Ar-40/Ar-39 muscovite dating of thrust activity: a case study from the Axial Zone of the Pyrenees
Résumé
The Pyrenees belt features well exposed deformation structures that make it ideally suited to study thrust faults. In this paper, we present the first 40Ar/39Ar age of the Pic de Port Vieux thrust (PPVT) located in the south-western part of the Pyrenean Axial Zone. The PPVT is a secondary thrust related to the major Gavarnie thrust (GT) with a footwall comprising Upper Cretaceous limestones, and a hanging wall made of Lower-Triassic pelites and sandstones. A vertical transect in the fault hanging wall was investigated to characterize the deformation mechanism and to date the thrust activity by using 40Ar/39Ar step-heating technique on muscovite.
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.