Three-Dimensional S-Velocity Structure of the Crust in the Southeast Margin

Journal of Asian Earth Sciences 148 (2017) 210–222

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Journal of Asian Earth Sciences

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Full length article Three-dimensional S-velocity structure of the crust in the southeast margin MARK of the Tibetan plateau and geodynamic implications ⁎ Hengchu Penga, Haiyan Yanga, Jiafu Hua, , José Badalb a Department of Geophysics, Yunnan University, 2 North Green Lake Rd., Kunming, Yunnan 650091, PR China b Physics of the Earth, Sciences B, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain

ARTICLE INFO ABSTRACT

Keywords: The lower crustal flow model is one of several competing models considered to interpret the growth and ex- P-wave receiver functions pansion of southeastern Tibet. However, the dynamic processes involved in the evolution and deformation of the Two-step inversion Tibetan plateau remain poorly understood due to the absence of reliable geophysical observations. In this study, Bootstrap technique 159 earthquakes of magnitude Ms ≥ 6.2 were selected, which were recorded by 50 permanent broadband Crustal shear velocity structure stations deployed in southwest China, and 1873 pairs of P-wave receiver functions with high signal-to-noise ratio Intracrustal low-velocity zone were isolated. On the basis of a linearized inversion algorithm, a two-step inversion procedure was implemented Lower crustal flow Southeastern margin of Tibet that not only reduced the dependence of the inversion results on the initial models, but also provided a statistical estimation of the solution. Thus, we obtained an accurate 3D image of the S-wave velocity structure of the crust and uppermost mantle in the southeast margin of the Tibetan plateau. Our results reveal that an extensive intracrustal low-velocity zone spreads beneath southwest China, and further suggest that a lower crustal flow coming from eastern Tibet is blocked by the Jinshajiang-Red River fault to the west and the Xiaojiang fault to the east and extends largely by the Sichuan-Yunnan diamond-shaped block. This crustal flow along the southeastern margin of Tibet does not appear to be restricted to two narrow low-velocity channels as was advanced, but rather reflects the southeastward extrusion of crustal material as a way of tectonic escape.

1. Introduction blocks along large strike-slip faults (Molnar and Tapponnier, 1975; Tapponnier et al., 1982, 2001), (2) continuous deformation (Houseman In the last 45 Ma the convergence between India and Eurasia has and England, 1986; Yang and Liu, 2013), and (3) lower crustal flow caused at least 1500 km of shortening and dramatic thickening (Royden et al., 1997; Clark and Royden, 2000; Shen et al., 2001). The (∼80 km) of the crust beneath the collision zone (Molnar and lower crustal flow model, which suggests that the ductile lower crust Tapponnier, 1975; Armijo et al., 1986; England and Molnar, 1997), and flows from central Tibet to SW China (Royden et al., 1997, 2008; Clark the topography has uplifted by more than 4 km. Southwest China and and Royden, 2000; Klemperer, 2006), can reasonably explain the ab- specifically the southeastern margin of Tibet is a tectonic transition sence of substantial shortening of the young upper crust in southeastern zone between the uplifted Tibetan plateau to the west and the stable Tibet. Nevertheless, this model is still a subject of lively debate due to Yangtze Platform to the east (Fig. 1) and plays a key role in accom- the non-uniqueness and limited resolution of the geophysical observa- modating the eastward expansion of the plateau. Global Positioning tions. System (GPS) observations indicate that the upper crust movement in In general, a mechanically weak lower crust can be reflected by the this area bifurcates due to the resistance of the cold and rigid Sichuan geophysical observations of an intracrustal low-velocity zone (hereafter

Basin. One movement is northeastward along the western margin of the named IC-LVZ), low electric resistivity, high VP/VS ratio and high heat basin, while the other is southeastward and extends into the Sichuan- ﬂow, all of which implies the existence of partial melting and reduced Yunnan diamond-shaped block. This latter movement of the upper crust viscosity (Chen et al., 2015a,b). Low-velocity zones in the lower crust in then undergoes a clockwise rotation around the Eastern Himalayan SW China have been detected by deep seismic soundings (Zhang et al., Syntaxis (Gan et al., 2007). To explain the mechanism of surface de- 2005a,b; Wang et al., 2007), body wave tomography (Wang et al., formation in the southeastern margin of Tibet, several models have 2003), surface wave tomography (Yao et al., 2008, 2010; Zheng et al., been proposed, including the following: (1) lateral extrusion of rigid 2017) and inversion of P receiver functions (Xu et al., 2007; Li et al.,

⁎ Corresponding author. E-mail addresses: [email protected] (J. Hu), [email protected] (J. Badal). http://dx.doi.org/10.1016/j.jseaes.2017.09.004 Received 10 March 2017; Received in revised form 1 September 2017; Accepted 2 September 2017 Available online 07 September 2017 1367-9120/ © 2017 Elsevier Ltd. All rights reserved. H. Peng et al. Journal of Asian Earth Sciences 148 (2017) 210–222

