Climate, exposed source-rock lithologies, crustal uplift and surface erosion: a theoretical analysis calibrated with data from the Alps/North Alpine Foreland Basin system

Paleofloristic data imply that paleoclimate changed in the Swiss Alps at th e Oligocene/Miocene boundary from humid and hot conditions toward a climate with high temperature and low humidity. The aridization is associated with a change in depositional pattern from alluvial fans to lakes and floodplai ns, suggesting decreasing sediment discharge. A further 25-40% decrease of sediment discharge occurred at ca. 20 Ma when the orogenic core of the Alps became exposed to the surface. We applied a surface processes model to exp lore potential controls on the pattern of sediment discharge and on the evo lution of the Alpine drainage basin. The model is based on the presumption that the rates of fluvial incision into bedrock are proportional to shear-s tress exerted by the flowing water. The model results imply that the paleoc limate change resulted in an instantaneous decrease of sediment discharge a nd a vertical topographic growth until steady-state conditions between eros ional and crustal mass flux are established. However, exposure of the cryst alline core of the Alps at ca. 20 Ma is likely to have resulted in the 25-4 0% decrease of sediment discharge and the reorganization of the drainage pa ttern from an orogen-normal to an orogen-parallel orientation of dispersion .