⁴⁰Ar/ ³⁹Ar dating of Penninic Front tectonic displacement (W Alps) during the Lower Oligocene (31–34 Ma)

Abstract

Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the 40Ar/39Ar 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∼280 °C, while 40Ar/39Ar 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 40Ar/39Ar 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.

Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

“…There is some evidence that the motion of the PDC thrust and of the associated Pelvoux, Meije and Combeynot thrusts occurred during earliest Oligocene times. The Combeynot shear band yielded a lowermost Oligocene 40 Ar/ 39 Ar age (Simon-Labric et al, 2009). Soft sediment deformation is recorded in the very top of the Paleogene sediments at several localities, indicating top-to-the NW displacements associated with the PDC thrust: to the SE of the Pelvoux, its Meso-Cenozoic hangingwall sequence is overlain by a thick Tertiary olistostrome (« complexe d'Orcières », Debelmas et al, 1980) beneath the Embrunais Basal Thrust.…”

“…Pervasive D1 transport lineations (Choukroune et al, 1986) have been re-arranged by arcuate bending during later episodes (Rosenbaum & Lister, 2005). Afterwards, the renewed tectonic activity is marked by the activation of major, west directed crustal thrusts in the Western Alps during early Oligocene (Badertscher & Burkhard, 1998;Tricart et al, 2000;Fügenschuh & Schmid, 2003;Simon-Labric et al, 2009), which cross-cut the previous nappes stack. Coeval eastward underthrusting of the Pelvoux massif is dated 31-33 Ma (Simon-Labric et al, 2009).…”

“…The deformation in its footwall at sites 1 and 4 indicate a NW-ward transport. The PDC thrust has been deformed during several younger Alpine events, as shown by (i) severe overprint by top-to-the-west shearing in the footwall of the Crustal Pennine Thrust (site 11: Pêcher et al, 1992 ;Butler, 1992;Simon-Labric et al, 2009), (ii) long wavelength folding ( fig. 4), (iii), eastward dip caused by differential uplift of the Pelvoux massif with respect to Internal Nappes during the Neogene (Tricart et al, 2000) and (iiii) offset of the thrust surface by several late orogenic NE-SW dextral strike-slip faults.…”

“…5 The Oligocene revolution: westward extrusion and formation of the arc 5.1 Deformation in the External Zone: Main Alpine, west-directed stacking (D3) The eastern part of the Dauphiné zone was underthrust eastwards beneath the Crustal Pennine Thrust from early Oligocene onwards (Simon-Labric et al, 2009). This is recorded by the dominantly top-to-the west intense shear in the footwall of the CPT at the eastern edge of the Pelvoux massif (Butler, 1992), and approximately 30° diverging transport directions on both sides of the Pelvoux culmination are observed ( fig.…”

The structural history of the Mont Blanc massif with regard to models for its recent exhumation

“…Kinematics along the Penninic thrust is still a matter of debate, especially regarding the latest history. Initial activity on the Penninic thrust has been described to be of Eocene-Oligocene age during underthrusting of the European passive margin beneath the Penninic domain (Ceriani et al 2001;Simon-Labric et al 2009). Strong, approximately NW-directed shear dominated along the Penninic thrust in the northern parts of the study area.…”