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Reconstruction of Miocene geodynamics in the Central Alps using detrital garnet geochemistry in sandstones of the Swiss foreland basin

The geodynamics in the Central Alps during the Miocene were majorly characterized by the exhumation of crystalline basement, the so-called external crystalline massifs. Their exhumation had a major impact on the evolution of relief, distribution of drainage networks and sediment-generation. The timing of their first surficial exposure and their total thickness still remains debated. The Swiss Molasse Basin contains ~35 to 13 Ma old clastic deposits that were mostly derived from the immediate hinterland and contain valuable information about its tectonic and climatic evolution. For this thesis, this sedimentary archive is used to trace the exhumation of the external crystalline massifs. In particular detrital garnet geochemistry is used to investigate 21.8 to 15 Ma old alluvial fan deposits of three different fan systems of the Swiss Molasse Basin. In this context, unusually grossular- and spessartine-rich garnets supplied from greenschist-facies metamorphic granitoid rocks serve as a unique proxy for the sediment supply from the external crystalline massifs. Their first appearance in a 15 Ma old sample in the Napf Fan indicates an at least partial exposure to the surface at that time. These garnets can neither be found in the alluvial fans to the east nor to the west, suggesting a relatively stable N/NW-directed drainage system since Miocene times. Another species of extremely grossular-rich garnets of so far unknown provenance has been found in 16.5-17 Ma sandstone of the Napf Fan, indicating a highly dynamic hinterland evolution during the Miocene, which is contrasting previous findings based on more traditional provenance proxies. (Poster)


Daniela Krieg, Laura Stutenbecker
TU Darmstadt, Germany
GeoKarlsruhe 2021