quantifying landscape differences across the tibetan ... · and relief structure, within a sound...

Quantifying landscape differences across the Tibetan plateau:

Implications for topographic relief evolution

J. Liu-Zeng,1,2 P. Tapponnier,2 Y. Gaudemer,2 and L. Ding1

Received 28 August 2007; revised 4 September 2008; accepted 14 October 2008; published 31 December 2008.

[1] We quantify the bulk topographic characteristics of the Tibet-Qinghai plateau withspecific focus on three representative regions: northern, central, and southeastern Tibet.Quantitative landscape information is extracted from Shuttle Radar TopographyMission digital elevation models. We find that the morphology of the Tibetan plateau isnonuniform with systematic regional differences. The northern and central parts of theplateau are characterized by what we suggest to call ‘‘positive topography,’’ i.e., atopography in which elevation is positively correlated with relief and mean slope. A majorchange from the internally drained central part of Tibet to the externally drained part ofeastern Tibet is accompanied by a transition from low to high relief and from positiveto ‘‘negative topography,’’ i.e., a topography where there is an inverse or negativecorrelation between elevation and relief and between elevation and mean slope. Relief ineastern Tibet is largest along rivers as they cross an ancient, eroded plateau margin at highangle to the major strike-slip faults, the Yalong-Yulong thrust belt, implying strongstructural control of regional topography. We propose that the evolution of river systemsand drainage efficiency, the ability of rivers to transport sediments out of the orogen,coupled with tectonic uplift, is the simplest mechanism to explain systematic regionaldifferences in Tibetan landscapes. Basin filling due to inefficient drainage played amajor role in smoothing out the tectonically generated structural relief. This mode ofsmoothing started concurrently with tectonic construction of the relief, as most clearlyillustrated today in the Qilian Shan-Qaidam region of the northeastern plateau. Inthe interior of Tibet, further ‘‘passive’’ filling, due to internal drainage only, continued tosmooth the local relief millions of years after the cessation of major phases of surfaceuplift due to crustal shortening. Thus, diachronous beveling at high elevation produced thelow-relief surface of the high plateau. In southeast Tibet, headward retreat of externaldrainages brought back ‘‘in’’ the global ocean base level, first disrupting then interruptingthe relief-reduction process. It produced a transitional topography by dissecting the ‘‘old’’remnant plateau surface, which introduced younger and steeper incision of this hithertopreserved high base level. This provides a unifying mechanism for the formation ofthe low-relief plateau interior, and for the origin of the high-elevation, low-reliefrelict surface in southeastern Tibet. Our analysis brings forth the importance of surfaceprocesses, in particular drainage efficiency, in shaping plateau morphology andlandscape relief. Such key processes appear to have been mostly ignored in numericalmodels of plateau deformation. Our results also cast doubt on and provide a morerealistic alternative to the fashionable contention that a continuous preuplift, low-reliefsurface first formed at low elevation, extending all the way to the South China Seashore, before being warped upward in the late Miocene-Pliocene by lower crustalchannel flow.

Citation: Liu-Zeng, J., P. Tapponnier, Y. Gaudemer, and L. Ding (2008), Quantifying landscape differences across the Tibetan

plateau: Implications for topographic relief evolution, J. Geophys. Res., 113, F04018, doi:10.1029/2007JF000897.

1. Introduction

[2] The Tibetan plateau is the world’s highest and largestorogenic plateau. It has a low-relief internally drainedinterior, flanked in the north and south by steep-sided edges[Fielding et al., 1994], the southern one including Earth’shighest summits with elevations greater than 8 km. Toward

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, F04018, doi:10.1029/2007JF000897, 2008

1Institute of Tibetan Plateau Research, Chinese Academy of Sciences,Beijing, China.

2Laboratoire de Tectonique, Institut de Physique du Globe, Paris,France.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2007JF000897

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the southeast and northeast, the plateau shows more gentlytapered topography, providing escape routes for some of thelargest Asian rivers.[3] Continuum models of Tibet plateau deformation have

been primarily motivated by these general topographicfeatures and their gravitational implications, and sometimesbased solely upon them. For instance, the low-relief interiorof the plateau has often been taken as evidence of viscousflow in the lower crust, a mechanism thought necessary tosmooth out irregularities in topography and crustal thick-ness [Bird, 1991; Shen et al., 2001; McKenzie and Jackson,2002]. At the plateau scale, these continuum models,whether the vertically averaged ‘‘thin viscous’’ sheet orthe decoupled weaker lower crust flow classes of models,view the lateral growth of Tibet as being driven by storedgravitational potential energy, through ‘‘concentric’’ out-ward spreading occurring more or less simultaneously alongall edges [e.g., England and Houseman, 1986, 1989;Royden et al., 1997; Shen et al., 2001; Jimenez-Munt andPlatt, 2006]. In such models, the southeastern plateau,like the northeastern plateau, would have been rising fastthroughout the late Cenozoic to the present-day. In partic-ular, the more gently sloping topography in southeasternTibet has been taken as evidence for tilting of a low-elevation, low-relief surface driven by recent and ongoinglower crustal channel flow [Clark and Royden, 2000;Clark et al., 2005, 2006]. In all these models, the currenttopography and landscape morphology of the plateau areinferred to be a straight forward reflection of deep seatedlower crust and mantle processes. By contrast, the impor-tance of surficial erosion processes in shaping the landscapeis downplayed.[4] Block models that include geological and tectonic

evidence make quite different predictions, in particular, thatthe plateau has grown mostly toward the northeast, instepwise fashion [Lacassin et al., 1997; Meyer et al.,1998; Metivier et al., 1998; Tapponnier et al., 2001; Pareset al., 2003]. On the basis of strike-slip driven mountaingrowth and not just foreland thrust migration, Tapponnier etal. [2001] proposed a ‘‘three-step’’ model of sequentialuplifting (Figure 1a): south Tibet would have been the firstpart of the plateau to rise during the Eocene. The regionnorth and east of the Kunlun range would be the youngestpart of the plateau, and would have risen fast due to crustalshortening, between the Pliocene and present. The region inbetween would have gone through its main phase of upliftbetween the Eocene and the Miocene. The southeastern partof the plateau would have been shortened and upliftedcoevally with south Tibet mostly before the Miocene, as itwas being extruded southeastward along the Red River andXianshuihe faults. In the stepwise model, intermontane‘‘bathtub sediment infilling’’ is an integral part of elevationgain and relief smoothing, a process essential to the buildingof plateau morphology [Meyer et al., 1998; Metivier et al.,1998; Tapponnier et al., 2001]. However, while suchsurface processes are clearly at work in shaping the topog-raphy of northern Tibet, it has been less straightforward toassess what role they might have played, if any, in centraland southeastern Tibet.[5] Understanding the coevolution of plateau topography

and relief structure, within a sound geological and tectonicframework, is a starting point to assess the validity of

models addressing plateau uplift and growth. In this paper,we engage in a quantitative description of the bulk land-scape characteristics of different regions of the Tibetanplateau. Our morphometric analysis of topography acrossthe plateau demonstrates the existence of systematic andmeaningful regional differences. The northern and centralparts of the plateau can be characterized as regions of‘‘positive’’ topography, i.e., where there is a positive corre-lation between relief and elevation, and between local slopeand elevation. The change from the internally drainedcentral plateau to the externally drained part of easternTibet is accompanied by a transition from positive to‘‘negative’’ topography, i.e., with inverse correlation be-tween elevation, relief and slopes. Fluvial relief in easternTibet is largest across a NE-trending topographic step thatmay mark an ancient, eroded rim of the plateau. Regionslocated southeast of this step do not show negative topog-raphy. We propose that the evolution of river systems anddrainage efficiency, the ability of rivers to transport sedi-ments out of the orogen, coupled with tectonic uplift,provides a robust unifying mechanism to explain both theway in which the plateau morphology and relief wereshaped and the systematic regional differences observed inthe Tibetan landscape.[6] Similar techniques of large-scale topographic analysis

have been applied to various regions [e.g., Ahnert, 1970,1984; Koons, 1989; Ohmori, 1993; Brozovic et al., 1997;Brocklehurst and Whipple, 2004]. An overall study of thetopography of the Tibet plateau was previously performedby Fielding et al. [1994] and Fielding [1996] using a �90 mresolution digital elevation model (DEM). Their data (notpublicly available at the time) was 100 times better inresolution than ETOPO5 (5 min latitude/longitude griddedelevation data). These authors focused mainly on theflatness of Tibet, however, without characterizing regionaldifferences, even though the total area of Tibet is almost halfthat of the contiguous United States. Yet inspection ofTibet’s morphology shows that the plateau exhibits signif-icant variations in interior relief. Besides, it is an assem-blage of tectonic terranes with different Phanerozoichistories that likely responded differently to the straingenerated by the India/Eurasia collision, and that likelyrose diachronously since �55 Ma ago [e.g., Lacassin etal., 1996; Meyer et al., 1998; Tapponnier et al., 2001]. Withthe advent of freely accessible Shuttle Radar TopographyMission DEMs, which provide public access to a spatialresolution similar to that used by Fielding et al. [1994], it isnow possible to explore such questions in greater detail onthe basis of a uniform high-quality digital data set.

