Stress gradients around porphyroclasts: palaeopiezometric estimates and numerical modelling

Based on strain gradients commonly found around porphyroclasts, the mechanical interaction of porphyroclasts and recrystallized mylonite matrix in a retrograde amphibolite-facies shear zone of the Ivrea-Verbano Zone, Italy, was investigated. Differential stresses inferred from recrystallized grain size and dislocation density indicate stress concentrations at the porphyroclast-matrix interface of about 1.7–2.0 with respect to the average in the matrix. The palaeopiezometric stress data are compared to the results of a finite-element model composed of two rheologically different phases. In this model, the local distribution and concentration of stresses around porphyroclasts mostly depend on clast-matrix coupling and the effective viscosity contrast of the two phases. The bulk strength and work hardening of the composite system increases with interface coupling. Both the modified stress field and the reduced average grain size around porphyroclasts may result locally in instabilities, and promote strain localization and a change in the dominant deformation mechanism.