Mass balance from alluvial fans and estimation of denudation rate in the chineses Tian Shan
Résumé
Continental surfaces are incessantly reworked through erosion and sedimentation. Markers of erosion within drainage areas are often scarce and temporary, but at the outlet of mountain belts, more continuous and perennial records of deposition can be found in alluvial fans. These fans are constructed by the deposition through time of the coarse part of sediments transported by rivers. The volume of sediments trapped in alluvial fans can then be used in order to reconstruct sediment fluxes coming out from their catchment areas and the associated denudation rates. Quantifying such denudation rates is of great interest for the study of mass transfer. It is also necessary to understand relief dynamics, as well as the influence of tectonic and climate on this dynamics. We propose here a complete method to estimate erosion fluxes from alluvial fans in a specific area, the northern piedmont of the Tian Shan in China. Along the piedmont of this range, series of fans of different ages are clearly identified. In particular, Holocene fans (which are active for 10 000 years) can be recognized on satellite images and DEM. Pleistocene abandoned fans (which were active before 10 000 years ago) are also well preserved in the landscape, and easily identifiable on the same data. These fans have been deeply incised during the last deglaciation (about 10 000 years ago), and therefore, their basal surface can be observed. In this specific area, it is thus possible to obtain field constraints on the real thickness of those fans. First, we draw a morpho-sedimentary map of the fans and their drainage basins. Then, we went on the field to estimate the Pleistocene fan thickness wherever it is possible. Finally, based on this data set, and on geometrical considerations, we built isopach maps of the fans and calculate their volumes. These 3D reconstructions can then be compared to the geometrical relationships classically used to assess alluvial fan volumes from their upper surface only. Holocene and Pleistocene denudation rates of ten drainage basins can be derived from these volumes, allowing a sink to source investigation, for the periods of fan activity. In north Tian Shan, these erosion rates can be compared with other values calculated from bedload measurements and cosmogenic data. We show that the present-day denudation rates derived from sediment records are biaised by the formation of canyons about 10 000 years ago, and that the real denudation rate of the range is quite low, contrary to what has been proposed based on cosmogenic data.