Gondwana Research 22 (2012) 1060–1067

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Lithospheric structure and crust–mantle decoupling in the southeast edge of theTibetan Plateau

Jiafu Hu, Haiyan Yang, Xingqian Xu, Limin Wen, Guangquan Li ⁎Department of Geophysics, Yunnan University, 2 North Green Lake Rd., Kunming, Yunnan 650091, PR China

⁎ Corresponding author. Tel.: +86 871 5032983 22.E-mail address: guangquanli74@gmail.com (G. Li).

1342-937X/$ – see front matter © 2012 International Adoi:10.1016/j.gr.2012.01.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 October 2011Received in revised form 13 January 2012Accepted 19 January 2012Available online 31 January 2012

Handling Editor: A. Aitken

Keywords:S receiver functionsMoho and LAB depthsCrust–mantle decouplingYunnanEastern Tibet

Convergence between the Indian plate and the Eurasian plate has resulted in the uplift of the Tibetan Plateau,and understanding the associated dynamical processes requires investigation of the structures of the crustand the lithosphere of the Tibetan Plateau. Yunnan is located in the southwest edge of the plateau and adja-cent to Myanmar to the west. Previous observations have confirmed that there is a sharp transition in mantleanisotropy in this area, as well as clockwise rotations of the surface velocity, surface strain, and fault orienta-tion. We use S receiver functions from 54 permanent broad-band stations to investigate the structures of thecrust and the lithosphere beneath Yunnan. The depth of the Moho is found to range from 36 to 40 km be-neath southern Yunnan and from 55 to 60 km beneath northwestern Yunnan, with a dramatic variationacross latitude 25–26°N. The depth of the lithosphere–asthenosphere boundary (LAB) ranges from 180 kmto less than 70 km, also varying abruptly across latitude 25–26°N, which is consistent with the sudden changeof the fast S-wave direction (from NW–SE to E–W across 26–28°N). In the north of the transition belt, thelithosphere is driven by asthenospheric flow from Tibet, and the crust and the upper mantle are mechanicallycoupled and moving southward. Because the northeastward movement of the crust in the Burma micro-plateis absorbed by the right-lateral Sagaing Fault, the crust in Yunnan keeps the original southward movement.However, in the south of the transition belt, the northeastward mantle flow from Myanmar and the south-ward mantle flow from Tibet interact and evolve into an eastward flow (by momentum conservation) asshown by the structure of the LAB. This resulting mantle flow has a direction different from that of the crustalmovement. It is concluded that the Sagaing Fault causes the west boundary condition of the crust to be dif-ferent from that of the lithospheric mantle, thus leading to crust–mantle decoupling in Yunnan.

© 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Yunnan is located in southwestern China and adjacent to Myanmarand the eastern Himalayan syntaxis to the west. The convergence be-tween the Indian plate and the Eurasian plate has been persisting overthe last 45 Ma. As a result, the crust in the collision zone has been short-ened by at least 1500 km (Molnar and Tapponnier, 1975; Armijo et al.,1986; England and Molnar, 1997; Yin, 2000). At the meantime, thecrust is thickened to twice of the normal (~80 km) beneath the collisionzone, and the topography is uplifted by more than 4 km. The eastwardtectonic escape flow from Tibet may move southeastward into Yunnan(Royden, 1996; Clark and Royden, 2000; Klemperer, 2006; Royden etal., 2008). SKS anisotropy measurements (Wang et al., 2008) and GPSdisplacement vectors (Zhang et al., 2004; Gan et al., 2007) confirmedthat the crust (and possibly the lithosphere) beneath eastern Tibet is es-caping eastwards, and then redirected southeastward to Yunnan afterencountering the rigid Sichuan Basin (Fig. 1). Near latitude 26°N, the

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motion of the crust begins to diverge into two parts, one in the south-westward direction and the other in the southeastward (Zhang et al.,2004; Gan et al., 2007). Seismic events recorded at permanent stationsin Yunnan showed that the crust is comprised of a brittle, seismically ac-tive upper-to-middle crust and a warm, aseismic lower crust. The litho-spheric structure has been modified during the Mesozoic–Cenozoic(Wu and Zhang, 2011). Many geophysical investigations, including sev-eral wide-angle seismic profiling (Zhang et al., 2005a,b), body/surfacewave tomography (Hu et al., 2008a; Li et al., 2008a; Chen et al., 2010)and P receiver function surveying (Hu et al., 2005, 2008b) showed astrong lateral variation in crustal thickness that decreases from 60 kmbeneath northern Yunnan to 30 km beneath southern Yunnan. Gener-ally, it is thought that the collision between the Indian plate andthe Eurasian plate and the subduction of the Burma micro-plateare responsible for the dramatic change in crustal thickness in Yun-nan, as well as the frequent earthquakes and the strong volcanic ac-tivity in this region (Hu et al., 2008a; Lei et al., 2009).

As shear wave travels through an anisotropic media, it will splitinto two orthogonally polarized components (fast S and slow S).The time delay between the fast S and the slow S provides informa-tion regarding anisotropic strength and thickness of the anisotropic

Published by Elsevier B.V. All rights reserved.