Fig. 1. Topographic map showing the broadband stations deployed in southeastern China (black and red inverted triangles) and the major regional faults (solid lines). All stations are identified by a three-letters code. In particular, the red triangles denote stations revealing an intracrustal low-velocity zone. Right lines A-A′, B-B′ and C-C′ illustrate the locations of the three vertical cross-sections included in Fig. 10. The yellow areas delimit the two crustal flow channels mapped by joint inversion of receiver functions and surface wave dispersion (Bao et al., 2015). The rec- tangle in the inset in the top left corner shows the location of the study region and the major faults (red lines) across the southeastern Tibetan plateau and nearby zones. White arrows describe the crustal flow channels obtained by magneto-telluric profiling (Bai et al., 2010). Abbreviations: F1, Longmenshan fault; F2, Xianshuihe fault; F3, Lijiang- Jinhe fault; F4, Xiaojiang fault; F5, Jinshajiang-Red River fault; F6, Lancangjiang fault; F7, Jiali-Nujiang fault; F8, Sagaing fault; EHS, Eastern Himalayan Syntaxis; SYDSB, Sichuan-Yunnan diamond-shaped block.

2008). These studies suggest that there exists a large-scale low-velocity Lijiang-Jinhe fault (Chen et al., 2013)? (2) Is there a large-scale crustal zone in the lower crust beneath SE Tibet. Chen et al. (2013) argued that flow to the west of Jinshajiang-Red River fault (Bai et al., 2010; Bao weak materials of the lower crust within the plateau flow south- et al., 2015; Li et al., 2016)? (3) Are there actually two crustal low- eastward and reach only up to the Lijiang-Jinhe fault. However, recent velocity channels driving the lower crustal flow throughout the south- studies, which utilize techniques such as magnetotelluric measurements eastern margin of Tibet (Bai et al., 2010; Bao et al., 2015; Li et al., (Bai et al., 2010) and joint inversion of receiver functions and Rayleigh 2016)? wave dispersion (Bao et al., 2015; Li et al., 2016), have emphasized that the lower-crustal flow occurs only along the Xianshuihe-Xiaojiang fault and the Jiali-Nujiang fault to the east and southeast of Tibet, respec- 2. Tectonic framework tively (see Fig. 1). Our study region is adjacent to Myanmar and the Eastern Himalayan Syntaxis (EHS, Fig. 1). Previous studies (Xu et al., Tapponnier et al. (1982, 1986) suggested that the northward motion 2007; Li et al., 2008; Liu et al., 2014; Bao et al., 2015) have indicated of the Indian plate, relative to the Eurasian plate, caused the south- that the crustal thickness of this region decreases dramatically eastward extrusion (with respect to South China) of large continental from > 60 km in the north to ∼30 km in the south. However, due to fragments. The eastern Tibetan region developed a series of Cenozoic the limitations of the inversion algorithms and its dependence on initial strike-slip faults that divide the eastern Tibet margin into several tec- models, it sometimes happens that different or even contradictory re- tonic blocks and accommodate the collisional strain, such as the Xian- sults may be provided for the same study region. shuihe fault, Lijiang-Jinhe fault, Xiaojiang fault, Jinshajiang-Red River To address the evidence for lower crustal flow, first we isolated P- fault, Lancangjiang fault, Jiali-Nujiang fault and Sagaing fault (Fig. 1). wave receiver functions from the available seismic data recorded by the The early deformation of the southeastern margin of Tibet is dominated permanent broadband stations deployed in SW China (Yunnan region by the lateral extrusion of Indochina along the Jinshajiang-Red River and adjacent areas). We then developed a two-step inversion approach fault, and possibly by the later extrusion of the South China block along to reduce the dependence on the initial model and so obtain a more the reactivated Red River fault (Schoenbohm et al., 2006). However, ca. – fl accurate solution. Third, we estimated the uncertainty of the solution 9 13 Ma (Clark et al., 2005b), the lower crustal material began to ow using a bootstrap technique (Efron and Tibshirani, 1991). Our interest into the southeastern margin of Tibet, causing crustal thickening and is focused on giving a precise answer to each of the following questions: passive surface uplift (Clark and Royden, 2000). During the last 4 Ma, (1) Is the lower crustal flow coming from central Tibet truncated by the the crust to the west of the Xianshuihe-Xiaojiang fault system has been undergoing a clockwise rotation relative to the crust in South China. As