2. Methods

[7] Our regional scale landscape analysis of the Tibetanplateau is carried out using Shuttle Radar TopographyMission (SRTM) DEMs. The SRTM (http://www2.jpl.nasa.gov/srtm/index.html) collected elevation data over 80% ofEarth’s land area by means of radar interferometry. The dataset is at present the best quality, freely available, digitalelevation model with 3 arc sec resolution and consistentaccuracy outside the United States of America. We use theunedited version, which contains holes or voids, especially in

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areas with very steep terrain, water bodies and sand dunefields.[8] The geomorphic indices we discuss in some detail are

elevation histograms, local relief, and local slope. Thetopographic hypsometry quantifies the distribution of ele-vations within a region. We take the local relief to be theelevation difference between the highest and lowest pointsin 10 � 10 km square sampling windows, assuming that a

10 km window size scale captures the most common ridge-valley spacing. Local relief is widely accepted as a basicparameter that reflects the interplay between tectonics anderosion [e.g., Burbank, 1992; Montgomery, 1994; Whippleet al., 1999]. It is a measure of roughness in topography atkilometer scale. The value of local relief has been shown todepend on the scale or window size, possibly as a self-affinefractal [Ahnert, 1984; Weissel et al., 1994]. Our choice of a

Figure 1. (a) Regions chosen for bulk landscape characteristics analysis, superimposed on ‘‘three-Tibet’’ model of principal plateau building epochs. From Tapponnier et al. [2001]. Reprinted withpermission from AAAS. Boxes N1–N4 are samples of the northern plateau, C1–C5 are samples of theplateau interior, and E1–E6 are samples of the SE Tibet. Major rivers draining from the plateau areshown in green. (b) Map of slopes in the plateau [from Fielding et al., 1994]. Red line delineates currentinternally drained area. Blue lines trace boundary between shallow and steep slopes within externallydrained area. Analysis boxes are also shown.


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10 km window size is consistent with those made inprevious studies, such as Montgomery and Brandon[2002] and Clark et al. [2006], who chose a �10 kmdiameter circular window, though 20 and 5 km squaresampling window sizes have also been used [Ahnert,1970; Sobel et al., 2003]. There still lacks a thoroughassessment of the exact effect of sampling window size.Despite potential biases related to scale dependence, how-ever, a comparison between regions can be performed and ismeaningful for the general, first-order exploration under-taken in this paper, provided the same horizontal windowscale is kept for all regions.[9] We capture the local slope from the average gradient

in 3 � 3� cell grids, whose dimensions are converted intometers and adjusted for latitude dependence. The calculatedlocal slope is averaged over a 200 m scale, hence under-estimates the actual slope. In addition, points with no data(NaNs) in the grid are ignored and not taken in account inthe averaging. While elevation histograms are a measure ofthe overall relief at the regional scale, local slopes charac-terize relief at the smallest scale (�200 m), and local reliefat an intermediate scale. Together, these three parametersthus quantify relief at three different spatial scales.[10] We target three different regions of the Tibet plateau,

in which we further separate subregional areas as analysisboxes (Figure 1): (1) northern Tibet, including the QilianShan region (N1 to N4); (2) Tibet’s interior, including bothinternally drained (C1 to C3) and externally drained regions(C4 and C5); and (3) southeastern Tibet, i.e., the areabounded to the north and east by the Xianshuihe fault (E1through E6) with one box south of the Red River-AilaoShan zone in the Shan Thai plateau. Each box is 2 by3 degrees in size, except for N1 (3 by 3 degrees) and N2.The selection of rectangular analysis boxes is somewhatarbitrary and they span multiple drainage basins. Such large,arbitrary boxes may not capture fine local variations [e.g.,Brocklehurst and Whipple, 2004], but are appropriate forregional to mesoscale quantification [Brozovic et al., 1997;Montgomery et al., 2001], which is the spatial scale ofinterest here. In all subregions, nondata points make up only2% of the surface, or less, except for E3, where this per-centage is somewhat higher (5.6%).

3. Morphological Characteristics of SelectedRegions

3.1. Northern Tibetan Plateau

[11] Northern Tibet, north of the Kunlun range andQaidam basin and south of the Hexi corridor and Gobidesert, consists of rugged roughly parallel mountain ranges,separated by subparallel intermontane basins (Figure 2a).The ranges are usually bounded by growing folds and activethrusts [e.g., Tapponnier et al., 1990b; Meyer et al., 1998;van der Woerd et al., 2001]. The drainage network is notwell developed in this area, because of the arid to semiaridclimate (annual rainfall is 50–400 mm/a, with a maximumof 400–600 mm/a along the Qilian Shan range front [WorldMeteorological Organization, 1981]). A few mountain riversdrain north into the Hexi corridor and Ala Shan platform.Other rivers flow parallel to the orientation of mountainranges and thrusts, some to the northwest into the Tarim basin.The externally drained Yellow River catchment only pene-

trates into the easternmost part of the region. Tributariesof the Yellow River are few, and do not extend far fromthe trunk.[12] In map view, the local relief, on length scales of

�10 km, mimics closely the topography (Figure 2b). Areasof relatively high local relief coincide with mountain ranges,and low local relief with intermontane basins. Furthermore,since the mountain ranges of the region are bounded byactive thrust faults, and continue to rise as large-scale rampanticlines [Tapponnier et al., 1990b; Meyer et al., 1998; vander Woerd et al., 2001], local relief is clearly controlled bythese faults. Hence, the local, actively growing relief of theQilian Shan area can be directly linked to ‘‘structural relief.’’[13] Another important feature in the local relief distri-

bution is that the largest local relief is localized along theplateau’s mountain rim. Inside and away from this rim,toward the Qaidam and Gonghe basins, the relief is smaller,though locally high along the Yema Shan, Danghe NanShan, and Qaidam Shan. Two NE–SW transects illustratethe spatial correlation between elevation, local relief andslope (Figure 3). In both transects, the areas of highestelevation are located 100–200 km inward from the plateauedge but correspond to more moderate, rather than to thelargest, local relief. This illustrates the fact that reliefdevelopment results from the competition between reliefgrowth by tectonic uplift and relief reduction by erosionalmass wasting. That local relief is generally smaller insidethe plateau despite higher elevation suggests that reliefreduction continues in these areas of increased regionaltopography at a rate that has probably outpaced that ofrelative uplift by thrusting.[14] Elongated intermontane basins, perched at elevations

of 3000–4000 m, are prominent in the Qilian Shan regionallandscape. They usually correspond to the local maxima inthe elevation histograms (Figure 4). Because the basins areflat or with only gently dipping slopes, the mean slope at thecorresponding elevations is less than it would be if suchbasins did not exist. This is clear in Figure 4 where theelevation histograms and average slope distributions are onaverage anticorrelated. The mirror images of the elevationhistogram and mean slope distributions are a clear indica-tion of the dominance of intermontane basins at �3000 and4000 m. Above �4000 m, the average slope increasesmonotonically with elevation. The dominant areal extentof basins is also clear on the slope histograms. These areexponential-type distributions, characteristic of depositionaltopography [Montgomery, 2001]. About 50% of the localslopes are gentler than 15�.[15] That such intermontane flats, which are characterized

by high elevation, low local relief and gentle slope, are themain component, in areal extent, of the regional topography,ahead of the rugged mountain ranges, suggests that theirdevelopment is a key factor of the regional topographicevolution. Not only do they provide a sedimentary record ofuplift and erosion in nearby mountain ranges but, as shownin the next section, they offer a clue to the origin of thelarger and flatter areas that are ubiquitous in the interior ofthe highest part of Tibet.

3.2. Tibet Interior

[16] The most striking feature of Tibet’s interior is itsgreat flatness [e.g., Fielding et al., 1994]. Tibet is in fact


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Figure 2. (a) Topography and major faults of northern Tibetan plateau. This region is generallycharacterized by narrow, linear mountain ranges separated by intermontane basins. The Yellow River isthe only externally draining river that flows across the eastern part of the region, cutting at high angleacross several ranges. Active faults are modified after Meyer et al. [1998] and Tapponnier et al. [2001].(b) Map of local relief using �10 � 10 km size moving window with 25% overlap. In northern Tibet,areas of high relief are mainly associated with active faults. To the northwest of Qinghai Lake is a high-elevation, low-relief region that is tectonically ‘‘quieter’’ and not yet invaded by external drainage. Insetshows the histograms of local relief in subregions.


