2A - from margin to ocean

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


From a Margin to an Ocean

 

Organization and contacts:

Gianreto Manatschal (manat@unistra.fr), Thierry Dumont (thierry.dumont@ujf-grenoble.fr)

 

Sites and aims of the 2 days field trip

On our 2 days field trip we will visit some of the famous localities where remnants of the ancient Alpine Tethys ocean and the distal European/Briançonnais margin can be studied directly in the field. The two field trips are located in the vicinity of Montgenèvre (see photograph). Both field trips will start and end in Montgenèvre. On our field trips we will see rocks belonging to different rift domains of the paleo-European rifted margin including the Briançonnais, Pre-Piemontais and Piemonte units preserving locally original, pre-Alpine tectono-stratigraphic-magmatic relationships of the former margin and adjacent embryonic oceanic crust. Although in detail the tectonic relationships between the different visited areas are complex and debated, the aim of the 2 field trips is to: 1) visit outcorps preserving remnants of the former margin/oceanic domains, 2) compare the rock associations and relationships to present-day magma-poor rifted margins, including comparisons with seismic sections, and 3) discuss the evolution of these units from the formation of the margins to their integration in the present day Alpine nappe stack. An itinerary of the two excursions is given in the following figures.

 

Field Trip I:
Chenaillet Ophiolite

Field trip I will bring us to the Chenaillet Ophiolite exposed at the French/Italian border south of Montgenèvre. The upper unit of this ophiolite was not subducted and represents one of the best-preserved ocean floor sequences exposed in the Western Alps. The ophiolite is only weakly affected by Alpine deformation. It still preserves primary oceanic structures that can be compared to those drilled for instance at ODP Site 1277 offshore Newfoundland. The aim of this field trip will be to study the tectono-magmatic evolution of an embryonic oceanic domain.

 

 

Field Trip II:
Briançonnais and Pre-Piemontais Units of the distal European margin

Field trip II will be devoted to the architecture of the most distal European margin preserved in the Briançonnais and Pre-Piemontais units north of Montgenèvre. The excursion will focus on the tectono-sedimentary evolution of the distal margin during final rifting. During the excursion, we will see the stratigraphic relation between pre-, syn- and post-rift sediments of the distal European margin and the relation to extensional allochthons and reactivated, rift-related detachment faults. The importance of the rift-related structure during the Alpine reactivation will be discussed. Panoramic views will enable to discuss the different units forming the nappe pile of the western Alps as well as their petro-stratigraphic content.