Abstract Paleofloristic data imply that paleoclimate changed in the Swiss Alps at the Oligocene/Miocene boundary from humid and hot conditions toward a cli- mate with high temperature and low humidity. The arid- ization is associated with a change in depositional pat- tern from alluvial fans to lakes and floodplains, suggest- ing 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 ex- plore potential controls on the pattern of sediment dis- charge and on the evolution of the Alpine drainage basin. The model is based on the presumption that the rates of fluvial incision into bedrock are proportional to shear- stress exerted by the flowing water. The model results imply that the paleoclimate change resulted in an instan- taneous decrease of sediment discharge and a vertical to- pographic growth until steady-state conditions between erosional and crustal mass flux are established. However, exposure of the crystalline core of the Alps at ca. 20 Ma is likely to have resulted in the 25–40% decrease of sedi- ment discharge and the reorganization of the drainage pattern from an orogen-normal to an orogen-parallel ori- entation of dispersion. Keywords Surface processes model · Paleotopography · Denudation · Surface erosion · Source rocks and erosion Introduction The drainage pattern of the Swiss Alps evolved from an orogen-normal to the present-day orogen-parallel orienta- tion (Rhein and Rhone valleys; Fig. 1), at a time when the Alps are considered to have experienced a phase of con- structive growth (Schlunegger 1999). However, despite the formation of enhanced relief, the volume of sediment preserved in the stratigraphy of the North Alpine Fore- land Basin (NAFB) suggests that there was a significant decrease in the discharge of sediment from the Central Alps at ca. 20 Ma compared with the preceeding 10 My (Schlunegger 1999; Kuhlemann et al. 2000). The aim of this paper is to explore possible controls on the Early Miocene decrease of surface erosion rates in the Central Alps. Also, we formulate hypotheses for why the drainage pattern changed from an orogen normal to an or- ogen-parallel orientation as is observed presently (Rhone and Rhein valleys; Fig. 1). The parameters that are consid- ered are changes in rates and pattern of crustal uplift, ex- posed source-rock lithologies and paleoclimate (e.g., Whipple and Tucker 1999; Kühni and Piffner, in press). In a first step of our analysis, we present a synthesis of the geodynamic evolution of the Central Alps including the structural development of the orogen, the geometric evolu- tion of the Alpine drainage basin, and estimated variations in surface erosion rates or discharge of sediment to the NAFB. Furthermore, data regarding temporal and spatial variations in exposed source-rock lithologies and paleocli- mate (both of which influence surface erosion) are extract- ed from published data on the petrographic evolution of the deposits in the NAFB, and from paleobotanic data of tem- porally calibrated fossil sites. In the final step of our analy- F. Schlunegger ( ✉ ) · J. Melzer Institut für Geowissenschaften, Friedrich-Schiller-Universität, Burgweg 11, 07749 Jena, Germany G.E. Tucker University Lecturer in Geocomputation and Fellow of Brasenose College, School of Geography and the Environment, Oxford University, Mansfield Road, Oxford OX1 3TI3, UK Present address: F. Schlunegger, Dept. of Earth Sciences. ETHZ, Sonneggstr. 5, CH-8092 Zürich, Switzerland e-mail: fritz.schlunegger@erdw.ethz.ch Int J Earth Sci (Geol Rundsch) (2001) 90:484–499 DOI 10.1007/s005310100174 ORIGINAL PAPER F. Schlunegger · J. Melzer · G.E. Tucker Climate, exposed source-rock lithologies, crustal uplift and surface erosion: a theoretical analysis calibrated with data from the Alps/North Alpine Foreland Basin system Received: 21 March 2000 / Accepted: 4 November 2000 / Published online: 2 June 2001 © Springer-Verlag 2001 Fig. 1 A Geological map of the Central Swiss Alps and large- scale stratigraphy of the Molasse Basin which borders the Alps to the north. I, II, III Deposits of the Speer-Hörnli, Rigi-Höhronen and Honegg-Napf palaeorivers; AR Aiguille Rouge massif; Mo Mont Blanc massif; Aar Aar massif; Go Gotthard nappe. B Tec- tonic section across the Central Alps of eastern Switzerland and northeastern Italy. NAFB North Alpine Foreland Basin. (Modified after Schmid et al. 1996) ▲