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best described as a mosaic of flat basins, separated bymountain ranges tens to hundreds of kilometers apart(Figure 5). At a more detailed level, close inspection revealsgeomorphic differences both between the northern andsouthern areas of the high plateau, and between internallydrained and externally drained regions.[17] Figure 6 shows the north–south differences in to-

pography and local relief inside the internally drainedplateau interior. The primary factor for high relief here isactive normal faulting. Such faulting typically producesstructural relief in the 1–2 km range [Armijo et al., 1986;see also Taylor et al., 2003; Kapp et al., 2008]. Away fromthe faults, the relief is generally less than 900 m. Asecondary factor is fluvial incision, as revealed by a patchof moderate local relief in the SE corner of C1: this smallarea is within the uppermost reach of the externally drainedYarlung Tsangbo River catchment. Toward the north, localrelief decreases, from 500 to 900 m in C1 to 300–500 m inC2, past the Bangong suture, then to an even narrower 200–300 m range in C3, north of the Tanggula Mountains. Theelevation histograms show an asymmetric distribution with

a relatively abrupt low-elevation cut-off at 4000–4500 mand a longer tail toward high elevations slightly above6000 m (Figure 7). The 4000–4500 m elevations corre-spond to low-lying, low-relief basins and piedmonts. Theyrepresent base levels for local drainages, and include lakesand sediment-floored flats. The regional morphology is bestdescribed to result from sediment ‘‘flooding’’ in the closedbasins that nearly ‘‘submerged’’ former peaks (Figure 5).These basins have average slopes of generally less than 7�,and large areal extents, making up 63–65 % of the totalsurface area. Slope distributions show that the mean slopeincreases with elevation. Slopes are gentle to flat at lowrelative elevation, but generally become larger as elevationincreases, up to around 15� on average. The increasing trendof slope toward high topography is similar to that observedin the Qilian Shan region of NE Tibet (Figure 4). Thedifference, however, is that most topographic highs are lesssteep, with slopes 10� gentler than in the Qilian Shanregion.[18] In summary, the internally drained Tibet interior is

characterized by positive topography, i.e., with elevation

Figure 3. Local relief, elevation, and slope along two transects across Qilian Shan region. Locations oftransects are shown in Figure 2a. Horizontal lines at 5000 m for elevation and 1000 m for local relief arearbitrarily chosen for references. Regions of highest relief are associated with faults. Along A-A0, inparticular, the highest relief is near the edge of the plateau, which is also the locus of frontal thrusts andactive uplift. Moving inward from the plateau edge on this section, regions of highest elevation areassociated with more moderate local relief.


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Figure 5. Photos showing general geomorphic characteristics in Tibetan plateau interior: (a and b) flatbasins and far apart steep mountain ranges, as compared with (c) that in the Qaidam basin.

Figure 4. Thick dark lines with error bars show average slope as a function of elevation for regionsN1–N4. The linear increasing trend of average slope with elevation is interrupted by local minimaat midaltitudes, because of flat intermontane basins. Gray dashed lines show histograms of elevationdistribution. Intermontane basins correspond to local maxima in elevation histogram. Also shown areslope histograms of subregions in the northern Tibetan plateau, showing dominance of slopes of lessthan 10�.


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positively correlated with relief and slope (Figure 8). Lowelevations are exclusively located in close basins, whichhave low or flat relief and gentle slope. At the other end ofthe spectrum, higher-elevation hills or ‘‘small’’ mountainsthat stand above the flat basins show greater relief.

[19] Internally and externally drained zones differ mainlyin slope gradient. The most prominent difference is seen atthe low end of the topographic distribution. In the internallydrained zones, the 4000 m elevation areas consist of closedbasin flats where sediments accumulate, often in and around

Figure 6. (a) Topography and (b) local relief, with active faults, of selected regions in Tibet plateauinterior (between 30–36�N and 85–88� E). The region is mostly internally drained, with a mean elevationof 4700 m and relief less than 1 km, except near active normal faults.


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lakes, with slopes that can be as small as �2�. But in theexternally drained zones, relative lowlands are associatedwith slopes in excess of 5–10�(e.g., C5, Figure 7), indicatinga shift from depositional to erosional regime [Montgomery,2001]. Low-elevation surfaces are most affected by thisshift. The overall effect on topography is a slight change inthe regional elevation distribution. Compared to the asym-

metrical distribution in internally drained zones, the eleva-tion histogram in externally drained zones shows moresymmetry. This is because incision introduces a low topog-raphy end ‘‘tail’’ to the distribution. The difference betweenthe two externally drained regions (C4, C5) is that C4 hashigher mean elevation (by �300 m), with a significantfraction of lowland flats still unaffected by river down

Figure 7. Average slopes (dark solid lines with error bars) increase nearly monotonically with elevationin typical internally drained regions (C3 and C4). Externally drained regions show increased slopes at4000 m elevation and below. Also shown are histograms of elevation distribution in each region (graydashed lines) with modal elevations indicated. Depending on location within the plateau interior,elevations of 4300–4900 m have maximal areal extent, with average slopes of 5–10�. Areas withelevations higher than 5500 m have steeper slopes, on order of 15–25� in general. As in northern Tibet,the slope histogram is dominated by shallow slopes, of less than 10�.

Figure 8. Local relief, elevation, and local slope along 650 km long N-S profile at longitude 83.6�.Location of profile is shown in Figure 6. The plateau interior is still characterized by ‘‘positive’’topography, with high elevation corresponding to high relief and steeper slope and vice versa.


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cutting, likely due to its position closer to the internallydrained interior. This difference likely illustrates the transi-tional landscape in the upper reach of external drainagesthat penetrate headward into the plateau from the east andsoutheast.

3.3. Eastern and Southeastern Tibetan Plateau

[20] The southeastern part of Tibet, between the easternHimalayan syntaxis and the Sichuan basin, is a region ofrugged topography with well-developed drainage and valleynetworks (Figure 9). Three of the largest rivers of Asia, theJinsha Jiang (Yangtze), the Lancang Jiang (Mekong), andthe Nu Jiang (Salween), flow out of the high plateau acrossthis region, following nearly parallel courses, and carvingdeep gorges into the terrain. Between these large rivers, themean elevation decreases gradually southeastward from 4–5 km to 2–3 km, across a NE-trending approximately 200–250 km wide zone that roughly coincides with the Yalongthrust belt. Southeast of this belt, the average elevation ismainly between 1 and 3 km, with 2–3 km high surfacepatches that loose continuity, shrink and finally vanishaltogether southeastward. The relatively gentler topographicgradient across SE Tibet has been interpreted to result fromthe uplift, due to lower crustal channel flow, of a low-elevation and low-relief ‘‘relict’’ surface that once extendedcontinuously, without major disruption, over thousands ofkilometers from the interior of Tibet to the South China Sea[e.g., Clark and Royden, 2000; Clark et al., 2006].[21] Clark et al. [2006] used topographic indices, among

other criteria, to characterize such a regional ‘‘preuplift’’surface. But these indices, often single values for the entireregion, were mainly based on the coarser �1 km resolutionGTOPO30 DEM [Clark, 2003]. Below we assess the spatialvariation of the morphological characteristics of the regionaltopography. Such variations define a topographic step and‘‘slope break’’ (Figure 9), which seems to have been over-looked in previous studies and appears to mark an erodedplateau rim. The region northwest of this rim has a differenttopographic signature from that to the southeast.[22] In the map of local relief (Figure 10), linear high-

relief swaths generally follow the courses of the large rivers.Along any given river course, the relief is largest in themiddle reaches, roughly between 25 and 31�N latitudes, anddecreases both up and downstream. In the upper reaches,from latitude 31�N northward into Tibet, relief along therivers starts decreasing to vanish into the background relief.A similar, more rapid decrease is observed along thedownstream reaches in the relative low-lying lands ofYunnan south of 26�N. In particular, the Jinsha river andthe major tributaries of Yangtze, such as the Yalong Jiangand Dadu He cut deepest across a NE-trending zonecoinciding with the Yalong-Yulong thrust belt. Interestingly,both the Jinsha and Yalong Jiang make sharp north directedhairpin bends as they cut across this belt. If, as long inferred[e.g., Clark et al., 2004], such bends formed as a result ofriver capture, then deformation along the thrust belt likelyplayed a role.[23] Between these major river valleys are areas of

relatively subdued relief [Clark et al., 2006]. These areas,however, correspond to low-relief terrains of different originand age. For example, the relatively flat surface between theLancang and Jinsha rivers north of 30�N is partly occupied

by the perched Gongjue and Changdu Paleocene-Eocenebasins (Figure 11), which remained relatively uneroded byeither river [Bureau of Geology and Mineral Resources ofXizang Autonomous Region, 1993]. The low-relief Dao-cheng surface between the second-order tributaries of theJinsha around 29�N caps large bodies of slightly moredissected granite and granodiorite that were intruded intothe Triassic flysch during the early Jurassic [Bureau ofGeology and Mineral Resources of Sichuan Province,1991]. Part of this surface above 4000 m was occupied bya small ice cap during the Last Glacial Maximum, andprevious maxima [Luo and Yang, 1963; Derbyshire et al.,1991; Li et al., 1991]. Along the southeast side of theYalong thrust belt, the Yanyuan basin occupies a prominentrelatively flat surface area of �450 km2. It is filled with 1–2 km of Paleogene red molasse and up to �650 m ofNeogene coal-bearing fluviolacustrine deposits. [Bureau ofGeology and Mineral Resources of Sichuan Province, 1991;Xu et al., 1992]. Southeast of the Yalong thrust belt, low-relief terrain caps parts of the folded Cretaceous-lower-Tertiary red beds of the Chuxiong basin.[24] The composite NW- to SSE-trending swath profile of

