Tectonophysics 465 (2009) 128–135

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Tectonophysics

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Crustal thickening and lateral extrusion during the Indo-Asian collision:A 3D viscous flow model

Youqing Yang ⁎, Mian LiuDept. of Geological Sciences, University of Missouri, Columbia, MO 65211, USA

⁎ Corresponding author.E-mail address: yangyo@missouri.edu (Y. Yang).

0040-1951/$ – see front matter. Published by Elsevier Bdoi:10.1016/j.tecto.2008.11.002

a b s t r a c t

a r t i c l e i n f o

Article history:

The ∼2000 km crustal shor Received 26 March 2008Received in revised form 21 October 2008Accepted 3 November 2008Available online 12 November 2008

Keywords:Crustal thickeningLateral extrusionIndo-Asian collisionHimalayan–Tibetan plateauAsian tectonics

tening from the Indo-Asian collision in the past 40–70 million years has beenaccommodated mainly by crustal thickening in the Himalayan–Tibetan Plateau and lateral extrusion ofblocks of Asian continents. However, the spatiotemporal evolution of crustal thickening and lateral extrusion,hence the far-field impact of the Indo-Asian collision on Asian tectonics, remains controversial. Here wepresent a 3D viscous flow model that simulates the partitioning of crustal material between thickening andlateral extrusion during the Indo-Asian collision. The model assumes conservation of crustal mass, and isconstrained by the history of the Indo-Asian plate convergence and by the present-day topography of theHimalayan–Tibetan Plateau. The results show that much of the early collision was absorbed by crustalthickening within the Himalayan–Tibetan plateau. However, lateral extrusion of crustal material hasgradually become dominant in the past ∼10–20 Myr, indicating increasing influence of the Indo-Asiancollision on Asian tectonics.

Published by Elsevier B.V.

1. Introduction

The Indo-Asian collision in the past ∼40–70 Myr has caused about2000 km crustal shortening (Yin and Harrison, 2000). Some collisionwas accommodated by crustal thickening in the Himalayan–TibetanPlateau and surrounding regions (England and Houseman, 1986); therest may be accommodated by lateral extrusion of blocks of Asiancontinent along numerous strike-slip fault systems (Tapponnier et al.,1982; Peltzer et al., 1989; Replumaz and Tapponnier, 2003) and byductile lower crustal flow out of Tibet (Clark and Royden, 2000) (Fig. 1).

How the shortened crust was partitioned between crustal thickeningand lateral extrusion during the Indo-Asian has important implicationsfor the diffuse Asian tectonics, yet the answers from previous studies areinconclusive. In a continuum mechanical model that approximates theAsian continent as a viscous thin sheet indented by a rigid Indian plate,England and Houseman (1986, 1988) have shown that the collision isaccommodatedmainly through crustal thickening in central Asia, mostlywithin the Himalayan–Tibetan plateau. Conversely, using an analogueexperiment of indentation of a plasticene Asian continent, Tapponnierand co-workers show far-reaching influence of the collision throughlateral extrusion of blocks of Asian continent (Tapponnier et al., 1982).Subsequent studies have narrowed the gap between these end membermodels (Houseman and England, 1993), and the perceptions of howcrustal thickens and extrudes have evolved. The lateral extrusion couldoccur in formsofbothdiscrete translationof crustal blocksanddistributed

.V.

extrusion of crustal material (Kong and Bird, 1996), or by lateral flow oflower crustal material out of the collision zone (Clark and Royden, 2000).On the other hand, the Himalayan–Tibetan Plateau may have risen indiscrete steps (Tapponnier et al., 2001). Nonetheless, the partitioning ofstrain between crustal thickening and lateral extrusion, and moreimportantly, the spatiotemporal evolution of such strain partitioning,remain controversial (Tapponnier et al., 2001; Wright et al., 2004).

The tectonic implications can be profound. When most of the Indo-Asian collision is absorbed by crustal thickening in the Tibetan Plateau,its impact would be limited to the plateau and surrounding regions(England and Molnar, 1997). Conversely, when the collision isaccommodated mainly by lateral extrusion of Asian continent, itsimpact on Asian tectonics would be immediate and far reaching. Hencehow the crustal shortening is partitioned between crustal thickeningand lateral extrusion controls both the rise of the Himalayan–TibetanPlateau and Cenozoic tectonics in much of the central and eastern Asia.

In this study we first estimate the amount of crustal shortening andthe portion accommodated by crustal thickening in the Himalayan–Tibetan plateau, based on the history of the Indo-Asian collision and thepresent crustal volume in the plateau.We then use a three-dimensional(3D) viscous flow model to simulate the spatiotemporal portioning ofthe shortened crust between crustal thickening and lateral extrusion.

2. The amount of crustal shortening and thickening

The relatively well-defined plate convergence history and thepresent-day crustal volume of the Himalayan–Tibetan Plateau permitsome estimation of the portioning of crustal mass between thickening

Fig. 1. Topography and simplified tectonic boundaries of the Himalayan–Tibetan Plateau and surrounding regions. The inset shows the movement of the Indian plate relative to stableEurasian. The fault zones bounding the plateau, together with the dashed lines, enclose the area of present Himalayan–Tibetan Plateau over which crustal volume in Fig. 5 iscalculated. Black dots shows the locations used in Fig. 6. H stands for Hindu-Kush; K for Kunlun, and G for Gonghe.

