The boundary between the Indian andAsian tectonic plates below TibetJunmeng Zhaoa, Xiaohui Yuanb, Hongbing Liua, Prakash Kumarc, Shunping Peia, Rainer Kindb,d,1, Zhongjie Zhange,Jiwen Tenge, Lin Dinga, Xing Gaoa, Qiang Xua, and Wei Wanga

aKey Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China;bDeutsches GeoForschungsZentrum (GFZ), 14473 Potsdam, Germany; cNational Geophysical Research Institute, Hyderabad 500 007, India; dFreieUniversität, 12249 Berlin, Germany; and eInstitute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

Edited by Barbara A. Romanowicz, University of California, Berkeley, CA, and approved May 17, 2010 (received for review February 23, 2010)

The fate of the colliding Indian and Asian tectonic plates below theTibetan high plateau may be visualized by, in addition to seismictomography, mapping the deep seismic discontinuities, like thecrust-mantle boundary (Moho), the lithosphere-asthenosphereboundary (LAB), or the discontinuities at 410 and 660 km depth.We herein present observations of seismic discontinuities withthe P and S receiver function techniques beneath central and wes-tern Tibet along two new profiles and discuss the results in connec-tion with results from earlier profiles, which did observe the LAB.The LAB of the Indian and Asian plates is well-imaged by severalprofiles and suggests a changingmode of India-Asia collision in theeast-west direction. From eastern Himalayan syntaxis to thewestern edge of the Tarim Basin, the Indian lithosphere is under-thrusting Tibet at an increasingly shallower angle and reachingprogressively further to the north. A particular lithospheric regionwas formed in northern and eastern Tibet as a crush zone betweenthe two colliding plates, the existence of which is marked by hightemperature, low mantle seismic wavespeed (correlating with latearriving signals from the 410 discontinuity), poor Sn propagation,east and southeast oriented global positioning system displace-ments, and strikingly larger seismic (SKS) anisotropy.

Tibetan lithosphere ∣ receiver functions ∣ anisotropy

It has long been recognised that the Tibetan plateau was createdby the collision of the northward moving Indian plate and the

relatively stationary Asian plate, which began about 50 million yrago (1). However, the mode of deformation of the mantle litho-spheres (2) remained largely unknown. A fundamental questionis whether the postcollision convergence of India and Asia, esti-mated at >2;000 km (3, 4), was accommodated by homogeneousthickening or plate subduction (2). Global positioning systems(GPS) measurements have shown that at present an eastward mo-tion dominates the surface deformation of northern and easternTibet (5). GPS and seismic anisotropy (6) indicate extrusion alsoof the deep Tibetan lithosphere to the east and southeast. Mostsurface wave studies revealed a thick lithosphere beneath muchof the plateau (7–12), whereas body wave tomography observedthe subducted Indian mantle lithosphere characterized by highwavespeed, in contrast to the Asian mantle lithosphere (13–15).Recently a high resolution P travel time tomographic study (15)imaged the high velocity Indian lithosphere in western Tibet belowthe entire plateau down to 300–400 km depth. In eastern Tibet,however, the front of the Indian plate is located south of the Yar-long–Zangbo Suture (YZS) (15). Relatively slow wave speeds arefound in theuppermantle below the central andnortheasternpartsof the plateau. Modeling indicates that the Tibetan part of thelithosphere originated from the progressive accretion of a numberof continental or island-arc type blocks before India came into di-rect contact with Asia (16) or stepwise subduction of the Asianplate (17). The Tibetan lithosphere is warm and weak in the northand northeast and therefore tends to deformwithin the frameworkof the Indo–Asian collision. High-frequency Sn waves are severelyattenuated in the uppermost part of the mantle (18). Barron and

Priestley (19) conclude from frequency dependent Sn propagationin Tibet that the lithosphere below the entire plateau is still intactand that there might be only in northern Tibet indications of a hotsub-Moho upper mantle with some melt. Liang and Song (20)obtain from Pn tomography a seismic model that is consistent withunderthrusting in the south, shortening in the north and extrusionin the east and southeast of Tibet. All this information aboutnorth-central and eastern Tibet leads to the conclusion that thiszone may act as a deformable crush zone between the collidingplates.

