Alpine thermal evolution in the central and the western Alps

Summary In the western and central Alps, high-pressure assemblages dated as Eoalpine are distributed in the Pennine, Piemonte and Austro-Alpine units. The starting point of metamorphism must have been at low temperatures as both pre-Alpine metamorphic basement and Mesozoic sediments have been involved. In those parts of the Sesia-Lanzo zone where no later Tertiary high temperature overprint modified or obliterated the early Alpine history, the Eoalpine peak of metamorphic pressure was reached at about 110 Ma, the temperature peak at 85 Ma and a cooling down to 300°C at 60 Ma. The radiometric evidence gives insight into the Cretaceous orogenic events which also include penetrative deformation, nappe thrusting and uplift. The Pennine and Austro-Alpine units are both considered to have been located south of the Jurassic oceanic gap and to have formed parts of the fragmented northern margin of Gondwana. The Tertiary Mesoalpine and Neoalpine metamorphic events in the western Alps are separated in time by 32–30 Ma magmatic intrusions. In the central Alps both events are distinguished on the basis of an additional schistosity connected with the second event. The Neoalpine ages are considered as resulting from the Mesoalpine thermal conditions outlasting the Mesoalpine time and, in the Simplon region, as recrystallization ages related to late mylonitization. The convergence rates during the Cretaceous orogenesis were one order of magnitude greater than during the Tertiary orogenesis. Sedimentation in some Pennine units continued through Eoalpine times until Palaeogene; this is interpreted as resulting from the imbrication of blocks of different evolution in Tertiary times.