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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union

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Earth Surf. Dynam., 5, 125-143, 2017
http://www.earth-surf-dynam.net/5/125/2017/
doi:10.5194/esurf-5-125-2017
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
21 Feb 2017
Autogenic versus allogenic controls on the evolution of a coupled fluvial megafan–mountainous catchment system: numerical modelling and comparison with the Lannemezan megafan system (northern Pyrenees, France)
Margaux Mouchené1,a, Peter van der Beek1, Sébastien Carretier2,3, and Frédéric Mouthereau2 1Université Grenoble Alpes, CNRS, ISTerre, CS-40700, 38058 Grenoble, France
2GET, Observatoire Midi Pyrénées, Université de Toulouse, CNRS, IRD, 14 avenue E. Belin, 31400 Toulouse, France
3Department of Geology, FCFM, Universidad de Chile, Santiago, Chile
anow at: Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA 70118, USA
Abstract. Alluvial megafans are sensitive recorders of landscape evolution, controlled by both autogenic processes and allogenic forcing, and they are influenced by the coupled dynamics of the fan with its mountainous catchment. The Lannemezan megafan in the northern Pyrenean foreland was abandoned by its mountainous feeder stream during the Quaternary and subsequently incised, leaving a flight of alluvial terraces along the stream network. We use numerical models to explore the relative roles of autogenic processes and external forcing in the building, abandonment and incision of a foreland megafan, and we compare the results with the inferred evolution of the Lannemezan megafan. Autogenic processes are sufficient to explain the building of a megafan and the long-term entrenchment of its feeding river on time and space scales that match the Lannemezan setting. Climate, through temporal variations in precipitation rate, may have played a role in the episodic pattern of incision on a shorter timescale. In contrast, base-level changes, tectonic activity in the mountain range or tilting of the foreland through flexural isostatic rebound do not appear to have played a role in the abandonment of the megafan.

Citation: Mouchené, M., van der Beek, P., Carretier, S., and Mouthereau, F.: Autogenic versus allogenic controls on the evolution of a coupled fluvial megafan–mountainous catchment system: numerical modelling and comparison with the Lannemezan megafan system (northern Pyrenees, France), Earth Surf. Dynam., 5, 125-143, doi:10.5194/esurf-5-125-2017, 2017.
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Short summary
The Lannemezan megafan (northern Pyrenean foreland) was abandoned during the Quaternary and subsequently incised. We use numerical models to explore possible scenarios for the evolution of this megafan. We show that autogenic processes are sufficient to explain its evolution. Climate may have played a second-order role; in contrast base-level change, tectonic activity and flexural isostatic rebound do not appear to have influenced its evolution.
The Lannemezan megafan (northern Pyrenean foreland) was abandoned during the Quaternary and...
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