211 H. Peng et al. Journal of Asian Earth Sciences 148 (2017) 210–222 shown in Fig. 1, our study region is situated at the junction of the Te- thys-Himalayan tectonic domain and the circum-Pacific tectonic domain. The Jinshajiang-Red River fault in eastern Tibet (F5 in Fig. 1)isa primary fault that consists of several segments trending to the north- west. This fault has one end on the east side of EHS and extends southeastward to Vietnam (Wang et al., 1998). The Xianshuihe fault (F2), with a long-term slipping rate of 7.5–11.1 mm/yr (Zhang, 2013), originated in eastern Tibet and connects with the Xiaojiang fault (F4) on its southeast end. The left-lateral Xianshuihe-Xiaojiang fault system is one of the most active faults in southeastern Tibet. Several strong earthquakes of magnitude greater than 7.0 have occurred throughout this fault system during the last 300 years (Allen et al., 1991), the largest being the 1833 Ms. 8.0 Songming earthquake, which caused a ∼90-km-long surface rupture. The left-lateral Xianshuihe-Xiaojiang fault and the right-lateral Jinshajiang-Red River fault delimit the Sichuan-Yunnan diamond- shaped block (hereafter named by its acronym SYDSB), which is regarded as the main escape block from central Tibet (Wang and Burchfiel, 2000; Tapponnier et al., 1982, 1986). GPS observations (Gan et al., 2007) have confirmed that SYDSB appears to move southeastward more rapidly than the adjacent crust. As one of the most seismically active areas in China, SYDSB is divided into two parts by the Fig. 2. Locations of the teleseismic events used in this study on a worldwide map. The Lijiang-Jinhe fault (F3), which is a large transverse fault trending to the earthquakes with epicentral distance between 30° and 90° are used for computation of northeast with a slipping rate of ∼3 mm/yr (Zhang, 2013). receiver functions. The dark gray circles indicate the events, while the black triangle in fi The right-lateral Sagaing fault in Myanmar (F8, see inset in Fig. 1) the center of the gure marks the approximate intermediate location of the seismic stations used. acts as the subduction boundary of the Burma micro-plate and South- west China, and the west boundary of the Tengchong volcanic region (close to station tec, see Fig. 1). This volcanic region plays a key role in radial component seismogram, the P-to-S conversions and multiples the transition from compression in the Himalayas to clockwise rotation produced by the interfaces beneath the receiver can be isolated. By around EHS (Xu et al., 2015a,b) and is characterized by rift-related modeling the timing and amplitude of the converted arrivals, a least- volcanic activity, a high geothermal gradient and low seismic-wave squares inversion of the receiver functions can constrain the velocity velocity in the crust and uppermost mantle (Lei et al., 2009). With 68 contrast and depth of each interface (Langston, 1979; Ammon and volcanoes, 145 hot springs and 25 volcanic craters and cones (Jiang, Randall, 1990). Due to the strong non-linearity inherent to the geo- 1998; Jiang et al., 2003), this region has preserved abundant records of physical inversion problem, the linearized inversion method is ex- the tectonic-magmatic evolution that reflect the continental breakup, tremely sensitive to the choice of the initial model (Ammon and subduction, collision, and the post-collision process. Randall, 1990). Such dependency is quite relevant for solving the problem of inversion of receiver functions, which can be largely de- 3. Data, method and synthetic experiments termined by the frequency (Cassidy, 1992). A low-frequency (α = 1.0) receiver function mainly presents the conversions from the Moho, so an 3.1. Data acquisition initial model with limited a priori information can constrain the non- uniqueness of the inversion. Although the result of the inversion of this All seismic data used in this study were collected by means of 50 low-frequency receiver function is imprecise, it contains sufficient permanent broadband stations (Fig. 1) deployed in SW China since constraints as to invert the corresponding high-frequency (α = 2.5) 2000. We selected 159 earthquakes of magnitude Ms. ≥ 6.2 and with receiver function. Therefore, a two-step inversion method can be pro- epicentral distances between 30° and 95° (Fig. 2), and we adopted an posed to reduce the dependence on the initial model. First, a tentative iterative approach in the time domain (Ligorría and Ammon, 1999)to solution can be obtained from a low-frequency receiver function to isolate P receiver functions. Compared with the frequency-domain constrain the overall crustal and upper mantle structure (first step of the water-level approach (Clayton and Wiggins, 1976), the iterative de- two-step approach). Second, using this solution as the initial model, we convolution method in the time domain is more stable for noisy data. can refine the previous structural details by inverting the corresponding Moreover, a Gaussian low-pass filter, whose bandwidth is controlled by high-frequency receiver function (second step of the two-step ap- the Gaussian parameter α, is introduced to reduce the amplifi cation of proach). the high-frequency signal when the deconvolution method is applied (Langston, 1979; Ammon, 1991). As a consequence of the two-step 3.3. Numerical experiments inversion procedure introduced in the following section, we calculated both low-frequency (α = 1.0) and high-frequency (α = 2.5) receiver As an example, we took a relatively simple seismic velocity model functions for the same teleseismic event. To guarantee the data quality, (black solid line in Fig. 3A). The response of this layered structure to a we manually checked each waveform and discarded the receiver nearly vertical incident P-wave is calculated by the reflection matrix functions with weak PS or indistinguishable reverberations at the Moho. approach (Kennett, 1983). The low-frequency (α = 1.0) and high-fre- In this way we achieved 1873 pairs of receiver functions. quency (α = 2.5) P receiver functions were isolated by iterative deconvolution in the time domain (Ligorría and Ammon, 1999) and re- 3.2. Two-step inversion of receiver functions garded as observed waveforms (Fig. 3B and C). Because of the low frequency, only the conversion phases generated at the Moho can be Receiver function analysis (Langston, 1979) is a straightforward and correctly reflected by the receiver function with α = 1.0, while the commonly accepted method to study crustal and upper-mantle struc- intracrustal arrivals are weakened or even unrecognizable (solid curve ture. By deconvolving the vertical component seismogram from the in Fig. 3B).

