NAPPE GEOMETRY IN THE WESTERN SWISS ALPS

Detailed geological mapping during the last 20 years in the Western Sw iss Alps has shown clearly that most of the lower basement nappes are fold nappes possessing normal and inverted limbs. Moreover their cores are made of strongly deformed gneisses indicating that important duct ile strain took place during the formation of the fold nappes. It is t herefore probably wrong to imagine deep basement nappes as rigid slice s as often actually claimed, especially when interpreting seismic prof iles. True 'brittle type' thrust nappes involving basement rocks only occur in the internal and upper parts of the belt. Cover nappes, on th e contrary, are in most parts of the Alpine belt thrust sheets followi ng more or less the rules of thin-skinned tectonics. Many basement fol d nappes lost part of their sedimentary cover during or just before th eir formation, by decollement along ductile horizons. The result is th at many cover thrust nappes in the external part of the Alps are direc tly related to their original basement fold nappes.