Fig. 2. Conceptual model for crustal mass balance. The control volume is defined by thepresent-day Himalayan–Tibetan crust, consisting of Vo, the initial crustal volume, andVt, the crustal volume added to the plateau by collision. Vin and Vout are the volumes ofcrustal mass entering and leaving the control volume, Vin−Vout=Vt.

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and lateral extrusion during the Indo-Asian collision (Dewey et al.,1989; Le Pichon et al., 1992; Molnar et al., 1993). If we use the present-day Himalayan–Tibetan Plateau as the control volume, we canestimate how much crustal material has been squeezed into thiscontrol volume since the beginning of the collision, and how much ofthe added crustal mass has been retained for plateau building. The restmust then be moved out of the control volume by lateral extrusion orother processes (see below).

The crustal volume of the present-day Himalayan–Tibetan Plateauis given by its surface area and the crustal thickness. At present theplateau may be defined along the Altyn Tagh fault (ATF) to its north,the Main Boundary Thrust (MBT) to its south, the Pamir plateau to itswest, and the Longmenshan thrust and the Xiaojiang–Red River Faultsto its east (Fig. 1). The area of the plateau so defined is ∼3×106 km2.The crustal thickness within the Himalayan–Tibetan Plateau may beconstrained by gravity and seismic results, which indicate that theplateau is generally in the Airy-type of isostasy (Houseman andEngland, 1996; Fischer, 2002). Seismic tomography and deep seismicsounding show that the Moho reaches 70–74 km beneath the Tibetanplateau (Li et al., 2006; Sun and Toksoz, 2006). Receiver functionanalyses show that the crust thickens from 50 km beneath the Qaidambasin to 80 km beneath the south Tibet (Kind et al., 2002; Shi et al.,2004). However, the coverage of seismological studies is insufficient toconstrain the crustal thickness of the entire plateau. So we use theelevation of the plateau and assume the Airy isostasy to estimate thecrustal thickness; the total crustal volume of the present-dayHimalayan–Tibetan Plateau thus calculated is 2.0×108 km3, assuming1) uniform crustal and mantle density of 2800 kg/m3 and 3300 kg/m3,respectively, and 2) initial elevation near the sea level (this serves asthe upper bound for crustal thickening within the plateau).

This crustal volumeof the present Tibetan Plateau includes the initialcrustal mass, Vo, and crustal thickening from the collision, Vt (Fig. 2). Vowould be 0.9×108 km3 if the initial crustal thickness was 30 km, or1.35×108 km3 if the initial crustal thicknesswas45 km. The initial crustalthickness is poorly constrained, but likely lies between 30 and 45 km, arange encompassing the crustal thickness of the India shield and mostlowland Asian continent. Accordingly,1.1–0.65×108 km3 of crustal mass(Vt in Fig. 2) has been added to build today's Tibetan Plateau.

The total volume of the crustal material pushed into the controlvolume by the Indian indenter during the collision (Vin in Fig. 2) maybe estimated from the collisional history between the Indian andEurasian plates, which is relatively well constrained from marinemagnetic anomalies (Acton, 1999; Schettino and Scotese, 2005)(Fig. 1). Because the continental margin of the Indian plate beforethe collision is not well constrained, we define the northern edge ofthe Indian intender along the MBT, the current collision front.Similarly, we assume the Altyn Tagh fault to be the northern boundaryof the collision zone. In doing so, we have lumped the effects ofcontinental margins of the Indian and the Asian plates, both poorlyconstrained, into the uncertainties of the initial crustal thickness.Uncertainties of other parameters, such as the inception time of thecollision, probably have less impact than that of the initial crustalthickness. Assuming the crust was initially 30–45 km thick, weestimate that the volume of crust that has entered the control volumesince 50 Ma is 1.8–2.7×108 km3.

Given that Vt=1.1–0.65×108 km3, or 61–24% of the Vin, the crustalmass entered the control volume, the rest 39–76% of Vin must be

Fig. 3. Finite element mesh and boundary conditions of the numerical model. The Indianindenter moves northward at 40 mm/yr near the western corner and 51 mm/yr at theeastern corner, with rigid transformation and rotation along the whole edge. Thedashed line shows the location of the Indian intender at present.

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accommodated by lateral extrusion and other processes, which mayinclude crustal thickening in the surrounding areas and erosion.

In this calculation we assumed mass conservation of the crustalmaterial. Some studies have found evidence for ecologites in the bottomof crust beneath southernTibet (Jacksonet al., 2004;Hetenyi et al., 2007).The potential eclogitizationmay significantly reduce crust volume in oldorogenic belts; however, eclogitization generally takes hundreds ofmillion years, so the amount of crustal mass affected by eclogitization inyoung orogens like the Tibetan plateau is likely insignificant (Fischer,2002). Similarly, although igneous intrusion or underplating may addcrustal mass, their impact is likely small when compared with the largeuncertainty in the pre-collision configuration of Indian and Tibetancontinental boundaries and initial crustal thickness.