Fig. 1. Topographic map, regional faults (blue solid lines), and major tectonic units.Faults: F1 – Nujiang Fault; F2 – Lancangjiand Fault; F3 – Jinshajiang-Red river Fault;F4 – Lijiang-Jinhe Fault; F5 – Xianshuihe–Xiaojiang Fault; F6 – Longmenshan Fault;F7 – Sagaing Fault; F8 – Naga hills and Arakan hills Fault. Tectonic units: I, eastern YunnanBlock; II, Sichuan–Yunnandiamond-shapedBlock; III, Indochina block; IV, Yunnan–Burma–Thailand Block. The red arrows indicate the direction of the GPS velocity vectors relative tothe stable Eurasia (Zhang et al., 2004; Gan et al., 2007), while the dark lines represent shearwave splitting of core phases propagating through the earth interior whose fast polariza-tion directions are illustrated at each network station by segments of length proportionalto the time delay (Lev et al., 2006). The red triangles denote the stations,with theupper let-ters indicating the station names: TEC – Tengchong; ZOD – Zhongdian; LIJ – Lijiang; YUL –Yunlong. EHS: eastern Himalayan syntaxis; MBT:main boundary thrust. The inserted smallmap is a geographic map for the study region..

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layer. Typically, the fast S-wave direction is interpreted as the lattice-preferred orientation of olivine crystals in lithosphere induced bymantle flow (Silver and Chan, 1991). That is, after millions of years,those long and thin olivine crystals tend to arrange along mantleflow, so that the shear modulus in the flow direction is larger thanthat in the orthogonal direction. Thus the fast S-wave direction pro-vides a good indicator of the flow direction.

In the last decades, several studies of the seismic anisotropy in theTibetan Plateau (Flesch et al., 2005; Lev et al., 2006; Sol et al., 2007;Wang et al., 2008) have been conducted, aiming to determine thelevel of mechanical coupling between the crust and the mantlethere. These studies consistently revealed that the fast S-wave di-rection in eastern Tibet is in a clockwise rotation around the easternHimalayan syntaxis, and inferred that the crust and the lithosphericmantle there are mechanically coupled. However, in central andsouthern Yunnan, the orientation discrepancy between the mantle an-isotropy and the surface strain suggests that the crust and the mantleare decoupled (Flesch et al., 2005; Lev et al., 2006; Sol et al., 2007;Wang et al., 2008). As shown in Fig. 1, across the southeastern flank ofthe plateau (south of latitude 26°N in central Yunnan), the fast S-wave is uniformly in the E–W direction, almost perpendicular to theGPS velocity vector (in the N–S direction). This strange phenomenonwas interpreted in different ways (Sol et al., 2007): (1) the crust andthe mantle are decoupled as proposed by Flesch et al. (2005); (2) themantle deformation is in 3D (rather than simply in the 2D horizontalplane), influenced by the rollback of the lithosphere beneath Burma;or (3) the observed anisotropy is from the asthenospheric flow. Dueto the absence of the fine structure of the lithosphere, the cause of thephenomenon remains debated. The associated questions include: whythis phenomenon occurs on the same side of a tectonic plate? Doesthe decoupling occur at the Moho or at the lithosphere–asthenosphereboundary (LAB)? We believe that an investigation of the structures ofthe crust and the lithosphere in Yunnan would help to answer thesequestions, as well as to explore the origin of the phenomenon.

LAB divides the rigid lithosphere from the weak asthenosphereand its depth is a key for understanding plate movement. In thepast, the P/S travel time (e.g., Huang et al., 2002; Wang et al.,2003), as well as the surface-wave velocity dispersion (e.g., Huet al., 2008a) was used to invert for the 3D velocity structure ofthe crust and upper mantle beneath Yunnan and its vicinity.These results, however, could not provide information regardingthe lithospheric thickness due to the low resolution and few broad-band stations.

Over the last decades, the receiver function technique has beendeveloped into one of the most powerful tools to investigate seismicdiscontinuities in crust and upper mantle. The P receiver functiontechnique (Langston, 1977; Vinnik, 1977) looks for the Ps convertedwaves generated by seismic discontinuities beneath a station, andthe depths of the discontinuities can be determined by the travel-time delay between the direct P wave and the converted phase Ps.Nonetheless, the depth of LAB estimated by P receiver function suffersfrom the interference from the multiple reverberation phases PsPs+PpSs in crust. In contrast, S receiver function (SRF) attempts to isolatethe Sp wave generated at a discontinuity (Farra and Vinnik, 2000;Yuan et al., 2006). The advantage of SRF is that the Sp convertedwave travels faster than the direct S wave and thus is separated fromthe later S reverberations (Vinnik et al., 2003, 2004; Yuan et al., 2006).This technique overcomes the aforementioned problem encounteredby the P receiver function and is frequently used to identify the LAB(Li et al., 2004; Kumar et al., 2005a,b; Li et al., 2007; Rychert et al.,2007; Rychert et al., 2010). The disadvantage of the SRF is that S wavehas a lower frequency (than P wave) due to its strong attenuation inmantle, such that it cannot resolve fine crustal structures.

In this paper, we shall use the SRF technique to investigate thedepths of the Moho and the LAB in Yunnan, to resolve the relevantdynamic processes, and to explain the strange phenomenon just in-troduced. The number of broad-band stations deployed in Yunnanhas been rapidly increased in the last years, which facilitates finishingthis task.

2. Geological setting

As shown in Fig. 1, Yunnan situates in the junction of the Tethy-Himalayan tectonic domain and the circum-Pacific tectonic domain,where the E–W structures in Tibet bend into the N–S direction. TheN–S right-lateral Sagaing Fault in Myanmar acts as the subductionboundary of the Burma micro-plate against southwestern China, aswell as the west boundary of the Yunnan–Burma–Thailand Block. Inwestern Yunnan, the Tengchong volcano cluster, located in the east-ern margin of this collision zone and comprised of 40 volcanoes(Jiang, 1998), extends 90 km from the north to the south and 50 kmfrom the west to the east. It is active with the latest eruption happen-ing in 1609. Numerous hot springs exist in this volcanic area that ex-hibits high geothermal gradient and low seismic-wave velocity in thecrust and uppermost mantle (Lei et al., 2009). Yin (2000) thought thisarea is characterized by rift-related volcanic activity, being associatedwith the subduction of the Burma micro-plate (Wang and Huangfu,2004). Geologic investigations revealed that strong eruptions ofmagma had occurred from the Pliocene or Miocene to Quaternary(Jiang, 1998; Wang and Huangfu, 2004).