Figure 12 clearly shows the coincidence between theYalong-Yulong thrust belt and a 200–250 km wide topo-graphic ‘‘step.’’ The steepest part of this profile, with amean elevation drop of 2400 m from �4200 m to �1800 m,across a series of 3–4 parallel Cenozoic thrust faults (e.g.,Lijiang-Muli and Jin He-Jian He thrust faults), unambigu-ously defines the southeastern margin of the high Tibetplateau. The Lijiang-Muli fault shows evidence of Quater-nary activity, with a still prominent thrust component [Xianget al., 2002; Shen et al., 2005]. In addition to this relativelylong wavelength slope break, a minor gentle decrease inmean elevation is observed toward the northwest, possiblydue to regressive fluvial erosion by the Mekong andYangtze drainages. Along the swath profile, the reliefincreases toward the plateau edge.[25] The low-relief terrain northwest of the Yalong thrust

belt is quite different from that to the southeast. In sub-regions on the plateau (E1–E3), there is a marked localminimum in slope hypsometry around 4200 m elevation(Figure 13). This minimum’s mean slope is �10� less thanthat at other elevations. This altitude coincides with themodal elevation of these subregions (Figure 13). A similarpattern with a local slope minimum close to the modalelevation is also found in the northwest Himalaya, at 4000–5000 m elevation [Brozovic et al., 1997]. It has beeninterpreted to reflect extensive glacial scouring of thelandscape coupled with vigorous freeze-thaw action be-tween 4000 and 6000 m [Brozovic et al., 1997], rather thanthe signature of a relict plateau surface. But it is clear thathere the surface at 4200 m elevation with minimum slopesof �20� is simply the continuation of the plateau interiorhigh base level. It may have been locally modified andfurther smoothed by glacial action since, as mentionedabove, ice caps of significant size appear to have occupiedareas northwest of Daocheng and Yajiang. But while therelative flatness and high elevation between 4000 and5000 m, close to the ELA (equilibrium line altitude) duringthe LGM or previous maxima [Derbyshire et al., 1991; Li etal., 1991], may have facilitated the accumulation of stagnant


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Figure 9. Smoothed topography (1 km resolution) of southeastern Tibetan plateau. Active faults areshown by dark lines; thick lines are major faults [after Lacassin et al., 1998; Wang et al., 1998;Tapponnier et al., 2001; Deng et al., 2003; Xu et al., 2003]. Red lines are main tectonic boundaries[Chengdu Institute of Geology and Mineral Resources, 2004; Pan et al., 1990]. ALS-RRF, Ailaoshan-Red River fault; JF, Jiali fault; JJF, Jinhe-Jianhe fault; KLF, Kunlun fault; LF, Longquan Shan fault; LMF,Lijiang Muli fault; LTF, Litang fault; NTF, Nanting fault; SF, Sagaing fault; WDF, Wanding fault; XSH-XJF, Xianshuihe-Xiaojiang fault; YJF, Yunijin fault.


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Figure 10. Map of local relief in southeast Tibetan plateau. Thick white lines are major river courses.Dark lines indicate major active faults. Regional relief is mainly controlled by fluvial incision withhighest elevation differences chiefly along the largest rivers. High relief is also prominent along majoractive faults, for example, the Xianshuihe or Red River faults. The moving window size (10 � 10 kmsquare) we used for local relief calculation is slightly larger than the �65 km2 circular window in Clark etal.’s [2006] analysis. Fault names as in Figure 9.


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ice, the primary origin of the broad plateau surface cannotbe solely due to local ice abrasion.[26] Southeast of the Yalong-Yulong belt, down below

the plateau edge, the variation of slopes as a function ofelevation is quite different from those in the highlands to thenorthwest. Slopes tend to be more uniform at most eleva-tions. The relatively flattest areas in subregion E5 are atelevations around 1850 m. This corresponds to the Chuxiongsedimentary basin. But a continuation of this surface at thiselevation does not show up in the adjacent subregion E6.

[27] Two E–W profiles illustrate further the difference intopography between regions northwest and southeast of theYalong-Yulong thrust belt. Profile A-A0, just north of 30 N,shows clearly that the elevation is inversely correlated withlocal relief and with slopes (Figure 14). High elevationcorresponds to relatively small relief and gentler slope,whereas low elevation corresponds to large relief and steepslope. This pattern, which we call negative topography, isnot observed in other parts of the plateau. It thus providesstrong evidence for a remnant high and flat surface that is in

Figure 11. Locations of main Mesozoic and Cenozoic basins and intrusives in eastern and southeasternTibet. Modified from Chengdu Institute of Geology and Mineral Resources [2004], Bureau of Geologyand Mineral Resources of Yunnan Province [1990], and Bureau of Geology and Mineral Resources ofSichuan Province [1991]. Fault names as in Figure 9.


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the process of being actively dissected by fluvial incision,but is still preserved in patches. By contrast, as illustrated onprofile B-B0, negative topography, characteristic of a dis-sected remnant surface, is not observed in regions south ofthe plateau edge. South of the Yalong-Yulong belt at24.4�N, there is no distinctive inverse correlation betweenelevation and local relief or between elevation and slope.

4. Interpretation and Model of TopographicDevelopment of Tibetan Plateau

[28] Our morphometric analysis and comparison of thecharacteristics of topography in three typical regions of theTibet plateau shows clear variations in local relief and slope/elevation patterns. In SE Tibet, average slopes at allelevations are generally steep, mostly greater than 10�. Thisregion is different from both the internally drained, rela-tively stable plateau interior and the Qilian Shan zone ofactive thrusting and mountain growth in the northeasternplateau. There, slopes are dominantly gentler than 10�,indicating that depositional processes prevail.

4.1. Formation of the Low-Relief Surface in thePlateau Interior

[29] The low-relief interior morphology of Tibet and itsclose association with internal drainage indicate that drain-age efficiency plays a key role in relief smoothing. A

present-day clue to relief smoothing is observed in theQilian Shan area of the northeastern plateau (‘‘bathtub’’basin-filling hypothesis of Meyer et al. [1998], Metivier etal. [1998], and Tapponnier et al. [2001; see also Sobel et al.,2003]). Intermontane catchments are either disconnectedfrom or only partially connected with external drainagetoward the foreland. Small catchments and short riversperform only short-distance mass transport from topographichighs to adjacent basins. While vigorous fluvial downcutting by external drainage systems may enhance ormaintain relief [Molnar and England, 1990; Gilchrist etal., 1994; Montgomery, 1994; Whipple et al., 1999], therestricted NE Tibet mass transfer process ‘‘erode high andfill low’’ reduces regional relief instead.[30] In the Qilian Shan region, relief reduction through

basin filling starts concurrently and in competition with, thegrowth of tectonic relief along thrust-bounded mountainranges, as indicated by regional basin development [Meyeret al., 1998]. Such growth, as well as bathtub infilling,raises the average elevation, such that the combination ofboth processes results in high elevation and low relief.Inward from the plateau rim, relief reduction will in generalprevail, because of slower tectonic uplift than along theplateau-bounding ranges and more effective intermontanebasin filling, by rivers that are more likely to becomecompletely cut off from their foreland outlet. Such aninterplay accounts well for the landscape evolution of

Figure 12. Topographic swath profile (�100 km width) across plateau margin to the east and southeastof Sichuan basin. Location of swath is shown on Figures 9 and 10. (top) Curves represent maximum,minimum, and mean elevation profiles projected onto axis of swath. Minimum elevation profile samplesparts of long profiles of the Lancang-Mekong, Yangtze, and Red rivers. (bottom) Relief differencebetween maximum and minimum elevation profiles along swath. The maximum relief (�3000 m) occurswhere the mean elevation starts to drop. Toward the plateau interior, the relief decreases linearly to auniform low value (�800 m) in the upper reaches of the external drainage catchments. Southeastwardfrom the plateau margin relief decreases to 1000–1500 m in the Chuxiong basin, then increases againfarther south, likely because of an incision by the Red River catchment and to differential tectonicexhumation along the Red River fault and Ailao Shan shear zone [Leloup et al., 1995, 2001].


