The Alps and their geodynamic state during the Late Oligocene. (a) Geologic map of the Alps and the neighbouring sedimentary basins23. The map also shows the location of the Rigi megafan (denoted as R) that was sourced from the area surrounding the Lepontine area (denoted as L) in the Central Alps of Switzerland. D denotes the major Alpine drainage divide between c. 30–20 Ma. (b) Simplified geologic map of the Rigi area, situated on the northern side of the Alps22. Please see Fig. 1a for location of map. (c) Schematic section through the Alps23 illustrating the geodynamic situation between c. 30–20 Ma. At that time, the Alps were made of the orogenic lid constituted by the Austroalpine nappes, and the crystalline core that assembled lithologies of the Penninic nappes. Slab breakoff beneath the rear of the Alps resulted in a period of fast rock uplift and in the formation of a highly elevated topography. Please see Fig. 1a for location of section. (d) 3D perspective on the Alps and the Molasse foreland basin illustrating the geodynamic situation between c. 30–20 Ma. Streams with sources in the Central Alps deposited large megafans at the thrust front. These were laterally bordered by bajada fans with sediment sources situated in the frontal Alpine nappes. The discharge within the Molasse foreland basin was oriented towards the NE at that time. Figures a) through c) have been drawn based on Schlunegger and Castelltort23 using Illustrator 15.1.0 licensed to Uni Bern.