Mantle shear zones and their effect on lithosphere strength during continental breakup

Abstract

The Erro-Tobbio lherzolite in the ophiolitic Voltri Massif of northwestern Italy includes several thrusted fragments of lithospheric mantle which were first exhumed to the ocean floor during Jurassic rifting and breakup, and at a later stage became emplaced in the Alpine collisional stack during Tertiary convergence between Africa and Europe. Coherent slices of these mantle rocks contain several sets of major shear zones generated during the Jurassic rift evolution. One such shear zone, several kilometres wide and formed at temperatures between 920 and 1040°C, is transected by up to 200 m wide, ultra-fine-grained hydrated mylonite zones formed at temperatures in the range 990-550°C. All these structures are cut by MORB-type gabbroic and basaltic dykes. The microstructures of the mylonite zones are interpreted to reflect progressive, reaction-related grainsize reduction plus localization of the deformation during the early stages of continental breakup. In view of experimental evidence that wet olivine rocks weaken considerably with decreasing grainsize, in response to a change from grainsize-insensitive dislocation creep to grainsize-sensitive creep mechanisms, it is proposed that shear localization and allied grainsize reduction may have resulted in a drastic decrease in strength of the upper mantle during rifting. In order to obtain an order of magnitude estimate of this rheological effect, we present a layered rheological model of the Piemonte-Ligurian lithosphere, based on the observed microstructures (i.e., grainsizes) and pressure-temperature data, and including appropriate rheological laws for grainsize-sensitive and -insensitive creep in wet olivine. The model calculations suggest strength values for the uppermost mantle up to four orders of magnitude lower than those expected for homogeneous deformation exclusively controlled by dislocation creep of dry olivine.