Résumé
In orogens, most fault zones have polyphase tectonic activity under ductile and/or brittle conditions, which renders challenging their analysis in terms of kinematics, metamorphism, fluid flow and time evolution and may lead to different interpretations of their role during the orogenic evolution. Therefore, 40Ar/39Ar dating of the deformation is an efficient tool to decipher tectonic events in fault zones. This is particularly true in the Axial Zone of the Eastern Pyrenees where few geochronological studies have focused on these fault zones. The Têt and Py faults are major NE- SW trending crustal faults that have accommodated the complex exhumation/cooling history of adjacent crustal blocks, Mont-Louis, Canigou and Carança massifs, as highlighted by thermochronological data (Milesi et al., 2022). These faults record deformation from slightly above the ductile-brittle transition to brittle conditions as witnessed by the presence of thick fault gouges that partially rework low temperature mylonites. In this work, we report 40Ar/39Ar illite/muscovite dating on fault zones showing different activity phases at the brittle-ductile transition characterized by the presence of gouges with variable mixture of polytypes and with different phyllosilicate generations. For these fine-grained materials, we used a sample encapsulation technique to evaluate the amount of 39Ar lost by recoil during irradiation before dating the phyllosilicates. The new dates provide a scenario of fault motion between the early Eocene to the early Miocene that allows a comparison with those predicted by low-temperature thermochronological data and thermal modeling and shows a close link between fault activity and exhumation of adjacent crustal blocks.