Sedimentary records imply that continent-continent collision occurred later in the Eastern than in the Western Alps.

The change from a deep-marine, underfilled Flysch to a terrestrial and/or shallow-marine, overfilled Molasse stage of basin evolution is probably one of the major steps in the evolution of a foreland basin. Chrono-stratigraphic and sedimentologic data from the north Alpine foreland basin (NAFB), situated on the northern margin of the Alps, document that such a shift occurred at c. 30 Ma in the western (Swiss and German) part of the basin and c. 10 My later in the eastern (Austrian) segment. We relate these basin-parallel differences in the basin’s evolution to an orogen-parallel variation in subduction tectonics, that itself appears to be conditioned by the segmentation of the European plate during the Mesozoic phase of spreading preceding the build-up of the Alps (Schlunegger and Kissling, 2022). During the Mesozoic, the transition from the continental European plate to its extended margin farther South was most likely offset by a left-lateral fault in the vicinity of Munich, separating the future depositional realms of the NAFB into a western and an eastern segment. As a consequence, during the construction of the Alps from 35 Ma onward, continent-continent collision occurred earlier in the Western Alps (c. 32-30 Ma) than in the Eastern Alps (c. 20 Ma). This collision resulted in the delamination of the subducted European oceanic lithosphere from its continental counterpart beneath the Western Alps. As a consequence, the European continental plate beneath the Western Alps experienced a rebound, thereby causing the build-up of the Alpine topography and the increase in sediment supply to the foreland basin. This is recorded in the Western NAFB by a shift from Flysch- to Molasse-type of sedimentation at 30 Ma. Farther to the East, however, the subducted oceanic lithosphere slab of the European plate was still attached to the European continental plate, with the consequence that Flysch-type of sedimentation still prevailed in the Austrian part of the basin. The situation of sedimentation in an underfilled basin persisted until c. 20 Ma when the Austrian (eastern) part of the NAFB changed from a Flysch- to a Molasse-type of basin evolution. This is the main reason why we propose that continent-continent collision most likely occurred 10 My later in the Eastern Alps than in the Western Alps. Schlunegger, F., Kissling, E. (2022). Slab load controls beneath the Alps on the source-to-sink sedimentary pathways in the Molasse Basin. Geosciences, 12, 226.