THE TERTIARY STRUCTURAL AND THERMAL EVOLUTION OF THE CENTRAL ALPS - COMPRESSIONAL AND EXTENSIONAL STRUCTURES IN AN OROGENIC BELT

The Western Alpine Are has been created during the Cretaceous and the Tertiary orogenies. The interference patterns of the Tertiary structur es suggest their formation during continental collision of the Europea n and the Adriatic Plates, with an accompanying anticlockwise rotation of the Adriatic indenter. Extensional structures are mainly related t o ductile deformation by simple shear. These structures developed at a deep tectonic level, in granitic crustal rocks, at depths in excess o f 10 km. In the early Palaeogene period of the Tertiary Orogeny, the m ain Tertiary nappe emplacement resulted from a NW-thrusting of the Aus troalpine, Penninic and Helvetic nappes. Heating of the deep zone of t he Upper Cretaceous and Tertiary nappe stack by geothermal heat flow i s responsible for the Tertiary regional metamorphism, reaching amphibo lite-facies conditions in the Lepontine Gneiss Dome (geothermal gradie nt 25 degrees C/ km). The Tertiary thrusting occurred mainly during pr ograde metamorphic conditions with creation of a penetrative NW-SE-ori ented stretching lineation, X(1) (finite extension), parallel to the d irection of simple shear. Earliest cooling after the culmination of th e Tertiary metamorphism, some 38 Ma ago, is recorded by the cooling cu rves of the Monte Rosa and Mischabel nappes to the west and the Surett a 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 int rusion 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 m ovements along the Tonale and Canavese Lines (Argand's Insubric phase) . Subsequently, the center of main updoming migrated slowly to the wes t, reaching the Simplon region some 20 Ma ago. This was contemporaneou s with the westward migration of the Adriatic indenter. Between 20 Ma and the present, the Western Aar Massif-Toce culmination was the cente r of strong uplift. The youngest S-vergent backfolds, the Glishorn ant icline 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-Si mplon Line represents a late retrograde manifestation in the preexisti ng ductile Simplon Shear Zone. This fault zone is still active today. The Oligocene-Neogene dextral transpression and extension in the Simpl on area were concurrent with thrusting to the northwest of the Helveti c nappes, the Prealpes (35-15 Ma) and with the Jura thin-skinned thrus t (11-3 Ma). It was also contemporaneous with thrusting to the south o f the Bergamasc (> 35-5 Ma) and Milan thrusts (16-5 Ma).