (NIB) and the Yangtze craton (e.g., Xu et al., 2007). Seismic tomographic models suggest that significant lithospheric heterogeneity exists (Huang et al., 2002; Li et al., 2008; Yao et al. 2008; Lei et al., 2009; Huang et al., 2015; Yang et al., 2015). Body wave (Li et al., 2008; Huang et al., 2015) and surface wave tomography (Yang et al., 2015) both reveal extensive slow velocity in the upper mantle beneath the Tengchong volcano (TV), northern part of the Indochina block (NIB) and the eastern Yangtze craton (EYC). However, the interpretations for the anomaly are not consistent among these studies. Huang et al. (2015) interpreted the low velocity anomaly as the extruded mantle flow from the Tibetan Plateau and proposed the
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Corresponding author. E-mail address: [email protected] (Y.V. Fu).
https://doi.org/10.1016/j.pepi.2018.04.002 Received 21 March 2017; Received in revised form 4 April 2018; Accepted 5 April 2018 Available online 06 April 2018 0031-9201/ © 2018 Elsevier B.V. All rights reserved.
mantle extrusion in southeast Tibet. In contrast, Li et al. (2008) and Yang et al. (2015) suggested that the low velocity of TV was related to the subduction beneath the Burma arc. For the west of the Yangtze craton (WYC), there are different features in several models. Li et al. (2008) found fast upper mantle P wave velocity in the WYC. Yao et al. (2008) also imaged a similar fast velocity in the WYC from Rayleigh wave tomography. However, Huang et al. (2015) observed a low velocity anomaly of P wave in the upper mantle of the WYC. The main difference among these studies is the geometry of the velocity anomaly. More information is needed to constrain the anomaly. Surface wave tomography has been widely used to determine the structure of the Earth on both global and regional scales (e.g. Ritzwoller et al., 2002; Bensen et al., 2009; Fu et al., 2010). Most of these studies have concentrated exclusively on Rayleigh wave. The relative weak and complex waveform of Love wave and correspondent method restriction result in the limited application of Love wave data. However, Love wave can provide independent information about the subsurface structure, especially the radial anisotropy. Recent studies have shown that the two-plane-wave (TPW) method could also be used for Love wave through some modifications, although it is originally developed for Rayleigh waves (Li and Li, 2015; Fu et al., 2015; 2016). Fundamental mode Love wave amplitude on the transverse component is
Physics of the Earth and Planetary Interiors 279 (2018) 15–20
Y.V. Fu et al.
Fig. 2. Sensitivity kernel for Love wave phase speeds at periods of 25, 50, 71, and 100 s. The kernels are calculated based on the shear wave velocity model AK135 (Kennett et al., 1995) using the method of Saito (1988).
Fig. 1. Map of tectonics and seismic stations in the southeastern Tibetan Plateau. The broadband stations from the temporary ChinArray and the permanent China Digital Seismic Array are indicated by blue and red triangles, respectively. Large white triangle shows the Tengchong volcano (TV). The yellow stars are the locations for the examples in Fig. 5. The blue circle is the station 53202. TP, Tibetan Plateau; BMP, Burma micro-plate; IB, Indochina Block; YC, Yangtze Craton; SC, South China; NIB, northern Indochina Block; WYC, western Yangtze Craton; EYC, eastern Yangtze Craton; RRF, Red River fault; XF, Xiaojinhe fault; XXF, Xianshuihe-Xiaojiang fault.
determined and then used as initial models in obtaining the 2-D phase velocities with the grid interval of 0.5° in the center and 0.75° at the edge. The standard deviation of phase velocity at each grid is calculated from the model covariance matrix. The resulting bandwidth of Love wave phase velocity measurements has the sensitivity to SH wave velocity from the surface to a depth of ∼150 km in the upper mantle (Fig. 2). Phase velocities provide integrated information over a broad depth range (Fig. 2). We take a two-step inversion of Love wave phase velocity for 3-D SH wave velocity structure. We first obtain the 1-D average SH wave velocity model from the average phase velocity dispersion curve. Then, we invert phase velocities at each grid point to obtain 3-D SH wave velocity, taking the 1-D average SH wave velocity model as the initial model. Since the inversion is somewhat non-linear and highly underdetermined due to the limitations of Love wave vertical resolution, the resultant model depend strongly on the initial model. In order to obtain an appropriate initial model for the 1-D inversion, we modified the AK135 reference model (Kennett et al., 1995) based on previous regional models in the study area (Bao et al., 2015; Huang et al., 2015) as the initial model. It contains 16 layers from the surface to 410 km depth with a three-layer crust. The crustal thickness is constrained from receiver functions (Li et al., 2014) and kept as a constant during the inversion since it has a very small effect on the velocity (Fu et al., 2016). We only invert shear wave velocity for each layer. P wave velocity is scaled to shear wave velocity with a constant VP/VS ratio of 1.73. The density does not change in the inversion since its effect on Love wave phase velocity is too small. The layer thickness ranges from 10 to 50 km. The model parameters are slightly damped by assigning prior standard deviations in the diagonal terms of model covariance matrix. A general estimate of these values can be made based on the results obtained from previous inversions of fundamental mode surface wave data. Yu and Mitchell (1979) found that the standard deviation of the velocity structure is on the order of 0.02–0.06 km/s depending on the ray path coverage. Since there are quite few earthquakes from west, we choose to use a conservative value of 0.05 km/s as the errors in shear wave velocity. The correlation between two different depths plays an
decomposed to x and y components in a local coordinate system for each earthquake. The modified TPW method provides a new way to measure long period Love wave phase velocity which is inverted for 3-D SH wave velocity model of the upper mantle. The SH wave velocity from Love wave can provide additional constraints on the lithospheric evolution. With the completion of the ChinArray in this region, phase velocity maps of Love wave with high resolution can be developed to better constrain the structure in the crust and uppermost mantle for the southeastern Tibet. In this study, we extend the work of Han et al. (2017) to SH wave velocity measurement. Han et al. (2017) adopted the modified TPW technique to estimate the Love wave phase velocity at periods of 20–100 s in southeastern margin of the Tibetan Plateau. Fundamental mode Love wave data from the ChinArray which consists of seismic stations at an interval of 35 km combined with the permanent China National Seismic Network is used to construct Love wave dispersion maps on a 0.5° × 0.5° grid. The model presented here is a 3-D volume of isotropic SH wave velocity inverted from the Love wave phase velocity maps (Han et al., 2017). It is the first time to obtain the SH velocity in the upper mantle with high resolution in southeastern margin of the Tibetan Plateau. This new model will provide useful information to estimate the radial anisotropy in the upper mantle and to constrain the deformation mechanism for the southeastern Tibet.