3. The 3D finite-strain flow model

To simulate the spatiotemporal partitioning between crustalthickening and lateral extrusion during the Indo-Asian collision, wedeveloped a3Dviscousflowmodel (Fig. 3). Thismodel approximates theAsian continent as a power-law viscous plate indented by a stiff Indianplate, similar to previous viscous thin-sheet models (England andHouseman,1988). Themodel can include vertical variations of rheology.For this study we included a relatively weak lower crust 20-km belowthe surface to simulate the first-order effect of lower crustal flow(Royden,1996; Clark and Royden, 2000). We also considered first-orderrheological contrasts among the Indian indenter, the Himalayan–Tibetan orogenic zone, and the stiff Tarim and the South China blocks(Fig. 3). The major fault zones bounding these tectonic units arerepresented in the model with simplified geometry (Fig. 1) andsimulated as weak zones using a special fault element (Goodmanet al., 1968). The faults within the interior of the Tibetan plateau are notincluded. Their role is probably largely confined to the plateau (Kirbyet al., 2007). The evolution of the finite strain is calculated using thefinite element method (Yang et al., 2003).

The model includes only the crust, which is our primary concernhere. The crustal material subducted into the mantle, if happened, islikely minor (Houseman et al., 2000). Deformation in the mantle hasnomajor effects on the crustal volume of the Tibetan Plateau, althoughbuoyancy forces in the mantle may have contributed to the elevationof the central-northern Tibet (England and Houseman, 1989; Molnaret al., 1993). Nonetheless, most part of the high Tibetan Plateau isisostatically compensated by crustal roots (Fischer, 2002).

The geological boundary conditions are simplified in the model(Fig. 3). The southern boundary moves northward at 40 mm/yr at thewestern end of the Indian indenter front, with a 0.22°/Myr counterclockwise rotation around a pole at thewestern endof the indenter. Thisis an approximation of the trajectories of the Indian plate derived frommarine magnetic anomalies (Patriat and Achache, 1984; Ali andAitchison, 2005). The western side of the model domain is fixed in thenormal direction and free-slip in the tangential direction, reflectinggeological conditions imposed by the Pamir plateau. The Himalayan–TibetanPlateau is boundedby the Tarimblock to thenorth and theSouthChina block to the east, both assumed fixed in the model. Doing so isjustifiable because their motion is insignificant in comparison to that ofthe Indian Indenter in the Cenozoic (Replumaz and Tapponnier, 2003).More importantly, it is the relativemotion between the Indianplate andthese bounding blocks that controls crustal shortening during thecollision. On the surface and bottom of the model domain, the Winklerspringmattress (Desai, 1979) is applied to simulate topographic loadingand isostatic restoring force (Williams and Richardson, 1991).

In this model, the Indian indenter bulldozes crustal material in thecollisional zone, causing both crustal thickening in its front, and lateralextrusion in the form of crustal flow away from the collisional zone.Because of the rigid blocks surrounding the Himalayan–TibetanPlateau, the main exits for the lateral extruding crustal mass are onthe southwestern corner where crustal mass flows into Indochina, andon the northeastern side of the plateau (Clark and Royden, 2000).Accordingly, we used viscous-piston elements on these boundaries(indicated by pistons in Fig. 3), where the viscous resistance isproportional to the flow velocity at the boundaries. When theresistance coefficient is set to zero, most excessive crustal materialwould flow out of the collision zone. With a resistance coefficient of2.8×106 Pa s/m or higher, these boundaries act like retaining wallsthat trap all crust material within the plateau.We adjusted the viscousresistance of these elements to preserve the right amount of crustalmaterial required by that of today's Himalayan–Tibetan Plateau. Inmost cases the required resistance coefficient is 5×103 Pa s/m at theIndochina boundary and 2.8×105 Pa s/m on the Gobi boundary.

The initial configuration of the collision zone is constructed byrestoring the location of the Indian indenter relative to the Asiancontinent (Fig. 3). Given the uncertain initial crustal thickness of thecollision zone (Yin and Harrison, 2000), we tested different valuesranging from 30 km to 45 km in the model.

The model assumes the rheology of the power-law fluid. Weexplored different rheology with the power index varying between 1and 10. The optimal models have a power index of 3 and effectiveviscosity of 6×1022 Pa s (defined at the strain rate of 10−15 s−1) for theupper curst (top 10 km), 4×1022 Pa s for the middle crust (10–20 kmdepth), and 1×1019–4×1022 Pa s for the lower crust, within thecollision zone. For the Indian indenter, the Tarim block, and the SouthChina block, we used a high effective viscosity of 4×1023 Pa s for theentire crust. The boundary faults are simulated as 20-km wide zonesof a relatively low viscosity, 3×1019 Pa s in most cases. Increasing theviscosity of the fault zone would simulate strong coupling across thefault. The fault zone would be effectively shut off (fully locked), if itsviscosity takes the value same as that of the ambient crust. Geologicalstudies indicate that the Altyn Tagh fault and the Longmenshan faultwere active at the early stage of the Indo-Asian collision (Yin et al.,2002; Burchfiel et al., 2008), so we included these faults from thebeginning of the modeling. On the other hand, the Main BoundaryThrust was probably formed quite late, but is included from thebeginning. This simplification may lead to overestimate of the role ofstrike-slipping during the early stages of collision.

