Late Miocene to present-day fluvial incisions in the Western Pyrenees: The record of local or regional uplift?

International audienceThe Pyrenean Range results from the collision between Eurasia and Iberia plates that started during the Santonian. The Pyrenean orogeny is considered to end in Early Miocene times, since from that period onwards, Iberia shows no more differential motion with respect to the rest of Europe and becomes part of the Eurasian plate. This coincided with the slowing down of African-Eurasian convergence. Nevertheless, the Pyrenean relief still displays an unexpected strong relief and a combination of various geological methods highlights the occurrence of local Upper Miocene to present-day uplifts in the Pyrenees (low temperature thermochronological dating, sedimentology, karstology, palynology, palaeoaltimetry). While it has already been studied extensively in the Eastern Pyrenees, fewer works refer to the Western Pyrenean uplift. Depending on the method considered, the timing and the vertical amplitude of this uplift differs.We are initiating a geomorphological study of the incisions in the Western Pyrenees in order to constrain this post-shortening uplift. We focused on the Aspe Valley, where Miocene highly elevated – low relief erosion paleosurfaces observed as relicts on top of the interfluves have been strongly dissected. This dissection was achieved in several steps, including a low erosion episode, followed by an acceleration of erosion which could be attributed to the Late Miocene to actual Western Pyrenean uplift event. The sharp incisions are highlighted by erosion triangular facets which cannot be attributed to glacial erosion as they occur beyond the last glacial deposits. We attempt to reconstitute the landscape prior to this late incision in order to quantify the subsequent erosion and to try to link it with foreland deposits.Evidence of a Late Miocene to present-day uplift is well described in the Eastern Pyrenees and at a larger scale, in the Central Massif, the peri-alpine area and in northern and central Iberia thus suggesting an uplift at a lithospheric scale. However, discrepancies in the uplift rates indicated by already published data are found at a kilometric scale in the Western Pyrenees, which leads us to consider a possible contribution of both local and regional processes to explain the recent incisions in the Aspe Valley.This work is funded and carried out in the framework of the BRGM-TOTAL project Source-to- Sink