DETRITAL ZIRCON (U-TH)/HE ANALYSES OF THE SWISS MOLASSE BASIN

The Northern Alpine Foreland Basin, also known as the Swiss Molasse Basin (SMB), represents one of the classic peripheral foreland basins in the world. Many studies have focused on the sedimentology, biostratigraphy, magnetostratigraphy and structure of the hinterland; these studies have influenced many geologic concepts concerning flexural subsidence, foreland basin formation and tectonically controlled facies transitions. To better understand the timing of thrusting, thermal evolution of the hinterland, depositional response of the basin due to tectonic loading, and sediment provenance, we are using double dating of detrital zircon (U-Th)/He and U-Pb analyses of the SMB deposits around Luzern, Switzerland. The stratigraphic units are the established Ultrahelvetic Flysch (UHF), North Helvetic Flysch (NHF), Lower Marine Molasse (LMM), Lower Freshwater Molasse (LFM), Upper Marine Molasse (UMM), and Upper Freshwater Molasse (UFM). The UHF, NHF, and LMM were deposited as submarine fans in front of the northward advancing Alpine thrust front in an underfilled basin. Preliminary zircon (U-Th)/He (ZHe) ages for the UHF range from ~300-120Ma, but the NHF and LMM ZHe show resetting of ages due to overthrusting. The reset ZHe ages of the NHF and LMM range from ~30-10Ma. The LFM, UMM, and UFM were deposited in a filled to overfilled basin. The LFM has a maximum depositional age of ~30Ma, and ZHe ages between ~330-30Ma indicating erosion of the thrust sheets and deposition of the sediments with a brief lag time. UMM samples also show a short lag time between exhumation, ZHe ages ~250-20Ma, and deposition at ~20Ma. The final stage ~16-9Ma of basin filling, prograding alluvial fans, LFM, record ZHe ages ~280-14Ma. Collectively these data record rapid exhumation and erosion of the Alpine fold-and-thrust belt with a brief lag time due to ZHe ages approximately the same as the depositional ages; however, all of the units also show a large variability in the ZHe ages representing broad source areas.