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Fig. 3. Results obtained from a synthetic experiment. (A) True model (black solid line), initial two-layer model (gray solid line) and solutions obtained by two-step inversion: after the first step (black dotted line) and after the second step (red solid line). The gray dashed line represents the model obtained by one-step inversion. (B) and (C) Fitting of the observed and synthetic low-frequency (α = 1.0) and high-frequency (α = 2.5) waveforms corresponding to the first step (B) and the second step (C) of the two-step inversion, respectively. (D) Fitting of the observed and synthetic high-frequency (α = 2.5) waveforms after one-step inversion. The parameter α controls the bandwidth of the Gaussian low-pass filter used in the calculation process.

We started from a simple initial two-layer model consisting of a 40- the solution obtained by one-step inversion (gray dashed line in km-thick uniform crust over a homogeneous half-space (gray solid line Fig. 4A) deviates widely from the true model and is comparatively in Fig. 3A). The Vs velocity values for the crust and upper mantle were worse. According to the results obtained from such a multilayered earth set to the global averages of 3.5 km/s and 4.7 km/s, respectively. The model, it is evident that the two-step inversion method gives greater H-κ algorithm (Zhu and Kanamori, 2000) was applied to estimate the reliability than the one-step inversion method. Moho depth. The whole structure was divided into a series of 2-km- thick layers for modeling. Other mechanical parameters were de- 3.4. Valuation of uncertainty termined with the help of empirical relationships (Berteussen, 1977). Using the linearized inversion algorithm (Ammon and Randall, 1990), A seismic station is usually able to provide more than one receiver the solution obtained from the low-frequency receiver function function for inversion. The amplitude and relative time delay of a (α = 1.0) after the first step of the two-step procedure already seems a converted phase are affected not only by the velocity structure below suitable result (black dotted line in Fig. 3A). This solution was used as the station, but also by the ray parameter and back azimuth of the in- the initial model for the second step of the two-step inversion, which coming wave. Therefore, a group of receiver functions recorded at one allowed determine the refined solution and therefore a more accurate station always exhibits undeniable diversity. There are two popular Vs velocity structure, i.e. closer to the true model (red and black solid methods of dealing with multiple waveforms recorded at one station. lines in Fig. 3A). After this, the observed and synthesized high-fre- One of them is to use a single velocity model to fit all the individual quency (α = 2.5) waveforms show an excellent fit(Fig. 3C). Using the receiver functions (Sun et al., 2014). If the azimuthal anisotropy of the same simple initial model (gray solid line in Fig. 3A), the solution ob- study region is weak and all seismic events have similar epicentral tained directly by one-step inversion of the high-frequency receiver distances, then a unique velocity model can explain the main phases function (gray dashed line in Fig. 3A) is obviously different from the generated from the Moho leaving aside the intracrustal conversions. If true model. Even so, the fit between the observed and synthesized high- not, the other method is to stack all waveforms recorded at one station frequency (α = 2.5) waveforms still reaches 98% (Fig. 3D). in a single receiver function for inversion (Xu et al., 2007; Bao et al., Ammon and Randall (1990) introduced a more complex velocity 2015). But for a given interface, both the amplitude and relative time model that included a 4-km-thick low-velocity layer (black solid line in delay of a converted arrival vary with the incidence angle of the wave, Fig. 4A) to demonstrate the non-uniqueness of the linearized inversion. which is controlled by the slowness. This forces to eliminate the de- Here, we also used this model as the true model to test the reliability of pendence of the traveltime on the epicentral distance or the slowness, the two-step inversion method. Using again the simple initial two-layer which is known as moveout correction (Dueker and Sheehan, 1997, model (gray solid line in Fig. 3A), we repeated all the previous opera- 1998). Unlike the time delay, the amplitude is inaccessible. So, before tions involved in this similar numerical experiment. The corresponding stacking waveforms, the time delay variation with the slowness (or the velocity profiles, as well as the observed and synthesized waveforms are epicentral distance) must be corrected using a reference epicentral shown in Fig. 4A–D, respectively. As can be seen, the two-step inversion distance. However, the difficulty in setting the value of the ray para- method provided satisfactory results (red solid line in Fig. 4A), while meter to perform moveout correction and then the stacking of

