OLIGO-MIOCENE DENUDATION OF THE CENTRAL ALPS: EVIDENCE FROM THE FORELAND BASINS

To reconstruct the Oligo-Miocene denudation history of the Central Alps we combined data from geochronological, sedimentological and isotope studies on the foreland basin sediments. The following results can be summarized: During Oligocene times mainly sedimentary cover nappes and Austroalpine basement units were exposed in the Central Alps. Increasing sediment accumulation rates in the foreland basin reflect the build-up of a relief. At the Oligocene-Miocene boundary the onset of lateral extrusion processes caused enhanced exhumation of the hinterland, leading to the simultaneous exposure of the Penninic lower plate over large areas of the Central Alps at 21 Ma. Contemporaneously, a drastic decrease of sediment accumulation rates reflect the collapse of the relief. Since 13 Ma Lower Penninic units of the Lepontine Dome were exposed to the surface. The Lepontine Dome was at least partly exhumed by tectonic denudation. Fission track ages of zircons derived from these Lower Penninic units yield lag times of ~7 Ma (lag time=cooling age minus sedimentation age). This is in agreement with lag times reported by Bernet et al. (2001), suggesting a steady state exhumation of the Central Alps from at least 15 Ma on, i.e. equilibrium between crustal convergence and denudation. Sediment accumulation rates, however, show a pronounced increase of erosion rates after 6-5 Ma. To retain steady state this increase of erosive denudation must be counterbalanced by an increase of the crustal convergence rate for the same period of time, which is clearly not the case. We therefore suggest that the constant cooling rates reflect the compensation of decreasing tectonic denudation rates by increasing erosive denudation rates and the Central Alps were not exhumed at a steady rate before 6-5 Ma.