A large number of broadband passive source seismic experi-ments have been conducted in the Tibetan plateau over the last20 yr. Most of them are located in southern central and easternTibet. We conducted in the period from September 2005 to No-vember 2007 two densely spaced seismic profiles in the westernpart of the plateau (Fig. 1), thus filling gaps. Within the recordingperiod 478 teleseismic earthquakes with high signal-to-noise ratiowere recorded by approximately 150 stations. We calculated Pand S receiver functions from the seismic records. The P receiverfunction technique is a conventional seismic method routinelyused to investigate the Earth’s structure. A description of themore recent S receiver function technique can be found; e.g.,in Yuan et al. (21). The essential point of this technique is thatseismic waves from far away penetrate the Tibetan plateau frombelow. At interfaces with changing physical properties (seismicimpedance) mode conversion occurs from one wave type to an-other (i.e., compressional-to-shear or shear-to-compressional).The converted seismic waves are weak but can be made visiblewith techniques to obtain an image of discontinuities in the entireupper mantle below Tibet.

We describe here results from our two recent profiles and fromother experiments (INDEPTH, 1991–1992 PASSCAL, Tien Shanand Karakoram) (22–24). We herein call our recent profiles thewest and central lines. The station locations are shown in Fig. 1,as well as in Fig. S1, where the piercing point locations from theMoho, the lithosphere-asthenosphere boundary (LAB), and the410 and 660 km discontinuities are also indicated. Location ofearthquakes used are shown in Fig. S2. Fig. 2 shows the S receiverfunctions along the two recent profiles down to about 350 kmdepth. The P receiver functions down to 800 km depth are shownin Fig. 3. We did not show S receiver functions to a depth largerthan about 300 km, because receiver functions of S phases are notvery suitable to image the mantle transition zone.

Author contributions: J.Z. and H.L. designed research; J.Z., H.L., S.P., R.K., Z.Z., J.T., L.D.,X.G., Q.X., and W.W. performed research; J.Z., X.Y., H.L., P.K., Q.X., and W.W. analyzeddata; and J.Z., X.Y., and R.K. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Freely available online through the PNAS open access option.1To whom correspondence should be addressed. Email: kind@gfz-potsdam.de.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1001921107/-/DCSupplemental.

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ResultsThe Tibetan Moho can clearly be seen in the S (Fig. 2) and P(Fig. 3) receiver functions along the west and central lines. Weused the IASP91 global reference model (25) for the conversionof differential times into depths. The obtained depths are rela-tively insensitive to absolute velocities because the differentialtimes depend largely on velocity differences. In the S and Preceiver function sections (Figs. 2 and 3) the depth of the Mohoincreases from approximately 50 km at the southern edge of Tibetto approximately 80 km beneath the central plateau and de-creases again to shallower depth to the north. TheMoho suddenlyshallows by approximately 20 km across the border of Tibet to theTarim Basin, which is seen in P and S receiver functions. The ele-vation change across the Altyn–Tagh Fault (ATF) (Fig. 2A) con-tributes only about 10% to the Moho step observed there. SimilarMoho offsets are also observed at the border to the Qaidam andSichuan basins (26–29). The Moho offsets at the northern andeastern borders of Tibet are significant features and they indicatethat the dominant mode of crustal shortening is pure shear thick-ening without much underthrusting. This is very different fromsouthern Tibet. In southern Tibet, besides the general under-thrusting of the Indian crust below the Tibetan crust along theMain Himalayan Thrust, underthrusting of the Indian lower crustbelow the Tibetan crust is also observed north of the YZS in anextended region (22, 30–32).