2. Data and method Phase velocity maps at each period are inverted from the amplitude and phase of fundamental mode Love wave recorded by the stations shown in Fig. 1 (Han et al., 2017). The modified TPW method is used to simultaneously solve for the incoming wave field and phase velocity. The 1-D average phase velocities at 12 periods from 20 to 100 s are 16
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With the average shear wave velocity as start value, we perform the same 1-D inversion at each map point. Fig. 5 shows the 1-D SH wave velocity model at three points located in different tectonic units (Tibetan Plateau, NIB and EYC). These models indicate various velocity structures. The shear wave velocity of the crust at the point in the Tibetan Plateau (red line in Fig. 5) is significantly slower than those at the points in the NIB (blue line in Fig. 5) and the EYC (green line in Fig. 5). At the depths of 70–110 km in the upper mantle, the velocity in the EYC (green line in Fig. 5) is much slower than those at the other two points (red and blue line in Fig. 5). The Tibetan Plateau has a slow crust while the eastern part of Yangtze craton has a slow upper mantle. 3.2. 3-D SH wave velocity The maps of shear wave velocity perturbation at six layers from the surface to 110 km in the upper mantle are shown in Fig. 6. Fig. 7 displays three cross sections with locations the same as those of SV wave velocity from Rayleigh wave tomography by Fu et al. (2017) for comparison. The anomalies are relative to the average shear wave velocity at corresponding depths in the entire area (red line in Fig. 4b). A prominent low-velocity anomaly appears in the Tibetan Plateau (TP) in the crust and uppermost mantle (Figs. 6a–d, 7b and c). This observation is consistent with the low velocity of SV wave from ambient noise tomography by Bao et al. (2015) and Rayleigh wave earthquake tomography by Fu et al. (2017). The TV is characterized by slow velocity from lower crust to the depth of 110 km (Figs. 6c–f, 7a and b). The NIB shows a variable velocity pattern with depth from low velocity anomaly at 25–70 km to high velocity anomaly at 70–110 km (Figs. 6c–f, 7b). In contrast, P wave model (Huang et al., 2015) and SV wave model (Fu et al., 2017) found a low velocity extending down to the depth of 150 km beneath the NIB. High velocity anomaly in the upper mantle is imaged beneath the western Yangtze craton (WYC) (Figs. 6d–f, 7a–c). However, the EYC displays different feature, strong low velocity anomaly in the upper mantle with the strength increasing with depth (Figs. 6d–f, 7a–c). Similar pattern of SV wave velocity was observed from Rayleigh wave tomography (Fu et al., 2017). The observation of high VSH beneath WYC in the upper mantle is different from a slow P wave velocity observed by Huang et al. (2015).
Fig. 3. Resolution kernels of shear velocity from the resolution matrix for the reference model at five layers with median depths at 18, 58, 100, 140, and 160 km.
important role in balancing model variance and resolution. We choose a correlation coefficient of 0.4 between adjacent layers in this study after testing several different values. The synthetic phase velocities and partial derivatives of phase velocities with respect to model parameters are calculated using the method of Saito (1988). The inversion seeks to minimize the difference between the observed and model-predicted Love wave phase velocity. The robustness and resolution of shear wave velocity with depth are estimated through the model resolution matrix. 3. Results
4. Discussion
The 1-D average shear velocity model is inverted from the averaged Love wave phase velocity from 20 to 100 s in southeastern margin of the Tibetan Plateau. The depth resolution of shear wave velocity model from such 1-D inversion can be determined according to the resolution matrix. The peak values of the resolution matrix for the 1-D shear wave velocity model generally become smaller at greater depths (Fig. 3), indicating the resolution of shear wave velocity decreases with increasing depth. Although the peak values reduce heavily at a great depth, we can still clearly recognize it at 110 km, suggesting a reasonable resolution at this depth. We therefore focus on the velocity structure above 110 km depth. We perform a series of 1-D inversions with the average shear wave velocity model as a start model to build up the 3-D model.