4. Model results

Based on the history of plate convergence, we conducted a series offorwardmodels to simulate the partitioning of crustalmaterial between

Fig. 4. Snapshots of the simulated Indo-Asian collision and the rise of the Himalayan–Tibetan Plateau. For this case the viscosity is 1×1021 Pa s for the lower crust in the orogenic zone; other rheological values are given in the text. All fault zonesbounding the Tarim and Shichuan-South China block, and theMain Boundary Fault are simulatedwith a 20 kmwide zone of low viscosity (1×1020 Pa s). Viscous resistance at the exit boundaries (shown by pistons in Fig. 3) is proportional to thevelocity at the boundaries; the resistance coefficient is 5×103 Pa s/m at the Indochina boundary, 2.8×105 Pa s/m on the Gobi boundary. The inset in (c) compares the predicted topography (thick lines) with that of present Tibetan plateau.

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Fig. 5. (a) Crustal volume telescoped by the Indian indenter and that absorbed by crustalthickening within the Himalayan–Tibetan Plateau. Gray bands show the range withdifferent initial crustal thickness (marked by the numbers in kilometer). (b) Estimate oferoded crustal material from the Himalayan–from Metivier et al. (1999). (c) Theproportion of crustal thickening, lateral extrusion, and erosion, averaged over 5-Myrbins, in accommodating the crust telescoped by the Indian indenter, calculated with a35-km thick initial crust in the collisional zone.

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thickeningwithin the Himalayan–Tibetan Plateau and lateral extrusion.The acceptable models should reproduce the present-day crustalvolume and general topography of the Himalayan–Tibetan Plateau.Fig. 4 shows snapshots of one of the simulations. As the Indian platecollided with the Asian continent, crust was thickened near the Indianindenter and then spread northward, as in previous thin-sheet models(England and Houseman,1986; England and Houseman,1988). Becauseof the collisional zone narrows to its west (a shorter than averagedistance between thewestern Indian indenter and the Tarim block), thewestern portion of the Himalayan–Tibetan Plateau rises faster andhigher, and spreads eastwards. Lateral extrusion, in the form of crustalflow out of the collisional zone, depends on the crustal viscosity and theboundary conditions. We found that flow in the weak lower crust isinstrumental in reproducing the strikingly flat Tibetan plateau (Fig. 4c),as suggested by Zhao and Morgan (1987).

In the model the Indian indenter moves at a constant rate of 40–51mm/yr, thus its 2700-km long northern edge moves 3.7–5.4 km3 ofcrustal material per year toward today's Himalayan–Tibetan Plateau,depending on the assumed initial crustal thickness (30–45 kmin the model). Over the past 50 Myr, this process has squeezed out1.8–2.7×108 km3 crustal mass (Fig. 5a), as in the aforementionedestimation. The portion accommodated by crustal thickening in thepresent Tibetan Plateau by a given time during the collision can becalculated by volume integration over the area enclosed by the Pamir,the ATF, the Longmenshan Fault and the MBT, whose positions weretraced in the model. The total amount of the added crustal mass forplateau building, however, is constrained by today's crustal volume(2.0×108 km3). Depending on the initial crustal thickness, in themodel the viscous resistance along the “exit” boundaries (shown aspistons in Fig. 3) is adjusted to retain the right amount of crustal mass.We found that, within the range of 30–45 km thick initial crust, crustalthickening absorbed much of the collision in the early history of thecollision, but its role has gradually declined with time. The relativeproportion between crustal thickening and lateral extrusion dependson the initial crustal thickness. A thicker initial crust would requireless crustal thickening within the Himalayan–Tibetan Plateau, andconsequently more lateral extrusion in the later periods, but the sametime-dependent trend of partitioning between thickening and lateralextrusion are independent of the initial crustal thickness.

The influxof crustal material not accommodated by plateau buildingmust be absorbed by lateral extrusion, either by translation ofcontinental blocks along strike-slip faults or by crustal flow out of theTibetan Plateau, and by other processes, mainly erosion. This part hasincreasedwith time (Fig. 5a). The contribution fromerosion is difficult todetermine, but some rough estimates may be derived from Cenozoicsediments in basins around central Asia (Metivier et al., 1999) (Fig. 5b).Subtracting the contribution from erosion gives the estimate of lateralextrusion. Fig. 5c shows the proportion of crustal thickening, lateralextrusion, and erosion at selected time intervals from a model with35 km initial crustal thickness. Given the uncertainties of initial crustalthickness and the rate of erosion, Fig. 3c is meant only to illustrate thegeneral trend of partitioning among the various processes. The outflowof crustal mass was less than 13% of the inflow in the first 10 Myr butover 100% in the last 5Myr. The negative contribution from crustalmassadded to the plateau in the past ∼5 Myr means that the total crustalvolume of the Himalayan–Tibetan Plateau, as defined in Fig. 1, isshrinking as the indentation continues to shrink the surface areaenclosed by those boundary faults in Fig. 1. However, the loss of surfacearea and crustal volume does not necessarily mean lose of crustalthickness and elevation. Because within the plateau, either adding newcrustalmass from outside or having internal shortening can both lead tocrustal thickening and hence uplift.

The predicted uplift history of the plateau is shown in Fig. 6.Everywhere within the Tibetan Plateau, the uplift accelerated withtime. The uplift history varies from place to place; it was fastest inthe western part and slowest in the northeastern part of the plateau.Nonetheless, most part of the plateau rose above 3 km about 10–20Myrago, broadly consistent with the timing of initiation of the widespreadnorth-trending grabens in the Tibetan Plateau (Coleman and Hodges,1995; Yin, 2000).