Besides the volcanic activity in western Yunnan, the tectonics inYunnan is also controlled by the nearly N–S faults such as the Xian-shuihe–Xiaojiang Fault, Jinshajiang-Red river Fault, and LancangjiandFault that separate Yunnan into four major geological units, namely,eastern Yunnan Block (I), Sichuan–Yunnan diamond-shaped Block(II); Indochina Block (III); Yunnan–Burma–Thailand Block (IV). Previ-ous studies suggested that the Sichuan–Yunnan diamond-shapedBlock should be separated from Tibet (Molnar and Tapponnier,1975; Armijo et al., 1986; Avouac and Tapponnier, 1993; Englandand Molnar, 1997; Wang and Burchfiel, 2000; Yin, 2000). The left-

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lateral Xianshuihe–Xiaojiang Fault forms the eastern boundary of theSichuan–Yunnan diamond-shaped Block that rotates clockwisearound the eastern Himalayan syntaxis (Avouac and Tapponnier,1993; Wang and Burchfiel, 2000). Its western boundary appears tobe controversial, though is generally thought to be the right-lateralJinshajiang-Red river Fault. Seismicity in Yunnan is higher than inother regions of China. Although the area of Yunnan is only 4.1% ofmainland China, 18.6% earthquakes with M>7.0 in mainland Chinahappened in this region since BC 1900. Over the last 30 years, morethan 20 strong earthquakes with magnitudes above M 7.0 occurredin Yunnan, distributed along the NE and NW fault belts, such as theXiaojiang Fault, the Red river Fault, the Lancangjiang Fault, etc.

3. Data and method

We calculate SRFs from 54 permanent stations in Yunnan (Fig. 2a).To reveal the presence of the Moho and LAB, S and SKS phases from40 teleseismic events with magnitudes greater than Ms 6.3 and epi-central distances between 60–85°or 85–165°(Fig. 2b) are employed.

The main steps to get SRFs are as follows:

(1) Seismograms are selected based upon high signal-to-noiseratio and the different reference phases (S, SKS or ScS) areidentified using the J-B travel time table. The waveforms arecut from 100 s before the reference phase to 20 s after. Thenthe time axis is reversed such that the Sp conversion has posi-tive arrival time. (2) The ZNE components are rotated into theZRT coordinates, and a transformation (Reading et al., 2003) isadopted to isolate the P component from the S component. (3)Finally, the S component is deconvolved from the P to obtainthe SRF; the deconvolution is performed in the time domain(Ligorría and Ammon, 1999). The effect of the source andpath has been removed effectively and the conversion struc-ture beneath the station is constrained in the function.

SRF samples a broader area than P receiver function. At a givendepth, the piercing points of the Sp conversions in the SRF are fartheraway from the stations than those of the Ps conversions in the P re-ceiver function. In the case of strong lateral heterogeneity, SRF maylose coherence in individual traces and the results may be biased byvarying locations of the piercing points. For this reason, a stackingprocedure is often applied to the SRF. Before that, moveout correctionmust be conducted to the individual traces to remove the dependenceof the time delay on epicentral distance. An IASP91 model (Kennett

Fig. 2. (a) Topographic map of Yunnan and location of the seismic stations (red triangles). MIII, Indochina block; IV, Yunnan–Burma–Thailand Block. Regional faults (blue lines): F1 – NuFault; F5 – Xianshuihe–Xiaojiang Fault; (b) Epicentral locations of the teleseismic events u

and Engdahl, 1991) and a reference epicentral distance of 67°areused for this purpose.

4. Results

4.1. Examples of SRFs

To demonstrate the reliability of our SRFs, we present the SRFs atfour stations as shown in Fig. 3. To display the SRF in the same way asP receiver function, we reverse the time axis, such that the Sp con-verted phases appear at a positive time, and reverse the amplitude,such that a positive amplitude indicates a positive velocity gradientwith depth and vice versa. The rightmost trace in each panel repre-sents the summation at the station. Although the individual traces ap-pear not quite clear, the Sp phase in the stacked trace is very clear.The positive and negative phases in the stacked trace are interpretedas coming from the Moho and LAB, respectively.

In order to equalize the different contributions of the different re-cords in the summation, all traces are normalized, filtered by a 5 slow-pass filter and moveout corrected to the reference distance 67°using the IASP91 model. As shown in Fig. 3, the number of individualSRFs varies from 19 at YUL and ZOD to 29 at LIJ (Fig. 1). The stackedtraces show mainly two phases, one positive and one negative. Thepositive phase at 6.0–8.8 s is from the Moho. Beneath stations LIJand ZOD, it occurs at 7.4 s and 8.8 s, respectively, indicating theMoho is very deep beneath the three-river area (where the Nujiangriver, Lancangjiang river, and Jinshajiang-Red river converge), proba-bly due to the collision from the Indian plate. However, this phase isat only 5 s at station TEC that is located in the Tengchong volcanoarea. Following this positive phase, a negative phase occurs at11.2 s. Beneath Tengchong, with consideration of the previous resultby Lei et al. (2009), this negative phase indicates a shallow LAB atdepth 70 km. Although the four stations in western Yunnan are veryclose to each other and located near latitude 26°N, the depths of theMoho and the LAB differ significantly among them.

