The timing of and nature of greenschist facies deformation and metamorphism in the upper Pennine Alps

The 40Ar/39Ar and Rb-Sr ages on micas and U-Pb ages for sphenes and amphiboles from the upper Pennine Alps were studies and showed that greenschist assemblages developed at circa 44 Ma in the Zermatt-Saas zone of the footwall and at circa 40 Ma in the thrust zone. Rapid cooling inferred for the Zermatt-Saas ophiolites in the mid-Eocene may document either the extensional unroofing of the ophiolite over the underlying Monte Rosa massif or their tectonic emplacement.