5. Discussion

The 2000–2500 km northward motion of the Indian plate relativeto Eurasian plate in the past 40–70 Ma implies that up to 2.7×108 km3

crustal material between the two plates has been telescoped. About61–24% of this crustal mass have been accommodated by thickeningwithin the Himalayan–Tibetan Plateau, the rest has to bemoved out ofthe collision zone, either by lateral translation of blocks of the Asiancontinent or by crustal flowout of the Himalayan–Tibetan Plateau, andby other processes such as erosion.

Fig. 6. Predicted uplift history of selected locations (shown in inset) in the Tibetan Plateau.

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The current crustal motion in the Himalayan–Tibetan Plateau andsurrounding regions are shown by the Global Positioning System(GPS) measurements (Fig. 7). The GPS velocities represent mainlyshort-term strain rates, and it is unclear how the crustal motion varieswith depth. Nonetheless, if we take the GPS velocities as anapproximation of the average motion of the whole crustal blocks,we may estimate the crustal mass flux in and out of the Himalayan–Tibetan Plateau enclosed by the simplified boundaries in Fig. 7.Through each boundary segment themass flux is given by the product

Fig. 7. GPS velocity relative to stable Eurasia (data from Zhang et al. (2004)). The error ellipsinflux and efflux of crustal material are calculated. See text for details.

of the length of the segment, the local crustal thickness, and thevelocity of crustal motion normal to the segment, extrapolated fromthe GPS data. We found that the total influx of crustal material fromthe Himalayan front is 3.3–3.7 km3/yr, depends on the average crustalthickness of the Indian indenter; the total efflux from other sides ofthe Tibetan Plateau is 54–70% of the influx, close but lower than ourestimated rate of extruding crustal mass (Fig. 5c). One possibleexplanation is the faster lower crustal flow than at the earth's surface.In our model the rates of lower crustal flow can be orders of

es are 95% confidence intervals. The line segments enclose the plateau over which the

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magnitude higher that the surface velocity. Royden and co-workersappealed to such lower crustal flow to explain the rise of eastern Tibetwhere surface shortening is insufficient (Clark and Royden, 2000;Royden et al., 2008).

The spatiotemporal partitioning of the telescoped crustal materialis simulated in a viscous flow model, constrained by 1) the history ofthe Indo-Asian convergence, and 2) the topography (or the crustalvolume) of the present Himalayan–Tibetan Plateau. Satisfying theseconstraints requires trade-offs between free or less constrained modelparameters. For example, weakening the boundary fault zones in themodel would enhance lateral extrusion, but then higher viscousresistance must be applied at the Indochina and the Gobi boundaries(indicated by pistons in Fig. 3) to retain enough crustal material toreproduce the present Tibetan topography.

Our finite strain model is a first-order approximation. Manyparameters are poorly constrained and may change the details of themodel predictions. One major uncertainty is the initial crustalthickness of the collision zone. Some part of the Tibetan Plateauprobable stood high with a thick crust at the beginning of the Indo-Asian collision (Murphy et al., 1997; DeCelles et al., 2002; Rowley andCurrie, 2006; Yin, 2006). In this case less of the telescoped crustalmaterial would be needed to build the present Tibetan Plateau,consequentlymore lateral extrusionwould be predicted. The effects ofmany other uncertainties, such as the initial geometry and structure ofthe margins of the Indian intender and the Asian continent, as well asthe paleo-positions of the Indian indenter (Ali and Aitchison, 2005),can be lumped into the large range of initial crustal thickness exploredin the model. As shown in Fig. 5a, while the details vary with thedifferent initial crustal thickness, the general trend of partitioning ofcrustal mass between thickening and lateral extrusion remains thesame.

Other uncertainties include the boundary conditions. In the modelwe hold both the South China and the Tarim blocks fixed, althoughthey both have moved away from the Indo-Asian collisional zonethrough the Cenozoic. However, their moving histories are not wellconstrained. Besides, their motion is of second order relative to that ofthe Indian indenter, thus their impact on the model is limited. Erosionis not included in the model, and the estimated rates of erosion areassociated with large intrinsic uncertainties (Metivier et al., 1999). Weincluded erosion in Fig. 5 to show the complete partitioning of thetelescoped crustal material. Additionally, some of the lower crust ofthe Indian plate may have experienced eclogitization and subductedinto the mantle (Le Pichon et al., 1992). The amount, while hard toconstrain, is unlikely to be significant to alter the general results in thisstudy.

With all the uncertainties, themodel consistently predicts that mostof the Indo-Asian collision was accommodated by crustal thickeningwithin the Himalayan–Tibetan Plateau during the early stages of thecollision. However, the role of crustal thickening declineswith time, andis gradually replaced by lateral extrusion of crustal material. Theunderlying physics for this trend is straightforward. In the model, thedriving force for lateral crustal flow is the gradient of gravitationalpotential energy between the Tibetan Plateau and the surroundinglowland. As the Tibetan Plateau rises, the gravitational force increases,hence the lateral crustal extrusion intensifies (England and Molnar,1997;Konget al.,1997). Furthermore, crust thickening increases theflowchannel of the weak lower crust, hence promoting lateral extrusion.

