40Ar/39Ar geochronology of syn-kinematic phengite reveals the tectonic history of underthrusted European crust (W Alps): a synthesis

Rolland, Yann1

1EDYTEM, Université de Savoie Mont Blanc, La Bourget du Lac, France

The major problem of 40Ar/39Ar geochronology applied to deformation is often to find sufficiently large crystals that grew in a clear relationship to the deformation context, and that have been preserved in the following tectonic history.

The dating of white mica syn-kinematic fibres (phengite) in complement to their PT conditions (obtained by combined mineralogical mapping for instance) allows the dating of deformation stages at a depth constrained by PT calculations and along certain geothermal conditions. Kinematics of related shear zones provide the polarity of tectonic motions. The integration of such data into the tectonic framework of a geodynamic zone allow deciphering along-belt variations, which relate to strain propagation and changes in the stress field through time and space.

The External Crystalline Massifs (ECM) of the Alps provide a nice case example of a crustal domain in which this technique was successfully applied to the dating of brittle-ductile motions at mid-crustal levels (10-15 km). The Western Alps are a curved mountain belt that accommodated tectonic motions related to subduction dynamics in the Mediterranean domain after a brief period of continent-continent collision of the Apulian microblock with Eurasia. Subsequently, the timing and the along-belt variations of deformations has been progressive through time.

In this presentation, I propose a synthesis of analyses conducted at the External Alps scale to investigate the timing of shear zone deformation since the onset of Eocene-Oligocene collision. The data include 40Ar/39Ar stepwise dating of synkinematic phengites, the associated shear zone kinematics and thermobarometric constraints on the depth and temperature of deformation. The advent of new dating approaches, based on U-Pb dating of syn-kinematic allanite, of hydrothermal vein monazite, and more recently, of calcite from veins and fault gouges, do confirm the timing of deformation suggested by the obtained 40Ar/39Ar ages on micas.

These dating constraints are used to discuss the timing of deformation of underthrusted European crust related to changes in tectonic modes during the Alps’ tectonic history. These changes are related, at the larger scale, to the evolution of the Mediterranean domain, i.e., to the rapid underthrusting of the external part of European plate, dated in the ECM at 35-32 Ma, followed by the onset a mainly dextral strike-slip context. This strike-slip context is controlled by the anticlockwise rotation of Apulia, notably driven by the Apulian roll-back.


Biography

Yann has worked on many orogen examples, starting from his PhD work on the Himalaya-Karakoram, to the Alps, Caucasus, Tien Shan… He develops approaches coupling geochronology and geodynamics. He has used the Ar/Ar chronometer applied to dating shear zone activity since his post-doc at the ANU in 2001.

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