Shear zones in the upper mantle: A case study in an Alpine Iherzolite massif

The Erro-Tobbio upper-mantle lherzolite of northwest Italy, emplaced in the Alpine suture zone during collision of the European and Adriatic plates, has preserved a set of major shear zones formed at temperatures in the range 800-1040 °C during the period of Jurassic rifting and breakup and development of the Piemonte-Ligurian oceanic basin. These structures clearly demonstrate localization of the deformation in the Piemonte-Ligurian upper mantle during lithosphere stretching and breakup.