Timing of syn-orogenic extension in the Western Alps revealed by calcite U-Pb and hematite (U-Th)/He dating

• We combined hematite and calcite geochronology to document fault and fluid activity. • Fault and fluid histories range from ∼13 Ma to ∼<0.8 Ma indicating a westward-propagating extension. • Two fluid regimes related to the faults are evidenced with a transition from hydrothermal fluid to meteoric fluids. • Syn-orogenic extension was synchronous with foreland compression and controlled by trans -crustal thrusting. Understanding fault activity over time provides valuable insights for reconstructing the tectonic history of an orogen, assessing seismological risks and understanding mineralization processes. In the Western Alps, one of the main controversies in existing tectonic models is the understanding of syn -orogenic extension. Seismological evidence shows widespread extensional deformation related to the reactivation of major lithospheric structures, such as the Penninic Frontal Thrust (PFT). However, the onset age and origin of extension are still debated due to the lack of suitable geochronological data. Fault hematite and calcite geochronology as well as clumped isotope data can be used to relate fluid regimes to fault activity. The analysis of calcite brecciae from extensional faults above the PFT shows that two distinct fluid regimes were present. The first regime, occurring before 2 Ma is associated with upwelling of deep fluids and is recorded by fault calcite at a temperature > 110°C. The second fluid regime is characterized by a meteoric signature and temperatures around 36 °C, representing crystallization since 2 Ma. This study presents a new model for the Miocene tectonic history of the Western Alps that combines (U-Th)/He and U-Pb geochronology on fault hematite (13.3 ± 0.8 to < 0.8 Ma) and calcite (5.3 ± 0.6 Ma). Results demonstrate a progression of extensional fault activity from east to west, from the Middle Miocene (ca. 13 Ma) to the Quaternary. The onset of extension in the inner part of the belt coincides with the development of the fold and thrust belt in the western Alpine foreland. Our new model proposes that extension occurs in the hanging wall of a large top-to-the-west thrust, known as the Alpine Frontal Thrust. This thrust, located to the west of the External Crystalline Massifs gives rise to their uplifting and extension at the rear.