Using the Dispersion of Apatite (U-Th-Sm)/He Single Grain Ages to Unravel the Burial and Exhumation History of the Foreland Basin of the Central Alps

Abstract

Dispersed single-grain ages are a common phenomenon in detrital thermochronometry datasets that are often challenging to interpret. In this study, we showcase how a thermochronological forward modelling approach can be applied to such a complex dataset from the distal Swiss Molasse Basin. Despite extensive thermochronological research, the basin's exhumation history, magnitude and driving processes remain a subject of scientific debate. We present a large new apatite (U-Th-Sm)/He (AHe) dataset from a densely sampled deep exploration borehole extending beyond the actual Molasse basin fill into the underlying sedimentary sequence to provide more robust constraints on the exhumation history. AHe ages of over 100 grains range between 4 and 30 Ma in the upper 500 m and between 3 and 80 Ma below 1300 m, respectively. This is counterintuitive as, given the partial resetting of the shallow data, total reset would be expected at depths exceeding approximately 600 m. To arrive at a single consistent thermal history, we use a forward thermochronological modelling approach that allows us to test the influence of different provenance histories and distinguish between cooling phases associated with changes in heat flow vs. changes in exhumation. We find that a total of approximately 1100 m of Neogene exhumation, already starting at around 11 Ma, reconciles all available data. Exhumation continued throughout the Late Miocene but was initially not accompanied by significant cooling, suggesting a compensation by gradual increase of heat flow. At around 5 Ma, heat flow rises sharply towards the anomalously high present-day values of 120 mW m⁻². We argue that this discrepancy between the onset of exhumation and the onset of cooling may be responsible for previously differing estimates for the exhumation history of the basin. Furthermore, we infer geodynamic processes as the primary driving force for basin-wide exhumation, as it best explains its early onset.