2B - recent alpine tectonics

Field trip 2B (18th – 19th September 2015)


W-Alpine neotectonics and brittle deformation

 

 

organization and contact :

Christian Sue - csue@univ-fcomte.fr , Audrey Bertrand (to be comfirmed)

 

 

Aims of the field trip

The briançonnais area, South-East of the Pelvoux massif, is carved by a dense fault network, which corresponds to the last tectonic event in the alpine history. Indeed, this fault network postdates all the alpine compression-related structures such as nappe piles, folds, schistosities and cleavages related to the different compressional phases. The late fault system is made of normal faults, which locally bear witness of transcurrent reactivation. They can be observed at every scale (kilometric to centimetric) in the field. In terms of paleostress this extension is radial to the belt close to the Crustal Penninic Front and multidirectional in the eastern part of the internal zones. The paleostress field of the strike-slip phase is coherent with the extensional one and compatible with dextral shear along longitudinal faults. The current tectonics has also been analyzed using seismotectonics, which allowed the recognition of several active faults. The ongoing tectonics is coherent with the observed brittle deformation. Indeed, the fault network observed on the field controls the present-day seismicity. During this 2-days field trip we will follow the longitudinal fault system along the Durance Valley, and southward up to the High-Ubaye area. The first day will concern the role of the Penninic Front in the late alpine brittle extension, and its relationship with the High-Durance fault. The second day will be dedicated to neotectonic observations along the Serennes Fault system.

 

Day 1

We will follow the High-Durance fault system along the Durance Valley, South of Briançon. The idea is to investigate the relationships between the Oligocene Penninic thrust, and the recent (still active) extensional fault system. We will in particular visit the key area of “La Pousterle Pass”. The discussion will concern the Penninic front reactivation, the timing of the extension, and the inter-relations between recent extension and strike-slip. We will also discuss on the extensional directions, enlarging the geodynamic interpretations to the NW-Alps. The current activity of this fault system, and its observation through seismotectonics and geodesy will be integrated to this field trip. This first day will end at Guillestre, with some considerations on the relations between active faulting and fluids flow in the upper crust.

 

Day 2

We will follow the Serennes fault system, South-East of Guillestre. Two options are planned, depending on the weather conditions. We could walk from Vars ski-resort toward the Mortice-Houerts summits (about 3100m). This excursion would ask for proper conditions, but would allow to observe several neotectonic indications on the Serennes Fault system and related transverse faults, including discussion on sackung dynamics. Alternatively, we could drive in the Escreins Valley, to observe large fault planes associated to the Serennes fault strike-slip, and complement these observations close to the Serennes village, which was stroke by the 1959 (Ml=5.3) earthquake.