4.1. Crustal block rotation model or flow model in the southeast TP Geodetic measurements (Chen et al., 2000; Shen et al., 2005) and geological study (Wang and Burchfiel, 2000) reveal a clockwise rotation around the eastern Himalayan Syntaxis, implying the crustal deformation in the southeast TP is dominated by block rotation through major strike-slip faults (Tapponnier et al., 1982). However, seismic tomography (Chen et al., 2014; Bao et al., 2015) and magnetotelluric study (Bai et al., 2010) observe a low velocity/resistivity layer in lower crust, implying a ductile lower crust, suggesting that northward advancement of the Indian plate results in broadly distributed deformation of southeastern Tibet. These two models suggest rather different crustal deformation patterns. Our SH wave velocity result shows various geometry of the low velocity zone with the depth in the crust. These low velocity could result from the elevated temperature and/or partial melt since the regional surface heat flow is high (Hu et al., 2000). In the upper and middle crust, the channel like low velocity zones are observed along the major faults (XF, XXF and RF) (Fig. 6a and b). However, the low velocity in the lower crust becomes broad (Fig. 6c). Our observation agrees with that of SV velocity from ambient noise (Bao et al., 2015) and Rayleigh wave data (Fu et al., 2017). The transformation of the geometry for the low SH wave velocity anomalies from channel-like to broad region probably indicate different deformation pattern in the middle and lower crust. The coexistence of the block rotation model and crustal flow model could occur
3.1. 1-D SH wave velocity The final 1-D reference model is obtained by inverting the average phase velocities in the study area (red line in Fig. 4b). The average SH wave velocity model is well constrained with standard deviation from 0.025 to 0.035 km/s. The predicted dispersion curve from this model fits the Love wave observations well (red line in Fig. 4a). Compared with the SV wave velocity model from Rayleigh wave (Fu et al., 2017, blue line in Fig. 4b), SH wave velocity is significantly larger in the middle and lower crust, and at the depths of 90–110 km, implying the existence of strong positive radial anisotropy (VSH > VSV). 17
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Fig. 4. Love wave average dispersion curve and associated best fitting model (red). The correspondent results of Rayleigh wave (blue) are from Fu et al. (2017) for comparison. (a) Observed (dots) and predicted (line) dispersions from the best fitting model. Error bars represent two standard deviations. (b) Shear wave models. The black line represents a modified AK135 model used as an initial model in the inversion. The width of the shaded area shows the standard error of isotropic shear wave velocity in each layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) Observed (triangles) and predicted dispersion curves and (b) associated best fitting models at three grid points that are shown in Fig. 1 (yellow stars). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Rayleigh and Love wave, respectively. Our high SH wave velocity is imaged at the depths of 70–110 km in the NIB, while Fu et al. (2017) observed slow SV wave velocity from Rayleigh wave tomography. Positive radial anisotropy (VSH > VSV) probably exist below the depth of 70 km. Three scenarios have been proposed to explain the slow SV and P wave velocity (Huang et al., 2015; Fu et al., 2017; Zhang et al., 2017). The first two are both about the subduction of Indian plate beneath the Burma. The difference of these two models is the existence of slab tearing. Huang et al. (2015) suggests that the fluid-induced partial melting from the stagnant slab in the transition zone results in the slow SV and P wave velocity (Huang et al., 2015), while Zhang et al. (2017) propose that the low seismic velocity anomaly is probably caused by the upwelling of hot and buoyant sub-lithospheric mantle that has been entrained beneath the sinking lithosphere of the Indian plate and is now escaping through the slab tearing. In these two cases, vertical flow should exist and therefore negative radial anisotropy (VSH < VSV) could be observed. The third possible cause for the low velocity is the upwelling of warm mantle during the lithospheric extension associated with the slab rollback of the Australian plate beneath the Sumatra trench (Yang et al., 2015).
simultaneously in the southeast TP at different depths. The north-south shortening of the Tibetan Plateau is accommodated by block extrusion at shallow depth and diffused deformation by thickening and weakening of the lower crust. How these two factors interact and what the geometry or inter-connectivity of low velocity zone is require further quantitative geodynamic modeling. 4.2. Positive radial anisotropy (VSH > VSV) at depths of 70-110 km beneath the NIB Radial anisotropy is attributed to the lattice preferred orientation of anisotropic materials caused by strains associated with deformation. It can provide some of the most direct evidence for deformation and flow within the Earth's interior. Radial anisotropy manifests itself as the difference in the speeds of horizontally and vertically polarized, horizontally propagating S waves (VSH and VSV, respectively). It is inferred from the simultaneous analysis of dispersion of Love and Rayleigh wave. Since Rayleigh wave is mainly sensitive to SV wave and Love wave is to SH wave, we can qualitatively determine the radial anisotropy by comparing the VSV and VSH which are separately inverted from 18
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Fig. 6. Maps of shear wave velocity perturbation in the crust and upper mantle. The perturbation is relative to the average value of each layer indicated in the lower left corner of each map.
The horizontal flow due to the slab rollback produces the lithospheric extension and positive radial anisotropy (VSH > VSV) beneath the NIB. Our observed high VSH and slow VSV support this hypothesis.
velocity anomalies with various geometry at different depths in the crust are imaged in the Tibetan Plateau. This variation from channellike at shallow depths to a broadened pattern in the lower crust could suggest that the crustal deformation is dominated by block rotation in the upper part and ductile flow in the lower part. The high SH wave velocity anomaly and low SV wave below 70 km depth in the northern Indochina block imply the existence of positive radial anisotropy, reflecting horizontal layering of anisotropic materials due to the past and recent rifting activities associated with the slab rollback along the Sumatra-Andaman Sea subduction zone. More quantitative analysis of the radial anisotropy is needed to constrain the geodynamics of this region in the future.
