Distribution of active rock uplift along the eastern margin of the Tibetan Plateau: Inferences from bedrock channel longitudinal profiles

Abstract

[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

Geochronology of the Baye Mn oxide deposit, southern Yunnan Plateau: Implications for the late Miocene to Pleistocene paleoclimatic conditions and topographic evolution

“…The incision rates for the Nanpie River are also comparable to the results calculated from 10 Be exposure and 14 C dating of the Quaternary fluvial terraces on the eastern Tibetan Plateau ranging from 200-800 m. Myr -1 (Ouimet et al, 2009;Godard et al, 2010;Hetzel, 2013). The river incision rates inferred from the Pleistocene to Holocene terraces on the eastern margin of the Tibetan Plateau range from 200-500 m. Myr -1 at low gradient channel rivers to 1000-3000 m. Myr -1 at high gradient channel rivers (Kirby et al, 2003), with the former being similar to the incision rate of the Nanpie River. This similarity may 21 reflect the fact that the relict landscape at Baye (Fig.…”

“…12a). The incision rates (235-416 m. Myr -1 ) calculated for the Nanpie River are consistent with the values estimated from zircon and apatite (U-Th)/He and apatite fission-track dating results in the eastern Tibetan Plateau (Kirby et al, 2003;Clark et al, 2005;Ouimet et al, 2010;Duvall et al, 2012). The incision rates are 250-500 m. Myr -1 for the Yalong and Dadu River gorges between 9-13 Ma (Clark et al, 2005), 200-600 m. Myr -1 for the Tsangpo, Salween, Mekong, Yalong, Yellow, and Yangtze River gorges between 4-11 Ma (Duvall et al, 2012), and ca.…”

“…This value is comparable to the measured incision rates (10-90 m. Myr -1 ) by cosmogenic 26 Al dating of the Wudingshan caves (with burial ages of 2.54 and 0.69 Ma) in close proximity of the northern end of the Red River (Schoenbohm, 2004b). The incision rate estimated from sites B-D, however, are much lower than those obtained from the Nanpie River and other deeply incised rivers over the Yunnan Plateau (Allen et al, 1984;Kirby et al, 2003;Clark et al, 2005;Ouimet et al, 2010;Godard et al, 2010;Duvall et al, 2012;Hetzel, 2013), implying distinct drainage incision in different time periods, likely due to changes in climatic conditions or staged tectonic uplift of the plateau (Schoenbohm et al, 2004a). It is noticeable that there is an absence of alluvium sediments over saprolites at the lower-elevation sites C and D, indicating that the relict terraces of the Nanpie River have experienced different stages of drainage incision since ~3 Ma.…”

Spatial analysis of stream power using GIS: SLk anomaly maps

“…This analysis was very useful in determining the parameters in these small sub-basins but not for inflection analysis of the main streams. Kirby et al (2003) made a very valuable analysis of the concavity index and the steepness index (both extracted by this well established areaslope relation) in streams of the Tibetan Plateau. In their study, Kirby et al (2003) needed to perform different regression analyses in the rivers that presented clear breaks in the slopearea line regression.…”

“…Kirby et al (2003) made a very valuable analysis of the concavity index and the steepness index (both extracted by this well established areaslope relation) in streams of the Tibetan Plateau. In their study, Kirby et al (2003) needed to perform different regression analyses in the rivers that presented clear breaks in the slopearea line regression. So, these results suggest that slope-area analysis is not the best way to examine gradient differences within the same river but may be a very valuable tool for the analysis and comparison of individual basins.…”