4.2. Mapping of Moho and LAB depths

To get the topography of the Moho and LAB, the arrival times ofthe phases from the Moho and LAB are read from the stacked SRFand then converted to depth through the IASP91 model. This may in-troduce an error up to 10 km in the LAB depth because the resolutionof SRF is lower than that of P receiver function (Li et al., 2007). As

ajor tectonic units: I, eastern Yunnan Block; II, Sichuan–Yunnan diamond-shaped Block;jiang Fault; F2 – Lancangjiand Fault; F3 – Jinshajiang-Red river Fault; F4 – Lijiang-Jinhesed in this study within the 30°–140° epicentral distance range (small circles).

Fig. 3. The stacked traces and the individual traces of the SRFs at four stations (LIJ, TEC, YUL and ZOD). The individual traces are aligned randomly, having been moveout corrected tothe reference epicentral distance 67°.

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such, the choice of a reference velocity model is not quite crucial (Liet al., 2007). In some areas there may be an intra-crustal phaseappearing in the P receiver functions (Hu et al., 2005), but the SRFmay not be able to resolve that due to its lower frequency and res-olution. Finally, using the depths of the Moho and the LAB beneatheach station, two contour maps are plotted in Figs. 4 and 5, in whichthe depths beneath the stations are observed, while the contoursare got by a spline interpolation and GMT software. The depth

Fig. 4. Distribution of the Moho depth (in km). The triangles, the dash lines, and the dotline represent the stations, the faults, and the transition belt, respectively.

outside the seismic net may not be accurate, thus being discardedfrom analysis.

As shown in Fig. 4, the contour map of the crustal thickness indi-cates a strong lateral variation in Yunnan, increasing from the SE tothe NW. In southern Yunnan, the crustal thickness ranges from 36to 40 km, only 5 km thicker than that from the P receiver functions

Fig. 5. Same as Fig. 4, except for the LAB depth and that the shadowed area has a LABdepth smaller than 100km.

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(Hu et al., 2005). The crustal thickness ranges from 55 to 60 km innorthwestern Yunnan, almost the same as that from the P receiverfunctions. Overall, the variation in crustal thickness is consistentwith the variation trend of the surface topography (Fig. 1).

Except for the low resolution results from P- and S-wave tomogra-phy (Huang et al., 2002; Wang et al., 2003) and from surface wave(Hu et al., 2008a; Li et al., 2008b), there is no result regarding thedepth of the LAB in Yunnan. Due to the short operation history ofthe Yunnan Seismic Net, it is not possible to select many seismicevents. Though, since we select the waveforms with high signal-to-noise ratio, the Sp phases from the Moho and LAB in the stacked traceare very clear as shown in Fig. 3. In Fig. 5, the LAB depth should be accu-rate due to a highdensity of stations. The regionwith a LABdepth smallerthan 100 km is plotted in grey. Compared with the crustal thickness, thedepth of the LAB varies more dramatically, with the largest depth reach-ing 180 km and the smallest depth less than 80 km.

In central Yunnan, a shallower LAB zone of more than 100 kmwidth extends from the Tengchong volcano area, across theJinshajiang-Red river Fault and Xiaojiang Fault, and finally into east-ern Yunnan (Fig. 1). The latest P time tomography has also confirmedthat a low-velocity anomaly beneath the Tengchong volcano areadown to depth ~400 km, extending in the northeast direction (Leiet al., 2009). That result agrees well with the thin lithosphere ob-served by our study. In addition, we observe a sharp transition inthe depths of the Moho and the LAB occurring near latitude 26°N,which is well revealed by the dense contours in Figs. 4 and 5. Also,the contour with the LAB depth at 80 km in the lower-left part ofFig. 5 clearly shows the diaper of the lithosphere by an upwellinghot asthenosphere there.

Fig. 6. (a) Location of the piercing points of the Sp phase at depth 120 km, represented by thebin is 1×1°. (b) Bin stacking of the SRF along AB; (c) Bin stacking of the SRF along CD; (d) Bstacking, all traces are corrected to the reference epicentral distance 67°.

4.3. Common conversion point (CCP) sections

To investigate the lateral variations in depth, we construct threedepth sections crossing the area where most of the stations are locat-ed, by CCP stacking along three profiles and projecting the individualSRFs to the depth. The locations of the Sp piercing points at depth120 km for all events are computed using the IASP91 model. Fig. 6shows the data coverage in the study region and the position of theprofiles that are labeled as AB, CD and EF. It also shows the stackedSRFs for the section along AB, the section along CD, and the sectionalong EF. The sections are formed by stacking individual traces withthe piercing points at depth 120 km within a window of 1×1°. Thewindow is moved by 0.5° for the next summation. Before summation,all traces are corrected to the referential distance 67°.

As shown in Fig. 6, two clear phases are visible, a positive phaseand a negative phase, interpreted as from the Moho and the LAB, re-spectively. Along AB and CD (Fig. 6b, c), the variation of the Mohodepth is relatively homogeneous. In Fig. 6d, along the fault towardsoutheastern Yunnan, the crustal thickness thins from 65 km underthe three-river area to 36 km under southeastern Yunnan. The causefor a small slope in the Moho depth (more gradual than in Fig. 4) isthat our CCP stacking is based upon the Sp from the LAB, instead ofthat from the Moho. If the CCP stacking were based on the Sp fromthe Moho, we would get a more accurate Moho depth. Actually, theCCP stacking based upon the Sp from the LAB should result in a verybroad sampling area for the Sp from the Moho. For this reason, the oc-currence of the gradually (worse) varying Moho depth in Fig. 6 is notthat surprising. In Yunnan, the lateral variation of the LAB is dramatic.Especially, in Fig. 6c a strong LAB phase occurs beneath the Tengchong

crosses. The lines AB, CD and EF denote the profiles along which the size of the stackingin stacking of the SRF along EF. The stacking bin is 1×1° with 0.5° overlapping. Before

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volcano area where the depth of the LAB is less than 100 km. This couldbe explained in this way: the subduction of the Burma micro-plate (Leiet al., 2009) causes an upwelling of the asthenosphere with high tem-perature and low velocity in this area, which tends to push the LAB up-ward and increases the material/velocity contrast across the LAB.

