Cosmogenic ¹⁰Be dating of Danube terraces in Hungary

Quaternary development of the Danube valley in the Pannonian Basin is of crucial importance for the understanding of the landscape evolution and neotectonics of the region. The Danube River is the only river crossing the uplifting Transdanubian Range (TR), therefore providing a unique opportunity for the quantification of the Quaternary river incision/uplift rate of the TR. The TR is a low altitude (up to 750 m) mountain range composed to Palaeozoic to Mesozoic basement between the two major sub-basins of the Pannonian Basin System. Accordingly, for a better understanding of the structural evolution of the area, it is necessary to calculate the vertical movements and to distinguish between tectonic and climatic forces in landscape evolution. The existing terrace chronology – the so called “traditional terrace system” – in the Danube valley was based on geomorphological, sedimentological and palaeolontological data. These data, however, allow only a relative chronology, which is valid at certain river sections and does not provide numerical ages of the terrace horizons. Exposure age dating of the Danube terraces has started in the axial zone of the TR, where cosmogenic 3He was used to determine the age of andesite strath terraces. These data showed that Danube terraces, and connected uplift of the TR are significantly younger then it was suggested before. Instead of the late Pliocene – early Pleistocene onset of the uplift suggested by the traditional terrace system, a middle Miocene beginning was proposed with an uplift rate above 1 mm/y. This result is indicative of ongoing tectonic deformation of the interior of the Pannonian Basin and rises the need for absolute age determination of river terraces at other valley sections, too. Accordingly, as a continuation, our study now focuses on gravel terraces upstream of the Danube Bend. Here earlier studies, using relative chronological data of the terracegravel suggest early Pleistocene age of the highest terrace horizons. We apply cosmogenic ¹⁰Be for age determination, aiming at finding evidences on the young vertical movements at several sections of the Danube valley. Sampling occurred along depth profiles using all particles involved in the cosmogenic nuclide production and allowed determining the exposure time denudation rate pairs for each locality. Sample preparation occurred at CEREGE-CNRS, Aix en Provence and AMS measurements occur at ASTER, the French National Facility, CEREGE. Measurements and calculations are on their way, our first results show that the formation of the highest horizons occurred in middle Pleistocene times (ca. 600ka) in contradiction to the formerly suggested earliest Pleistocene age. The lower horizons also appear to be younger than their age based on previous relative chronologies. The first denudation rates calculated by the depth profile data suggest slow denudation of the flat terraces surfaces, the average values ranging around 4-6 m/My. Exposure ages are in accordance with the previous ³He study, that is to say, the dated gravel terraces also appear to be younger than it was suggested by previous works. These results suggest that considerable vertical movements occur in the lithosphere of the Pannonian Basin and ongoing deformation contradicts the theoretical tectonic stability of the basin interior.