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1 Department of Earth Sciences, Universität Basel, Bernoullistrasse 30, CH-4056 Basel, Switzerland
2 Laboratoire de Géologie, École Normale Supérieure, UMR 8538, 24 rue Lhomond, F-75005 Paris, France
3 LGCA, Université J. Fourier, 1381, rue de la Piscine, F-38400 St-Martin d'Hères, France
4 Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Strasse 24/H25, D-14476 Potsdam-Golm, Germany
5 DRO/GEM, B.P. 70, F-29280 Plouzané, France
Petrographic and structural studies on metasedimentary rocks of the Valaisan domain from the Engadine window (Switzerland) to the Petit St. Bernard (France) show that this entire domain was subjected to high pressure–low temperature conditions during the Alpine orogeny. The pressure and temperature conditions increased from east to west. The metasedimentary rocks of the Engadine window and the Grisons area are characterized by blueschist-facies conditions (12–13 kbar, 350–400 °C, defined by Mg-carpholite and chloritoid occurrences). Toward the west, in the Petit St. Bernard and Versoyen units, the metamorphic conditions reached the eclogite facies (15– 16 kbar, 500 °C). The shortening direction associated with the high-pressure metamorphic event was oriented northwest- southeast all over the Valaisan domain. This direction is compatible with the direction of convergence between the European and Apulian plates in early Tertiary time (65–35 Ma).
In the Eastern and Central Alps, blueschist-facies rocks occur below the Austroalpine nappes over an area of 300 x 20 km2 (from the Tauern window to the Grisons area) and have a thickness around 10 km. This volume of blueschist-facies rocks is in contrast with eclogite-facies rocks of Western Alps that form only a small slice with a thickness of 2 to 5 km. We interpret the difference in volume and metamorphic conditions from east to west as being due to a change of the subduction style. In the east, we assume the formation of a large wedge with a thickness of 40–50 km, such that rocks underwent blueschist metamorphism and were exhumed before collision between the European and Apulian plates. In the west, the Petit St. Bernard and the Versoyen units were metamorphosed and extruded in a subduction channel above the subducting slab. The data further allow us to conclude that the Valaisan domain was an ocean that was independent of the Piemontese Ocean and that had an important place in the youngest phase of the evolution of the Alps before the collision.
Key Words: Alps • carpholite • chloritoid • exhumation • high-pressure metamorphism
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