5. Discussion

As S wave propagates through an anisotropic media, it will splitinto two orthogonally polarized components (fast S and slow S). Insoutheastern Tibet, the fast S-wave direction may be interpreted tobe induced by either asthenospheric flow around the eastern Himala-yan syntaxis or strain in the lithosphere. The anisotropic source wasinferred to reside primarily in depth 60 to 160 km and is located inthe lithospheric mantle (Flesch et al., 2005; Sol et al., 2007). In south-eastern Tibet, the fast S-wave direction is parallel to the maximumshear direction of the surface deformation field. In eastern Tibet, al-though previous studies (Royden et al., 1997; Clark and Royden,2000) argued that there exists a weak lower crust flow to facilitatecrust–mantle decoupling, observation indicated that the crust andthe mantle are strongly coupled, which implies a vertically coherentdeformation (Flesch et al., 2005; Lev et al., 2006; Sol et al., 2007;Wang et al., 2008).

Yunnan is located just outside of eastern Tibet, where the strike-slip faults are the most prominent tectonic feature. Those faults areapproximately in the NS direction, almost parallel to the direction ofthe GPS velocity vector (Fig. 1). However, the fast S-wave directionis inconsistent with the GPS velocity direction (Fig. 1). Especially,across latitude 25–26°N in central Yunnan, the fast S-wave directionis oriented in the EW direction, approximately perpendicular to thefault orientation.

Previous studies (e.g. Hu et al., 2005; Li et al., 2008b) indicated theN–S varying trend in crustal depth in Yunnan. Nonetheless, theycould not provide the fine lateral variation of the Moho depth dueto the few stations. Because the converted phase from the LAB in P re-ceiver functions may be masked by the reverberated phases in crust,there is few research to get the LAB in Yunnan except the low resolu-tion tomography (e.g. Lei et al., 2009). Our study uses SRF to investi-gate the lithospheric structure in Yunnan, the results of whichindicate that the crustal thickness beneath Yunnan increases fromthe SE to the NW, well matching the topography variation. We findthat near latitude 25 to 26°N, the crustal thickness increases abruptly

Fig. 7. Rose diagram for the P axis from focal mechanisms in different areas at different depthick lines with arrows indicates the direction of the NUVEL-1 absolute plate motion (APM),Fault; F3 – Jinshajiang-Red river Fault; F4 – Lijiang-Jinhe Fault; F5 – Xianshuihe–Xiaojiang Fa

from ~40 km beneath the southern Yunnan to 50–60 km beneath itsnorthern part (Fig. 4), being consistent with the variation of the man-tle anisotropy (Fig. 1).

To analyze the tectonic stress field in the region, we use the focalmechanism provided by the USGS to plot the P-axis distribution atdifferent depth for different areas. As shown in Fig. 7, when thefocal depth is less than 50 km (Fig. 7a), the P axis is dominant in theNW–SE in the central and eastern Yunnan, while it is dominant inthe NE–SW from the Jinshajiang-Red river Fault to the SagaingFault. This is consistent with the GPS vector at the surface but quitedifferent from the direction of the plate movement. For the Burmamicro-plate, the direction of the P axis is close to that of the absoluteplate movement, whether in the crust or in the lithosphere. However,the P-axis direction beneath the Yunnan–Burma–Thailand Block ro-tates from the NNE at shallow depth (Fig. 7a) toward the NE as thefocal depth increases (Fig. 7b), being more consistent with the direc-tion of the absolute plate movement. Due to the absence of deepearthquakes in central and southern Yunnan, the P-axis directioncould not be obtained. It appears that the crustal movement of theBurmamicro-plate toward the NNE direction is absorbed by the Saga-ing Fault. The change of the P-axis direction with depth indicates thatthe crust and the upper mantle are controlled by two different tecton-ic stress fields.

Velocity measurements around Sichuan Basin by Yao et al. (2008)and Li et al. (2008a) have indicated that the velocity down to at least200 km depth beneath the basin and eastern Himalayan syntaxis is 4%higher than that beneath eastern Tibet. Therefore, the basin and thesyntaxis act as two cold rigid barriers as shown in Fig. 8. That figurepresents a geodynamic model for the study region. The mantle ex-truding from the channel between the Sichuan Basin and the easternHimalayan syntaxis moves southward, interacts with the northeast-ward mantle flow from Myanmar, and eventually forms an eastwardflow.

Finally, we discuss the Tengchong volcano. The volcano is an ac-tive one located in southwestern China neighboring the Indianplate. A local high-resolution tomography revealed a low velocityanomaly in the lithosphere down to a depth of 85 km. Also, a promi-nent low velocity zone is visible down to depth 300 km (Zhao, 2007;Zhao and Ohtani, 2009), indicating that the volcano is likely to becaused by the eastward subduction of the Burma microplate. In theTibetan Plateau, the volcanic evolution was caused by an extensionalenvironment due to the rollback of the subduction slab (Xia et al.,

th. (a) The focal depth is less than 50 km; (b) The focal depth is larger than 50 km. Thewhile the dash lines represent the regional faults: F1 – Nujiang Fault; F2 – Lancangjiandult; F6 – Longmenshan Fault; F7 – Sagaing Fault; F8 – Naga hills and Arakan hills Fault.

