GeodynAlps

Some informations about the geodynamics of the Alps

One ocean?
Two oceans?

  • The Alpine orogen formed during a long process of convergence and collision between the European and Adria continental domains during Mesozoic and Cenozoic times (Trüpy, 1975; Le Pichon et al., 1988; Dewey et al., 1989; Escher et al., 1997). The intervening oceanic crust progressively deformed and partially accreted to the continental margins; it is now part of the Penninic nappes, which are sandwiched between units of the overlying Austroalpine nappes and rocks of the underlying European domains. The occurrence of high pressure–low temperature metamorphism (HP-LT) in major parts of the oceanic and adjacent continental units is now well known from many parts of the Alps (see for review Bearth, 1974; Goffe ́ and Chopin, 1986; Frey et al., 1999). Nevertheless, different opinions exist about the age and paleogeographic position of the HP-LT units (Cretaceous vs. Tertiary, one or two sites of subduction, eastward or westward vergence) partly due to conflicting age determinations of HP metamorphism (see Chopin and Maluski, 1980; Hunziker et al., 1992; Ducheˆne et al., 1997; Gebauer, 1999) and partly due to a fragmentary view of each paleogeographic domain.
    Even though it is generally accepted that the Piemontese Ocean was subducted southward or eastward under the Apulian plate and that eclogites and blueschists of the Piemontese nappe resulted from this subduction. In contrast, the significance of the Valaisan Ocean is still controversial and has been interpreted in different ways.


    • It is frequently considered as a small oceanic basin (Figure 1) closing toward the west and joining the main Piemontese realm in the east (Polino et al., 1990; Dercourt et al., 1993). Thus, the Briançonnais domain is seen as a continental swell, which was separated from the European continent by a Valaisan basin underlain by continental crust. Such a view is based on the stratigraphic similarity of the sedimentary cover in some basins to the south and in the Versoyen area to the north (Lemoine et al., 1989). In this case, the HP metamorphic rocks of the Valaisan trough are interpreted as allochthonous Piemontese tectonic slices thrust over the Briançonnais continent into the Valaisan realm (Schürch, 1987; Scharer et al., 2000).







    Figure 1 : Comparison of different models for the paleogeographic evolution of the Alps. (A) Model with one ocean (after Dercourt et al., 1985, 1993). The Valaisan domain represents a continental basin between the European continent and the Briançonnais area. In this model, the Briançonnais domain is a part of Europe. 150 Ma- Opening of the Valaisan rift and deposition of the sediments. 110 Ma- Apulia and Austroalpine nappes move northward; closure of the Piemontese Ocean begins. 65 Ma- Collision between Europe and Austroalpine nappes. 45 Ma (not depicted)- The Piemontese Ocean closed completely during Lutetian time. 30 Ma- Deposition of Swiss molasse on the European margin.

    Sometimes, however, the Valaisan realm is interpreted as an open ocean basin (Figure 2) that extended all the way from the Eastern Alps (Tauern window) (see discussion in Froitzheim et al., 1996) to the Pyrenees (Frisch, 1981; Stampfli,993; Stampfli and Marchant 1997; Schmid et al., 1997a). The great similarity in geochemical data from bauxites (Poinssot et al., 1997) and continuity of stratigraphic data (Tempier, 1987) from the Briançonnais to the Iberian platform support the hypothesis of a separate Iberia-Briançonnais microcontinent. The HP metamorphism of the Valaisan domain is a result of the its subduction under the Briançonnais microcontinent (Oberhänsli, 1994).






    Figure 2 :Model with two oceans (after Frisch, 1981; Schmid et al., 1990; Stampfli, 1993; Stampfli et al., 1998). The Valaisan domain represents a real ocean that separated the Briançonnais domain from the European continent. 150 Ma- The Piemontese Ocean had opened, and the Briançonnais domain was still joined to Europe. 110 Ma- Opening of the Valaisan Ocean from the Eastern Alps to the Pyrenees. The Piemontese Ocean was closing by southward subduction. 70 Ma- End of the closure of the Piemontese Ocean and continuing southward subduction of the Valaisan Ocean under the Briançonnais (since 90 Ma). 40 Ma- European distal margin (Adula; location in Fig. 2) is removed by subduction; the Valaisan was not completely closed.



    To cite this pagee
  • Bousquet R., Goffé B., Vidal O., Oberhänsli R., Patriat M., 2002, The tectono-metamorphic history of the Valaisan domain from the Western to the Central Alps: New constraints on the evolution of the Alps, GSA Bulletin; v. 114; no. 2; p. 207–225