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northeastern Tibet, where relief becomes smaller as eleva-tion increases from the Gobi rim toward the plateau interior(Figure 2). In the highest internally drained part of centralTibet, amalgamated closed basins form a coalescent andrelatively uniform high-elevation base level. The restricteddrainage catchments are effective in further smoothing reliefat even higher elevation. This is in a way perhaps similar to‘‘high-altitude peneplanation,’’ which is thought to explainsmaller low-relief surfaces within orogenic belts [e.g.,Babault et al., 2005, 2007].[31] Our topographic analysis thus supports the inference

that, instead of having been raised after formation at lowelevation, the low-relief surface of the Tibetan plateauformed by diachronous beveling at high elevation.

[32] Low-relief interior morphology associated with in-ternal drainage appears to be typical of other continentalorogenic plateaus. Particularly clear examples are the Ira-nian plateau and the Puna-Altiplano plateau in SouthAmerica [Meyer et al., 1998; Sobel et al., 2003; Garcia-Castellanos, 2007]. Though smaller in area and lower inmean elevation than Tibet, the latter is a high (�3800 m)low-relief orogenic plateau with internal drainage, largelakes or swamps (e.g., Titicaca, Salar de Ulyuni, Salar deCoipasa) and aridity [e.g., Jordan and Alonso, 1987; Isacks,1988; Alonso et al., 1991; Masek et al., 1994; Vandervoortet al., 1995]. An erode-high, fill-low type of surface process,i.e., intermontane bathtub-filling and piedmont growth dueto inefficient drainage evacuation of sediments, is thought to

Figure 13. Average slope and elevation histograms, plotted as a function of elevation, for regions E1–E6. Average slope is steeper than 20� at almost all elevations in E1–E3. Local minimum at �4200 mcorresponds to remnant plateau surface. At or south of former plateau edge (E4, E5, and E6), slope andelevation curves are markedly different from those in E1–E3. Also shown are histograms of slopedistribution in each region. Elevation histograms of regions C4–C5 and E1–E3 show a gradual changewith a decreasing percentage of elevations at 4000–4500 m concurrent with an increasing percentage oflower elevation.


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Figure

14.

Localrelief,elevation,andlocalslopealongtwoE–W

profilesat

latitude30.2�(A

-A0 )andlatitude24.4�(B-

B0 );locationsgiven

onFigures9and10.Profile

A-A

0showsthecharacteristic

signature

of‘‘negative’’topography,

high

elevationscorrespondto

lowrelief

andshallowslopes

whilelowelevationsareassociated

withhighrelief

andsteepslopes.

Thelow-relief,high-elevationremnantplateau

surfaceis

clear.Profile

B-B

0 ,south

oftheplateau

edge,

showsnosuch

negativetopography.


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have played a critical role in shaping the Altiplano subduedlandscape [e.g., Riller and Oncken, 2003; Sobel et al., 2003;Hilley and Strecker, 2005]. Therefore, there is growingconsensus that surface processes play a major role in thegeomorphic evolution of orogenic plateaux, with high baselevels connected to short local drainage systems being thekey condition to maintain high-elevation, low-relief plateaumorphology.

4.2. A Model of Topographic Evolution Linkingthe Three Regions of Tibetan Plateau

[33] The lateral growth of the Tibet plateau since theonset of collision between India and Eurasia implies that itstopography is not in steady state flux. Clearly therefore, thelandscape morphology reflects not only the current climaticand tectonic setting and surficial processes, but also theirhistory [e.g., Howard, 1997]. An evolutionary sequence ofplateau development emerges from the quantitative com-parison and characterization of geomorphic differencesbetween the three parts of Tibet: northeastern plateau,plateau interior and southeastern plateau.[34] A connection between these three regions may be

inferred from the variation of mean slope as a function ofelevation. Figure 15a shows that the topography of centralTibet is characterized by a monotonic increase of meanslope with elevation above 4000 m. This is similar to whatis observed in northeastern Tibet at elevations above that ofintermontane basins. Both northeastern and central Tibetshow positive topography. In Figure 15b, a comparison ofthe slope-elevation functions for southeastern Tibet andcentral Tibet shows the gradual transition from the Cregions of smooth high topography to the E regions withdegraded topography. Despite steeper slopes at all eleva-tions and the change to negative topography induced by theexternal drainage ‘‘reconquest’’ of formerly isolated basins,the remnant plateau surface is still preserved, as demon-strated by the relative flatness at 4000–4500 m.

[35] Figure 16 illustrates a simple interpretation of thegradual changes in the slope-elevation functions betweenthe three regions. In cross-sectional view, northern Tibet hasa steep rim and hanging intermontane basins separated byrugged, towering mountain ranges. Due to the inefficientdrainage network that progressively becomes internal, sedi-ments eroded from the high mountains fill the basins andflatten the low elevation end of the spectrum. Depending onthe period during which internal drainage is maintained, themountain top slopes should become smoother with time.Later on, as tectonic uplift at the nm slows down or stopsaltogether or possibly due to wetter climate, external drain-age grows back vigorously headward into the plateau in-terior [Hilley and Strecker, 2005]. The subdued ‘‘protected’’topography of the isolated, high base level succumbs toincision, which starts at the relatively lowest elevationswhere steep slopes reappear. In this interpretation, thedrainage evolution, from external to restricted then internaldue to mountain rim growth, then back to external throughheadward conquest, is the key factor linking the threeregions of the Tibetan plateau. Each region may in factbe taken to reflect a time step in plateau building, from birthto demise.[36] Seen from this timescale point-of-view, the Qilian

Shan and Qaidam regions, where active crustal shorteningand thickening occur prominently over a surface area ofabout 5.4 � 105 km2 [e.g., Meyer et al., 1998], provide atypical example of plateau landscape at an early stage oftopographic development under relatively dry climate con-ditions (annual rainfall �400 mm/a and less [World Meteo-rological Organization, 1981]). Regional topography ischaracterized by relatively large structural relief, still rela-tively low mean elevation (�2700m, 1500 m lower thanTibet proper), and a steep outer rim. Subparallel narrowmountain ranges bounded by active thrusts and growingfolds isolate long and narrow, often rhomb-shaped inter-montane basins.

Figure 15. Comparison of mean slope: as a function of elevation (a) between northeastern plateauand plateau interior and (b) between plateau interior and southeastern plateau. HPBL is the high plateaubase level.


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[37] The plateau interior represent the next evolutionstage of plateau morphology, essentially driven by internaldrainage with vanishingly small fault uplift or even tectonicquiescence, as the plateau grows and the deformation frontmoves outward. The region undergoes further relief smooth-ing while the mean elevation keeps increasing. Highlandsbecome more isolated and separated by wider basins.Highland disintegration brings their summits down whileponding of the resulting debris and infilling of adjacentlowlands raises the high-elevation base level to even greaterheight. As in the Qaidam and Qilian Shan, diffusive slopeand short fluvial transport are the dominant modes of masswasting. The characteristic ‘‘positive topography’’ remains(Figure 15a). The internal drainage smoothing process cancontinue for protracted periods, as long as no outbound riverpenetrates into the high, isolated base level and tectonicquiescence prevails.[38] Internal relief smoothing can later be disrupted by

resumed tectonic deformation as is observed with normalfaulting in southern Tibet, but more efficiently and com-monly by the invasion of external drainage. Our studysuggests that such vigorous headward invasion providesthe most plausible link between the geomorphic signatures

of the interior, and of the southeastern marginal part of theplateau. Southeastern Tibet would in fact represent the thirdelevation stage of plateau topography: the ultimate, demiseperiod. Intensive fluvial down cutting by external drainagesattacks and dismembers the elevated and smoothed plateausurface thus far protected by isolation. The Sichuan andnorthern Yunnan regions of SE Tibet are characterized bymean elevation lowering and renewed relief buildup. Suchfluvially driven increase of relief, or ‘‘negative topogra-phy,’’ is radically different from the tectonically controlledstructural relief of the NE plateau. The fluvial incisiondominance is clear from the slope-elevation functions(Figure 15b). Down cutting associated with fluvial incisionstarts at low elevation, and progresses into the highlandsthrough headward retreat and capture. This eventuallyincreases the slopes at all elevations, though remnants ofthe plateau surface are preserved for a time. The ‘‘flat’’highland limit that subsists 50–400 km east of the internaldrainage divide in east-central Tibet attests to such recentrapid headward conquest of the plateau by the upper catch-ments of the Yellow, Jisha, Lancang and Nu rivers (Figure 1).