Fig. 8. A geodynamic model for the study region. The left is the surface with the deformation field represented by black arrows, while the right is the topography of the astheno-sphere. SB, Sichuan Basin; EHS, eastern Himalayan syntaxis; TEC, Tengchong volcano area. The two blue pillars represent two cold rigid barriers beneath the EHS and the SB, respec-tively. The red solid line denotes the transition belt of mantle anisotropy, while the red dash line is the extent of the right figure on the surface.

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2011). However, the focal mechanism indicated that in the Yunnan–Burma–Thailand Block, most of the faults are strike-slip in the NE–SW direction (Hu et al., 2008a). This fact illustrates that in this area,the shear force in the horizontal plane wins over the shear stress in-duced by the upwelling magma onto the bottom of the lithosphere(that is mechanically equivalent to a virtual extensional stress in thehorizontal plane).

6. Summary and conclusions

Often, viscous asthenosphere is the dominant driving source forthe movements of lithosphere, and it is relatively new and active. To-mography of the LAB may well reflect the current movement of as-thenosphere. Comparatively, lithosphere is an old rigid solid thatbetter preserves the trace and footprint of older tectonics, althoughthe geothermal events associated with the upwelling of astheno-sphere and the resulting diaper of the lithosphere may modify theolder lithosphere. Ignoring those small-scale geological processes onthe surface of the Earth, the crust is a relatively intact layer for globaltectonics. The crust, the lithosphere, and the asthenosphere are differ-ent in history and character. At regional scale, this discrepancy in thevertical direction presents different boundary conditions for the dif-ferent layers, and therefore, we should consider the boundary condi-tions when investigating regional tectonics.

Our observation with SRFs from 54 permanent broad-band sta-tions indicates that the depth of the Moho beneath southern Yunnanranges from 36 to 40 km and from 55 to 60 km beneath northwesternYunnan. The depth of the LAB ranges from 180 km to less than 70 km,with a abrupt variation across latitude 25–26°N that is consistentwith the sudden change of the fast S-wave direction (from NW–SEto E–W across latitude 26°N).

Mathematically speaking, the structure of the crust and litho-sphere is a historic integral of the actions of asthenospheric flowonto the lithosphere. However, the formation of the above regionaltectonic features is also controlled by the regional boundary condi-tions, which should be considered when exploring the origin of theabove transition. The escape flow from Tibet is very likely to slowdown and accumulate near latitude 25–26°N in northern Yunnandue to the resistance from the rigid Sichuan Basin and eastern Hima-lan syntaxis, which results in the thickening of the lithosphere andthe uplift of the surface in the north of latitude 26°N. There, the lith-osphere is driven by asthenospheric flow from Tibet, and the crustand the upper mantle are mechanically coupled and both move tothe south. In contrast, in the south of the transition belt, the right-

lateral Sagaing Fault makes the northeastward movement of thecrust in the Burma micro-plate change into the north direction(thus facilitating the southward movement of the crust on the eastside). However, in the deep lithosphere where the fault stops to act,the hot asthenospheric flow associated with the subduction of theBurma micro-plate continues to push the mantle lithosphere towardthe northeast. The northeastward mantle lithosphere from Myanmarwould meet the southward mantle lithosphere (extruding from thechannel between the Sichuan Basin and the eastern Himalayan syn-taxis) near latitude 26°N, where the two mantle flows interact intoan eastward motion, according to momentum conservation (approx-imately). Thus the crust and the lithospheric mantle are decoupled inthat they have different directions. Therefore, the Sagaing Fault absorbsthe crustal movement from Myanmar, making the west boundary con-dition for the crust in Yunnan being different from that for the mantle,which eventually leads to crust–mantle decoupling in Yunnan.

The bottom of lithosphere is heated and removed by the underly-ing hot asthenospheric flow, so that the resulting thinner lithospherecan reflect the direction of the asthenospheric flow (Hu et al., 2011),thus indicating the direction of the lithospheric mantle flow (becausethese two flows are often coupled). The shallow LAB in the upperright of Fig. 5 clearly showed an eastward mantle flow near the tran-sition belt, which together with other data, supports the above phys-ical analysis of the origin of the crust–mantle decoupling in Yunnan.

Acknowledgements

This research was sponsored by the National Science Foundationof China under contract U0933602 and the National Basic ResearchProgram of China (973 Program, 2011CB808904). Special thanks toEditor-in-Chief, Professor M. Santosh, Associate Editor, Professor AlanAitken, and two anonymous reviewers for their constructive suggestions.

References

Armijo, R., Tapponnier, P., Mercier, J.L., Han, T.L., 1986. Quaternary extensioninsouthern Tibet: field observations and tectonic implications. Journal of Geophys-ical Research 91, 13803–13872.

Avouac, J.P., Tapponnier, P., 1993. Kinetic model of active deformation in central-Asia.Geophysical Research Letters 20, 895–898.

Chen, Y., Badal, J., Hu, J.F., 2010. Love and Rayleigh wave tomography of the Qinghai–Tibet Plateau and surrounding areas. Pure and Applied Geophysiccs 167,1171–1203.

Clark, M.K., Royden, L.H., 2000. Topographic ooze: building the eastern margin of Tibetby lower crustal flow. Geology 28, 703–706. doi:10.1130/0091-7613(2000)28b703:TOBTEM>2.0.CO;2.

1067J. Hu et al. / Gondwana Research 22 (2012) 1060–1067

England, P., Molnar, P., 1997. Active deformation of Asia: from kinematics to dynamics.Science 278, 647–650.

