Controls on the Post-Orogenic Evolution of the Lannemezan Megafan in the Northern Pyrenean Foreland: Insights from Cosmogenic Nuclide Exposure Dating and Morphometric Analysis

Abstract:

Fluvial megafans are large, low-gradient geomorphic features characterized by sandy and muddy sediments deposited by distributive sheet-flows. They 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 253 ± 28 ky to 288 ± 12 ky, 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 MIS7, MIS5 and post-LGM cold-to-warm transitions. Geomorphic analysis of the stream network reveals that the river profile concavity increased through time and 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 chi proxy (χ) analysis. We propose that this pattern results from flexural isostatic rebound in response to increased erosion rates within the range due to Quaternary glaciations.