Controls on Quaternary incision of the Northern Pyrenean foreland: Chronological and geomorphological constraints from the Lannemezan megafan, SW France

Alluvial fans and megafans hold a key position in the sediment routing system and are particularly sensitive to changes in geodynamic and climatic forcing, as well as autogenic fluvial processes. The Lannemezan megafan in the Northern Pyrenean foreland (SW France) was built during Miocene to Pliocene times and subsequently abandoned as the stream network deeply entrenched the foreland. We report new cosmogenic nuclide (10Be, 26Al) exposure dates for the abandoned fan surface and a series of alluvial terraces along the Neste and Garonne rivers. Our results show that abandonment of the fan occurred at or before ~ 300 ka, and we suggest that this abandonment is the result of the autogenic dynamics of the river system leading to the capture of the feeding Neste River by the Garonne. However, the incision episodes that produced the terrace levels are concomitant to major climatic shifts. Detailed analysis of stream network morphometry does not reveal knickpoints or patterns in the steepness index and chi-values that could be linked to either active tectonic features, base-level change or network reorganization. We therefore suggest that fan abandonment and long-term incision occurred through autogenic processes and that climate shifts may have modulated the incision, allowing for the formation and persistence of alluvial terraces.