Middle Miocene onset of thrusting along the basal décollement of the Jura Mountains: Evidence from carbonate U-Pb dating and clumped isotope thermometry

During its late-stage evolution, the European Alpine orogen witnessed a northwest-directed propagation of its deformation front along an evaporitic basal décollement into the foreland. This resulted in the decoupling of the northern Alpine Molasse Basin from its basement and the formation of the Jura fold-and-thrust belt. Here, we present the first absolute age and temperature constraints on deformation along this major décollement by using combined carbonate U-Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and clumped isotope thermometry. We analyzed calcite veins associated with a secondary thrust fault branching off from the basal décollement in a deep borehole within the Swiss Molasse Basin providing evidence for three brittle deformation events related to Alpine contraction: Vein formation at ~14 Ma ago, thrust faulting with cataclasis ~13 Ma ago, and another vein formation event ~9 Ma ago. Clumped isotope data reveal vein calcite precipitation at temperatures of ~100 °C from fluids equilibrated with the host rock during all deformation intervals. These temperatures are in good agreement with temperature estimates from microfabrics of anhydrite mylonites in the basal décollement. Our data demonstrate that the propagation of Alpine deformation into its distal foreland occurred significantly earlier than previously inferred, at middle Miocene (Langhian) times at the latest, contributing to the debate about the late-stage geodynamics of the central Alps. The deformation sequence indicated by our results underpins fundamental kinematic models of viscous décollement-based foreland thrust belts. Beyond that, we demonstrate the potential of our methodological approach applied to foreland thrust systems in resolving the late-stage evolution of convergent orogens.