5. Conclusions The 3-D isotropic SH wave velocity model of the crust and upper mantle beneath the southeastern margin of the Tibetan Plateau is obtained from the fundamental mode Love wave phase velocities at the periods of 20–100 s, determined by the modified two-plane-wave method presented previously by Han et al. (2017). It is the first time the SH wave velocity structure has been constrained from the surface to approximately 110 km depth in the study area. Strong low SH wave 19
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Chen, M., Huang, H., Yao, H., van der Hilst, R., Niu, F., 2014. Low wave speed zones in the crust beneath SE Tibet revealed by ambient noise adjoint tomography. Geophys. Res. Lett. 41, 334–340. Chen, Z., Burchfiel, B.C., Liu, Y., King, R.W., Royden, L.H., Tang, W., Wang, E., Zhao, J., Zhang, X., 2000. Global positioning system measurements from eastern Tibet and their implications for India/Eurasia intercontinental deformation. J. Geophys. Res. 105, 16215–16227. Fu, Y.V., Li, A., Chen, Y.J., 2010. Crustal and upper mantle structure of southeast Tibet from Rayleigh wave tomography. J. Geophys. Res. Solid Earth 115, B12323. Fu, Y.V., Gao, Y., Li, A., Li, L., Shi, Y., Zhang, Y., 2015. Lithospheric shear wave velocity and radial anisotropy beneath the northern part of North China from surface wave dispersion analysis. Geochem. Geophys. Geosyst. 16, 2619–2636. Fu, Y.V., Gao, Y., Li, A., Li, L., Shi, Y., Zhang, Y., 2016. Origin of intraplate volcanism in northeast China from Love wave constraints. J. Geophys. Res. Solid Earth 121. http:// dx.doi.org/10.1002/2016JB013305. Fu, Y.V., Gao, Y., Li, A., Li, L., Zhang, Y., 2017. Lithospheric structure of the southeastern margin of the Tibetan Plateau from Rayleigh wave tomography. J. Geophys. Res. Solid Earth accepted. Han, F., Jia, R., Fu, Y. V., Chen, A., 2017. Love wave phase velocity maps of the southeastern margin of the Tibetan Plateau. submitted to Tectonophysics. 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., Zhang, S., 2002. Lithospheric structure and its relationship to seismic and volcanic activity in southwest China. J. Geophys. Res. 107, 2255–2268. Huang, Z., Wang, P., Xu, M., Wang, L., Ding, Z., Wu, Y., Xu, M., Mi, N., Yu, D., Li, H., 2015. Mantle structure and dynamics beneath SE Tibet revealed by new seismic images, Earth Planet. Sci. 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Three dimensional crust structure beneath SE Tibetan Plateau and its seismotectonic implications for the Ludian and Jinggu earthquakes. Seismol. Geol. 36 (4), 1204–1216. Liu, J., Liu, F., Wu, H., Li, Q., Hu, G., 1989. Three-dimension velocity image of the crust and upper mantle beneath the north-south seismic zone in China (in Chinese). Acta Geophys. Sin. 32, 152–161. Ritzwoller, M.H., Shapiro, N.M., Barmin, M.P., Levshin, A.L., 2002. Global surface wave diffraction tomography. J. Geophys. Res. 107, 4–13. Saito, M., 1988. DISPER80: A subroutine package for the calculation of seismic normalmode solutions. In: Doornbos, D.J. (Ed.), Seismological Algorithms: Computational Methods and Computer Programs. Elsevier, New York, pp. 293–319. Shen, Z., Lu, J., Wang, M., Burgmann, R., 2005. Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau. J. Geophys. Res. 110, B11409. Tapponnier, P., Peltzer, G., Le Dai, A.Y., Armijo, R., Cobbold, P., 1982. Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine. Geology 10, 611–616. Wang, E., Burchfiel, B.C., 2000. Late Cenozoic to Holocene deformation in southwestern Sichuan and adjacent Yunnan, China, and its role in formation of the southeastern part of the Tibetan Plateau. Geol. Soc. Am. Bull. 112, 413–423. Xiong, S., Zheng, H., Yin, Z., 1993. Two-dimensional crustal structure and its tectonic implications under the Lijiang-Panzhihua-Zhehai region (in Chinese). Acta Geophys. Sin. 36, 434–443. Xu, L., Rondenay, S., van der Hilst, R.D., 2007. Structure of the crust beneath the Southeastern Tibetan plateau from teleseismic receiver functions. Phys. Earth Planet. Int. 165, 176–193. Yang, T., Liu, F., Harmon, N., Le, K.P., Gu, S., Xue, M., 2015. Lithospheric structure beneath Indochina block from Rayleigh wave phase velocity tomography. Geophys. J. Int. 200, 1582–1595. Yao, H., Beghein, C., van der Hilst, R.D., 2008. 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Fig. 7. Vertical profiles of shear wave velocity perturbation in the crust and upper mantle. The locations of the profiles are shown in Fig. 6a. TP, Tibetan plateau; TV, Tengchong volcano; NIB, northern Indochina block; WYC, western Yangtze craton; EYC, eastern Yangtze craton.