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Fig. 4. Results obtained from a similar synthetic experiment. Same legend as in Fig. 3. waveforms persists, still more when not all the selected earthquakes interval determined after the second inversion (vertical strokes on both have comparable epicentral distances (Fig. 2). Taking into account the sides of the highest bar of each histogram in Fig. 7) is obviously nar- above reasons, a suitable way to make the best use of multiple wave- rower than that given by the first one. Although the 35 individual so- forms recorded at a specific station is to invert individual signals and lutions obtained after the second inversion step (Fig. 6B) seem to vary evaluate the corresponding solutions statistically. within a more or less wide range, they are statistically coherent because Since the size of the data set for each seismic station is generally of the narrow confidence interval deduced after the second inversion insufficient to statistically reveal the characteristics of the elastic step (Figs. 6C and 7). Our results reveal the Moho at ∼58 km depth and medium due to a limited observation period, we adopted a bootstrap two intracrustal low-velocity layers, all consistent with the result ob- method (Efron and Tibshirani, 1991) to expand the sample size. Taking tained by joint inversion of receiver functions and Rayleigh wave dis- station lij as reference (Fig. 1), we identified the original seismic data persion (Li et al., 2008). that had a clear direct P-wave and signal-to-noise ratio ≥20.0. We calculated the receiver functions with α = 1.0 and α = 2.5, checked 4. Results manually the quality of each waveform, and eventually acquired 35 pairs of receiver functions (Figs. 5A and 6A). As with inversion of The lateral distribution of the crust thickness in SW China is shown synthetic receiver functions, the same initial two-layer model was used in Fig. 8, where the depth iso-lines are contours calculated by spline with real data, consisting of a uniform crust over a homogeneous half- interpolation performed with the help of the Generic Mapping Tools space. The Vs velocity values for the crust and upper mantle were set to (GMT, Wessel and Smith, 1998). Therefore, the depth values near the the global averages and the H- κ algorithm was applied to estimate the margins of the study region are not reliable and are not taken into Moho depth. To meet the first inversion step, we individually inverted account in our analysis. Our results indicate that the Moho depth varies low-frequency (α = 1.0) receiver functions (Fig. 5B). Then, we re- between ∼30 km in the south and > 60 km in the north of SW China, sampled the Vs velocity of each layer with replacement, so that the size and are consistent with previous results (Xu et al., 2007; Bao et al., of the new sample is the same as that of the original data set, and we 2015). Furthermore, there is a relatively wide transition belt at ap- calculated the mean value of VS (Monte Carlo algorithm, Davison and proximately 26°N, through which the crust thickness varies by ap- Hinkley, 1997). We repeated the resampling routine 1000 times and proximately 15–20 km. calculated the expectation, standard deviation and confidence interval The inversion results also indicate the existence of IC-LVZs beneath for the 1000 re-samples. Then, the statistically most accurate solution 20 stations (Fig. 1), 17 of them located in the interior of SYDSB or in the (red solid line in Fig. 5C) was used as the initial model to carry out the surroundings, implying that the Xiaojiang fault (F4) and the Jin- second inversion step, i.e. to individually invert high-frequency shajiang-Red River fault (F5) are possibly two natural tectonic bound- (α = 2.5) receiver functions (Fig. 6A and B). The same resampling aries that are blocking and driving the lower crustal flow coming from process was executed once again to estimate the optimal VS velocity central Tibet. To the west of F5, there are only 3 stations (tec, bas and structure beneath the reference station (red solid line in Fig. 6C). yul) with IC-LVZs, which are close to the Tengchong volcanic region. In order to support the reliability of the statistic results, we show the There is not any IC-LVZ below the stations to the east of F4 (Fig. 1). probability density distributions of S-velocity related to four crustal The lateral variation of VS velocity at increasing depths of 6, 12, 18, fi layers at increasing depth. All results t well to the normal distribution 24, 30 and 38 km is shown in Fig. 9. The pattern of VS velocity dis- (Fig. 7). In addition, for each of the selected layers, the confidence tribution is quite complex and varies considerably with depth. At 6 and

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Fig. 5. Waveforms obtained after the first step of the two-step inversion of the low-frequency receiver functions (α = 1.0) recorded by station lij (see Fig. 1) and assessment of uncertainty. (A) Fitting between individual observed receiver functions (black color-filled curves) and synthetic receiver functions (black dashed curves). (B) Set of S-velocity models inverted from the individual receiver functions. (C) Statistically most accurate solution (best S-wave velocity model, red line) obtained by the bootstrap technique (Efron and Tibshirani, 1991) and 95% confidence interval (blue lines).

