Le Briançonnais, terrain exotique dans les Alpes ?


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Le Briançonnais, terrain exotique dans les Alpes ?
Eclogae Geologicae Helvetiae
Stampfli G.M.
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New reconstructions of the Western Alps from late Early Jurassic till
early Tertiary are proposed. These reconstructions use deep lithospheric
data gathered through recent seismic surveys and tomographic studies
carried out in the Alps. The present day position, under the Po plain,
of the southern limit of the European plate (fig. 1), allows to define
the former geometry of the Brianconnais peninsula.
The Brianconnais domain is regarded as an exotic terrane formerly
belonging to the European margin until Late Jurassic, then transported
eastward during the drift of Iberia (fig. 5). Therefore, on a present
day Western Alps cross section, a duplication of the European
continental margin can be recognized (fig. 10).
Stratigraphic and sedimentological data along a zone linking the
Pyrenean fracture zone to the Brianconnais, can be related to a rifting
event starting in Oxfordian time. This event is responsible for the Late
Jurassic till mid-Cretaceous drift of Iberia opening, first the northern
Atlantic, then the Gulf of Biscay. Simultaneously, the drift of the
Brianconnais will open the Valais ocean and close the Piemontese ocean.
The resulting oblique collision zone between the Brianconnais and the
Apulian margin generates HP/LT metamorphism starting in Early
Cretaceous. The eastward drift of the Brianconnais peninsula will
eventually bring it in front of a more northerly segment of the former
European margin. The thrusting of the Brianconnais unto that margin
takes place in early Tertiary (fig. 9), following the subduction of the
Valais ocean.
The present nappe pile results not only from continent/continent frontal
collision, but also from important lateral displacement of terranes, the
most important one being the Brianconnais. The dilemma of `'en echelon''
oceanic domains in the Alps is an outcome of these translations. A
solution is found when considering the opening of a Cretaceous Valais
ocean across the European margin, running out eastward into the
Piemontese ocean, where the drift is taken up along a former transform
fault and compensated by subduction under the Apulian margin (fig. 8).
In the Western Alps we are then dealing with two oceans, the Piemontese
and the Valaisan and a duplicated European margin. In the Eastern Alps
the single Piemontese ocean is cut by newly created oceanic crust. All
these elements will be incorporated into the Penninic structural domain
which does not represent a former unique paleogeographic area, it is a
composite accretionary domain squeezed between Europe and Apulia.
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20/10/2012 19:11
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20/08/2019 17:10
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