Quantifying Landscape Differences Across the Tibetan Plateau: Implications for Topographic Relief Evolution J

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, F04018, doi:10.1029/2007JF000897, 2008 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 with specific focus on three representative regions: northern, central, and southeastern Tibet. Quantitative landscape information is extracted from Shuttle Radar Topography Mission digital elevation models. We find that the morphology of the Tibetan plateau is nonuniform with systematic regional differences. The northern and central parts of the plateau are characterized by what we suggest to call ‘‘positive topography,’’ i.e., a topography in which elevation is positively correlated with relief and mean slope. A major change from the internally drained central part of Tibet to the externally drained part of eastern Tibet is accompanied by a transition from low to high relief and from positive to ‘‘negative topography,’’ i.e., a topography where there is an inverse or negative correlation between elevation and relief and between elevation and mean slope. Relief in eastern Tibet is largest along rivers as they cross an ancient, eroded plateau margin at high angle to the major strike-slip faults, the Yalong-Yulong thrust belt, implying strong structural control of regional topography. We propose that the evolution of river systems and drainage efficiency, the ability of rivers to transport sediments out of the orogen, coupled with tectonic uplift, is the simplest mechanism to explain systematic regional differences in Tibetan landscapes. Basin filling due to inefficient drainage played a major role in smoothing out the tectonically generated structural relief. This mode of smoothing started concurrently with tectonic construction of the relief, as most clearly illustrated today in the Qilian Shan-Qaidam region of the northeastern plateau. In the interior of Tibet, further ‘‘passive’’ filling, due to internal drainage only, continued to smooth the local relief millions of years after the cessation of major phases of surface uplift due to crustal shortening. Thus, diachronous beveling at high elevation produced the low-relief surface of the high plateau. In southeast Tibet, headward retreat of external drainages brought back ‘‘in’’ the global ocean base level, first disrupting then interrupting the relief-reduction process. It produced a transitional topography by dissecting the ‘‘old’’ remnant plateau surface, which introduced younger and steeper incision of this hitherto preserved high base level. This provides a unifying mechanism for the formation of the low-relief plateau interior, and for the origin of the high-elevation, low-relief relict surface in southeastern Tibet. Our analysis brings forth the importance of surface processes, in particular drainage efficiency, in shaping plateau morphology and landscape relief. Such key processes appear to have been mostly ignored in numerical models of plateau deformation. Our results also cast doubt on and provide a more realistic alternative to the fashionable contention that a continuous preuplift, low-relief surface first formed at low elevation, extending all the way to the South China Sea shore, before being warped upward in the late Miocene-Pliocene by lower crustal channel 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 1Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China. [2] The Tibetan plateau is the world’s highest and largest 2Laboratoire de Tectonique, Institut de Physique du Globe, Paris, orogenic plateau. It has a low-relief internally drained France. interior, flanked in the north and south by steep-sided edges [Fielding et al., 1994], the southern one including Earth’s Copyright 2008 by the American Geophysical Union. 0148-0227/08/2007JF000897 highest summits with elevations greater than 8 km. Toward F04018 1of26 F04018 LIU-ZENG ET AL.: LANDSCAPE DIFFERENCES IN TIBET F04018 the southeast and northeast, the plateau shows more gently models addressing plateau uplift and growth. In this paper, tapered topography, providing escape routes for some of the we engage in a quantitative description of the bulk land- largest Asian rivers. scape characteristics of different regions of the Tibetan [3] Continuum models of Tibet plateau deformation have plateau. Our morphometric analysis of topography across been primarily motivated by these general topographic the plateau demonstrates the existence of systematic and features and their gravitational implications, and sometimes meaningful regional differences. The northern and central based solely upon them. For instance, the low-relief interior parts of the plateau can be characterized as regions of of the plateau has often been taken as evidence of viscous ‘‘positive’’ topography, i.e., where there is a positive corre- flow in the lower crust, a mechanism thought necessary to lation between relief and elevation, and between local slope smooth out irregularities in topography and crustal thick- and elevation. The change from the internally drained ness [Bird, 1991; Shen et al., 2001; McKenzie and Jackson, central plateau to the externally drained part of eastern 2002]. At the plateau scale, these continuum models, Tibet is accompanied by a transition from positive to whether the vertically averaged ‘‘thin viscous’’ sheet or ‘‘negative’’ topography, i.e., with inverse correlation be- the decoupled weaker lower crust flow classes of models, tween elevation, relief and slopes. Fluvial relief in eastern view the lateral growth of Tibet as being driven by stored Tibet is largest across a NE-trending topographic step that gravitational potential energy, through ‘‘concentric’’ out- may mark an ancient, eroded rim of the plateau. Regions ward spreading occurring more or less simultaneously along located southeast of this step do not show negative topog- all edges [e.g., England and Houseman, 1986, 1989; raphy. We propose that the evolution of river systems and Royden et al., 1997; Shen et al., 2001; Jimenez-Munt and drainage efficiency, the ability of rivers to transport sedi- Platt, 2006]. In such models, the southeastern plateau, ments out of the orogen, coupled with tectonic uplift, like the northeastern plateau, would have been rising fast provides a robust unifying mechanism to explain both the throughout the late Cenozoic to the present-day. In partic- way in which the plateau morphology and relief were ular, the more gently sloping topography in southeastern shaped and the systematic regional differences observed in Tibet has been taken as evidence for tilting of a low- the Tibetan landscape. elevation, low-relief surface driven by recent and ongoing [6] Similar techniques of large-scale topographic analysis lower crustal channel flow [Clark and Royden, 2000; have been applied to various regions [e.g., Ahnert, 1970, Clark et al., 2005, 2006]. In all these models, the current 1984; Koons, 1989; Ohmori, 1993; Brozovic´etal., 1997; topography and landscape morphology of the plateau are Brocklehurst and Whipple, 2004]. An overall study of the inferred to be a straight forward reflection of deep seated topography of the Tibet plateau was previously performed lower crust and mantle processes. By contrast, the impor- by Fielding et al. [1994] and Fielding [1996] using a 90 m tance of surficial erosion processes in shaping the landscape resolution digital elevation model (DEM). Their data (not is downplayed. publicly available at the time) was 100 times better in [4] Block models that include geological and tectonic resolution than ETOPO5 (5 min latitude/longitude gridded evidence make quite different predictions, in particular, that elevation data). These authors focused mainly on the the plateau has grown mostly toward the northeast, in flatness of Tibet, however, without characterizing regional stepwise fashion [Lacassin et al., 1997; Meyer et al., differences, even though the total area of Tibet is almost half 1998; Metivier et al., 1998; Tapponnier et al., 2001; Pares that of the contiguous United States. Yet inspection of et al., 2003]. On the basis of strike-slip driven mountain Tibet’s morphology shows that the plateau exhibits signif- growth and not just foreland thrust migration, Tapponnier et icant variations in interior relief. Besides, it is an assem- al. [2001] proposed a ‘‘three-step’’ model of sequential blage of tectonic terranes with different Phanerozoic uplifting (Figure 1a): south Tibet would have been the first histories that likely responded differently to the strain part of the plateau to rise during the Eocene. The region generated by the India/Eurasia collision, and that likely north and east of the Kunlun range would be the youngest rose diachronously since 55 Ma ago [e.g., Lacassin et part of the plateau, and would have risen fast due to crustal al., 1996; Meyer et al., 1998; Tapponnier et al., 2001]. With shortening, between the Pliocene and present. The region in the advent of freely accessible Shuttle Radar Topography between would have gone through its main phase of uplift Mission DEMs, which provide public access to a spatial between

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