Fluvial Incisions in the North-Western Pyrenees (Aspe Valley, France): Dissection of a Former Planation Surface and Some Tectonic Implications

Abstract : The Pyrenean range results from the collision between the Iberian and Eurasian plates which started during the Santonian. Iberia has shown no more relevant motion with respect to Europe since Early Miocene times. In this regard, the Pyrenees can no longer be considered as an active plate boundary from Miocene onwards. Dissected remnants of a high elevation low relief surface (HELRS) are encountered all along the Pyrenean range. All authors agree on the tardi- to post-shortening age of this surface, but doubt remains on its original elevation. In particular, whether a substantial post-shortening uplift 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 evidence a Lower Miocene to present-day multi-stage history of dissection of the HELRS in the North-Western Pyrenees. Each incision stage is recorded by erosional triangular facets and associated stepped remnants of palaeovalleys. Compared analysis of the patterns of incisions characterizing (i) the Axial Zone of the range and (ii) its northern border allows to evidence differential vertical motion. We finally discuss the process(es) controlling the deduced uplift of the Western Pyrenean Axial Zone.