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

Abstract. In the Western Alps, Penninic frontal thrust (PFT) is main crustal-scale tectonic structure of belt. This transported high-pressure metamorphosed internal units over non-metamorphosed European margin during Oligocene (34–29 Ma). Following propagation compression toward foreland, PFT was later reactivated as an extensional detachment associated with development High Durance fault system (HDFS). inversion displacement along a major has been widely emphasized example collapse thickened collisional orogen. However, inception age remains unconstrained. Here, for first time, we provide chronological constraints on motion exhumed zone by applying U–Pb dating secondary calcites from cataclasite. The calcite cement and veins cataclasite formed after slip event, at 3.6 ± 0.4–3.4 0.6 Ma. Cross-cutting featuring last activity are dated 2.6 0.3–2.3 0.3 ?13C ?18O fluid signatures derived these suggest percolation deep-seated reservoir scale Alps. Our data evidence that reactivation initiated least ? 3.5 Myr ago phase 2.5 may result westward compressional deformation external combined exhumation crystalline massifs. this context, normal faults linked to eastward translation HDFS seismogenic zone, in agreement present-day seismic activity.