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

Abstract : 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.
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Submitted on : Wednesday, January 9, 2019 - 3:05:33 PM
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Ahmed Abd Elmola, Martine Buatier, Patrick Monie, Pierre Labaume, Pierre Trap, et al.. Ar-40/Ar-39 muscovite dating of thrust activity: a case study from the Axial Zone of the Pyrenees. Tectonophysics, Elsevier, 2018, 745, pp.412-429. ⟨10.1016/j.tecto.2018.09.004⟩. ⟨hal-01975717⟩



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