A negative signal below the Moho may be identified in bothsections in Fig. 2 within the time window of 10–30 s, indicatinga seismic discontinuity with a relatively sharp velocity reductiondownward. We interpret these negative phases as representingthe LAB. On the west line (Fig. 2A), the Indian LAB graduallydeepens from approximately 120 km depth in the south toapproximately 200 km depth in central Tibet. It then remains hor-izontal up to the data gap at 36 °N. Further north, the Asian LAB

is visible at a depth of approximately 150 km beneath the ATF.There is a jump of about 50 km to shallower depth from IndianLAB to the Asian LAB at the northern edge of Tibet along thewest line. This indicates that the northwestern boundary of Tibetcuts across the whole lithosphere and therefore supports themode of deformation there, which is expressed by strike-slipfaults. On the central line (Fig. 2B) the Indian LAB is deepeningfrom about 120 km depth at the southern end of the profile toabout 200 km below the Bangong–Nujiang Suture (BNS). TheAsian LAB is deepening from 120 km beneath the northernend of the profile to about 140 km below the BNS. There is also,like at the west line, a jump of about 50 km between Indian andAsian LAB. This jump occurs on the central line below the BNS,and on the west line between Jinsha–River Suture (JRS) andATF. These locations of the LAB jumps seem to mark thelocation of the northernmost extension of the Indian lithospherebeneath Tibet along the west and central lines. The BNS was alsodetermined as northern end of the Indian mantle lithosphere bythe neighboring INDEPTH profile (23) [called east line in Fig. 1(see also Fig. S3)].

In the P receiver functions in Fig. 3 besides the Moho, we alsosee clearly the conversions from the 410 and 660 discontinuities.Also, typical for P receiver function, crustal multiples are visible.These multiples may be useful for crustal studies, but they arealso corrupting the depth region from about 200–300 km forthe observation of direct conversions. Multiples from internalcrustal discontinuities may also corrupt shallower depth regions.There are also a number of blue regions (meaning negative ve-locity jumps) below the Moho in Fig. 3, which are probably notmultiples. The clearest structures are marked by a red dashed

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Fig. 1. Map of the Tibetan plateau showing the seismic profiles. The westand central lines are our recent profiles. East line is a compilation of twoFrench experiments, the INDEPTH profiles and the PASSCAL experiment from1991–1992 (22, 23). The TK Line (24) is an additional profile with LAB obser-vations used in this paper. Key: Main Boundary Thrust (MBT), Main CentralThrust (MCT), Yarlong–Zangbo Suture (YZS), Bangong–Nujiang Suture (BNS),Jinsha–River Suture (JRS), Altyn–Tagh Fault (ATF), and Qaidam Basin Fault(QBF). Blue and green bars with arrows mark LAB observations of the Indian(IML) and Asian (AML) plates, respectively. Yellow and red boxes mark obser-vations of the upper mantle discontinuities. Red parts of these boxes markunusually late arrivals of the converted phases from the 410 discontinuity.Black bars at the northern margins of Tibet at the west line (Figs. 2A and3A) and east line (28, 29) denote sharp steps in Moho depth. Red dashed linemarks zone of poor Sn propagation (18, 19).

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Fig. 2. S receiver functions, sorted by piercing points at the 150 km conver-sion depth. Positive amplitudes are coded red, indicating an increasing velo-city jump with depth; negative amplitudes are coded blue, indicating anegative velocity jump with depth. The Moho and LAB phases are markedby dashed lines. The elevation is plotted at the top of each section, alongwith the position of the major sutures and faults. The Indian LAB is deepen-ing from about 100 km depth in northern India to about 200 km depth belowthe JRS at the west line and also to about 200 km below the BNS at the centralline. The Asian LAB is only slightly south dipping from about 120 to 140 km atboth profiles. There are weak indications at both profiles that the Asian LABmight continue some distance in south direction above the Indian LAB.

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line. The continuous thin black lines mark the locations of theAsian LAB from the S receiver functions in Fig. 2. Both linesare overlapping to a large extent. This could mean that the AsianLAB reaches relative far to the south above the Indian LAB. Thiswould indicate a complicated interaction of the two lithospheres.However, these observations are still too vague to be final.