The increasing role of lateral extrusion is generally consistent withinitiation of widespread north-trending grabens in south and centralTibet (Armijo et al., 1986; Blisniuk et al., 2001), which has beenexplained either as gravitational spreading of the elevated TibetanPlateau (Dewey, 1988; Liu and Yang, 2003) or as a consequence oflateral extrusion of the Tibetan crust (Yin, 2000). Note that in ourmodel lateral extrusion is defined as crustal material flowing out ofthe presently defined Tibetan Plateau. Thus it includes both lateraltranslation of rigid blocks of Asian continents along strike-slip faults,

and ductile flow of crustal material that may have contributed tocrustal thickening in other parts of Asia. We cannot distinguish thesetwo processes frommodeling. It is conceivable that, when the Tibetancrust was relatively thin and strong during the early stages of collision,crustal flow is ineffective and block motion along strike-slip faults wasthe main form of extrusion. This may help explain early fault slipsalong some of the faults within and around the plateau, such as thelate Oligocene to early Miocene slip along the Red River fault (Gilleyet al., 2003). As the crust become thicker and presumably weaker,ductile flow in the lower crust would become more effective (Clarkand Royden, 2000). The increasing role of lower crustal flow isconsistent with late Miocene and younger uplift in eastern Tibet(Royden et al., 1997; Schoenbohm et al., 2006). Rapid changes oflithospheric rheology and boundary conditions, such as delaminationor convective downwelling of mantle lithosphere under Tibet(England, 1993), could cause departure from the gradual change ofcrustal thickening and lateral extrusion predicted in this model(Tapponnier et al., 2001).

The existence of a weak, ductile lower crust under the TibetanPlateau and its role in the plateau building have been challenged byrecent results of consistent GPS velocities and the fast polarizationdirection of split SKS waves (Flesch et al., 2005;Wang et al., 2008). Theapparently vertical coherence of deformation near surface and in theuppermost mantle has been interpreted as evidence for a strongcrust–mantle mechanical coupling that is unfavorable for theexistence of a weak lower crust (Bendick and Flesch, 2007; Wanget al., 2008). We suggest that the vertical coherence of lithosphericdeformation within the Tibetan Plateau may simply result from thetectonic boundary conditions. The Tarim block and the South Chinablocks are rigid structures with high seismic velocities extending toN200 km depth (Liu and Jin, 1993; Huang and Zhao, 2006), with theIndian plate indenting from the south and southwest, the crustal andmantle mass is forced to extrude southwestward and northeastward(Fig. 1). Hence the coherent surface and mantle deformation cannotexclude the existence of a weak lower crust. Royden et al. (2008) haveshown clearly reduced S wave velocities in the lower crust undereastern Tibet that are indicative of a weak lower crust.

The increasing role of lateral extrusion may also explain tectonicsin many regions in central and eastern Asia where early Cenozoictectonics show no clear link to the Indo-Asian collision (Northrupet al., 1995), whereas the impact of the collision in these regions isclear today (Avouac and Tapponnier, 1993; Zhang et al., 2003).

6. Conclusions

Major conclusions we may draw from this study include thefollowing:

1) In Indo-Asian collision has telescoped up to 2.7×108 km3 crustalmaterial between these two plates. About 61–24% of the shortenedcrustal material has been accommodated by crustal thickeningwithin the Himalayan–Tibetan Plateau; the rest has been accom-modated by lateral extrusion, either in form of lateral translation ofblocks of the Asian continent or by ductile flow in the lower crust,and by other processes such as erosion.

2) Most of the Indo-Asian collision was accommodated by crustalthickening within the Himalayan–Tibetan Plateau during the earlystages of collision. The plateau rose from south to north at thebeginning, and then from west to east. Most part of the TibetanPlateau likely reached an elevation of 3 km or more about15 million years ago.

3) As the Himalayan–Tibetan rises, lateral extrusion has graduallybecome the dominant way to accommodate the Indo-Asian collisionin the past 10–15Ma. The increasing role of lateral extrusion impliesbroader and stronger impact of the Indo-Asian collision on Asiantectonics.

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Acknowledgements

This work was supported by NSF grants EAR0207200 and 0710354,and grant 40228005 of the National Science Foundation of China. Wethank two anonymous reviewers and editor Sandiford for helpful andconstructive comments.

References

Acton, G.D., 1999. Apparent polar wander of India since the Cretaceous withimplications for regional tectonics and true polar wander. In: Radhakrishna, T.,Piper, J.D.A. (Eds.), The Indian Subcontinent and Gondwana: A Palaeomagnetic andRock Magnetic Perspective. Mem. Geol. Soc. India, pp. 129–175.

Ali, J.R., Aitchison, J.C., 2005. Greater India. Earth-Science Reviews 72 (3–4), 169–188.Armijo, R., Tapponnier, P., Mercier, J.L., Han, T.L., 1986. Quaternary extension in Southern

Tibet: field observations and tectonic implications. J. Geophys. Res. 91, 13803–13872.Avouac, J.P., Tapponnier, P., 1993. Kinematic model of active deformation in Central Asia.

Geophys. Res. Lett. 20 (10), 895–898.Bendick, R., Flesch, L., 2007. Reconciling lithospheric deformation and lower crustal flow

beneath central Tibet. Geology 35 (10), 895–898.Blisniuk, P.M., et al., 2001. Normal faulting in central Tibet since at least 13.5 Myr ago.

Nature (London) 412 (6847), 628–632.Burchfiel, C., et al., 2008. A geological and geophysical context for the Wenchuan

earthquake of 12 May 2008, Sichuan, People's Republic of China. GSA Today 18 (7),4–11. doi:10.1130/GSATG18A.1.

