Slab load controls beneath the Alps on the source-to-sink sedimentary pathways in the Molasse basin

The stratigraphic evolution of foreland basins has mainly been related to the tectonic development in the adjacent mountain belt, where thrusting and loading at the surface have been considered to contribute to the downwarping of the foreland plate. These concepts are mainly based on the results of numerical models that relate to the concept of an elastic plate overlying a fluid substratum such as the mantle. However, although high-resolution seismic tomography images became available since the early 2000s documenting the occurrence of lithospheric mantle slabs beneath the Alps, the controls of slab loads particularly on the development of the north Alpine foreland basin, or Molasse basin, has received less attention. Here, we relate the evolution of this basin to the subduction processes beneath the European Alps. At 30 Ma, the western and central (Swiss and German) portions of the Molasse basin experienced a change from deep marine to terrestrial conditions, which documents a change from the early underfilled Flysch to the overfilled Molasse stage of basin development. At the same time, however, deep marine conditions prevailed in the eastern (Austrian) part of the basin, which was the final sedimentary sink of the sediment routing system. We consider these conditions as response to the oceanic lithosphere slab-breakoff beneath the Central and Western Alps, where slab unloading together with buoyancy effects of the incipient crustal root resulted in a rebound of the plate, in a rise of the Alpine topography and thus in rapid erosion and the filling of the Swiss and German part of the Molasse basin. Farther east, however, at least part of the subducted oceanic slab remained attached to the European plate and downwarped the foreland plate, thereby maintaining a deep marine environment. Slab rebound in the central and western part of the Alps and ongoing downwarping of the plate in the east most likely resulted in an east-directed tilt of the foreland plate. We use this configuration to explain the east-directed dispersion of sediment between 30 and 20 Ma. The situation changed at 20 Ma, when the remnant oceanic slab beneath the Eastern Alps broke off, which resulted in a rebound of the plate. This rebound, however, was less than in the Central Alps 30 Ma ago because the buoyant crustal root was much smaller. Nevertheless, at 20 Ma, the eastern part of the basin started to respond to this unloading by a shift from deep underfilled to shallow marine filled and finally to terrestrial overfilled conditions. Beneath the Central and Western Alps, however, continuing rollback subduction after 30 Ma resulted in the situation where a continental mantle lithosphere slab was attached to the European plate at 20 Ma, resulting in a downwarping of the plate in its central and western portions. As a result, the foreland plate started to be tilted towards the west after 20 Ma, which explains the reversal in the drainage direction between 20 and 18 Ma where the western Molasse basin became the final sedimentary sink.