Évolution morphogénique des Pyrénées orientales : apports des datations de systèmes karstiques étagés par les nucléides cosmogéniques et la RPE

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 as a mature mountain range in spite of significant seismicity and evidence of neotectonics. Nevertheless, recent studies indicate 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 from the Pyrenean terrace systems do not exceed the middle Pleistocene. Thus, to enlarge the time span of this dataset, we studied alluvium-filled horizontal epiphreatic passages in limestone karstic networks. Such landforms are used as substitutes of fluvial terraces because they represent former valley floors. They record the transient position of former local base levels during the process of valley deepening. In addition to morphological observations, the burial durations of detrital sediments in 61 cavities of the Têt, Ariège, Aude and Agly valleys were determined using cosmogenic 26Al/10Be and 10Be/21Ne ratios. The results obtained allow us to document incision processes since the early Miocene (~ 16-13 Ma) and to study influences of external forcing and eustatism. In comparison with other studies, it appears that incision rates are probably higher in the central Pyrenees, suggesting an increased role for glaciers in the incision process. Moreover, a gradient would also be observable from north to south, the Spanish slope probably having been incised faster. However, the density of sub-horizontal levels on an elevation range, the geodynamical and paleo-climatic contexts, the reorganization of the drainage networks may lead to networks filling stories more complex than expected. The obtained results applying various suitable geochronological methods (26Al/10Be, 10Be/21Ne, ESR and OSL on quartz) sometimes evidence that these radiometric approaches may be limited when source and/or emplaced deposits are reworked. The validity of dosimetric methods in a mountainous context, and the possibility that not all of the quartz grains are bleached at the time of deposition will also be discussed.
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Amandine Sartégou. Évolution morphogénique des Pyrénées orientales : apports des datations de systèmes karstiques étagés par les nucléides cosmogéniques et la RPE. Géomorphologie. Université de Perpignan, 2017. Français. ⟨NNT : 2017PERP0044⟩. ⟨tel-01708921⟩

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