Extensional reactivation of the Penninic Frontal Thrust 3 Ma ago as evidenced by U-Pb dating on calcite in fault zone cataclasite

Abstract. In the Western Alps, the Penninic Frontal Thrust (PFT) is the main crustal-scale tectonic structure of the belt. This thrust transported the high-pressure metamorphosed internal units over the un-metamorphosed European margin during the Oligocene (34–29 Ma). The PFT was later reactivated as an extensional detachment associated with the development of the High-Durance extensional fault system. This inversion of tectonic motions on a major tectonic structure has been widely emphasized as an example of extensional collapse of a thickened collisional orogen. However, the inception age of the extensional inversion remains unconstrained. Here, for the first time, we provide chronological constraints on the extensional motion of an exhumed zone of the PFT by applying U-Pb dating on secondary calcites from a fault zone cataclasite. The calcite cement of the cataclasite, formed after the main fault slip event, is dated at 3.4 ± 1.5 Ma. Cross-cutting calcite veins featuring the last fault motions are dated at 2.6 ± 0.3 and 2.3 ± 0.3 Ma. δ¹³C and δ¹⁸O fluid signatures derived from these secondary calcites suggest fluid percolation from deep-seated reservoir at the scale of the Western Alps. Our data give evidence that the PFT extensional reactivation is active since at least ~ 3 Ma, at the crustal scale and results from the westward propagation of the compressional deformation related to the exhumation of External Crystalline Massifs. In this context, the exhumation of the dated extensional fault is associated with the eastward translation of the seismogenic zone controlled by the deep rooting of the High-Durance fault system.