Acknowledgments Data are from the Institute of Geophysics, China Earthquake Administration and requested through [email protected]. We thank the participants in the ChinArray for collecting the data. Most figures in this paper were made with General Mapping Tools (Wessel and Smith, 1998). Comments from the editor and two anonymous reviewers helped improve the manuscript. This work was supported by grant 41574042 from the National Natural Science Foundation of China. References Bai, D., Unsworth, M.J., Meju, M.A., Ma, X., Teng, J., Kong, X., Sun, Y., Sun, J., Wang, L., Jiang, C., Zhao, C., Xiao, P., Liu, M., 2010. Crustal deformation of the eastern Tibetan Plateau revealed by magnetotelluric imaging. Nat. Geosci. 3, 358–362. Bao, X., Sun, X., Xu, M., Eaton, D.W., Song, X., Wang, L., Ding, Z., Mi, N., Li, H., Yu, D., Huang, Z., Wang, P., 2015. Two crustal low-velocity channels beneath SE Tibet revealed by joint inversion of Rayleigh wave dispersion and receiver functions, Earth Planet. Sci. Lett. 415, 16–24. Bensen, G.D., Ritzwoller, M.H., Yang, Y., 2009. A 3-D shear velocity model of the crust and uppermost mantle beneath the United States from ambient seismic noise. Geophys. J. Int. 177, 1177–1196.
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Contents lists available at ScienceDirect
Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi
SH wave structure of the crust and upper mantle in southeastern margin of the Tibetan Plateau from teleseismic Love wave tomography
T
⁎
Yuanyuan V. Fua, , Ruizhi Jiab, Fengqin Hanc, Anguo Chena a
Institute of Earthquake Forecasting, China Earthquake Administration, Beijing, China Peking University, Beijing, China c Hebei University of Engineering, Handan, Hebei, China b
A B S T R A C T
The deep structure of southeastern Tibet is important for determining lateral plateau expansion mechanisms, such as movement of rigid crustal blocks along large strike-slip faults, continuous deformation or the eastward crustal channel flow. We invert for 3-D isotropic SH wave velocity model of the crust and upper mantle to the depth of 110 km from Love wave phase velocity data using a best fitting average model as the starting model. The 3-D SH velocity model presented here is the first SH wave velocity structure in the study area. In the model, the Tibetan Plateau is characterized by prominent slow SH wave velocity with channel-like geometry along strike-slip faults in the upper crust and as broad zones in the lower crust, indicating block-like and distributed deformation at different depth. Positive radial anisotropy (VSH > VSV) is suggested by a high SH wave and low SV wave anomaly at the depths of 70–110 km beneath the northern Indochina block. This positive radial anisotropy could result from the horizontal alignment of anisotropic minerals caused by lithospheric extensional deformation due to the slab rollback of the Australian plate beneath the Sumatra trench.
1. Introduction Regional investigation of the lithospheric structure in southeastern margin of the Tibetan Plateau dates back over 25 yr (e.g. Liu et al., 1989), since it is a unique place to study the interaction between the collision of the Burma microplate and Eurasian plate and the eastward or southeastward extrusion of material from the central and eastern part of the plateau (Fig. 1). Refraction studies have been conducted across some large strike-slip faults (e.g. Kan and Lin, 1986; Xiong et al., 1993). Receiver functions have provided valuable constraints on crustal thickness, which decreases gradually from the Tibetan Plateau to the northern part of the Indochina block (NIB) and the Yangtze craton (e.g., Xu et al., 2007). Seismic tomographic models suggest that significant lithospheric heterogeneity exists (Huang et al., 2002; Li et al., 2008; Yao et al. 2008; Lei et al., 2009; Huang et al., 2015; Yang et al., 2015). Body wave (Li et al., 2008; Huang et al., 2015) and surface wave tomography (Yang et al., 2015) both reveal extensive slow velocity in the upper mantle beneath the Tengchong volcano (TV), northern part of the Indochina block (NIB) and the eastern Yangtze craton (EYC). However, the interpretations for the anomaly are not consistent among these studies. Huang et al. (2015) interpreted the low velocity anomaly as the extruded mantle flow from the Tibetan Plateau and proposed the
⁎
Corresponding author. E-mail address: [email protected] (Y.V. Fu).
https://doi.org/10.1016/j.pepi.2018.04.002 Received 21 March 2017; Received in revised form 4 April 2018; Accepted 5 April 2018 Available online 06 April 2018 0031-9201/ © 2018 Elsevier B.V. All rights reserved.
mantle extrusion in southeast Tibet. In contrast, Li et al. (2008) and Yang et al. (2015) suggested that the low velocity of TV was related to the subduction beneath the Burma arc. For the west of the Yangtze craton (WYC), there are different features in several models. Li et al. (2008) found fast upper mantle P wave velocity in the WYC. Yao et al. (2008) also imaged a similar fast velocity in the WYC from Rayleigh wave tomography. However, Huang et al. (2015) observed a low velocity anomaly of P wave in the upper mantle of the WYC. The main difference among these studies is the geometry of the velocity anomaly. More information is needed to constrain the anomaly. Surface wave tomography has been widely used to determine the structure of the Earth on both global and regional scales (e.g. Ritzwoller et al., 2002; Bensen et al., 2009; Fu et al., 2010). Most of these studies have concentrated exclusively on Rayleigh wave. The relative weak and complex waveform of Love wave and correspondent method restriction result in the limited application of Love wave data. However, Love wave can provide independent information about the subsurface structure, especially the radial anisotropy. Recent studies have shown that the two-plane-wave (TPW) method could also be used for Love wave through some modifications, although it is originally developed for Rayleigh waves (Li and Li, 2015; Fu et al., 2015; 2016). Fundamental mode Love wave amplitude on the transverse component is
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Fig. 2. Sensitivity kernel for Love wave phase speeds at periods of 25, 50, 71, and 100 s. The kernels are calculated based on the shear wave velocity model AK135 (Kennett et al., 1995) using the method of Saito (1988).
