The Western Alpine Are has been created during the Cretaceous and the
Tertiary orogenies. The interference patterns of the Tertiary structures
suggest their formation during continental collision of the European and
the Adriatic Plates, with an accompanying anticlockwise rotation of the
Adriatic indenter. Extensional structures are mainly related to ductile
deformation by simple shear. These structures developed at a deep
tectonic level, in granitic crustal rocks, at depths in excess of 10 km.
In the early Palaeogene period of the Tertiary Orogeny, the main
Tertiary nappe emplacement resulted from a NW-thrusting of the
Austroalpine, Penninic and Helvetic nappes. Heating of the deep zone of
the Upper Cretaceous and Tertiary nappe stack by geothermal heat flow is
responsible for the Tertiary regional metamorphism, reaching
amphibolite-facies conditions in the Lepontine Gneiss Dome (geothermal
gradient 25 degrees C/ km). The Tertiary thrusting occurred mainly
during prograde metamorphic conditions with creation of a penetrative
NW-SE-oriented stretching lineation, X(1) (finite extension), parallel
to the direction of simple shear. Earliest cooling after the culmination
of the Tertiary metamorphism, some 38 Ma ago, is recorded by the cooling
curves of the Monte Rosa and Mischabel nappes to the west and the
Suretta Nappe to the east of the Lepontine Gneiss Dome.
The onset of dextral transpression, with a strong extension parallel to
the mountain belt, and the oldest S-vergent `'backfolding'' took place
some 35 to 30 Ma ago during retrograde amphibolite-facies conditions and
before the intrusion of the Oligocene dikes north of the Periadriatic
Line. The main updoming of the Lepontine Gneiss Dome started some 32-30
Ma ago with the intrusion of the Bergell tonalites and granodiorites,
concomitant with S-vergent backfolding and backthrusting and dextral
strike-slip movements along the Tonale and Canavese Lines (Argand's
Insubric phase). Subsequently, the center of main updoming migrated
slowly to the west, reaching the Simplon region some 20 Ma ago. This was
contemporaneous with the westward migration of the Adriatic indenter.
Between 20 Ma and the present, the Western Aar Massif-Toce culmination
was the center of strong uplift.
The youngest S-vergent backfolds, the Glishorn anticline and the Berisal
syncline fold the 12 Ma Rb/Sr biotite isochron and are cut by the 11 Ma
old Rhone-Simplon Line. The discrete Rhone-Simplon Line represents a
late retrograde manifestation in the preexisting ductile Simplon Shear
Zone. This fault zone is still active today.
The Oligocene-Neogene dextral transpression and extension in the Simplon
area were concurrent with thrusting to the northwest of the Helvetic
nappes, the Prealpes (35-15 Ma) and with the Jura thin-skinned thrust
(11-3 Ma). It was also contemporaneous with thrusting to the south of
the Bergamasc (> 35-5 Ma) and Milan thrusts (16-5 Ma).