Absolute Age and Temperature Constraints on Deformation Along the Basal Decollement of the Jura Fold-and-Thrust Belt From Carbonate U-Pb Dating and Clumped Isotopes

During its late-stage evolution, the European Alpine orogen witnessed a northwest-directed propagation of its deformation front along an evaporitic basal decollement 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 basal decollement using carbonate U-Pb LA-ICP-MS dating and clumped isotope thermometry. We analyzed calcite veins associated with a thrust fault branching off from the basal decollement in the distal Molasse Basin and slickenfibers from thrusts and strike-slip faults in the eastern Jura Mountains. Our U-Pb data provide evidence for tectonic activity related to Alpine contraction between 14.3 and 4.5 Ma ago. According to the oldest deformation ages, the propagation of Alpine deformation into the distal foreland along the basal decollement occurred earlier than previously inferred by biostratigraphy, at Middle Miocene (Langhian) times at the latest. Younger deformation ages between 11.3 and 4.5 Ma correspond very well in time with shortening in the Subalpine Molasse and the Central Alps, proving simultaneous tectonic activity along both thrust fronts; i.e., the Jura Mountains and the Subalpine Molasse. Clumped isotopes reveal vein calcite precipitation at temperatures between 53 and 104 degrees C from fluids with oxygen isotope compositions between -6.2 parts per thousand and +9.5 parts per thousand reflecting distinct burial settings. Combined, our U-Pb and clumped isotope data show that the burial conditions in the studied area remained constant between 14.3 and 4.5 Ma indicating that large-scale foreland erosion did not initiate before 4.5 Ma.

Automated summary

The late-stage evolution of the European Alpine orogen showed northwest-directed propagation of deformation front along an evaporitic basal decollement. The first absolute age and temperature constraints on deformation along this basal decollement have been determined using carbonate U-Pb LA-ICP-MS dating and clumped isotope thermometry. The findings suggest that Alpine deformation occurred earlier than previously thought, and the burial conditions in the studied area remained constant between 14.3 and 4.5 Ma.