Fig. 1. Map of tectonics and seismic stations in the southeastern Tibetan Plateau. The broadband stations from the temporary ChinArray and the permanent China Digital Seismic Array are indicated by blue and red triangles, respectively. Large white triangle shows the Tengchong volcano (TV). The yellow stars are the locations for the examples in Fig. 5. The blue circle is the station 53202. TP, Tibetan Plateau; BMP, Burma micro-plate; IB, Indochina Block; YC, Yangtze Craton; SC, South China; NIB, northern Indochina Block; WYC, western Yangtze Craton; EYC, eastern Yangtze Craton; RRF, Red River fault; XF, Xiaojinhe fault; XXF, Xianshuihe-Xiaojiang fault.
determined and then used as initial models in obtaining the 2-D phase velocities with the grid interval of 0.5° in the center and 0.75° at the edge. The standard deviation of phase velocity at each grid is calculated from the model covariance matrix. The resulting bandwidth of Love wave phase velocity measurements has the sensitivity to SH wave velocity from the surface to a depth of ∼150 km in the upper mantle (Fig. 2). Phase velocities provide integrated information over a broad depth range (Fig. 2). We take a two-step inversion of Love wave phase velocity for 3-D SH wave velocity structure. We first obtain the 1-D average SH wave velocity model from the average phase velocity dispersion curve. Then, we invert phase velocities at each grid point to obtain 3-D SH wave velocity, taking the 1-D average SH wave velocity model as the initial model. Since the inversion is somewhat non-linear and highly underdetermined due to the limitations of Love wave vertical resolution, the resultant model depend strongly on the initial model. In order to obtain an appropriate initial model for the 1-D inversion, we modified the AK135 reference model (Kennett et al., 1995) based on previous regional models in the study area (Bao et al., 2015; Huang et al., 2015) as the initial model. It contains 16 layers from the surface to 410 km depth with a three-layer crust. The crustal thickness is constrained from receiver functions (Li et al., 2014) and kept as a constant during the inversion since it has a very small effect on the velocity (Fu et al., 2016). We only invert shear wave velocity for each layer. P wave velocity is scaled to shear wave velocity with a constant VP/VS ratio of 1.73. The density does not change in the inversion since its effect on Love wave phase velocity is too small. The layer thickness ranges from 10 to 50 km. The model parameters are slightly damped by assigning prior standard deviations in the diagonal terms of model covariance matrix. A general estimate of these values can be made based on the results obtained from previous inversions of fundamental mode surface wave data. Yu and Mitchell (1979) found that the standard deviation of the velocity structure is on the order of 0.02–0.06 km/s depending on the ray path coverage. Since there are quite few earthquakes from west, we choose to use a conservative value of 0.05 km/s as the errors in shear wave velocity. The correlation between two different depths plays an
decomposed to x and y components in a local coordinate system for each earthquake. The modified TPW method provides a new way to measure long period Love wave phase velocity which is inverted for 3-D SH wave velocity model of the upper mantle. The SH wave velocity from Love wave can provide additional constraints on the lithospheric evolution. With the completion of the ChinArray in this region, phase velocity maps of Love wave with high resolution can be developed to better constrain the structure in the crust and uppermost mantle for the southeastern Tibet. In this study, we extend the work of Han et al. (2017) to SH wave velocity measurement. Han et al. (2017) adopted the modified TPW technique to estimate the Love wave phase velocity at periods of 20–100 s in southeastern margin of the Tibetan Plateau. Fundamental mode Love wave data from the ChinArray which consists of seismic stations at an interval of 35 km combined with the permanent China National Seismic Network is used to construct Love wave dispersion maps on a 0.5° × 0.5° grid. The model presented here is a 3-D volume of isotropic SH wave velocity inverted from the Love wave phase velocity maps (Han et al., 2017). It is the first time to obtain the SH velocity in the upper mantle with high resolution in southeastern margin of the Tibetan Plateau. This new model will provide useful information to estimate the radial anisotropy in the upper mantle and to constrain the deformation mechanism for the southeastern Tibet.
