A long history of ductile and brittle deformation of Eastern Pyrenees fault zones: coupling 40Ar/39Ar geochronology, chlorite geothermometry and structural observation

In orogenic settings, fault networks accommodate crustal deformation during the evolution of mountain ranges. Fault zones exhibit multiphase ductile and brittle activity, complicating their interpretation. Dating deformation in orogenic basement remains a challenge. In this study, we conducted structural and microstructural observations, coupled with ⁴⁰Ar/³⁹Ar dating on encapsulated mineral fractions and chlorite thermometry, on major fault zones in the Eastern Pyrenees. To overcome the presence of K-feldspar within fault gouge, a major issue for fault gouge argon dating, we present a method to estimate the contribution of two mixed K-rich phases from ⁴⁰Ar/³⁹Ar step heating. This method provides limit or maximum age for the deformation recorded by fault gouges in which illite polytypes are mixed with K-feldspar. Our results reveal a ductile-to-brittle transition between 40 and 35 Ma (Priabonian-Bartonian), characterized by 2M1 muscovite and evidence of strike-slip movement along the Py NE-SW fault and NW-SE secondary faults. The Py fault gouge contains muscovite formed at temperatures exceeding 200–250 °C. In the Têt NE-SW fault gouge, the coexistence of 2M1 and 1M illite polytypes suggests late-stage crystallization of 1M illite at 22.1 ± 1.4 Ma, at temperatures between 100 and 150 °C, as determined by chlorite thermometry. These findings confirm significant normal faulting activity on the Têt fault during the Oligo-Miocene, consistent with published low-temperature thermochronology data indicating early exhumation of the Canigou massif relative to the Carança massif, facilitated by normal displacement along the Py fault, and a later exhumation of both massifs in relation to the Têt normal fault activity.