We have assembled in Fig. 4 sketches of profiles across thecollision zones of the two plates based on observations of seismicdiscontinuities. Two older profiles [east line (23) and the TienShan–Karakoram (TK) line (24)] and our two recent profiles(west line and central line) are presented. Fig. 4 resemblessketches by Willett and Beaumont (2) who discuss possiblemodels of the collision of the two lithospheric plates. The crustis marked yellow in all profiles without any details or separationbetween the different plates, because details of the crustal inter-ior are not the aim of our present receiver function study. Theonly exception is the indicated sudden jumps in Moho depth

in northern and eastern Tibet (west line and east line). Thesesteps are also marked in Fig. 1 by black bars. The Moho is ob-served by both, P and S receiver functions. The Moho step indi-cated at the Qaidam Basin Fault (QBF in Fig. 4) north of the eastline was observed by Zhu and Helmberger (28) and Shi et al. (29).The Indian mantle lithosphere is marked blue and the Asianmantle lithosphere green in Fig. 4. All LAB observations are ob-tained with S receiver functions. The upper boundaries of thesubducting lithospheres are not observed, neither with P receiverfunctions, nor with S receiver function. There is only one excep-tion at the east line. Along this line additional positive disconti-nuities (velocity increase downward) have been observed in Preceiver functions above the LAB (22, 33). They are marked withblack dashed lines in Fig. 4 (east line). A summary of the LABobservations in Tibet is also shown in Fig. 1 (heavy blue and green

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Fig. 3. Migrated P receiver functions along west line (A) and along centralline (B). TheMoho and the discontinuities at about 410 and 660 km depth areclearly visible. Also crustal multiples appear at the apparent depth between200 and 300 km. The Moho step at the ATF is obvious (Fig. 3A). There areseveral blue structures below the Moho at both profiles, indicating negativevelocity jumps downward. The clearest ones are marked by red dashed lines.The Asian and Indian LAB (thin black lines) from Fig. 2 is also marked forcomparison. There seems to be a good correlation between Asian LAB inS and P receiver functions. There are, however, many more blue structuresin this depth region that remain unexplained. Multiples are always a severeproblem in P receiver functions, but not in S receiver functions.

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arrows). The northern extension of the Indian LAB beneathTibet ends abruptly at a depth of about 200 km near the ATFat the west line and near the BNS at the central and east lines.North of this jump the Asian LAB continues at a depth of about150 km and is shallowing slowly.

Fig. 4 makes also more clear how the mode of collision ischanging from western to eastern Tibet. The westernmost profile(TK line from Tien Shan to Karakoram) shows clearly dominat-ing Asian subduction. This is already known from seismicity. Inwestern Tibet (west line) Indian subduction is already moreemphasized than Asian subduction, but has a large horizontalcomponent and reaches very far north, almost to the Tarim Basin.Further east (central line and east line) Indian subduction isgetting steeper than Asian subduction.

DiscussionIn Fig. 5 are shown enlarged P receiver function details of theupper mantle discontinuities at 410 and 660 km depth along threeprofiles. The west and central lines are our new data. Data fromthe east line are from Kind et al. (22). On the west line both dis-continuities are close to the average value of the global IASP91model (thin solid lines). Both discontinuities are apparently dee-pening at the northern end of the central and east lines. This canbest be explained by a lower average velocity above 410 km depthin the north of both profiles. Regions with normal and late arrivaltime of the 410 discontinuity are marked in Fig. 1. It should benoted that the sudden changes in the thickness of the crust andmantle lithosphere have limited influence on the 410 arrival times[<0.5 s (Fig. S4 and Fig. S5)].

The relatively constant differential depths between the 410 and660 discontinuities along the west, central and east lines indicatethat the temperature in the mantle transition zone below mostparts of Tibet is constant. Laterally varying temperatures wouldchange the thickness of the transition zone due to opposingClapeyron slopes of the phase changes in the olivine system(34). The accuracy of this observation is about 10 km, whichcorresponds to about 70–100 °C, which would be the limit ofthe temperature stability. We conclude from these observationsthat present day lithospheric tectonics does not reach the mantle

transition zone below most parts of Tibet. This view is supportedby Li et al. (15) who see little connection between the northdirected upper mantle subduction of India and possible earlierdeeper and south directed subduction of the Tethyan ocean.