Clark, M.K., Royden, L.H., 2000. Topographic ooze: building the eastern margin of Tibetby lower crustal flow. Geology (Boulder) 28 (8), 703–706.

Coleman, M., Hodges, K., 1995. Evidence for Tibetan Plateau uplift before 14 Myr agofrom a new minimum age for east–west extension. Nature 374, 49–52.

DeCelles, P.G., Robinson, D.M., Zandt, G., 2002. Implications of shortening in theHimalayan fold-thrust belt for uplift of the Tibetan Plateau. Tectonics 21 (6).

Desai, C.S., 1979. Elementary Finite Element Method. Prentice-Hall, Englewood Cliffs, N. J.434 pp.

Dewey, J.F., 1988. Extensional collapse of orogens. Tectonics 7, 1123–1139.Dewey, J.F., Cande, S., Pitman, W.C., 1989. Tectonic evolution of the Indian–Eurasia

collision zone Ecol. Geol. Helv. 82, 717–734.England, P., 1993. Convective removal of thermal boundary layer of thickened

continental lithosphere: a brief summary of causes and consequences with specialreference to the Cenozoic tectonics of the Tibetan Plateau and surrounding regions.Tectonophysics 223, 67–73.

England, P.C., Houseman, G.A., 1986. Finite strain calculations of continental deforma-tion 2. Comparison with the India–Asia collision. J. Geophys. Res. 91, 3664–3676.

England, P., Houseman, G., 1988. The mechanics of the Tibetan plateau. Phil. Trans. R.Soc. Lond. A327, 379–413.

England, P.C., Houseman, G.A., 1989. Extension during continental convergence withspecial reference to the Tibetan plateau. J. Geophys. Res. 94, 17,561–17,597.

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

Fischer, K.M., 2002. Waning buoyancy in the crustal roots of old mountains. Nature 417(6892), 933–936.

Flesch, L.M., et al., 2005. Constraining the extent of crust–mantle coupling in centralAsia using GPS, geologic, and shear wave splitting data. Earth Planet. Sci. Lett. 238(1–2), 248–268.

Gilley, L.D., et al., 2003. Direct dating of left-lateral deformation along the Red Rivershear zone, China and Vietnam. J. Geophys. Res. 108 (B2).

Goodman, R.E., Taylor, R.L., Brekke, T.L., 1968. A model for the mechanics of jointed rock.American Society of Civil Engineers, New York, pp. 637–659.

Hetenyi, G., et al., 2007. Density distribution of the India plate beneath the Tibetanplateau: geophysical and petrological constraints on the kinetics of lower-crustaleclogitization. Earth Planet. Sci. Lett. 264 (1–2), 226–244.

Houseman, G., England, P., 1993. Crustal thickening versus lateral expulsion in theIndian–Asian continental collision. J. Geophys. Res. 98 (7), 12233–12249.

Houseman, G., England, P., 1996. A lithospheric-thickening model for the Indo-Asiancollision. In: Yin, A., Harrision, T.M. (Eds.), The Tectonic Evolution of Asia. CambridgeUniversity, pp. 3–17.

Houseman, G.A., Neil, E.A., Kohler, M.D., 2000. Lithospheric instability beneath theTransverse Ranges of California. J. Geophys. Res. 105 (B7), 16237–16250.

Huang, J., Zhao, D., 2006. High-resolutionmantle tomography of China and surroundingregions. J. Geophys. Res. 111 (9), B09305. doi:10.1029/2005JB004066.

Jackson, J.A., Austrheim, H., McKenzie, D., Priestley, K., 2004. Metastability, mechanicalstrength, and the support of mountain belts. Geology 32 (7), 625–628.

Kind, R., et al., 2002. Seismic images of crust and upper mantle beneath Tibet: evidencefor Eurasian plate subduction. Science 298 (5596), 1219–1221.

Kirby, E., et al., 2007. Slip rate gradients along the eastern Kunlun fault. Tectonics 26 (2).Kong, X., Bird, P., 1996. Neotectonics of Asia: thin-shell finite-element models with

faults. In: Yin, A., Harrison, T.M. (Eds.), The Tectonic Evolution of Asia. World AndRegional Geology Series. Cambridge University Press, pp. 18–36.

Kong, X., Yin, A., Harrison, T.M., 1997. Evaluating the role of preexisting weakness andtopographic distributions in the Indo-Asian collision by use of a thin-shellnumerical model. Geology 25, 527–530.

Le Pichon, X., Fournier, M., Jolivet, L., 1992. Kinematics, topography, shortening, andextrusion in the India–Eurasia collision. Tectonics 11 (6), 1085–1098.

Li, S.L., Mooney, W.D., Fan, J.C., 2006. Crustal structure of mainland China from deepseismic sounding data. Tectonophysics 420 (1–2), 239–252.

Liu, F., Jin, A., 1993. Seismic tomography of China. In: Iyer, H.M., Hirahara, K. (Eds.), SeismicTomography: Theory and Practice. Chapman and Hall, New York, pp. 299–310.

Liu, M., Yang, Y., 2003. Extensional collapse of the Tibetan Plateau: results of three-dimensional finite element modeling. J. Geophys. Res. 108 (8), 2361. doi:10.1029/2002JB002248.

