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Modélisation expérimentale de la dynamique long-terme des reliefs en réponse aux forçages climatiques et tectoniques

Abstract : Continental reliefs are dynamic interfaces between the lithosphere and the atmosphere where a great diversity of processes interact with internal geodynamics and external geodynamics. They are at the essence of the problem of interactions between climate and tectonics. A better understanding of this interactions suggests a better understanding of effects of climate and its variations on the morphology of the landforms and their erosion. We present in this work the results of an experimental study in laboratory of the dynamics of the reliefs, under the effect of tectonic forcing (uplift rate) and climatic (rainfall rate). From the development of a consistent database of experiments carried out under different conditions of uplift and precipitation, we show that the mean steady-state of the reliefs, their update time and their morphology, deduced from slope-area analyzis, are non-linear functions of the uplift, with a rainfall effect that needs to be specified. We show that the mean equilibrium altitude of the experimental landscape is controlled by the ratio between rainfall and uplift “water-to-rock ratio”, according to a nonlinear law constrained by more than 80 experimental data. The problem of the response of reliefs to climatic variations was approached by considering the effect of a decrease of rainfall rate on a landscape initially at steady-state, considering different durations of the decrease, with constant rate of rise. We show that more the deacreasing of rainfall lasts in time, , more the erosive response (erosion rate) is shifted in time and decreases in amplitude. This result implies that over very long time scales (> a few Ma), climatic variations probably have very little impact on erosion rates. We also show that this response is controlled in our experiments by an erosion threshold. We also discussed the impact of spatial rainfall variations on the mobility of river and ridge networks. We show that such variations leads to a dissymmetry of the valleys and a migration of the drainage networks, which can cause catches between catchments. These phenomena observed in the experiments are in agreement with the geomorphological characteristics of the valleys of the north-Pyrenean piedmont. These results highlight the role of climate, in this case precipitation, in the long-term dynamics of topographic surfaces.
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Submitted on : Friday, January 12, 2018 - 11:21:53 AM
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Bérangé Moussirou. Modélisation expérimentale de la dynamique long-terme des reliefs en réponse aux forçages climatiques et tectoniques. Géomorphologie. Universite Toulouse 3 Paul Sabatier (UT3 Paul Sabatier), 2017. Français. ⟨tel-01682334⟩



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