A new regional compilation of the drainage history in southeastern Tibet suggests that the modern rivers draining the plateau margin were once tributaries to a single, southward flowing system which drained into the South China Sea. Disruption of the paleo‐drainage occurred by river capture and reversal prior to or coeval with the initiation of Miocene (?) uplift in eastern Tibet, including ∼2000 m of surface uplift of the lower plateau margin since reversal of the flow direction of the Yangtze River. Despite lateral changes in course due to capture and reversal, the superposition of eastward and southward draining rivers that cross the southeastern plateau margin suggests that uplift has occurred over long wavelengths (>1000 km), mimicking the present low‐gradient topographic slope. Thus reorganization of drainage lines by capture and reversal events explains most of the peculiar patterns of the eastern plateau rivers, without having to appeal to large‐magnitude tectonic shear.

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[1] High topography in central Asia is perhaps the most fundamental expression of the Cenozoic Indo-Asian collision, yet an understanding of the timing and rates of development of the Tibetan Plateau remains elusive. Here we investigate the Cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the Sichuan Basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed. Temperaturetime paths inferred from 40 Ar/ 39 Ar thermochronology of biotite, multiple diffusion domain modeling of alkali feldspar 40 Ar release spectra, and (U-Th)/He thermochronology of zircon and apatite imply that rocks at the present-day topographic front of the plateau underwent slow cooling (<1°C/m.y.) from Jurassic times until the late Miocene or early Pliocene. The regional extent and consistency of thermal histories during this time period suggest the presence of a stable thermal structure and imply that regional denudation rates were low (<0.1 mm/yr for nominal continental geotherms). Beginning in the late Miocene or early Pliocene, these samples experienced a pronounced cooling event (>30°-50°C/m.y.) coincident with exhumation from inferred depths of $8 -10 km, at denudation rates of 1 -2 mm/yr. Samples from the interior of the plateau continued to cool relatively slowly during the same time period ($3°C/m.y.), suggesting limited exhumation (1 -2 km). However, these samples record a slight increase in cooling rate (from <1 to $3°C/m.y.) at some time during the middle Tertiary; the tectonic significance of this change remains uncertain. Regardless, late Cenozoic denudation in this region appears to have been markedly heterogeneous, with the highest rates of exhumation focused at the topographic front of the plateau margin. We infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable Sichuan Basin and thus marks the onset of deformation related to the development of the Tibetan Plateau in this region. The present margin of the plateau adjacent to and north of the Sichuan Basin is apparently no older than the late Miocene or early Pliocene ($5 -12 Ma).

[1] Current models of long-term river incision into bedrock suggest that the local rate of differential rock uplift should exert a primary control on the gradient of channel longitudinal profiles. However, discrimination of this effect from the influence of variations in substrate erodibility, sediment flux, precipitation, and transient changes in profile shape has proved difficult in practice. Here we investigate the controls on the spatial distribution of bedrock channel gradients adjacent to the Sichuan Basin in an effort to assess the degree and nature of active deformation along this margin of the Tibetan Plateau. Analysis of river longitudinal profiles utilizing a channel steepness index (a measure of profile gradient normalized for drainage area) reveals a zone of anomalously steep channels adjacent to the topographic front of the plateau margin. Channel profiles are systematically less steep in their headwater reaches on the plateau and in their lower reaches east of the plateau margin. Comparison of steepness indices to mapped lithologic variations reveals that lithology has only a limited influence on channel gradient in this field area. We observe no systematic relationship between steepness indices and upstream drainage area; channels of all size are steeper near the plateau margin. We argue that these systematic changes are not readily explained as a consequence of increased sediment flux or of orographic precipitation. We are led to conclude that steep channel profiles along the topographic front of the plateau reflect active differential rock uplift between this region and the foreland. Motivation[2] The recognition that mass redistribution by erosion represents a governing force in the tectonic evolution of orogenic systems [Beaumont et al., 1992;Molnar and England, 1990] spurred a decade of intensive research into the interrelationships between tectonic and surface processes. One of the primary outgrowths of this work is the understanding that much of the tempo and style of landscape evolution in active mountain belts is dictated by the processes of river incision into bedrock [e.g., Seidl and Dietrich, 1992;Tinkler and Wohl, 1998]. The bedrock channel network dictates critical relationships among relief, elevation, and denudation rate [Howard, 1994;Howard et al., 1994; and conveys signals of tectonic and climatic change across landscapes, effectively setting landscape response time . The rate of channel incision sets the lower boundary condition for hillslopes and thus fundamentally influences denudation rates across the landscape.[3] Current models of bedrock channel incision in tectonically active regions consider that channel gradients are set by a competition between the local rate of differential rock uplift (relative to a fixed, external base level) and channel incision rate [Howard, 1994]. Consequently, analysis of channel gradients and longitudinal profiles provides a promising means of exploring the spatial distribution of rock uplift in an actively deforming orogen

Abstract. We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1991-1998 for a network encompassing the eastern part of the Tibetan Plateau and its foreland. Relative to a Eurasian frame defined by minimizing the velocities of 16 GPS stations in Europe, central Asia, and Siberia, stations within all parts of the plateau foreland in south China move 6-10 mm/yr east-southeast, indicating that the eastward movement within the plateau is part of a broader eastward movement that involves the plateau and its eastern and northern foreland. North of the plateau, foreland stations move northeastward at -10 mm/yr, indicating that the northern boundary of the deformation zone lies north of the plateau. With this realization of a Eurasian frame, the velocity of the GPS station at Bangalore in southern India implies that the northward motion of India is 5-12 mm/yr slower than that predicted from the NUVEL-1A plate reconstruction. Viewed relative to the South China Block, stations of the northeast plateau, bounded on the north by the Qilian Shan and the Altyn Tagh fault, move NNE to NE with velocities ranging from 19 mm/yr within the plateau to 5-11 mm/yr in its foreland. The Altyn Tagh fault shows left-lateral slip of-10 mm/yr at 95øE and shortening across the fault of <5 mm/yr. Stations south and west of the Xianshuihe/Xiaojiang fault system define a crustal fragment rotating clockwise at -10 mrn/yr relative to the South China Block around the eastern Himalayan syntaxis. The GPS measurements indicate no significant shortening (< 3 mm/yr) within the Longmen Shan of the central eastern plateau and its adjacent foreland, although the Longmen Shan rise over 6 km in <100 km horizontal distance. Geological studies indicate that the deformational field was established diachronously in late Miocene to Pliocene time, was characterized by no east-west shortening of Tibetan crust, and has an inhomogeneous style of deformation resulting from a balance of different tectonic processes.