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The Penninic nappes or the Penninicum are one of three nappe stacks and geological zones in which the Alps can be divided. In the western Alps the Penninic nappes are more obviously present than in the eastern Alps (in Austria), where they crop out as a narrow band. The name Penninic is derived from the Pennine Alps, an area in which rocks from the Penninic nappes are abundant.
Contents
Of the three nappe stacks the Penninic nappes have the highest metamorphic grade. They contain high grade metamorphic rocks of different paleogeographic origins. They were deposited as sediments on the crust that existed between the European and Apulian plates before the Alps were formed. They are characteristically ophiolite sequences and deep marine sediments, metamorphosed to phyllites, schists and amphibolites.
Subdivision in the Western Alps
Four paleogeographic domains can be recognized in the Penninic nappes of the Western Alps:
The Piemont-Liguria Ocean and the Valais Ocean are, together with some other small oceanic basins, called Alpine Tethys Ocean or Western Tethys Ocean. The Tethys Ocean itself is sometimes considered to have begun east of the Apulian and African plates, but normally the Alpine Tethys is regarded as part of it.
Subdivision in the Eastern Alps
The following Penninic lithologies are found in the Hohe Tauern window, the Kőszeg Mountains and at the northern boundary of the Alps:
It is not clear which of these units can be correlated with the Penninic units of the Western Alps. Some of them are clearly Penninic, some clearly Helvetic, and some are disputed.
The oceanic trench deposits of the Penninic nappes are found through the Alps and called Bündner slates.
What is clear at least is that the Briançonnais terrane is not found in the Eastern Alps. The conclusion that can be drawn is that the microcontinent wedged out in the east in the Alpine Tethys Ocean. Some authors suggest the ophiolites that occur at the Hohe Tauern window must be correlated with the Piemont-Liguria terrane of the western Alps, because trench deposits such as radiolarites occur in both.