Chronologie de la mise en place du bassin versant de la Sarthe moyenne par datation ESR sur quartz optiquement blanchis via l’approche multi-centres

National audience; This study aims to establish a first chronological framework based on paleodosimetric ages of the Middle Sarthe Valley, located between the cities of Le Mans and Sable-sur-Sarthe. It is based on recent physical geography works (Chantreau, 2011) which made it possible to propose a first relative sequence of fossil alluvial formations. The data provided here enable us to date the formation of the successive alluvial terraces. Most of the alluvial terraces were sampled and systematically dated by the measure of Electron Spin Paramagnetic Resonance (ESR) of optically bleached quartz in order to establish the chronology of the system. The use of the multiple-centres approach suggests here the presence of mainly Cenomanian substrate sand mixed with the alluvial sands of the valley. The aluminium centres (Al), usually used to date alluvial systems but suboptimal due to its longest bleaching time, were incompletely bleached during sediment transport prior to deposition. Al ages are thus overestimated for almost all samples. Nevertheless, the use of titane centres (Ti-Li and Ti-H), whose bleaching is faster, indicates that the establishment of the hydrographic network of the Sarthe valley began about 850 ka ago. Together with a clear nested morphology, a total of eight terraces were identified and dated between the end of the Lower Pleistocene and the beginning of the Upper Pleistocene. We also notice the existence of a gap of 200 ka between the mid- and high-altitude terraces, and active Quaternary tectonics, with fault reactivation during the Middle or even the Upper Pleistocene.