The main deformation structures due to the Tertiary continental
collision in the Western Swiss Alps are ductile shear zones. Four main
shear zones can be recognized on the structural map, each characterised
by a different shear direction. The first D I shear zone with a X I, SE
(transverse) stretching direction has been created during the
under-thrusting towards the SE of the European plate under the Adriatic
plate. This took place mainly by ductile deformation of the upper part
of the European continental basement with the formation of the external
massifs basement folds and the Penninic foldnappes. The second D II
shear zone (Simplon ductile shear zone) is characterized by a XII
stretching, dipping from 0 to 30-degrees to the SW (longitudinal
stretching). It is approximately 10 km wide, and crosses the Alpine
nappes in an oblique direction. It corresponds to a relative SW
transport direction of the upper units together with the Adriatic plate.
This dextral transpression zone is also responsible for the stretching
parallel to the elongation of the Alpine belt. The third D III shear
zone is made of mylonites with a steep stretching direction and
corresponds to the hanging wall of the Canavese reverse fault. The D IV
shear zones, dextral wrench zones combined with underthrusting movement,
are characterised by a W and SW stretching direction. They were formed
during and after the S facing backfolding which for instance made the
Mischabel fold and the Boggioleto fold. Actually it occupies two narrow
areas of more ductile rocks between the Mischabel backfold to the N and
the Monte Rosa nappe to the S and allong the Canavese Line. These
dextral shear zones represent probably the western continuation of the
Tonale Line dextral wrench fault. The D I to IV ductile shear zone were
formed under greenschist and amphibolite facies conditions during the
Tertiary orogenic metamorphism. Their regional distribution is limited
to the metamorphic ductile zone representing the deep part of the Alpine
belt, between 10 and 30 km depth. The emplacement and orientation of
the shear zones was also directed by the geometry of the boundaries of
the European and Adriatic plates. The analysis of the superposed
Central Alpine shear zones permits thus to propose a model of the
history of the relative convergent and wrench movements which took place
between the European and Adriatic plates during the Alpine Tertiary
continental collision.