HR: 1340h
AN: T23C-1538    [Abstracts]
TI: Transient incision of the Yellow River: a response to drainage basin integration across the northeastern margin of the Tibetan Plateau
AU: * Kirby, E
EM: ekirby@geosc.psu.edu
AF: Penn State University, Department of Geosciences, University Park, PA 16802, United States
AU: Craddock, W
EM: bill.craddock@geosc.psu.edu
AF: Penn State University, Department of Geosciences, University Park, PA 16802, United States
AU: Harkins, N
EM: nharkins@geosc.psu.edu
AF: Penn State University, Department of Geosciences, University Park, PA 16802, United States
AB: Although the character, provenance and accumulation rate of detritus shed from active mountain ranges contains a rich archive of the interplay between tectonics, climate and erosion, the subsequent excavation of such basins offers additional insight into the processes that govern the topographic evolution of orogens. In northeastern Tibet, growth of the Tibetan Plateau was associated with a protracted period of basin development and sediment accumulation extending from Oligocene to Pliocene time. The present-day course of the Yellow River transects a number of these basins as it descends off the high plateau; from upstream to downstream these are the Tongde, Gong He, Guide, Xunhua, and Linxa basins. In the vicinity of the plateau margin, in the Linxia basin, the youngest portions of the stratigraphic record are characterized by an abrupt transition from lacustrine sediments to fluvial gravels that herald the onset of relatively rapid fluvial incision at ~1.7 Ma [Fang et al., 2003; Li et al., 1997]. Little is known, however, about the timing, rates and patterns of fluvial incision along the Yellow River as it flows through basins upstream of the plateau margin. We address this question through a combination of new constraints on the uppermost basin fill from cosmogenic isotope burial ages, analysis of tributary channel profiles, and volumetric reconstruction of the basin fill. The highest basins along the Yellow River (Gong He and Tongde) are characterized by a thick (>400 m) sequence of conglomerates and fluvial sands that appear to reflect filling by an ancestral Yellow River. Well preserved remnants of aggradational surfaces atop the basin fill allow us to reconstruct the volume of eroded material from these basins. Over a ~200 km long reach of the river, both the depth of fluvial incision and the total volume of eroded material decrease in the upstream direction, consistent with headward migration of a wave of incision. Moreover, cosmogenic radionuclide burial ages of several samples near the top of the basin fill indicate that aggradation ceased at approximately 0.5-0.7 Ma, and suggest subsequent incision rates of ~0.7 – 1.0 mm/yr in this region. Finally, upstream of these basins, the Yellow River has incised a deep canyon through bedrock in the Anyemaqen Shan; analysis of tributary profiles and strath terrace profiles reveal a transient wave of incision propagating into the higher elevations of the plateau. Ages of the highest terrace surfaces near the rim of the inner canyon suggest that the onset of incision occurred only in the past 0.15 Ma. Together these results indicate that incision along the Yellow River propagated upstream, integrating fluvial and lacustrine systems along the way. The wave of incision appears to have migrated a streamwise distance of >500km in the past 1.7 Ma, at average rates of nearly 300 km/Myr. We hypothesize that the breach of drainage boundaries was facilitated by 1) a change to a more erosive climatic condition and/or 2) the steady accumulation of potential energy during basin filling and growth of the plateau. Whereas the timing of incision along large rivers in eastern and southeastern Tibet reflects the development of high topography associated with the Tibetan Plateau, incision along the Yellow River appears to significantly postdate growth of the plateau in northeastern Tibet.
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1825 Geomorphology: fluvial (1625)
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: 2007 Fall Meeting