Farra, V., Vinnik, L., 2000. Upper mantle stratification by P and S receiver functions.Geophysical Journal International 141, 699–712.

Flesch, L.M., Holt, W.E., Silver, P.G., Stephenson, M., Wang, C.-Y., Chan, W.W., 2005.Constraining the extent of crust–mantle coupling in Central Asia using GPS, geo-logic, and shear-wave splitting data. Earth and Planetary Science Letters 238,248–268. doi:10.1016/j.epsl.2005.06.023.

Gan, W., Zhang, P., Shen, Z., Niu, Z., Wang, M., Wan, Y., Zhou, D., Cheng, J., 2007. Pre-sentday crustal motion within the Tibetan Plateau inferred from GPS measure-ments. Journal of Geophysical Research 112, B08416. doi:10.1029/2005JB004120.

Hu, J., Su, Y., Zhu, X., Chen, B., 2005. S-wave velocity and Poisson's ratio structure ofcrust in Yunnan and its implication. Science in China Series D: Earth Sciences 48,210–218.

Hu, J., Hu, Y., Xia, J., Chen, Y., Zhao, H., Yang, H., 2008a. Crust–mantle velocity structure of Swave and dynamic process beneath Burma Arc and its adjacent regions. Chinese Jour-nal of Geophysics 51 (1), 140–148.

Hu, J., Yang, H., Zhao, H., 2008b. Structure and Significance of S-wave velocity and Pois-son's ratio in crust beneath the eastern side of the Qinghai–Tibet plateau. Pure andApplied Geophysics 165, 829–845.

Hu, J., Xu, X., Yang, H., Wen, L., Li, G., 2011. S receiver function analysis of the crustaland lithospheric structures beneath Eastern Tibet. Earth and Planetary Science Let-ters 306, 77–85. doi:10.1016/j.epsl.2011.03.034.

Huang, J., Zhao, D., Zheng, S., 2002. Lithospheric structure and its relationship to seis-mic and volcanic activity in southwest China. Journal of Geophysical Research107, 2255. doi:10.1029/2000JB00013.

Jiang, C., 1998. Distribution characteristics of Tengchong area, Yunnan province. Jour-nal of Seismological Research 8, 107–120 (in Chinese).

Kennett, B.L.N., Engdahl, E.R., 1991. Travel times for global earthquake location andphase identification. Geophysical Journal International 105, 429–465.

Klemperer, S.L., 2006. Crustal flow in Tibet: A review of geophysical evidence for thephysical state of Tibetan lithosphere, in Channel Flow, Ductile Extrusion and Exhu-mation of Lower Mid-Crust in Continental Collision Zones. In: Searle, M.P., Law,R.D. (Eds.), Geological Society Special Publication, 268, pp. 39–70.

Kumar, P., Kind, R., Hanka, W., Wylegalla, K., Reigber, C., Yuan, X., Wölbern, I.,Schwintzer, P., Fleming, K., Dahl-Jensen, T., Larsen, T.B., Schweitzer, J., Priestley,K., Gudmundsson, O., Wolf, D., 2005a. The lithosphere–asthenosphere boundaryin the North-West Atlantic region. Earth and Planetary Science Letters 236,249–257. doi:10.1016/j.epsl.2005.05.029.

Kumar, P., Yuan, X., Kind, R., Kosarev, G., 2005b. The lithosphere–asthenosphere boundaryin the Tien Shan-Karakoramregion froms receiver functions: evidence for continentalsubduction. Geophysical Research Letters 32, L07305. doi:10.1029/200 4GL022291.

Langston, C.A., 1977. Corvallis, Oregon, crustal and upper mantle structure from tele-seismic P and S waves. Bulletin of the Seismological Society of America 67,713–724.

Lei, J., Zhao, D., Su, Y., 2009. Insight into the origin of the Tengchong intraplate volcanoand seismotectonics in southwest China from local and teleseismic data. Journal ofGeophysical Research 114, B05302. doi:10.1029/2008JB005881.

Lev, E., Long, M.D., van der Hilst, R.D., 2006. Seismic anisotropy in eastern Tibet fromshear wave splitting reveals changes in lithospheric deformation. Earth and Plane-tary Science Letters 251, 293–304. doi:10.1016/j.epsl.2006.09.018.

Li, X., Kind, R., Yuan, X., Wolbern, I., Hanka, W., 2004. Rejuvenation of the lithosphereby the Hawaiian plume. Nature 427, 827–829.

Li, X., Yun, X., Kind, R., 2007. The lithosphere–asthenosphere boundary beneath thewestern United States. Geophysical Journal International 170, 700–710.doi:10.1111/j.1365-246X.2007.03428.x.

Li, C., van der Hilst, R., Meltzer, A.S., Engdahl, E.R., 2008a. Subduction of the Indian lith-osphere beneath the Tibetan Plateau and Burma. Earth and Planetary Science Let-ters 274, 157–168.

Li, Y., Wu, Q., Zhang, R., Tian, X., Zeng, R., 2008b. The crust and upper mantle structurebeneath Yunnan from joint inversion of receiver functions and Rayleigh wave dis-persion data. Physics of the Earth and Planetary Interiors 170, 134–146.

Ligorría, J.P., Ammon, C.J., 1999. Iterative deconvolution and receiver-function estima-tion. Bulletin of the Seismological Society of America 89, 1395–1400.

Molnar, P., Tapponnier, P., 1975. Tectonics in Asia: consequences and implications of acontinental collision. Science 189, 419–426.

Reading, A., Kennet, B., Sambridge, M., 2003. Improved inversion for seismic structureusing transformed, S-wave vector receiver function: removing the effect of the freesurface. Geophysical Research Letters 30 (19). doi:10.1029/2003GL018090.

