Detrital thermochronology of the Alpine foreland basin in Central Switzerland: Insights into tectonic and erosion history of the North Central Alps

Abstract

This study systematically integrates zircon U-Pb geochronology and (U-Th)/He low-temperature thermochronology to correlate the Northern Alpine Foreland Basin (NAFB) sediment provenance and the tectonic and thermal evolution of the thrust stack. The Swiss Molasse Basin (SMB), a portion of the NAFB, is a wedge of Oligocene and Miocene clastic sediment bounded by the Central Alps and the Jura Mountains. The sedimentary units in central Switzerland form two upward coarsening mega-sequences that are divided into five major units: North Helvetic Flysch (NHF), Lower Marine Molasse (LMM), Lower Freshwater Molasse (LFM), Upper Marine Molasse (UMM), and Upper Freshwater Molasse (UFM). The LFM and UMM are separated by a flooding surface known as the Burdigalian transgression. By integrating zircon U/Pb and (U-Th)/He analyses on the same samples, even the same grains, with the established stratigraphic framework, this study correlates Central Alpine tectonic activity with the basin development to determine the exhumational history and the cause of the Burdigalian Transgression. The SMB deposits record the convergence and exhumational history of the Central Alps. U/Pb analyses reveal shifts in crystallization ages of the source material from older Caledonian and Cadomian aged material in the NHF and LMM to Variscan aged material in the LFM, UMM, and UFM. Also, a lack of Alpine derived volcanic material is shown from the U/Pb geochronology of the SMB sediments. The detrital zircon (U-Th)/He ages can be correlated with early Alpine imbrication and convergence from ~120-45 Ma. Alpine nappe stacking and exhumation is represented in the 35-14 Ma detrital zircon (U-Th)/He ages. Combining these zircon (U-Th)/He ages with the stratigraphic ages indicates episodic exhumation of the Central Alps occurred during the Oligocene and Miocene. At 20 Ma there is a decrease in depositional lag-time from 8 m.y. to 0 m.y. This decrease in lag-time requires an increase in exhumation that coincides with an increase in subsidence. The basin-ward shift of facies combined with a decrease in depositional lag-time indicates that tectonic loading caused the Burdigalian Transgression.