The Alps: a case of multiple collision

Abstract

Summary The numerous isotopic data accumulated during the last decade throughout the Western Alps, make it tempting to draw a picture of polyphase Alpine tectono-metamorphic history constrained by these data. Gabbroic precursors of the Mesozoic Piémont ophiolites in the Dent Blanche Nappe, point to a thinning of the continental crust by the beginning of the Triassic. Biotite cooling ages of ∼ 180 Ma, in the Valpelline-IIDK-Ivrea Unit, set an older limit to the beginning of Alpine nappe emplacement. The younger limit is given by Cretaceous ages on high-pressure paragenesis in the Sesia basement and the Piémont ophiolites. In both, this first eo-Alpine tectono-metamorphic cycle was overprinted by an Eocene-Oligocene greenschist-facies metamorphism and a recumbent schistosity. Miocene mica cooling ages, as well as apatite fission track ages, can be linked to the Miocene décollement and ‘rétrocharriage’ phase in the Western Alps, followed by isostatic re-equilibration and denudation.