Intramontane basin opening and erosive unloading: A 10Be-supported model for the Usia drainage basin, southern Pyrenees.

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Intramontane basins are common features of many mountain ranges. These topographic depressions function as sediment traps during the syn- and postorogenic evolution of a mountain chain. Hence, studying the sedimentary archives and morphogenetic development of intramontane basins provides (i) crucial insights into erosion-sedimentation processes in mountain ranges, and (ii) conduces to assess their sensitivity to changing environmental conditions. The proposed research project is aimed at unravelling the processes of basin opening and excavation of intramontane drainage basins. Using the example of the Usia catchment (southern Pyrenees), a particular focus is set on the causes and consequences of severe late-stage erosion phases. The Usia basin is a sub-basin of the Eocene Ainsa depression (Aragon, Spain) - a large piggyback basin sandwiched between south-Pyrenean thrust sheets. The basin is filled with Eocene marine sediments and consolidated synorogenic debris and conglomerates.An unpublished pilot study was carried out by the applicant in the Usia basin which is drained towards the Cinca main river valley at its passage through the Ainsa depression. Field observations revealed systems of valley terraces and residual erosion surfaces that are deeply dissected by gullies and canyons. The Usia outlet canyon is deeply cut into the Los Molinos thrust front and represents a dramatic landscape feature that is relevant to the base level and opening history of the Usia drainage basin. Within the basin, alluvial terrace successions witness a series of intense late stage erosion episodes. Preliminary observations and terrace exposure dating point to an unexpected young age (18,2 +1,3/-2,8 ka) and exceptional magnitude of recent basin excavation. The projects objectives are to clarify (i) the opening mechanisms for the Usia drainage basin, (ii) the timing and controls of late stage erosion and terrace aggradation phases, and (iii) to assess the relative impacts of tectonic and climatic (e.g. hydrological) parameters on intramontane sediment fluxes. We will apply state of the art digital terrain analysis in combination with field surveys and cosmogenic nuclide exposure dating. The expected outcomes concern (i) an erosion model for the Usia drainage basin and (ii) short- and long-term erosion rates for an highly erodible catchment. Both will provide a solid reference data set for future studies on intramontane basin systems in mid-latitude mountain ranges.