RATES AND PATTERNS OF FLUVIAL INCISION ALONG THE YELLOW RIVER IN NORTHEASTERN TIBET: INFERENCES FROM PLEISTOCENE – HOLOCENE TERRACE SEQUENCES

The sensitivity of fluvial systems to downstream changes in tectonic forcing provides a means of investigating active deformation in regions where stratigraphic markers are absent. The upper reaches of the Yellow River display pronounced changes in character as the river flows off the Tibetan Plateau, transitioning from a broad, meandering river on the plateau to a steep, incised bedrock river through the Anyemaquen Shan. Immediately downstream of this gorge are a series of Tertiary basins that are actively being evacuated by the Yellow River and its tributaries. We seek to test whether incision across the Anyemaqen Shan represents a response to basin excavation or whether differential rock uplift is occurring across the range. We surveyed the heights of a series of strath and alluvial terraces near the center of the range, and downstream of the gorge, where the river occupies a broad alluvial valley but shows evidence of ~800 m of incision into Quaternary alluvium. Minimum terrace surface ages are determined using radiocarbon dating of charcoal and terrestrial shell fragments extracted from fluvial gravels and OSL dating of silt lenses and loess accumulations capping straths. These ages permit the calculation of incision rates along the river. Preliminary data suggest that incision rates in the center of the Anyemaquen Shan have been as high as 2 mm/yr since the Late Pleistocene and Holocene. Within this region, channel long profiles of tributaries to the Yellow River exhibit high steepness (ks) values. In contrast, incision rates downstream of the gorge in the Tongde basin are somewhat lower 1-1.5mm/yr). Tributaries adjacent to this lower incision rate site display lower ks values as well. Additional samples pending analysis are expected to refine our incision rate estimates. Taken together, our preliminary incision rate and long profile data suggest that higher rates of fluvial incision within the Anyemaqen Shan is a manifestation of increased rates of rock uplift within the range.