Fig. 6 shows results of seismic (SKS) splitting measurementsalong the west and central lines and a collection of available pre-vious measurements (6, 35–40). Anisotropy in the mantle derivedby shear wave splitting is usually explained by orientation of oli-vine crystals in the direction of mantle flow. Strongest anisotropy(with delay time >1 s) is observed in eastern Tibet and north ofthe northern Lhasa Terrane (between YZS and BNS) in centralTibet. The interpreted mantle flow directions agree well withsurface displacement obtained from GPS measurements. Muchsmaller delay times (many <0.5 s) or null splitting are observedaround the YZS on the central line, on the entire west line and atthe western and northern edges of Tibet. The southern part of thecentral and east line and the region of the west line are, however,also the region where Tibet is underlain by the Indian mantlelithosphere. That means the region of strong anisotropy is alsothe region of the deformable crush zone south of the Asianlithosphere, which is observed with a number of parameters(see introductory text).

To conclude, the LAB and the Moho below Tibet are wellobserved. Essential resulting components of the India–Asia colli-sion are the following: (i) Crustal shortening is in the south accom-modated by underthrusting of the Indian crust below the Asiancrust that may reach further north than the YZS. In north Tibet,crustal shortening is accommodated by homogeneous crustalthickening. (ii) The boundary between the Indian and Asian litho-spheres below Tibet runs roughly from the western Tarim Basin tothe Eastern Himalayan Syntaxis (which is in very good agreementwith tomographic results by Li et al. 15). There is a sharp depthstep between lower boundaries of both lithospheres along thisline. The Indian lithosphere reaches about 200 km depth whereasthe Asian lithosphere reaches only about 150 km depth along thisline. Also the inclination of both lithospheres might be changingfrom west to east. The Asian LAB is dipping shallower from westto east whereas the Indian lithosphere seems to steepen fromwestto east. (iii) The location of the deformable crush zone in north-central and eastern Tibet is confirmed by SKS anisotropy obser-vations and slow average velocities in the upper mantle obtainedfrom arrival times from the 410 discontinuity. The geometry of theIndian and Asian plate collision may also explain the difference in

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Fig. 5. Details of the structure of the 410 and 660 discontinuities. Significantdepression of the 410 and 660 km discontinuities can be clearly seen in northTibet on the central and east lines. Thin black lines are theoretical values ofthe IASP91 model. Blue, green, and red bars indicate locations of the differ-ent lithospheric units shown in Fig. 4.

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surface topography betweenwest and east Tibet. Themore ruggedand higher topography in west Tibet can be supported by the rigidmantle lithosphere there, whereas to the east the lithosphere isweaker due to the existence of the crush zone. Of course manydetails are still elusive, especially little seems to be known aboutthe lithospheric boundaries in the northeast. As a technicalcomment we would like to mention that data quality could besignificantly improved by future experiments with much closerstation spacing (3–5 km).

MethodsWe used the P and S receiver function method to identify and to map dis-continuities of elastic parameters and densities below Tibet. This is a standardseismic technique now. It utilizes signals from far away earthquakes that are

converted from the P mode to the S mode, or vice versa at material disconti-nuities below a seismic station. The structure of these discontinuities leads togeological conclusions. We deployed relatively dense temporary networks ofautonomously recording seismic stations for a time period of 2 yr to record asufficient number of large earthquakes. More details of the method aregiven in SI Text.

ACKNOWLEDGMENTS. We thank Jim Mechie and Stephan Sobolev for com-ments. This research is supported jointly by the Talent Project of the ChineseAcademy of Sciences (CAS), the Innovation Project of the CAS (Grant kzcx3-sw-143), and the National Natural Science Foundation (Grant 40652002).A number of instruments that we used were provided by the Institute ofGeology and Geophysics, CAS. We also acknowledge the support of theDeutsche Forschungsgemeinschaft.

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