Deciphering landscape evolution with karstic networks : a Pyrenean case study [+ Corrigendum in Quater. Geochronol. 2020, vol. 57, art. 101050, 2 p.]

Abstract

The rates and chronologies of valley incision are closely modulated by the tectonic uplift of active mountain ranges and were controlled by repeated climate changes during the Quaternary. The continental collision between the Iberian and Eurasian plates induced a double-vergence orogen, the Pyrenees, which has been considered a mature mountain range despite significant seismicity and evidence of neotectonics. Nevertheless, recent studies indicated that the range may have never reached a steady state. One option for resolving this controversy is to quantify the incision rates since the Miocene by reconstructing the vertical movement of geometric markers such as fluvial terraces. However, the few available ages for the Pyrenean terrace systems do not exceed the Middle Pleistocene. Thus, we study alluvium-filled horizontal epiphreatic passages in limestone karstic networks to increase the span of this dataset. Such landforms are used as substitutes of fluvial terraces because they represent former valley floors. These features record the transient position of former local base levels during the process of valley deepening. The Tet river valley (southern Pyrenees) is studied near the Villefranche-de-Conflent limestone gorge, where 8 cave levels have been recognized over a vertical height of 600 m. In this setting, already published Al-26/Be-10 cosmogenic burial data were limited to the last similar to 5 Ma. This work extends this initial dataset through the acquisition of cosmogenic Be-10/Ne-21 data, which should enable us to reconstruct a more complete valley incision chronology. We also revise the Al-26/Be-10 record for the lowest portion of the valley. The obtained quantitative results are surprising relative to certain geological evidence, and the limitations of such an approach in the investigated geodynamic context are discussed. In particular, sampling within a detrital deposit (the Escaro Formation) and in the riverbeds suggests that the Al-26/Be-10 ratios that are associated with the lower cave levels could have been inherited by pre-burial episodes. Similarly, pre-burial catchment denudation rates that do not exceed hundreds of cm/ka are probably tainted by the same amalgamation processes