Relationships between subduction tectonics beneath the Alps and the source-to-sink sedimentary pathways in the Molasse basin

Abstract

The stratigraphic development of foreland basins has mainly been related to surface loading in the adjacent orogens, whereas the controls of slab loads on these basins have received much less attention. This has also been the case for the Molasse basin situated on the northern side of the European Alps. Here we relate the evolution of this basin between Geneva (Switzerland) and Linz (Austria) to the subduction processes beneath the European Alps (Schlunegger and Kissling, 2022). At 30 Ma, the western and central portions of the basin (between Geneva and Munich) experienced a change from deep marine (underfilled Flysch stage) to terrestrial conditions (overfilled Molasse stage), while the eastern part in Austria remained a deep Flysch-type of basin and the final sedimentary sink. This is considered as response to oceanic lithosphere slab-breakoff beneath the Central and Western Alps, which resulted in a rise of the Alpine topography, in an increase of surface erosion rates and sediment discharge, and finally in the overfilling of the basin west of Munich. Beneath the Eastern Alps, however, the subducted oceanic slab remained attached to the European plate and down-warped the plate in the East, thereby controlling the east-directed routing of the clastic material and maintaining the Austrian part of the basin in underfilled conditions. The situation changed at 20 Ma, when an oceanic slab breakoff beneath the Eastern Alps resulted in a rebound of the European plate in the East. Beneath the Central and Western Alps, however, the buoyant crustal rocks of the European continental plate continued to be delaminated from the mantle lithosphere, which itself was further subducted by c. 60 km between 30 Ma (time of oceanic slab breakoff beneath the Central/Western Alps) and 20 Ma (Schmid et al., 1996). Because in the central/western part of the Alps, the mantle slab of the continental lithosphere remained attached to the European plate at 20 Ma, the foreland plate continued to be down-warped in its central and western portions. Accordingly, while in the Austrian Molasse basin, the facies changed at 20 Ma from deep underfilled (Flysch-type of sedimentation) to terrestrial filled/overfilled conditions (Molasse sedimentation), the central and western Molasse basin became the final sedimentary sink and remained in the Molasse stage of basin evolution. As a further consequence, the drainage direction in the basin axis changed from an E-directed material transport prior to 20 Ma to a W-directed sediment discharge thereafter. We thus propose that slab loads beneath the Alps were presumably the most important drivers for the development of the Molasse basin at the basin scale.