12 km depths, the distribution of the smallest VS values (< 3.3 km/s) is of ∼10 km, but is < 10 km around the Tengchong volcanic region. coherent and mainly represents the thick sedimentary cover in the However, at 38 km depth, this IC-LVZ is obviously blocked by the basins of the region, such as the Kunming Basin (hlt), Tonghai Basin Jinshajiang-Red River fault. On the other hand, the low VS velocity (toh), Baoshan Basin (bas), Yongsheng Basin (yos) and Heqing Basin around the Xiaojiang fault extends from the surface down to ∼40 km

(heq). In addition, the high VS velocity (∼3.8 km/s) around station tec depth, suggesting that this fault could deepen up to the Moho. is likely related to the solidiﬁed magma intrusions and the residual Fig. 10 presents three velocity cross-sections that cut the study re- magma in the interior of the cooled volcanic material channels. The gion vertically from the surface to the upper mantle. These sections southwestern border of the Sichuan Basin is characterized by low-ve- show the spatial correlation between the crustal thickness, the low- locity anomalies that extend from the surface down to ∼18 km depth velocity zones and the major strike-slip faults. Along the proﬁle A-A′,it due to the presence of thick sediments. At 18 and 24 km depths, even at is clear that the crustal thickness varies from ∼40 km at Lushui (lus) to

30 km, most of the southern SYDSB exhibits high VS velocity (> 3.7 ∼60 km in the middle of SYDSB, then decreases to ∼40 km in the km/s) except around the Xiaojiang fault. At ∼38 km depth, a low-ve- southwest margin of the Sichuan Basin. Two low-velocity zones are locity anomaly (< 3.5 km/s) spreads clearly by the central part of revealed by this proﬁle; one is at ∼23 km depth to the west of the SYDSB, and seems to extend southwestward following the course of the Jinshajiang-Red River fault (F5) and the other is at ∼35 km depth to Jinshajiang-Red River fault. This low-velocity layer reaches a thickness the east of this fault. The proﬁle B-B′ also shows that these two IC-LVZs,

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Fig. 6. Waveforms obtained after the second step of the two-step inversion of the high-frequency receiver functions (α = 2.5) recorded by station lij (see Fig. 1) and assessment of uncertainty. Same legend as in Fig. 5. which differ by about 10 km in depth, are still separated by F5. The IC- used, P receiver functions can resolve 2–5-km-thick layers, with the LVZ to the west, which may be a magma chamber, originates at ∼10 reverberations covering laterally a radius of approximately 1–1.5 times km depth and extends down to the lower crust in the Tengchong vol- the depth of the reflecting interface (Cassidy, 1992). The combined use canic region (around station tec). Beneath this IC-LVZ, the VS velocity of the bootstrap technique with the two-step inversion method has al- in the upper mantle is rather low (4.1–4.2 km/s), and the Moho is in- lowed us to reduce the dependence of the solution on the initial model distinguishable. In contrast, further south (profile C-C′), both the upper and to evaluate the uncertainty of the solution. and lower crust appear well defined, and the Moho depth varies gra- The lower crustal flow model can provide a satisfactory inter- dually from west to east between ∼34 km and ∼48 km. pretation for the topography of the eastern margin of Tibet (Royden et al., 1997, 2008; Clark and Royden, 2000; Klemperer, 2006). Rock mechanics laboratory experiments predict that there may be a weak 5. Discussion ductile zone within the middle or lower crust due to a thick crust or moderately high geothermal gradient (Goetze and Evans, 1979; Brace Reliable data and accredited working methods are inescapable and Kohlstedt, 1980; Kirby, 1983). Our results make clear the existence prerequisites to develop quality geophysical research. In our case, all 50 of a wide and complex IC-LVZ in SW China, which agrees with the permanent broadband seismic stations deployed in SW China are in- findings from previous studies (Xu et al., 2007; Yao et al., 2010; Bao stalled on bedrock or hard soil within tunnels and are rarely disturbed et al., 2015). Nevertheless, this extensive IC-LVZ is not found at a by human activities, which is a guarantee of data with higher signal-to- constant depth in the study region, so the ∼10 km depth fluctuation noise ratio than those provided by portable stations. As for the method

