Detailed geological mapping during the last 20 years in the Western
Swiss 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 ductile
strain took place during the formation of the fold nappes. It is
therefore probably wrong to imagine deep basement nappes as rigid slices
as often actually claimed, especially when interpreting seismic
profiles. True `brittle type' thrust nappes involving basement rocks
only occur in the internal and upper parts of the belt. Cover nappes, on
the contrary, are in most parts of the Alpine belt thrust sheets
following more or less the rules of thin-skinned tectonics. Many
basement fold nappes lost part of their sedimentary cover during or just
before their formation, by decollement along ductile horizons. The
result is that many cover thrust nappes in the external part of the Alps
are directly related to their original basement fold nappes.