4.3. Origin of the High-Elevation, Low-Relief RemnantSurface in Southeastern Tibet

[39] The relict high-elevation, low-relief, and low-slopeplateau surface of SE Tibet clearly extends out from theplateau interior (Figures 9 and 10). In contrast with previousinferences of a remnant, formerly continuous low-elevationsurface having reached as far south as Vietnam (20�N)without major disruption prior to the uplift of Tibet [Clark etal., 2006], our quantitative analysis suggests that the Tibetplateau surface terminates along a 200 km wide, NE-trending topographic edge, which coincides with theYalong-Yulong or Jinhe-Jianhe (in Chinese literature) belt.Though eroded, this fold and thrust belt, which comprisesseveral parallel ridges, likely represents the Eocene-Miocenestructural barrier that once separated the SE plateau regionfrom the Chuxiong Cretaceous-Eocene foreland basin[Leloup et al., 1995; Lacassin et al., 1996, 1997]. TheYalong range formed during the Cenozoic [Pan et al., 1990;Xu et al., 1992] as a result of crustal shortening on south-eastward younging thrusts (Z. Q. Xu, personal communica-tion, 2005). Synmetamorphic deformation on the YulongShan decollement was dated at �35 Ma [Lacassin et al.,1996], while39Ar/40Ar cooling ages indicate later exhuma-tion by folding at �17 Ma. The uplift of the range is alsorecorded in the sediments of the Yanyuan piggyback basin,on the hanging wall of the Jinhe-Jianhe thrust. Still preservedare 1–2 km of Paleogene red molasses unconformablycapped by a several hundred meters thick fault-boundedwedge of fluvial, coal-bearing Neogene clastics [Bureau ofGeology and Mineral Resources of Sichuan Province, 1991;Xu et al., 1992; Li et al., 2001].[40] That the high and relatively flat SE plateau surface

did not continue south of the Yalong thrust belt is supportedby several lines of evidence. Topographic parameters NWand SE of the thrust belt are markedly different. The slope-elevation relationships in Figure 15b clearly show thegradual transition from the internally drained, to the shal-lowly incised, then highly dissected regions, as oneapproaches the Yalong belt from the northwest. Despiteincreasing erosion, remnants of the plateau surface are still

Figure 16. Cartoons illustrating sequential buildup ofplateau relief, reflecting interplay between sedimentation,drainage efficiency, and crustal shortening, as well aspossible evolutionary links between northern plateau, plateauinterior, and southeastern plateau, as a function of age.


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recognizable from the 4000–4500 m HPBL (high plateaubase level) trough in the slope-elevation functions, up to28�N southward. But there is no trace of them SE of theYalong belt (E5 and E6). The topography there lacks thecontrast between a relatively flat highland and steeplyincised river valleys seen northwest of the belt (Figure 14).In fact, Clark et al.’s [2006] best examples of low-reliefsurfaces are from this NW region. To the southeast, there isno quantitative evidence; such a surface is questionable atbest (e.g., Figures 14 and 15b).[41] Although Tertiary sedimentary records in SE Tibet

are few, perhaps due to extensive fluvial erosion, the largestTertiary basins are localized along and southeast of theLongmenshan-Yalong-Yulong thrust belt (Figure 11). Thedepositional environments were clearly different northwestand southeast of the plateau edge. To the northwest, theCretaceous-Eocene basins share some common features.They are narrow and elongated, intermontane basins whereinternal drainage led to diverse paleocurrent flow directions,apparently under depositional and climatic conditions sim-ilar to those currently acting in the interior of Tibet [Yi et al.,2000; Horton et al., 2002; Liu and Wang, 2001; Liu et al.,2001; Zhou et al., 2003]. In contrast, southeast of theplateau edge (i.e., Lanping-Simao, Chuxiong, and Jinggubasins further to the southeast), sedimentary records ofsimilar ages indicate open, broad alluvial fan and fan delta,or large lake environments [Zhou et al., 2003], as befits aforeland piedmont. Besides, the appearance of coarse con-glomerates, indicative of proximal deposition, occurs laterin the southeast than in the northwest [Zhou et al., 2003].Such differences in depositional environments across aregional topographic rim are analogous to those nowobserved across the Qilian Shan, south of the Hexi corridorforeland.[42] Rocks exposed on either side of the Yalong-Yulong

thrust belt imply different amounts of denudation. To thenorthwest and on the plateau, large granite plutons that likelycrystallized below 10 km depth [Mattauer et al., 1992] areexposed at the surface over hundreds of square kilometers,whereas to the SE, the Jurassic-Cretaceous sedimentarycover is extensive. Large early Tertiary basins are preserved,and there are only small leucrogranite plugs alongmajor faults,such as the Xianshuihe and Red River faults (Figure 11).Therefore, denudation plateauward from the Yalong-Yulongthrust belt must have been at least �10 km greater thanto the southeast, requiring a substantial amount of tectonicstructural relief across this belt, probably since at least theearly Miocene (�20 Ma [Lacassin et al., 1996]). In turn, the�2 km high topographic step observed today across it islikely a residual of a formerly higher plateau rim.[43] Thus, contrary to previous suggestions of a now

uplifted surface that formed at low elevation [Clark et al.,2006], both geological and quantitative geomorphic evi-dence suggest that the surface of the southeastern part of theTibet plateau NW of the Yalong-Yulong belt formed at highelevation. It was part of the elevated base level of thecurrent southern plateau interior, beveled at 4000 m or morein the Eocene-Oligocene, at a time when it was largelyunconnected with external drainage systems. Dissection ofthis Sichuan high plateau surface is now in an advancedstage, due to particularly aggressive headward retreat of the

modern large river catchments into a plateau interior, fromwhich they had long been excluded.[44] This interpretation requires that the modern external

drainages of the Jinsha, Lancang, and Nu Jiang did notreach into the plateau while its surface formed, a viewradically different from previous interpretations surmisingantecedence of these rivers [e.g., Hallet and Molnar, 2001].Studies by Horton et al. [2002] indeed suggest that thepresent headwaters of the Lancang Jiang south of Yushuwere established during the Neogene, well after the devel-opment of the red-bed basins. In the Hoh Xil basin (90–94�E and 34–36�N), part of which is currently drained bythe Jinsha Jiang, widely distributed lacustrine carbonates ofearly Miocene age (�23.0–16 Ma) and sandstones includ-ing organic rich shales [Yi et al., 2000; Liu and Wang, 2001;Liu et al., 2001; Zhenhan et al., 2008] imply restrictedinternal drainage until at least 15 Ma. Roughly coeval lakedeposits were found in regions to the east (Z. Liu, personalcommunication, 2004), but the evidence becomes patchier,possibly because of later removal by erosion. This would bebroadly consistent with the mid- to late Miocene (12–7Ma)termination of planation long inferred by Armijo et al.[1986] and Shackleton and Chang [1988].

5. Discussion

5.1. Processes Responsible for the Low-ReliefTopography of Tibet’s Interior

[45] The interior flatness of the highest part of the Tibetplateau has often been taken as evidence of viscous flow inthe lower crust, a mechanism thought necessary to smoothout irregularities in topography and crustal thickness [Bird,1991; Fielding, 1996; Shen et al., 2001; McKenzie andJackson, 2002]. The possibility of a combined effect oflower crustal flow and passive sediment infilling cannot beruled out, as has been proposed to account for the subduedrelief in the Altiplano plateau [Lamb and Hoke, 1997].Importantly however, several recent tomographic studies[Kind et al., 2002; Vergne et al., 2002; Wittlinger et al.,2004] confirm the existence of steps and relief of the Mohointerface beneath the plateau.[46] That surface processes play an important role in

smoothing the surface of the Tibetan plateau has beendismissed by some [e.g., Fielding et al., 1994; Fielding,1996] in part because of low erosion rates associated withthe aridity of the current climate. The rather small amountsof denudation, as indicated by widely preserved outcrops ofTriassic to middle Cretaceous sediments in most places[e.g., Tapponnier et al., 1981, 1986; Kidd and Molnar,1988; Dewey et al., 1988, Matte et al., 1996] have beentaken to support this in inference [e.g., Fielding et al.,1994]. However, Sobel et al. [2003] argued exactly theopposite in the Puna plateau, where there is a spatialcorrelation of low-relief landscape with dryness in theplateau interior and high relief with high precipitation nearthe windward plateau edges. Second, extrapolation ofpresent-day erosion rates back in time is uncertain, becauseerosion rates generally decrease as relief decreases fortectonically inactive areas [e.g., Ahnert, 1970; Montgomeryand Brandon, 2002; Schaller et al., 2001]. Clearly, thecurrent dry climate in Tibet is not typical of that of the past.For instance, during the Miocene, a humid to semihumid


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climate prevailed on the forested plateau, suggesting higherprecipitation rates than that at present [Bureau of Geologyand Mineral Resources of Xizang Autonomous Region,1993; Quade et al., 1989]. In any event, with shorteningand relief buildup having occurred in the early Cenozoic[Liu et al., 2003; Wang et al., 2004], there would have beenenough time to smooth the topography, even with slowerosion rates. The existence of the residual Tangula moun-tain range in central Tibet, with its Eocene granitic intru-sives [Roger et al., 2000], and of the 5 km deep HohxilPaleogene foreland sedimentary basin [Wang et al., 2004] tothe north suggest that the now subdued topography of thisrange once marked the former high-relief rim of the south-ern Tibet plateau in the early Tertiary [Roger et al., 2000;Tapponnier et al., 2001; Liu et al., 2003; Wang et al., 2004].In the interior of Tibet, the thickness of Neogene deposits,though quite variable, does reach up to 4 km [Bureau ofGeology and Mineral Resources of Xizang AutonomousRegion, 1993]. The existence of locally thick low-densitysediment accumulations in the basins of Tibet is alsosupported by regional seismic waveforms, teleseismic re-ceiver function and teleseismic tomographic experiments[e.g., Zhu et al., 1995, 2006; Wittlinger et al., 1996, 2004].