Metivier, F., Gaudemer, Y., Tapponnier, P., Klein, M., 1999. Mass accumulation rates inAsia during the Cenozoic. Geophys. J. Int. 137 (2), 280–318.

Molnar, P., England, P., Martinod, J., 1993. Mantle dynamics, uplift of the Tibetan plateau,and the Indian monsoon. Rev. Geophys. 31 (4), 357–396.

Murphy, M.A., et al., 1997. Did the Indo-Asian collision alone create the Tibetan plateau?Geology 25 (8), 719–722.

Northrup, C.J., Royden, L.H., Burchfiel, B.C., 1995. Motion of the Pacific plate relative toEurasia and its potential relation to Cenozoic extension along the eastern margin ofEurasia. Geology 23 (8), 719–722.

Patriat, P., Achache, J., 1984. India–Eurasia collision chronology has implications forcrustal shortening and driving mechanism of plates. Nature (London) 311 (5987),615–621.

Peltzer, G., Tapponnier, P., Amijio, R., 1989. Magnitude of late Quaternary left-lateraldisplacement along north edge of Tibet. Science 246, 1285–1289.

Replumaz, A., Tapponnier, P., 2003. Reconstruction of the deformed collision zonebetween India and Asia by backward motion of lithospheric blocks. J. Geophys. Res.108 (6), 2285. doi:10.1029/2001JB000661.

Rowley, D.B., Currie, B.S., 2006. Palaeo-altimetry of the late Eocene to Miocene Lunpolabasin, central Tibet. Nature 439 (7077), 677–681.

Royden, L., 1996. Coupling and decoupling of crust and mantle in convergent orogens:implications for strain partitioning in the crust. J. Geophys. Res. 101, 17,679–17,705.

Royden, L.H., et al., 1997. Surface deformation and lower crustal flow in Eastern Tibet.Science 276, 788–790.

Royden, L.H., Burchfiel, B.C., Hilst, R.D.v.d., 2008. The geological evolution of the TibetanPlateau. Science 321, 1054–1058.

Schettino, A., Scotese, C.R., 2005. Apparent polar wander paths for the major continents(200 Ma to the present day): a palaeomagnetic reference frame for global platetectonic reconstructions. Geophys. J. Int. 163 (2), 727–759.

Schoenbohm, L.M., Burchfiel, B.C., Chen, L.Z., 2006. Propagation of surface uplift, lowercrustal flow, and Cenozoic tectonics of the southeast margin of the Tibetan Plateau.Geology 34 (10), 813–816.

Shi, D., et al., 2004. Detection of southward intracontinental subduction of Tibetanlithosphere along the Bangong–Nujiang suture by P-to-S converted waves. Geology32 (3), 209–212.

Sun, Y.S., Toksoz, M.N., 2006. Crustal structure of China and surrounding regions fromP wave traveltime tomography. J. Geophys. Res. 111 (B3), 12.

Tapponnier, P., Peltzer, G., Le Dain, A.Y., Armijo, R., Cobbold, P., 1982. Propagatingextrusion tectonics in Asia; new insights from simple experiments with plasticine.Geology (Boulder) 10, 611–616.

Tapponnier, P., et al., 2001. Oblique stepwise rise and growth of the Tibet Plateau.Science 294 (5547), 1671–1677.

Wang, C.Y., Flesch, L.M., Silver, P.G., Chang, L.J., Chan, W.W., 2008. Evidence formechanically coupled lithosphere in central Asia and resulting implications.Geology 36 (5), 363–366.

Williams, C.A., Richardson, R.M.,1991. A rheological layered three-dimensional model ofthe San Andreas Fault in central and southern California. J. Geophys. Res. 96,16597–16623.

Wright, T.J., Parsons, B., England, P.C., Fielding, E.J., 2004. InSAR observations of low sliprates on the major faults of western Tibet. Science 305 (5681), 236–239.

Yang, Y., Liu,M., Stein, S., 2003. A 3-D geodynamicmodel of lateral crustalflowduringAndeanmountain building. Geophys. Res. Lett. 30 (21), 2093. doi:10.1029/2003GL018308.

Yin, A., 2000. Model of Cenozic east–west extension in Tibet suggesting a commonorigin of rifts in Asia during the Indo-Asia collision. J. Geophy. Res. 105 (9),21745–21759.

Yin, A., 2006. Cenozoic tectonic evolution of the Himalayan orogen as constrained byalong-strike variation of structural geometry, exhumation history, and forelandsedimentation. Earth-Sci. Rev. 76 (1–2), 1–131.

Yin, A., Harrison, M., 2000. Geological evolution of the Himalayan–Tibetan orogen. Ann.Rev. Earth Planet. Sci. 28, 211–280.

Yin, A., et al., 2002. Tectonic history of the Altyn Tagh fault system in northern Tibetinferred from Cenozoic sedimentation. Geol. Soc. Am. Bull. 114 (10), 1257–1295.

Zhang, Y.Q., Ma, Y.S., Yang, N., Shi, W., Dong, S.W., 2003. Cenozoic extensional stressevolution in North China. J. Geodyn. 36 (5), 591–613.

Zhang, P.Z., et al., 2004. Continuous deformation of the Tibetan Plateau from globalpositioning system data. Geology 32 (9), 809–812.

Zhao, W.-L., Morgan, W.J., 1987. Injection of Indian crust into Tibetan lower crust;a two-dimensionalfinite element model study. Tectonics 6 (4), 489–504.