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Fig. 7. Probability density distributions of S-velocity related to four crustal layers below station lij (see Fig. 1) at increasing depth (numbered 1, 9, 16 and 25). Two panels illustrate the results for each layer: the left one corresponds to the first step of the two-step inversion (α = 1.0), while the right one corresponds to the second step of the two-step inversion (α = 2.5). The parameter α controls the bandwidth of the Gaussian low-pass filter used in the calculation process. The envelope curve in each panel represents the normal distribution after data adjustment. The two vertical strokes on both sides of the highest bar of each histogram mark the 95% confidence interval, and σ is the standard deviation. implies different origins of the two IC-LVZs on both sides of the Jin- earthquake in northern Burma (21.00°N, 99.70°E) on September 22, shajiang-Red River fault (profile A-A′ in Fig. 10). P-wave tomography 1965 (black stars in Fig. 8). Also, the Ms. 6.6 earthquake occurred in (Lei et al., 2009) suggests that the IC-LVZ below the Tengchong vol- northern Laos (20.50°N, 100.80°E) on May 16, 2007, was followed by canic area is attributable to the eastward subduction of the Indian plate, the Ms. 6.4 earthquake in Ninger County (23.05°N, 101.02°E) on June rather than to the expansion of eastern Tibet. More recently, surface 3, 2007 (black circles in Fig. 8). These pairs of earthquakes with similar wave tomography (Wu et al., 2016) has revealed the existence of a magnitudes and dates may be reflecting the same dynamic processes. prominent low-velocity anomaly at mantle depths of ∼50–80 km, Seismicity is certainly high in SW China, according to the spatial dis- which could be caused by the upwelling of deep mantle materials. Our tribution of Ms. ≥ 6.0 earthquakes (1500 BC ∼ 2013, see Fig. 9); but results also reveal a low-velocity transition zone at the top of the upper however such pairs of earthquakes do not occur to the east of the Jin- mantle (profile B-B′ in Fig. 10). Therefore, the IC-LVZ to the west of the shajiang-Red River fault, implying that the IC-LVZs on both sides of this Jinshajiang-Red River fault may be related to the upwelling of hot fault may not to be dynamically related. In this situation, the IC-LVZ to material originating from the upper mantle (profile B-B′ in Fig. 10) and the west of the Jinshajiang-Red River fault (Bao et al., 2015; Li et al., not with a lower crustal flow throughout SYDSB, as some previous 2016) does not appear to be a trace of the crustal flow coming from studies argue (Bai et al., 2010; Bao et al., 2015; Li et al., 2016). eastern Tibet. To this respect, geological studies already highlight the Jinshajiang- Another IC-LVZ is observed further east and also along the Xiaojiang Red River fault as the natural tectonic boundary separating the fault (F4 in Fig. 1), which is consistent with the information given by Indochina block from the South China block (Tapponnier and Molnar, some seismic profiles that cut this fault (Chen et al., 2015a,b; Xu et al., 1977; Allen et al., 1984; Leloup et al., 2006). In fact, besides the low 2015a,b), but inconsistent with the existence of two crustal low-velo- electric resistivity observed in the zone (Bai et al., 2010), significant city channels driving the lower crustal flow throughout the southeast differences in crustal velocity on both sides of this fault have been margin of the Tibetan plateau (Bai et al., 2010; Bao et al., 2015; Li et al., found through many other geophysical studies based on body wave 2016). All broadband stations where low velocity has been detected tomography (Huang et al., 2002; Wang et al., 2003), surface wave to- (red triangles in Fig. 1) delimit this area geometrically. This picture mography (Yao et al., 2008), joint inversion of receiver functions and suggests that a large-scale crustal flow has penetrated across the Li- Rayleigh wave dispersion (Li et al., 2008), and Rayleigh-wave group- jiang-Jinhe fault and continues along the Xiaojiang fault in NW-SE di- velocity inversion (Fan et al., 2015). rection (Fig. 1). Geological studies also suggest that the Xiaojiang fault The subduction of the Indian plate can be dynamically reflected by forms the eastern boundary for the crustal escape material coming from the seismicity to the west of the Jinshajiang-Red River fault. For ex- the southeastern margin of Tibet (Royden et al., 1997; Wang and ample, the Ms. 6.1 earthquake occurred in the Jiangcheng area Burchfiel, 2000). There has never been an Ms. ≥ 6.0 earthquake to the (22.30°N, 101.40°E) on July 3, 1965, was followed by the Ms. 6.0 east of the Xiaojiang fault (Fig. 9). Therefore, the existence of a large-