5.2. Style and Age of Surface Uplift in SoutheasternTibet

[47] Clark and Royden [2000] interpreted the topographyof SE Tibet as a response to regional long-wavelength tiltlinked to thickening by lower crustal channel flow. Theonset of such regional warping was inferred to be lateMiocene or younger, coeval with the onset of rapid incisionof modern rivers [Clark et al., 2005; Kirby et al., 2002;Schoenbohm et al., 2004]. This interpretation is under-pinned by the inference that a low-relief preuplift surface,which formed at low elevation, on the order of a fewhundred meters only, did exist. Moreover that surface wouldhave been continuous for thousands of kilometers from aformer drainage divide somewhere west of eastern Tibet allthe way to the South China Sea [Clark et al., 2006]. Inactual fact, the evidence that ‘‘one’’ regionally extensivelow-relief surface existed at low elevation across southeast-ern Tibet and adjacent regions prior to the mid-Miocene isvery tenuous.[48] First, Clark et al.’s [2006] identification of such a

relict low-relief landscape is based on the Davisian conceptof ‘‘peneplain,’’ depicted to be the final result of landscapeevolution as it is beveled ‘‘to sea level’’ during long tectonicquiescence [Davis, 1899]. As pointed out by Ritter [1988],however, ‘‘because of this loose definition of peneplain,every geologist was free to establish his own meaning forthe term,’’ and ‘‘almost every major proposed peneplain hasbeen explained in different ways.’’ The revised definition byClark et al. [2006] and Widdowson [1997] of ‘‘a remnant ofa regionally significant paleolandscape with a low-relieftopographic surface, of initially erosional and/or depositionalorigin, that is associated with a protracted period of ero-sion’’ is just as loosely defined, and leaves even morefreedom in arguing for the existence of such a kind ofsurface. Several studies now show that low-relief surfacescan form at high elevations, provided a local base level canbe maintained at such elevations [e.g., Armijo et al., 1986;Meyer et al., 1998; Babault et al., 2005, 2007]. Thus, the

assumption that the low-relief surface visible in SE Tibetmust have been at low elevation prior to regional uplift canhardly be justified.[49] Second, offshore integrated sedimentation flux and

geochemistry in southeast Asia [Metivier et al., 1999; Clift,2006; Clift and Sun, 2006; Clift et al., 2006] does notsupport the existence of a protracted period of slow erosionbefore the mid-Miocene as would be in keeping with thehypothesis of a regional low-elevation, low-relief paleo-landscape at that time. These authors find that thecorresponding clastic sediment flux first peaked in the earlyto middle Miocene (24–11 Ma), and decreased consider-ably during the late Miocene (11–5Ma). Throughout theSouth China Sea, offshore of both the Red river and Zhuriver deltas, peak sedimentation rates were inferred to dateback to a similar but slightly older period (29–16 Ma) [e.g.,Gong and Li, 1997; Zhang and Hao, 1997; Zhong et al.,2004]. This Oligocene-Early Middle Miocene sedimenta-tion flux peak, which appears to be a robust feature, likelyindicates that the mean elevation in the headwaters regionsof east and southeast Asian rivers increased before, ratherthan after the mid-Miocene, as inferred by Clark et al.[2006].[50] Third, the diachronous ages of the proposed ‘‘low-

elevation low-relief’’ (LELR) surface of Clark et al. [2006]undermine the inference that this surface is unique. Corre-lation of patchy low-relief areas, up to hundreds of kilo-meters apart in highlands of different ages, into a singlesurface without discrimination between depositional anderosional origin is highly uncertain, even more so wheresuch areas are on opposite sides of major tectonic dividessuch as the Yalong-Yulong or Ailao Shan ranges. Conversely,it is hard to justify why the plateauward extent of theproposed preuplift low-relief surface would coincide withthe current outer limit of the internally drained part of Tibet,which is in fact flatter and more continuous than the LELRremnants. Our results show that there is no sharp boundarybetween the ‘‘remnant surface’’ and the internally drainedarea, whether of tectonic or geomorphic origin. In Fieldinget al.’s [1994] map of slopes, the boundary between steepand gentle lies east of the current internal drainage divide(Figure 1b). This suggests that the divide is ephemeral, itsposition being controlled by the degree of headward retreatof river catchments.[51] In short, topographic data, morphometric measure-

ments, and other evidence suggest that the high low-reliefsurface in Sichuan, which is clearly the continuation of thatin the plateau interior all the way to the Yalong-Yulongtopographic step, is better explained as a surface that formedat high elevation, rather than close to sea level prior to theLate Miocene. There is still no solid age constraint forsurface uplift in SE Tibet. Thermochronological data arequite diverse [e.g., Arne et al., 1997; Lacassin et al., 1997;Xu and Kamp, 2000; Leloup et al., 2001; Kirby et al., 2002;Gilley et al., 2003; Wallis et al., 2003; Clark et al., 2005;Lai et al., 2007; Godard, 2007; Richardson et al., 2008].But a Late Miocene initiation of regional surface uplift isclearly not in keeping with the integrated sedimentationbudget representing the net flux of eroded material fromSoutheast Asia into the surrounding marginal seas [Metivieret al., 1999; Clift et al., 2006]. Late Cenozoic increasedriver incision might just be one consequence of the estab-


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lishment of the monsoon climate system in East Asia sincethe Miocene [Sun and Wang, 2005].[52] Studies of the structure and tectonics of the region

imply that the topography of SE Tibet attained its presentheight or even greater height well before mid-Miocene. Theevidence includes:[53] 1. Geochronological and thermochronological stud-

ies north of the Red River that document a �36 Ma south-verging decollement exhumed in the core of the YulongShan [Lacassin et al., 1996] along the topographic stepsingled out in Figure 12. Uplift and growth of the YulongShan anticline continued up to 17 Ma [Lacassin et al.,1996].[54] 2. Preserved sedimentary basins inside the SE pla-

teau that record Paleogene shortening [e.g., Tapponnier etal., 2001, and references therein; Liu et al., 2001; Horton etal., 2002; Zhou et al., 2003].[55] 3. Widespread shortening and transpression in the

Chuxiong and Lanping-Simao basins during the Paleogene(Figure 11 [Pan et al., 1990; Tapponnier et al., 1986, 1990a;Leloup et al., 1995]).[56] As long argued by the above authors, there can be no

doubt that the major deformation episodes involving large-scale crustal shortening in SE Tibet took place during theEocene-Oligocene and earliest Miocene. It is highly unlikelythat the existence of south-directed shortening and of a steepsouth-facing topographic break along the Yulong Yalungbelt is a coincidence. Rather it has to reflect a causal link.Consistently, Moho depth underneath the Yulong belt has alarger gradient than that to the northwest and southeast[Teng et al., 2002]. Finally, it must be noted that themechanism proposed for explaining broad surface uplift,‘‘lower crustal channel flow’’ [Clark and Royden, 2000], isnot unambiguously supported by crustal anisotropy studies[Bhaskar et al., 2005, Lev et al., 2006; Sol et al., 2007].

