CONTROLS ON THE POST-OROGENIC EVOLUTION OF THE LANNEMEZAN MEGAFAN IN THE NORTHERN PYRENEAN FORELAND: INSIGHTS FROM COSMOGENIC NUCLIDE EXPOSURE DATING, MORPHOMETRIC ANALYSIS AND NUMERICAL MODELING

Fluvial megafans hold a critical position in the source-to-sink routing of sediments and are thus particularly sensitive to changes in the erosion-deposition dynamics through tectonic and/or climatic forcing. The Mio-Pliocene Lannemezan megafan (SW France) is the most prominent feature of the Northern Pyrenean foreland and was abandoned and incised at some point in the Quaternary. However, neither the precise age of megafan abandonment nor its driving mechanism within a stable post-orogenic context has been elucidated, and either tectonic, climatic or base-level controls have been proposed.

We report new cosmogenic exposure ages for the abandonment of the surface of the megafan as well as for a series of alluvial terraces produced during the incision. We show that the modern morphology of the foreland was acquired only recently, as the abandonment of the megafan happened at ~280 to 320 ka, much later than what was previously assumed (Early Pleistocene). The subsequent incision of the megafan appears to have been strongly influenced by glacial/interglacial cycles, as terrace abandonment ages correlate to the MIS9, MIS5 and post-LGM cold-to-warm transitions. Geomorphic analysis of the stream network shows no indications for systematic knickpoint generation or retreat, as expected if the river network responds to active tectonics or base-level change, respectively. We do evidence river network reorganizations through river captures using χ proxy analysis.

We explore the respective roles of autogenic versus allogenic forcing in the evolution of the Lannemezan megafan using a landscape evolution numerical model (CIDRE). Our results suggest that autogenic processes may have been sufficient to explain the long-term evolution (building, abandonment and incision) of the megafan but a climatic imprint cannot be ruled out.