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TheHelvetic nappes (German:Helvetische Decken) are a series ofnappes in the Northern part of theAlps and part of theHelvetic zone. They consist ofMesozoiclimestones,shales andmarls that were originally deposited on the southerncontinental margin of the European continent. During theAlpine orogeny they werethrust north over adécollement and at the same time were internallydeformed byfolding and thrusting.
The Helvetic nappes are thrust over theInfrahelvetic complex and the external massifs of the Alps (like theAarmassif orMont Blanc Massif). In Switzerland, Germany and Austria they are also thrust over theMolasse basin of the Alpineforeland. In turn, the Helvetic nappes were overthrust by thePenninic nappes from the south. In Switzerland these have beeneroded away at most places but in Germany and Austria they are still covering the Helvetic nappes. Due to this the Helvetic nappes only crop out as a thin band in those countries, which forms theNorthern Limestone Alps.
The Helvetic nappes consist of Mesozoicsedimentary rocks deposited on the former southern continental margin of the European plate. A narrow ocean, theValais Ocean, existed south of Central Europe in the Mesozoic. This later developed into aconvergent plate boundary where the European platesubducted beneath theApulian plate. Thesedimentary facies of the rocks from this age thus becomes deeper marine when the rocks were deposited further south. Therefore, the rocks of the Helvetic nappes have a shallower marine facies than the contemporaryBündner slates of the Penninic nappes; and a deeper facies than the rocks of the Infrahelvetic complex of Eastern Switzerland or contemporary sediments in theJura Mountains north of the Alps.