5.3. Evolution of Tibet Drainages and Plateau Growth

[57] The results of our morphometric analysis of thedifferent areas of Tibet that can be discerned on Fieldinget al.’s [1994] gradient map (Figure 1b) support a growthmodel of the Tibet plateau similar to that idealized inFigure 17. The basis for this model is the successiveisolation, by the outward shifting of mountain rims of theorogen, of distinct internal drainage areas, hence local baselevels disconnected from the global sea level. The model isconsistent with the geological and geophysical evidencediscussed by Meyer et al. [1998], Metivier et al. [1998], andTapponnier et al. [2001, and references therein] and with theresults of recent paleoelevation or paleoenvironmental stud-ies [e.g., Spicer et al., 2003; Rowley and Currie, 2006;DeCelles et al., 2007]. The primary emphasis of theschematic, three-stage evolution depicted Figure 17 is onthe age of high plateau topography in areas positioned in thepresent-day geographical reference frame. No attempt isthus made to reconstruct the deformation of the region,whether on the large thrusts or strike-slip faults that havegenerated such deformation throughout the collision zonesince �55 Ma ago. For the same reason, the faults are notrepresented. As food for thought, we speculate on possiblelocations of paleodrainage networks. Clearly, the elevationsdepicted are only indicative and areas away from the

Tibetan orogen have been arbitrarily assumed to be uni-formly low.[58] Three or four main competing mechanisms likely

controlled the drainage evolution at all times. The first,headwater advection toward the mountain rim’s foreland[e.g., Lave and Avouac, 2001; Godard, 2007] played amajor role in isolating the mountain hinterlands fromoutbound erosion, thus allowing for the formation of thehigh internally drainage and high base levels of the plateau.The second, headward catchment retreat is responsible forthe present-day negative topography of SE Tibet and for therelatively recent inward shift of the eastern internal drainagedivide from the flat highland boundary in Figure 1b. Today,this mechanism leads to significant isostatic rock uplift inthe border region between Sichuan and Yunnan. It is aprocess that was probably kept at bay by the first one duringmost of the initial stages of growth of the high plateau(�35–40 Ma). The third mechanism is drainage capturewhich probably occurred at different times between theonset of collision and the present, particularly along themid to lower catchments of the Jinsha, Lancang Jiang [e.g.,Brookfield, 1998; Clark et al., 2004] and Huang He, butalso recently in the upper catchments of these and otherlarge rivers. Capture has probably been largely orchestratedby headward catchment retreat (see van der Woerd et al.[2001], for instance, for small-scale, present-day examplesof this interaction). Finally, antecedence, across high moun-tain rims of large trunk rivers that had developed broadupstream catchments is also observed. The most strikingexamples appear to be those of the Indus and YarlungZangbo, across the western and eastern terminations ofthe Himalayan range at Nanga Parbat and Namche Barwa,respectively. We infer that both rivers had developed pale-ocatchments along the south slope of the Gandese rim of theEocene plateau (stage 35–40 Ma, Figure 17) that resembledthe present-day catchments of the Ganges and Punjab Indussouth of the Himalayas, and were large enough for the trunkrivers to keep crossing the latter range as it grew to toweringheights since the Early Miocene (stage 15 Ma and present,Figure 17). Note that this interpretation is different fromprevious inferences of river capture, particularly across theeastern syntaxis [e.g., Clark et al., 2004].[59] The recent work of Richardson et al. [2008], which

implies unusual exhumation of the surface of the Sichuanbasin, suggests that this basin may have once been inter-nally drained (stage 15 Ma, Figure 17) and filled withsignificant thicknesses of sediments of Oligo-Mioceneage, in much the same way as the Qaidam basin is today.Sediment infill removal, hence exhumation, would thenhave occurred by drainage ‘‘flushing,’’ as the powerfulYangtze catchment retreated headward across the fold beltsthat bound the basin to the east and northeast, then intosoutheastern Tibet (present, Figure 17). This would accountfor the inference of drainage ‘‘reversal’’ along the middlereaches of the Jinsha-Yangtze River, discussed by Clark etal. [2004] and Richardson et al. [2008], among others. Asimportantly, such a sequence of events might explain thepresent-day morphology of the basin floor, most of whichlooks erosional rather than depositional, and the prevailingevidence for drainage superimposition rather than anteced-ence throughout much of the basin.


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[60] Research and fieldwork targeted at testing inferencessuch as those depicted in Figure 17 has only begun. Itshould be boosted by the development of reliable, directpaleoaltimeters [e.g., Ghosh et al., 2006; Blard et al., 2006].

But we suggest that, as previously proposed by Meyer et al.[1998], Tapponnier et al. [2001], and Sobel et al. [2003],among others, surface processes associated with long-last-ing internal drainage may well turn out to be the main

Figure 17. Schematic three-stage growth of Tibetan topography, based on stepwise tectonic model ofTapponnier et al. [2001]. Topographic evolution is represented in present-day geographic referenceframe. No attempt is made at reconstructing deformation. Locations of advected or retreating drainagesystems are inferred at each stage. (top) Older (35–40 Ma) internally drained Eocene plateau, bounded byGangdese, Tanggula, and Yulong-Yalung ranges. (middle) Miocene (15 Ma) plateau, also internallydrained, rimmed by Kunlun, Longmen Shan, and Himalayan ranges. At that time, possibly internallydrained Sichuan basin may have been analogous to present-day Qaidam basin. (bottom) Present-dayplateau has grown further toward NE, while headward catchment retreat has flushed out Sichuansedimentary infill and is at work lowering average topography (small black arrows) and increasing reliefin ancient, southeastern part of plateau.


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mechanisms that lead to the ‘‘uplift’’ and areal growth ofother flat and horizontal plateaus around the world, be theymoderately high (e.g., Iran, Kalahari) or very high (e.g.,Altiplano).

6. Summary

[61] Our morphometric analysis of the topography inthree typical regions of the Tibet plateau demonstrates theimportance of drainage efficiency in controlling the plateaumorphology. Inefficient or internal drainage, short-distancesediment transport and deposition can effectively smoothout tectonically generated local relief, in areas wheredrainage headwaters are not connected to adjacent low-lying forelands. In the internally drained plateau interior,local relief is further reduced, except in the vicinity of activefaults. Both regions display positive topography. Highelevations correspond to large local relief and steep slope,and low elevations to small relief and gentle slope. Becauseof the persistence of such positive topography, ‘‘planation’’processes must have continued and are still going on in theplateau interior, from a starting point that was likely similarto that in the Qilian Shan. Clearly such planation can occurat any elevation, as long as the corresponding regionaldrainage base level is maintained at that altitude for sus-tained periods of time. Planation surfaces have no reason tobe near-sea-level, low-elevation objects.[62] Rejuvenation of drainages, i.e., the capture of inter-

nal drainage systems by external drainage can disrupt theplanation process and produce a transitional topography bydissecting an ‘‘old’’ remnant plateau surface, introducingyounger and steeper incision at that surface’s base level. Inthe southeastern plateau, where large rivers have carveddeep gorges, the regional topography is negative, with high-elevation terrain corresponding to smaller relief and gentlerslope than at low elevation. This remnant SE Tibet plateausurface appears to terminate along a degraded plateau rimthat coincides with a major Oligo-Miocene thrust belt, theYalong-Yulong range. The development of the great drain-ages, Jinsha, Lancang, and Nu Jiang, appears to have beendue to headward retreat, probably controlled primarily bythe slowing or termination of transpressional tectonics[Leloup et al., 1995] and correlative rim uplift, but possiblyalso in part by climate, as inferred in other areas [i.e., Sobelet al., 2003; Hilley and Strecker, 2005]. Renewed erosion ofthe plateau inward from this rim could thus have laggedconsiderably behind the construction of topography anduplift of the plateau, the preservation of high internaldrainage base levels leading in effect to a decoupling be-tween erosion and tectonics. Our analysis suggests that itshould be difficult, in general, to infer deep-seated crustal ormantle processes from current surface topography withoutfirst taking into account the time-integrated effect of uppercrustal deformation and surficial geomorphic evolution ontopography. In fact, the local incision of rivers should not bereadily used as a proxy of coeval uplift due to tectonicshortening or other deep-seated process. In support of ourconclusions, and at a more local scale, recent thermochro-nogical studies in the Longmen Shan [Godard, 2007;Richardson et al., 2008] suggest that the recent evolutionof this rim of Tibet may also be controlled by the westward

propagation of regressive erosion acting on an inheritedtopographic step.

[63] Acknowledgments. This work was funded by a grant from theMinistery of Education Nationale de Enseignement Superieur et de laRecherche and by the National Natural Science Foundation of China(NSFC 40625008) and Chinese Academy of Sciences Hundred Talentprogram. We benefited from help with coding from Paul Pinzuti andMathieu Daeron. We thank Rolando Armijo for an unpublished map ofinternal drained area in Tibet; Vincent Godard for comments on an earlierversion; Mark Allen, Ed Sobel, Phillippe Davy, and Associate Editor AlexDensmore for reviews that improved the content and clarity of themanuscript; and Anne Claire Morillon for drafting Figure 17. The photoin Figure 5b was taken by Qinghai Zhang.

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��L. Ding and J. Liu-Zeng, Institute of Tibetan Plateau Research, Chinese

Academy of Sciences, 18 Shuang Qing Road, P.O. Box 2871, Beijing100085, China. ([email protected])Y. Gaudemer and P. Tapponnier, Laboratoire de Tectonique, Institut de

Physique du Globe, 4 Place Jussieu, F-75005 Paris, France.


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quantifying landscape differences across the tibetan ... · and relief structure, within a sound...

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