The spatial and temporal scales and the geometry of fluid pathways in a collisional orogen are investigated using stable isotope analysis (O, C, and H) and ⁴⁰Ar/³⁹Ar dating of vein minerals formed at circa 11–16 Ma in the Mont Blanc and the Aar External Crystalline Massifs. In both massifs ⁴⁰Ar/³⁹Ar dating of veins adularia provides evidence for progressive crystallization from 16 to 9 Ma, and mainly at 11–12 Ma following veins opening during shear zone activity. The fluid flow duration thus ranges from 4 to 5 Ma in the two massifs. The δ¹⁸O values of vein quartz and calcite are similar to those of undeformed crystalline and sedimentary host rocks, suggesting rock buffering, while carbon isotope ratios of vein calcites fall into three compositional groups. A‐type veins have δ¹³C values that are buffered by the Helvetic metasediments, which suggests that these veins formed in a closed system from a locally derived CO₂‐rich fluid. The fluid in equilibrium with C‐type veins has depleted δ¹³C values similar to mantle‐CO₂, while the intermediate δ¹³C values of B‐type veins suggest mixing between the A‐type and C‐type fluids. These results are in agreement with crustal‐ to lithosphere‐scale upward vertical fluid flow along vertical shear zones related to the strike‐slip system bounding the Adriatic block since 16–20 Ma, connecting a deep‐seated fluid to some downward flow in the sedimentary cover of External Crystalline Massifs.

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 non-metamorphosed European margin during the Oligocene (34–29 Ma). Following the propagation of the compression toward the European foreland, the PFT was later reactivated as an extensional detachment associated with the development of the High Durance extensional fault system (HDFS). This inversion of tectonic displacement along 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 and veins of the cataclasite formed after the main fault slip event, at 3.6 ± 0.4–3.4 ± 0.6 Ma. Cross-cutting calcite veins featuring the last fault activity are dated at 2.6 ± 0.3–2.3 ± 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 provide evidence that the PFT extensional reactivation initiated at least ∼ 3.5 Myr ago with a reactivation phase at ∼ 2.5 Ma. This reactivation may result from the westward propagation of the compressional deformation toward the external Alps, combined with the exhumation of external crystalline massifs. In this context, the exhumation of the dated normal faults is linked to the eastward translation of the HDFS seismogenic zone, in agreement with the present-day seismic activity.