A 3D geological model of the Subalpine Molasse in Switzerland: Insights into its subsurface geometry and spatial evolution

The late-stage evolution of the Central European Alps is recorded by the Cenozoic sediments of the Swiss Molasse Basin (SMB). From a sedimentary perspective, deep marine Flysch sedimentation transitioned to shallow marine and terrestrial Molasse deposition around 30 Ma (Schlunegger & Kissling, 2022 and references therein). Shallow marine conditions were re-established around 20–18 Ma, after which terrestrial sedimentation continued until 10–5 Ma. From a tectonic perspective, the Flysch and Molasse deposits were continuously accreted by the advancing Alpine nappe stacks from late Eocene times onwards. The tectonic exhumation of the External Crystalline Massifs occurred around 22–20 Ma (Herwegh et al., 2023), after which the Molasse units were thrusted on top of each other and tilted towards the south thereby forming the Subalpine Molasse (Kempf et al., 1999). Around 16 Ma, southward-oriented back-thrusting resulted in the formation of a Triangle Zone (von Hagke et al., 2012) where Molasse units dip northwards, marking the structural transition from the flat-lying Plateau Molasse situated in the north, to the tilted and thrusted Subalpine Molasse in the south and adjacent to the Central Alps. The Subalpine Molasse units were then further thrusted and exhumed between 12–4 Ma (Mock et al., 2020).

Nowadays, large parts (c. 2/3) of the Subalpine Molasse are covered by the so-called Helvetic and Prealpine nappe stacks, preventing a complete exposure. Only a SW–NE oriented stretch is exposed, while subsurface information is fragmentary (e.g., from seismic surveys or deep wells). Despite extensive research, we lack an understanding about the present-day lateral and longitudinal geometry of the Subalpine Molasse adjacent to and beneath the Central Alps. In addition, the timing of initial thrusting and emplacement has not been fully resolved at the scale of the entire SMB. This study addresses these objectives through the construction of a large-scale 3D geological model of the Subalpine Molasse, particular of its major lithostratigraphic and tectonic boundaries. The model is based on the Tectonic Map of Switzerland and integrates an input dataset compiled from numerous published geological-mapping, seismic-survey, and drilling campaigns.

The model allows a revised interpretation of the geometry of the Subalpine Molasse in Central Switzerland. The homogenised map shows that the 39+ individual thrust sheets are laterally and frontally displaced by thrust- and fault-complexes, both along strike and across the SMB. We also observe that the steeply dipping (20–30°) frontal thrusts of the Flysch and Molasse units root in depths of 5–7 km below the exhumed External Crystalline Massifs. Where these basement massifs are absent, the frontal thrusts are more gently dipping (10–20°) and likely rooting in Mesozoic fault-zones. Furthermore, palinspastic restorations of reference cross-sections provide insights into the style of deformation and the evolution of thrusting within the Subalpine Molasse.

 

REFERENCES

Herwegh, M. et al. (2023). Wiley, London.
Kempf, O. et al., (1999). Int J Earth Sci., 88(2).
Mock, S. et al., (2020) Solid Earth, 11.
Schlunegger, F., & Kissling, E. (2022). Geosciences, 12(226).
Von Hagke, C. et al., (2012). Tectonics, 31(5).