2. Data and method Phase velocity maps at each period are inverted from the amplitude and phase of fundamental mode Love wave recorded by the stations shown in Fig. 1 (Han et al., 2017). The modified TPW method is used to simultaneously solve for the incoming wave field and phase velocity. The 1-D average phase velocities at 12 periods from 20 to 100 s are 16
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With the average shear wave velocity as start value, we perform the same 1-D inversion at each map point. Fig. 5 shows the 1-D SH wave velocity model at three points located in different tectonic units (Tibetan Plateau, NIB and EYC). These models indicate various velocity structures. The shear wave velocity of the crust at the point in the Tibetan Plateau (red line in Fig. 5) is significantly slower than those at the points in the NIB (blue line in Fig. 5) and the EYC (green line in Fig. 5). At the depths of 70–110 km in the upper mantle, the velocity in the EYC (green line in Fig. 5) is much slower than those at the other two points (red and blue line in Fig. 5). The Tibetan Plateau has a slow crust while the eastern part of Yangtze craton has a slow upper mantle. 3.2. 3-D SH wave velocity The maps of shear wave velocity perturbation at six layers from the surface to 110 km in the upper mantle are shown in Fig. 6. Fig. 7 displays three cross sections with locations the same as those of SV wave velocity from Rayleigh wave tomography by Fu et al. (2017) for comparison. The anomalies are relative to the average shear wave velocity at corresponding depths in the entire area (red line in Fig. 4b). A prominent low-velocity anomaly appears in the Tibetan Plateau (TP) in the crust and uppermost mantle (Figs. 6a–d, 7b and c). This observation is consistent with the low velocity of SV wave from ambient noise tomography by Bao et al. (2015) and Rayleigh wave earthquake tomography by Fu et al. (2017). The TV is characterized by slow velocity from lower crust to the depth of 110 km (Figs. 6c–f, 7a and b). The NIB shows a variable velocity pattern with depth from low velocity anomaly at 25–70 km to high velocity anomaly at 70–110 km (Figs. 6c–f, 7b). In contrast, P wave model (Huang et al., 2015) and SV wave model (Fu et al., 2017) found a low velocity extending down to the depth of 150 km beneath the NIB. High velocity anomaly in the upper mantle is imaged beneath the western Yangtze craton (WYC) (Figs. 6d–f, 7a–c). However, the EYC displays different feature, strong low velocity anomaly in the upper mantle with the strength increasing with depth (Figs. 6d–f, 7a–c). Similar pattern of SV wave velocity was observed from Rayleigh wave tomography (Fu et al., 2017). The observation of high VSH beneath WYC in the upper mantle is different from a slow P wave velocity observed by Huang et al. (2015).
Fig. 3. Resolution kernels of shear velocity from the resolution matrix for the reference model at five layers with median depths at 18, 58, 100, 140, and 160 km.
important role in balancing model variance and resolution. We choose a correlation coefficient of 0.4 between adjacent layers in this study after testing several different values. The synthetic phase velocities and partial derivatives of phase velocities with respect to model parameters are calculated using the method of Saito (1988). The inversion seeks to minimize the difference between the observed and model-predicted Love wave phase velocity. The robustness and resolution of shear wave velocity with depth are estimated through the model resolution matrix. 3. Results
4. Discussion
The 1-D average shear velocity model is inverted from the averaged Love wave phase velocity from 20 to 100 s in southeastern margin of the Tibetan Plateau. The depth resolution of shear wave velocity model from such 1-D inversion can be determined according to the resolution matrix. The peak values of the resolution matrix for the 1-D shear wave velocity model generally become smaller at greater depths (Fig. 3), indicating the resolution of shear wave velocity decreases with increasing depth. Although the peak values reduce heavily at a great depth, we can still clearly recognize it at 110 km, suggesting a reasonable resolution at this depth. We therefore focus on the velocity structure above 110 km depth. We perform a series of 1-D inversions with the average shear wave velocity model as a start model to build up the 3-D model.
4.1. Crustal block rotation model or flow model in the southeast TP Geodetic measurements (Chen et al., 2000; Shen et al., 2005) and geological study (Wang and Burchfiel, 2000) reveal a clockwise rotation around the eastern Himalayan Syntaxis, implying the crustal deformation in the southeast TP is dominated by block rotation through major strike-slip faults (Tapponnier et al., 1982). However, seismic tomography (Chen et al., 2014; Bao et al., 2015) and magnetotelluric study (Bai et al., 2010) observe a low velocity/resistivity layer in lower crust, implying a ductile lower crust, suggesting that northward advancement of the Indian plate results in broadly distributed deformation of southeastern Tibet. These two models suggest rather different crustal deformation patterns. Our SH wave velocity result shows various geometry of the low velocity zone with the depth in the crust. These low velocity could result from the elevated temperature and/or partial melt since the regional surface heat flow is high (Hu et al., 2000). In the upper and middle crust, the channel like low velocity zones are observed along the major faults (XF, XXF and RF) (Fig. 6a and b). However, the low velocity in the lower crust becomes broad (Fig. 6c). Our observation agrees with that of SV velocity from ambient noise (Bao et al., 2015) and Rayleigh wave data (Fu et al., 2017). The transformation of the geometry for the low SH wave velocity anomalies from channel-like to broad region probably indicate different deformation pattern in the middle and lower crust. The coexistence of the block rotation model and crustal flow model could occur
3.1. 1-D SH wave velocity The final 1-D reference model is obtained by inverting the average phase velocities in the study area (red line in Fig. 4b). The average SH wave velocity model is well constrained with standard deviation from 0.025 to 0.035 km/s. The predicted dispersion curve from this model fits the Love wave observations well (red line in Fig. 4a). Compared with the SV wave velocity model from Rayleigh wave (Fu et al., 2017, blue line in Fig. 4b), SH wave velocity is significantly larger in the middle and lower crust, and at the depths of 90–110 km, implying the existence of strong positive radial anisotropy (VSH > VSV). 17
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Fig. 4. Love wave average dispersion curve and associated best fitting model (red). The correspondent results of Rayleigh wave (blue) are from Fu et al. (2017) for comparison. (a) Observed (dots) and predicted (line) dispersions from the best fitting model. Error bars represent two standard deviations. (b) Shear wave models. The black line represents a modified AK135 model used as an initial model in the inversion. The width of the shaded area shows the standard error of isotropic shear wave velocity in each layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) Observed (triangles) and predicted dispersion curves and (b) associated best fitting models at three grid points that are shown in Fig. 1 (yellow stars). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Rayleigh and Love wave, respectively. Our high SH wave velocity is imaged at the depths of 70–110 km in the NIB, while Fu et al. (2017) observed slow SV wave velocity from Rayleigh wave tomography. Positive radial anisotropy (VSH > VSV) probably exist below the depth of 70 km. Three scenarios have been proposed to explain the slow SV and P wave velocity (Huang et al., 2015; Fu et al., 2017; Zhang et al., 2017). The first two are both about the subduction of Indian plate beneath the Burma. The difference of these two models is the existence of slab tearing. Huang et al. (2015) suggests that the fluid-induced partial melting from the stagnant slab in the transition zone results in the slow SV and P wave velocity (Huang et al., 2015), while Zhang et al. (2017) propose that the low seismic velocity anomaly is probably caused by the upwelling of hot and buoyant sub-lithospheric mantle that has been entrained beneath the sinking lithosphere of the Indian plate and is now escaping through the slab tearing. In these two cases, vertical flow should exist and therefore negative radial anisotropy (VSH < VSV) could be observed. The third possible cause for the low velocity is the upwelling of warm mantle during the lithospheric extension associated with the slab rollback of the Australian plate beneath the Sumatra trench (Yang et al., 2015).
