Impact du climat et de la tectonique sur l'évolution géomorphologique d'un piémont - Exemple du piémont Nord du Tian Shan depuis la fin du Pléistocène

Abstract : A piedmont consists in the transition zone between a mountain range and its foreland basin. We studied the northern piedmont of Eastern Tian Shan (NW China), in order to describe the influence of climate and tectonics on this zone and to quantify observed surface processes. Tectonics induces a long-term signal by uplifting the range, giving rise to a sedimentary flow toward the piedmont, and through the development of a fold-and-thrust belt. Climate has a short-term influence because its evolution follows the main global climatic cycles with some local modifications. During glacial times, some material is eroded in the high range but it is difficult to export from there. Sediment transport becomes easy at the transition to the interglacial, so that large alluvial fans emplace in the piedmont around 12 ka BP. They are then deeply entrenched by the rivers with incision rates ranging from 1 to 2.5 cm/yr over the Holocene. During this phase, wide terraces are first abandoned. Then, valleys become narrows without stopping river incision. We propose a modeling for one of these rivers over the Holocene, and so intend to inverse the decreasing trend of riverbed gradient shown by topographic profiles of terraces. This approach suggests that river discharge was divided by 3 since the mid-Holocene. This study is supported by the analysis of the lake into which the piedmont rivers flow. The lake surroundings show evidence of ancient shorelines corresponding to Holocene lake-levels. The lake-level may have been 50 m higher than today, attesting that the lake area was 3 times larger than present one. We compute a lake hydrological balance for present and mid-Holocene conditions, respectively. This calculation shows that the river inflow was 2.6 higher at 6000 BP. From this two complementary studies, we conclude that Holocene climate was regionally much wetter than today, as paleoclimatic models had already shown but not quantified.
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Applied geology. Université Paris Sud - Paris XI, 2002. French


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Submitted on : Friday, July 8, 2005 - 4:39:43 PM
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Blanche Poisson. Impact du climat et de la tectonique sur l'évolution géomorphologique d'un piémont - Exemple du piémont Nord du Tian Shan depuis la fin du Pléistocène. Applied geology. Université Paris Sud - Paris XI, 2002. French. <tel-00009710>

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