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Fig. 8. Lateral variation of the Moho depth determined by inversion of receiver functions. The scale for the Moho depth is shown on the right. Splitting vectors indicating both the fast- wave polarization direction and the time delay of the Pms-wave are also plotted (red bars) (Sun et al., 2013). The direction and length of the bars represent the fast-wave polarization direction and time delay, respectively. The rose diagram (bottom right corner) shows the dominant direction of the fast-wave polarization. The scale for splitting time is shown in the lower right corner. Black dashed lines represent regional faults. Black triangles mark the location of the seismic stations used in this study. Black circles and stars indicate the location of two pairs of earthquakes. scale crustal flow to the east of the Xiaojiang fault is quite unlikely. splitting, determined that the dominant fast-wave polarization direc- Instead, we suggest that the lower crustal flow coming from eastern tion is also NW-SE to the west of the Jinshajiang-Red River fault. Tibet is blocked by the Jinshajiang-Red River fault and the Xiaojiang Therefore, it seems unlikely that the lower crustal flow has penetrated fault, although it is flowing within the margins that SYDSB draws. the Jinshajiang-Red River fault and that the mid-lower crust beneath Transdimensional Bayesian seismic ambient noise tomography (Zheng the Indochina block is rotating clockwise around EHS, because then the et al., 2017) supports that the crustal flow in the southeast of Tibet is dominant fast-wave polarization should be NE-SW. So, we think that widely spread and does not appear to be restricted to two narrow low- the dominant polarization to the west of the Jingshjiang-Red River fault velocity channels, which is fully consistent with our results. reflects rather the early exclusion from East Tibet. Although GPS observations confirm that the crust in SW China is rotating clockwise around EHS, the truth is that these measurements only indicate the movement of the uppermost crust (Gan et al., 2007). 6. Conclusions The interpretation of GPS data depends on the reference frame and cannot necessarily constrain the dynamic behavior of the mid-lower In combination to the bootstrap technique to expand the sample crust (Yao et al., 2010). Pms-wave splitting vectors, which indicate both size, we have introduced a two-step inversion procedure to fit P receiver the polarization direction and the time delay of the split fast wave, functions, which not only reduces the dependence of the solution on the better reflect the deformation of the mid-lower crust (Chen et al., initial model but also provides a statistical evaluation of the solution. fi 2013). Focusing on the transition zone between the plateau and the Following this method, we have obtained the ne VS velocity structure South China and Indochina blocks, Chen et al. (2013) suggested that the of the crust in SW China and provided new evidence of an extensive IC- motion of the Central Yunnan sub-block is a southeastward extrusion as LVZ in the southeastern margin of Tibet. The crust is roughly divided a way of tectonic escape, and that the motion is controlled by the Ai- into two major layers: the upper crust that has a thickness of approxi- laoshan–Red River fault to the west and the Xianshuihe–Xiaojiang fault mately 10–15 km and the lower crust whose thickness increases up to to the east (F2, F4 and F5 in Fig. 1). Sun et al. (2013) studied the mid- 20–40 km. The Moho depth varies between ∼30 km in the south and lower crust anisotropy in southwestern Yunnan (red bars in Fig. 8) and more than 60 km in the north of SW China, and there is a relatively found that the dominant direction of the fast-wave polarization is NW- wide transition belt at approximately 26°N through which the crust SE (see rose diagram in Fig. 8), which is almost parallel to the Jin- thickness varies by approximately 15–20 km. Except for the Tengchong shajiang-Red River fault. Cai et al. (2016), using more than 300 tem- volcanic region, the near-surface seismic velocity structure exhibits low porary broadband seismic stations deployed in SE Tibet and Pms- VS values. On the other hand, there exists an extensive IC-LVZ in SW China

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Fig. 9. For interpretation of the references to color, mapping of S-wave velocity at increasing depths of 6, 12, 18, 24, 30 and 38 km. In these horizontal depth sections, black triangles mark the location of the seismic stations used in this study; black dashed curves represent major faults; circles denote the location of the seismic events of magnitude Ms ≥ 6 that occurred in the study region (from 1500 BC to 2013). Right lines A-A′, B-B′ and C-C′ illustrate the locations of the three vertical cross-sections included in Fig. 10. whose 3D image shows a very complicated pattern. The IC-LVZ to the Xiaojiang faults play an important role in accommodating lithospheric west of the Jinshajiang-Red River fault appears to be related to the deformation during the uplift and expansion of eastern Tibet. upwelling of hot mantle material caused by the eastward subduction of Finally, according to the information provided by Pms-splitting the Indian plate, rather than to the lower crustal flow coming from measurements, it seems unlikely that the mid-lower crust beneath the eastern Tibet. To the east of this major fault, the large-scale IC-LVZ Indochina block is rotating clockwise around EHS; the predominant originating from eastern Tibet can be regarded as the lower crustal flow polarization to the west of the Jingshjiang-Red River fault reflects ra- that once exceeded the Lijiang-Jinhe fault, continues its course in the ther the early exclusion from East Tibet. interior of the Sichuan-Yunnan block in NW-SE direction, and is blocked by the Jinshajiang-Red River fault to the west and the Xiaojiang fault to the east. Based on the occurrence of strong earthquakes along Acknowledgements the large strike-slip faults and the distribution of the IC-LVZ in the southeastern margin of Tibet, we believe that both the lower crustal The authors are grateful to the Editor-in-Chief, Prof. Mei-Fu Zhou, flow and the rigid extrusion along the Jinshajiang-Red River and Associate Editor, Dr. Dapeng Zhao and two anonymous reviewers for their insightful comments and constructive suggestions. The National

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Fig. 10. For interpretation of the references to color, vertical cross-sections along the profiles A-A′, B-B′ and C-C′ (see Figs. 1 and 9) showing the S-wave velocity distribution in the crust. The scale for S-velocity is shown on the right. The white curves describe the appreciable variation of the Moho depth. The black inverted triangles on top of each section precise the location of the stations intersected by the concerned profile. The vertical strokes on top of the sections indicate the geographical position of the major faults intersected by the profiles. Acronyms: F4, Xiaojiang fault; F5, Jinshajiang-Red River fault; F6, Lancangjiang fault; F7, Jiali-Nujiang fault.

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