simultaneously in the southeast TP at different depths. The north-south shortening of the Tibetan Plateau is accommodated by block extrusion at shallow depth and diffused deformation by thickening and weakening of the lower crust. How these two factors interact and what the geometry or inter-connectivity of low velocity zone is require further quantitative geodynamic modeling. 4.2. Positive radial anisotropy (VSH > VSV) at depths of 70-110 km beneath the NIB Radial anisotropy is attributed to the lattice preferred orientation of anisotropic materials caused by strains associated with deformation. It can provide some of the most direct evidence for deformation and flow within the Earth's interior. Radial anisotropy manifests itself as the difference in the speeds of horizontally and vertically polarized, horizontally propagating S waves (VSH and VSV, respectively). It is inferred from the simultaneous analysis of dispersion of Love and Rayleigh wave. Since Rayleigh wave is mainly sensitive to SV wave and Love wave is to SH wave, we can qualitatively determine the radial anisotropy by comparing the VSV and VSH which are separately inverted from 18
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Fig. 6. Maps of shear wave velocity perturbation in the crust and upper mantle. The perturbation is relative to the average value of each layer indicated in the lower left corner of each map.
The horizontal flow due to the slab rollback produces the lithospheric extension and positive radial anisotropy (VSH > VSV) beneath the NIB. Our observed high VSH and slow VSV support this hypothesis.
velocity anomalies with various geometry at different depths in the crust are imaged in the Tibetan Plateau. This variation from channellike at shallow depths to a broadened pattern in the lower crust could suggest that the crustal deformation is dominated by block rotation in the upper part and ductile flow in the lower part. The high SH wave velocity anomaly and low SV wave below 70 km depth in the northern Indochina block imply the existence of positive radial anisotropy, reflecting horizontal layering of anisotropic materials due to the past and recent rifting activities associated with the slab rollback along the Sumatra-Andaman Sea subduction zone. More quantitative analysis of the radial anisotropy is needed to constrain the geodynamics of this region in the future.
5. Conclusions The 3-D isotropic SH wave velocity model of the crust and upper mantle beneath the southeastern margin of the Tibetan Plateau is obtained from the fundamental mode Love wave phase velocities at the periods of 20–100 s, determined by the modified two-plane-wave method presented previously by Han et al. (2017). It is the first time the SH wave velocity structure has been constrained from the surface to approximately 110 km depth in the study area. Strong low SH wave 19
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Fig. 7. Vertical profiles of shear wave velocity perturbation in the crust and upper mantle. The locations of the profiles are shown in Fig. 6a. TP, Tibetan plateau; TV, Tengchong volcano; NIB, northern Indochina block; WYC, western Yangtze craton; EYC, eastern Yangtze craton.
Acknowledgments Data are from the Institute of Geophysics, China Earthquake Administration and requested through [email protected]. We thank the participants in the ChinArray for collecting the data. Most figures in this paper were made with General Mapping Tools (Wessel and Smith, 1998). Comments from the editor and two anonymous reviewers helped improve the manuscript. This work was supported by grant 41574042 from the National Natural Science Foundation of China. References Bai, D., Unsworth, M.J., Meju, M.A., Ma, X., Teng, J., Kong, X., Sun, Y., Sun, J., Wang, L., Jiang, C., Zhao, C., Xiao, P., Liu, M., 2010. Crustal deformation of the eastern Tibetan Plateau revealed by magnetotelluric imaging. Nat. Geosci. 3, 358–362. Bao, X., Sun, X., Xu, M., Eaton, D.W., Song, X., Wang, L., Ding, Z., Mi, N., Li, H., Yu, D., Huang, Z., Wang, P., 2015. Two crustal low-velocity channels beneath SE Tibet revealed by joint inversion of Rayleigh wave dispersion and receiver functions, Earth Planet. Sci. Lett. 415, 16–24. Bensen, G.D., Ritzwoller, M.H., Yang, Y., 2009. A 3-D shear velocity model of the crust and uppermost mantle beneath the United States from ambient seismic noise. Geophys. J. Int. 177, 1177–1196.
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