Topographic development of the Alps in response to slab breakoff. (a) Situation following slab breakoff. Incipient rise of the Alpine topography caused fluvial incision to initiate at the orogen front as revealed by petrographic investigations and palinspastic reconstructions23,24,25. This most likely reflects the occurrence of a dissected transient landscape at the orogen front, while the headwaters represented a flat-laying, non- to poorly dissected plateau as clast types from farther south are absent in pre 27 Ma-old deposits24. We suggest that the flat headwater reaches in the south, and the dissected Alpine landscape farther north were separated by a knickzone (K) where most of the erosional work was accomplished. We use this scenario to explain the relatively low fluxes and concentrations of supplied sediment. In the Molasse foreland basin, low sediment fluxes resulted in the establishment of a relatively flat fluvial environment with channels that were confined by a floodplain. See text for further explanations. (b) Ongoing uplift promoted headward retreat of knickzones and the shift of the erosional front towards the drainage divide (D) situated in the rear of the Alps, thereby reaching the region where crystalline rocks were exposed. At this stage, the Alpine landscape has reached a steady state situation where ongoing rock uplift is fully compensated by erosion18, and where the shape of the Alpine landscape has remained stationary. This mechanism explains the delayed arrival of crystalline clasts in the conglomerates and the increase in both the flux and the concentrations of supplied sediment. Sediment concentrations increased until this steady state situation was established. In the Molasse foreland basin, the ensemble of these processes finally caused the Rigi megafan to prograde. See text for further explanations. Please note that both figures have the same scale. Please also note that the topography is exaggerated for illustration purposes. The view on both illustrations is towards the SE.