Paired electron spin resonance and in situ produced terrestrial cosmogenic nuclide ages on a Pleistocene fluvial sequence of the Têt River, eastern Pyrenees (France): implications for Quaternary crustal uplift regimes. Session SS1-A13

International audienceSequences of alluvial strath-and fill-terraces record the long-term variation of fluvial transport regimes in response to Quaternary climatic changes. These landforms are also useful tools for quantifying river incision rates and inferring vertical uplift of the Earth’s crust. Age constraints on Pleistocene fluvial sequences also provide a potential for correlation with climatic controls on catchment sediment delivery. Land cover, precipitation intensity, and permafrost play key roles in catchment-scale sediment supply to river channels, and the major aggradational and incisional phases tend to occur in step with global 100 ka orbital cycles. It has also been established that incision phases occur during periods of climatic transition whereas aggradation coincides with periods of fluvial dynamic equilibrium. This evidence is mainly based on dated Pleistocene fluvial sequences in periglacial environments, i.e. in non-glaciated catchments characterized by the presence of continuous permafrost. Recent studies of North-Pyrenean glaciofluvial terraces have shown similar trends, but with cycles of aggradation and incision occurring in response to major stages of mountainside colonization by woodland vegetation belts rather than to the beat of advancing and retreating outlet glaciers. Pyrenean fluvial sequences have benefited from systematic mapping on both sides of the Pyrenean range, but the chronostratigraphy of these deposits is still largely based on relative criteria such as terrace staircase geometry and contrasts in clast weathering intensities or in soil types capping different generations of terrace treads. Published ages have nonetheless been obtained from OSL and in situ produced Terrestrial Cosmogenic Nuclide (TCN) vertical profiles on both sides of the range (Gallego, Cinca and Sègre rivers on the south side, and Ariège, Garonne, Neste, and Aspe on the north). Here we document the River Têt catchment (1400 km 2 ), which extends from the Pyrenean Axial Zone directly to the Mediterranean Sea via a succession of mid-to late Cenozoic intramontane sedimentary basins. Five generations of alluvial terrace were identified in the catchment based on clast weathering intensities and soil profile characteristics, and eight levels can be distinguished based on terrace staircase geometry. We present two age series obtained from ESR and TCN vertical profiles on the Têt terrace staircase. Modeled ages did not allow clear correlations between terrace level formation and marine isotope stages. Unlike data previously produced for the Ariège catchment, it was likewise not possible to construct a well-defined soil chrono-sequence, i.e. clear correlations between the age of a tread level and the duration required to produce its distinctive soil cover. However, the ages obtained were sufficiently robust to quantify the magnitude of Pleistocene incision of the landscape by the Têt River. On that basis, we infer the longitudinal variability of vertical crustal uplift between the Pyrenean Axial Zone, the foothills and the coastal plai