Deciphering landscape evolution with karstic networks: A Pyrenean case study

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 alluviumfilled 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 Têt 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 26 Al/ 10 Be cosmogenic burial data were limited to the last ~5 Ma. This work extends this initial dataset through the acquisition of cosmogenic 10 Be/ 21 Ne data, which should enable us to reconstruct a more 2 complete valley incision chronology. We also revise the Al/ 10 Be 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 26 Al/ 10 Be 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. Keywords Cave deposits, cosmogenic burial dating, 26 Al, 10 Be, 21 Ne, incision rates, Pyrenees. Highlights  Exceptional preservation of a Neogene canyon in the eastern Pyrenees is observed, with 8 epiphreatic cave levels  26 Al/ 10 Be and 10 Be/ 21 Ne burial durations are obtained for intrakarstic alluvial deposits in 8 cave levels  The post-Messinian inferred incision rates and the 10 Be-derived denudation rates cannot be used as proxies for quantifying landscape development because of possible amalgamation processes  The results indicate valley incision since the Burdigalian with possible pronounced fluctuations since the Messinian

Determining the Plio-Quaternary uplift of the southern French Massif Central; a new insight for intraplate orogen dynamics

“…For the Alps and Pyrenees mountains, isostatic adjustment due to erosion and glacial unloading has been recently quantified (Champagnac et al, 2007;Vernant et al, 2013;Genti, 2015;Chéry et al, 2016). Because the erosion rates measured in the Cévennes are similar to those of the eastern Pyrenees (Calvet et al, 2015;Sartégou et al, 2018a), we investigate by numerical modeling how an erosion-induced isostatic rebound could impact the southern Massif Central morphology and deformation.…”

“…Burial dating using TCN is now a common tool to quantify incision rates in a karstic environment (Granger and Muzikar, 2001;Stock et al, 2005;Moccochain, 2007;Tassy et al, 2013;Granger et al, 2015;Calvet et al, 2015;Genti, 2015;Olivetti et al, 2016;Harmand et al, 2017;Rovey II et al, 2017;Rolland et al, 2017;Sartégou, 2017;Sartégou et al, 2018a). This method relies on the differential decay of TCN in detrital rocks that were previously exposed to cosmic ra- diation before being trapped in the cave system.…”

Topographic and Faults Control of Hydrothermal Circulation Along Dormant Faults in an Orogen

“…Naturally, the relief would have been modified by erosion over the last 5 Ma. However, the 55‐m/Ma maximum erosion rates evaluated in the region (Calvet et al, ; Sartégou et al, ) represent a maximum incision of 350 m of the Têt valley. Moreover, the ice cover during the last glaciations was quite limited in the study area (Calvet et al, ; Delmas et al, ) and has had little effect on the topography (Jarman et al, ).…”

Mapping a geothermal anomaly using apatite (U‐Th)/He thermochronology in the Têt fault damage zone, eastern Pyrenees, France

“…For the Têt valley low incision rates between 1 and 25 m/m.y. since 6 Ma have been estimated by cosmogenic nuclides (Delmas et al, ; Sartégou et al, ).…”