Post-Eocene evolution of the North Alpine Foreland Basin and its response to Alpine tectonics

The post-flysch (Oligocene–Miocene) palaeogeographic evolution of the entire North Alpine Foreland Basin (NAFB) between Savoy (France) and Lower Austria is presented in eight sketch maps. The compilation considers the palinspastic evolution of the Alps. It includes intramontane deposits, which represent a continuous marginal facies of the NAFB during Rupelian to Early Burdigalian times. The facies distribution in the NAFB was driven by two major types of processes, which are related to the tectonic evolution of the Alpine orogen. The first type, representing tectonic processes at the thrust front of the Alps, directly influenced the facies distribution of the narrowing NAFB. The second type represents an indirect impact of Alpine uplift and tectonics to the NAFB, transformed by varying sediment discharge. A strong increase in sediment discharge due to uplift of the Alps is the major reason for the generally regressive coarsening- and shallowing-upward cycle from the Lower Marine to the Lower Freshwater Molasse (UMM, USM) between 33 and 21 Ma. The development of the “Burdigalian Seaway” at ∼20 Ma was coeval with a reduction of thrust advance rates in the western and central part of the NAFB. Shallow marine conditions (Upper Marine Molasse, OMM) prevailed for ∼3 million years. In the Eastern Alps, reduction of relief due to lateral (east–west-oriented) extension caused a strong reduction in sediment discharge. Closure of the “Burdigalian Seaway” around 17 Ma occurred during a phase of tectonic reorganisation in the Alpine orogen and is coeval with a short-term increase in sediment discharge. Between 17 and 12 Ma, the NAFB was constantly overfilled (Upper Freshwater Molasse, OSM), despite strongly decreasing sediment discharge. Termination of sedimentation in the (unfolded) NAFB occurred diachronously in an undramatic process. It started in the western NAFB in the course of uplift of the Swiss Jura Mountains after 11 Ma and reached Lower Austria around 6–5 Ma. Strong uplift of the Alps and the NAFB started at around 6 Ma in the Swiss and Western Alps and at 4–3 Ma in the Eastern Alps. The uplift was followed by reworking and erosion of more than 2 km of Molasse sediments in the western NAFB.