Restoring the source-to-sink relationships in the Paleogene foreland basins in the Central and Southern Alps (Switzerland, Italy, France): a detrital zircon study approach

Late Paleogene syn-tectonic magmatic products are known from sandstones contained in the North Alpine (NAFB) and South Alpine (SAFB) foreland basins and in the South Alpine pelagic Trento Basin. The generally turbiditic and mass-flow deposits grade up from marly hemipelagic deeper water series. The source and amount of the reworked volcanic materials have been in debate for a long time. As a proxy for the magma-derived input we use the U-Pb geochronology and geochemistry [Hf-176/Hf-177((t)) and Eu/Eu* ratios] of detrital zircons, and evaluate their temporal and genetic relationships with potential volcanic sources in the Periadriatic magmatic systems (Adamello, Bergell, Biella). The oldest volcanic sources (Lutetian-Priabonian) we identify are in the Trento Basin and Glarus NAFB. During most Rupelian, Bergell and Biella volcaniclastics were delivered to the NAFB in the Glarus, Alpe de Taveyanne and Haute-Savoie. Bergell and minor Adamello magmatic material were supplied to the Villa Olmo Conglomerate in the SAFB. During late Rupelian-Chattian in the entire NAFB, the influx of Paleogene volcanic material faded out. At the same time in the SAFB, the Como Conglomerate shows mixed input from Biella and Bergell. High numbers of old zircons (> 90%) in the NAFB document the asymmetry of the early Alpine orogenic wedge exposing large basement areas to the north of the Periadriatic intrusions. The syn-sedimentary right-lateral movement along the Periadriatic fault system is identified as the main driver of magmatic activity, uplift, and exposure to erosion and transport to the basins. On this base, the dynamics of the early Alpine drainage systems are reconstructed with new accuracy.