40Ar/39Ar muscovite dating of thrust activity: a case study from the Axial Zone of the Pyrenees

Petrographic observations confirm that pressure solution and recrystallization/dissolution processes in presence of fluids are the main mechanisms that controlled the chemical/textural changes in the fault core zone. Three groups of muscovite are identified in the fault zone i.e. detrital, diagenetic and newly-formed muscovites. The detrital and diagenetic muscovites are mainly present in the damage zone, while the newly-formed muscovite is abundant in the core zone and is synkinematic to the fault activity. The fractions 50-16 μm, 16-2 μm, 2-0.5 μm, <0.5 μm and the bulk of two samples from the core and damage zones were encapsulated and dated using the 40Ar/39Ar technique. All size-fractions of the core zone sample have younger ages compared to ones from the damage zone and the ages decrease with grain-size reduction. This indicates the strong effect of deformation in the core zone that enhanced the recrystallization of younger muscovite. The finest fraction (<0.5 μm) of the core zone sample has a plateau-like spectrum with a total gas age of 36.9 ± 0.2 Ma. This age is interpreted as the oldest age possible for the activity of the PPVT during the late Eocene (Priabonian) that coincides with the activity of the GT.