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. In the Western Alps, the Penninic frontal thrust (PFT) is the maincrustal-scale tectonic structure of the belt. This thrust transported thehigh-pressure metamorphosed internal units over the non-metamorphosedEuropean margin during the Oligocene (34–29 Ma). Following the propagationof the compression toward the European foreland, the PFT was laterreactivated as an extensional detachment associated with the development ofthe High Durance extensional fault system (HDFS). This inversion of tectonicdisplacement along a major tectonic structure has been widely emphasized asan example of extensional collapse of a thickened collisional orogen.However, the inception age of the extensional inversion remainsunconstrained. Here, for the first time, we provide chronologicalconstraints on the extensional motion of an exhumed zone of the PFT byapplying U–Pb dating on secondary calcites from a fault zone cataclasite.The calcite cement and veins of the cataclasite formed after the main faultslip event, at 3.6 ± 0.4–3.4 ± 0.6 Ma. Cross-cutting calcite veinsfeaturing the last fault activity are dated at 2.6 ± 0.3–2.3 ± 0.3 Ma. δ13C and δ18O fluid signatures derived fromthese secondary calcites suggest fluid percolation from deep-seatedreservoir at the scale of the Western Alps. Our data provide evidence that the PFTextensional reactivation initiated at least ∼ 3.5 Myr ago witha reactivation phase at ∼ 2.5 Ma. This reactivation may resultfrom the westward propagation of the compressional deformation toward theexternal Alps, combined with the exhumation of external crystalline massifs.In this context, the exhumation of the dated normal faults is linked to theeastward translation of the HDFS seismogenic zone, in agreement with thepresent-day seismic activity.

Extensional reactivation of the Penninic frontal thrust 3 Myr ago as evidenced by U–Pb dating on calcite in fault zone cataclasite