Fluvial incisions in the North‐Western Pyrenees (Aspe Valley): Dissection of a former planation surface and some tectonic implications

Dissected remnants of a high elevation low relief surface (HELRS) are encountered all along the Pyrenean range. All authors agree that the HELRS was shaped during the post-shortening evolution of the belt, but doubt remains on its original elevation. Notably, whether a post-shortening uplift event occurred after the generation of the Pyrenean planation surface is still debated. Based on a geomorphological study of the entrenchment of the Aspe River in the North-Western Pyrenees, we describe a post-Oligocene multi-stage dissection of the HELRS linked to a regional post-shortening uplift. Each incision stage is recorded by erosional triangular facets and associated stepped remnants of former smooth topographies. Compared analysis of patterns of incision in the Axial Zone and its northern border allows to evidence differential vertical motions. We discuss the forcing mechanism(s) that controlled this morphological evolution.