ZIRCON U-PB-HE DOUBLE DATING OF NORTHERN ALPINE FORELAND BASIN, SWITZERLAND – IMPLICATIONS FOR OROGENIC DYNAMICS AND SUBSIDENCE

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. Zircon is an ideal mineral for U-Pb and (U-Th)/He double dating since it is mechanically robust, has a high U-Pb closure temperature which records crystallization or metamorphism, and a high enough (U-Th)/He closure temperature to resist resetting by burial up to ca. 180ºC. The NAFB is a clastic wedge, composed of conglomerate, sandstone and shale deposited in the foredeep of the alpine orogen. The sedimentary units in central Switzerland form two upward coarsening mega-sequences that are divided into five major established units: North Helvetic Flysch (NHF), Lower Marine Molasse (UMM), Lower Freshwater Molasse (USM), Upper Marine Molasse (OMM), and Upper Freshwater Molasse (OSM). The USM and OMM are separated by a flooding surface known as the Burdigalian transgression. LA-ICP-MS U-Pb analyses provide crystallization ages of the rocks contained within thrust sheets, while zircon (U-Th)/He data provide the exhumation history of the hinterland. (U-Th)/He ages for the NHF and UMM are reset due to burial. The USM detrital zircon U-Pb ages are mostly Permian, with some Ordovician components. The USM (U-Th)/He ages range from 330 to 30 Ma. Deposits from the OMM show a strong Permian component, but also contain Proterozoic detrital zircon U-Pb ages. Zircon (U-Th)/He ages for the OMM range between 250 and 20 Ma. The uppermost depositional unit, OSM, has (U-Th)/He ages between the Permian and mid-Miocene, with U-Pb ages ranging from Oligocene to Proterozoic. The (U-Th)/He ages of the NAFB have been compared to existing biostratigraphic and magnetostratigraphic correlations. Brief lag-times require quick exhumation through the closure temperature, erosion of the source material, transport and deposition of the sediment, indicating an increase hinterland tectonic activity. There is a decrease in lag-time between the USM and the OMM from eight million years to less than two million years, and a lack of volcanic first cycle zircons, indicating the Burdigalian transgession is caused by increased hinterland tectonic activity and flexural subsidence of the foreland basin.