1A - Pelvoux massif

Field trip 1A (13th – 14th September 2015)

 

Collision kinematics, thermicity, and fluid flow in the External Alps

(Oisans/Pelvoux – Grandes Rousses, External Crystalline Massifs)

 

 

organization and contact:

Nicolas Belhasen (nicolas.bellahsen@upmc.fr), A. Verlaguet, Y. Rolland

 

This field trip in the external Western Alps aims at studying the structures attesting for Alpine collisional crustal shortening in the Grandes Rousses and Oisans/Pelvoux massifs. Along with this structural topic, we will look at the witnesses of fluid circulations in the Mesozoic cover, associated to the basement shear zones development. Finally, we will look at the data documenting the thermal history of the crust during its structural evolution, from burial to exhumation. The aim is to discuss the structural versus thermal evolution of the external zone, in order to better understand the rheology of the crust in such collisional context.

 

During day 1, we will visit outcrops where the distributed basement shear zones accommodating the Oligocene shortening can be observed. These outcrops are located in the Oisans and the Grandes Rousses massifs, where the cover was not significantly detached from the basement and where the inherited Liassic normal faults were not reactivated. The rheological implications will be discussed. At these outcrops, we will observe the Alpine paragenesis (greenschist facies) and discuss the age of these structures obtained by in situ Ar/Ar dating on syn-kinematic phengites, as well as their temperature (and pressure) of formation. Once the structure of this external crystalline massif is well imaged, we will focus on additional data about the thermal structure and evolution: RSCM (Tmax) data, chlorite and isotopic thermometry, thermochronology, thermopaleomagnetism from the recent literature.

This dataset shows that the collision occurred in 3 successive phases: underthrusting, distributed shortening, and then localized underplating. The thermal evolution is also characterized by a three-step evolution controlled by these kinematics. Such results will be compared to similar ones from the Mont Blanc massif.

 

During day 2, we will focus on the deformation and vein mineralization in the cover above a basement shear zone. We will study outcrops where the vein sequence can be discussed. We will then look at geochemical and microthermometric data that allow characterizing the scale of fluid circulation and mass transfer, through the basement/cover interface. We will show that, in the cover where no localized shear zone propagated from the basement, only small amounts of basement-derived fluids percolated in the vein network observed at the outcrop. By comparing these data with similar fluid circulation studies in the Mont Blanc and Aar massif, we will discuss the evolution of the fluid circulation scale with amount and style of deformation in the External Crystalline Massifs