Direct absolute dating of the Penninic Frontal Thrust tectonic motion is
achieved using the Ar-40/Ar-39 technique in the Pelvoux Crystalline
Massif (Western Alps). The dated phengites were formed syn-kinematically
in shear zones. They underline the brittle-ductile stretching lineation,
pressure-shadow fibres and slickensides consistent with underthrusting
of the European continental slab below the propagating Penninic Thrust.
Chlorite-phengite thermobarometry yields 10-15 km and T similar to 280
degrees C, while Ar-40/Ar-39 phengite ages mainly range between 34 and
30 Ma, with one younger age at 27 Ma. This Early Oligocene age range
matches a major tectonic rearrangement of the Alpine chain. Preservation
of prograde Ar-40/Ar-39 ages is ascribed to passive exhumation of the
Pelvoux shear zone network, sandwiched between more external thrusts and
the Penninic Front reactivated as an E-dipping detachment fault. Partial
resetting in the Low Temperature part of argon spectra below 24 Ma is
ascribed to brittle deformation and alteration of phengites.