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TheValais Ocean is a subductedoceanic basin which was situated between the continentEurope and themicrocontinentIberia or so calledBriançonnais microcontinent. Remnants of the Valais ocean are found in the western Alps and in tectonic windows of the eastern Alps and are mapped as the so-called "northPenninic" nappes.[1]
After the breakup ofPangaea in the earlyMesozoic age, the continents of Africa,South America, Europe, andNorth America began to move away from each other. The breaking up, orrifting, did not take place along one unbroken line; thus, at the southern edge of the European plate, the microcontinent Iberia also began to break away from Europe. In the western part of the rift that separated the two landmasses, oceanic crust was formed in what is at present theGulf of Biscay, while in the eastern part, the Valais Ocean was formed.
When, in theCretaceous period, Africa again began to move towards Europe, the Valais Ocean became sandwiched between the two continents. To the east, the Valais oceanic crust, together with a piece of Iberiancontinental crust (called theBriançonnais terrane),subducted beneath theApulian plate, a part of the Africantectonic plate that had begun to move independently. This process eventually led to the formation of theAlps. To the west, no subduction took place, but the Iberian plate moved against the European plate along a largetransform fault, which led to the formation of thePyrenees.
Fragments of Valais oceanic crust have beenobducted and can be found asophiolites in thePenninic nappes of the Alps.
The Valais Ocean was named after theSwisscantonValais.
In the easternAlps remnants of the Valais are confined totectonic windows and the northern margin of the Alps. They include oceanic sediments (e.g.radiolarites,turbidites) and oceanic crust (e.g.basalt,pillow lava). The windows in the overlyingAustroalpine nappes reveal the underlyingPenninic nappes. In theEngadin window, remnants are found in the Pfundser zone and in theTauern window the "Obere Schieferhülle"; at the northern margin of the Alps the remnants are calledRhenodanubic flysch.[1]
In the westernAlps, remnants of the Valais crop out in many areas ofSwitzerland andFrance. This includes theCheval noir unit, theVersoyen unit, theSion-Courmayeur Zone, theNiesen nappe, theSchlieren flysch, theAntrona unit, theWägital flysch and thePrätigau Bündnerschiefer.[2]