Royden, L., 1996. Coupling and decoupling of crust and mantle in convergent orogens:implications for strain partitioning in the crust. Journal of Geophysical Research101, 17,679–17,705. doi:10.1029/96JB00951.

Royden, L.H., Burchfiel, B.C., Rovert, W.K., King, R.W., Wang, E., Chen, Z., Shen, F., Liu, Y.,1997. Surface deformation and lower crustal flow in eastern Tibet. Science 276,788–790.

Royden, L.H., Burchfiel, B.C., van der Hilst, R.D., 2008. The geological evolution of theTibetan plateau. Science 321, 1054–1058. doi:10.1126/science.1155371.

Rychert, C.A., Rondenay, S., Fischer, K.M., 2007. P-to-S and S-to-P imaging of a sharplithosphere–asthenosphere boundary beneath eastern North America. Journal ofGeophysical Research 112, B08314.

Rychert, C.A., Shearer, P.M., Fischer, K.M., 2010. Scatteredwave imaging of the lithosphere–asthenosphere boundary. Lithos 120, 173–185. doi:10.1016/j.lithos.2009.12.006.

Silver, P.G., Chan, W.W., 1991. Shear wave splitting and subcontinental mantle defor-mation. Journal of Geophysical Research 96, 16,429–16,454.

Sol, S., Meltzer, A., Burgmann, R., van der Hilst, R.D., King, R., Chen, Z., Koons, P.O., Lev,E., Liu, Y.P., Zeitler, P.K., Zhang, X., Zhang, J., Zurek, B., 2007. Geodynamics of south-eastern Tibet from seismic anisotropy and geodesy. Geology 35, 563–566.doi:10.1130/G23408A.1.

Vinnik, L.P., 1977. Detection of waves converted from P to SV in the mantle. Physics ofthe Earth and Planetary Interiors 15, 39–45.

Vinnik, L., Kumar, M.R., Kind, R., Farra, V., 2003. Super-deep low velocity layer beneath theArabian plate. Geophysical Research Letters 30, 1415. doi:10.1029/2002GL016590.

Vinnik, L., Farra, V., Kind, R., 2004. Deep structure of the Afro-Arabian hotspot by S receiv-er functions. Geophysical Research Letters 31, L11608. doi:10.1029/2004GL019574.

Wang, E.C., Burchfiel, B.C., 2000. Late Cenozoic to Holocene deformation in southwest-ern Sichuan and adjacent Yunnan, China, and its role in formation of the southeast-ern part of the Tibetan Plateau. Geological Society of America Bulletin 112 (3),413–423.

Wang, C., Huangfu, G., 2004. Crustal structure in Tengchong volcanic-geothermal area,western Yunnan, China. Tectonophysics 380, 69–87.

Wang, C., Chan, W., Mooney, W., 2003. 3-D velocity structure of crust and upper mantlein southwestern China and its tectonic implications. Journal of Geophysical Re-search 108, 2442. doi:10.1029/2002JB001973.

Wang, C.Y., Lucy, M., Paul, G., Chang, L., Winston, W., 2008. Evidence for mechanicallycoupled lithosphere in central Asia and resulting implications. Geology 36,363–366. doi:10.1130/G24450A.1.

Wu, J., Zhang, Z.J., 2011. Spatial distribution of seismic layer, crustal thickness, and Vp/Vs ratio in the Permian Emeishan Mantle Plume region. Gondwana Research.doi:10.1016/j.gr.2011.10.007.

Xia, L., Li, X., Ma, Z., Xu, X., Xia, Z., 2011. Cenozoic volcanism and tectonic evolution ofthe Tibetan plateau. Gondwana Research 19, 850–866.

Yao, H., Beghein, C., van der Hilst, R.D., 2008. Surface wave array tomography in SETibet from ambient seismic noise and two-station analysis-II. Crustal and upper-mantle structure. Geophysical Journal International 173, 205–219. doi:10.1111/j.1365-246X.2007.03696.x.

Yin, A., 2000. Mode of Cenozoic east–west extension in Tibet suggesting a common or-igin of rifts in Asia during the Indo-Asian collision. Journal of Geophysical Research105, 21745–21759.

Yuan, X., Kind, R., Li, X., Wang, R., 2006. The S receiver function: synthetics and data ex-ample. Geophysical Journal International 165, 555–564.

Zhang, P.Z., Shen, Z., Wang, M., Gan, W., 2004. Continuous deformation of the TibetanPlateau from Global Positioning System data. Geology 32, 809–812. doi:10.1130/G20554.1.

Zhang, Z.J., Bai, Z.M., Wang, C.Y., Teng, J.W., Lv, Q.T., Li, J.L., Liu, Y.F., Liu, Z.K., 2005a. Thecrustal structure under Sanjiang and its dynamic implications: revealed by seismicreflection/refraction profile between Zhefang and Binchuan, Yunnan. Science inChina Series D: Earth Sciences 48 (9), 1329–1336.

Zhang, Z.J., Bai, Z.M., Wang, C.Y., Teng, J.W., Lv, Q.T., Li, J.L., Sun, S.X., Wang, X.Z., 2005b.Crustal structure of Gondwana and Yangtze typed blocks: an example by wide-wangle seismic profile from Menglian to Malong in Western Yunnan. Science inChina Series D: Earth Sciences 48 (11), 1826–1836.

Zhao, D., 2007. Seismic images under 60 hotspots: search for mantle plumes. Gondwa-na Research 12, 335–355.

Zhao, D., Ohtani, E., 2009. Deep slab subduction and dehydration and their geodynamicconsequences: evidence from seismology and mineral physics. Gondwana Re-search 16, 401–413.