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.
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