Shear zone memory revealed by in-situ Rb-Sr and 40Ar/39Ar dating of Pyrenean and Alpine tectonic phases in the external Alps

Abstract

The combination of Rb-Sr and ⁴⁰Ar/³⁹Ar dating methods with high-resolution mineralogical investigation allows deciphering the multiphase history of shear zones and serves as a tool for tectonic reconstructions. However, the interpretation of dates obtained by these two methods in relation to cooling, fluid circulation or to deformation-induced (re)crystallization remains controversial. Here, we apply the in situ ⁸⁷Rb/⁸⁷Sr and ⁴⁰Ar/³⁹Ar dating methods to shear zone minerals used together with step heating ⁴⁰Ar/³⁹Ar dating in several case studies with contrasting structural and metamorphic histories along the External Crystalline Massifs (ECMs) of the SW Alps. Our results emphasize polyphase deformation and highlight a variable behaviour of Rb-Sr and K-Ar systems in shear zones. This study provides new constraints about the timing, conditions and mechanisms of deformation in the southern ECMs of the Western Alps. The inherited crustal-scale Variscan shear zones have been reactivated several times in a thick-skin mode since the Late Cretaceous. An early N-S compressional phase impacted the SW Alps mainly at around ∼80–70 and ∼40 Ma in the Argentera and Pelvoux shear zones, but this signal is not documented further north. This signal is better preserved in the southern Pelvoux massif, with a Rb-Sr age of 79.7 ± 3.7 Ma. There is also a significant compressional deformation on the W-Alpine scale at 34–32 Ma, associated with underthrusting beneath the Penninic Frontal Thrust (PFT). Two compressional deformation pulses occur at 26 and 22–20 Ma in the southern Argentera Massif, corresponding to the onset of transpressive deformation induced by the anti-clockwise rotation of the Adriatic Plate. Finally, the last phase of deformation around 18–15 Ma concerns only the NW Alps, in the Beaufortain massif, which was buried under the Mont-Blanc massif, during the propagation of deformation leading to the development of the frontal fold and thrust belt of the SW Alps. Our results also show that the southern Pelvoux massif already reached a temperature around 300 °C at a depth of 10–15 km depth during the Late Cretaceous. This range of temperature and pressure conditions is broadly similar to that reached in the ECMs during the Cenozoic deformation stages.