The effect of microstructural and rheological heterogeneity on porphyroblast kinematics and bulk strength in porphyroblastic schists

The kinematic record and bulk viscous strength of polyphase rocks depend in part upon the relative strengths and distributions of rheologically distinct fabric elements. Here, we explore the effects of microstructural and rheological heterogeneity in porphyroblastic schists. Electron backscatter diffraction and petrographic analyses reveal asymmetric microboudinage of staurolite, indicating relative rotation of staurolite porphyroblasts synchronous with bulk non-coaxial strain. Boudinage and relative rotation both require porphyroblast–matrix shear coupling. Based on 2D optical observations, the extent of the coupling appears related to the initial and boudinaged staurolite grain shape and orientation as well as the geometry of heterogeneities such as mica domains or shear bands.We designed 2D finite element numerical models to assess the role of microstructural variation and rheological heterogeneity on the degree of porphyroblast–matrix shear coupling and bulk viscous strength. Model results indicate that the bulk strength of a three-phase system comprising inclusion, weak domain, and matrix is sensitive to the relative proximity of weak and strong domains, particularly at high viscosity contrasts (i.e. ηmatrix/ηweak > 10). The threshold for bulk weakening below the matrix strength occurs over a narrow range of weak domain viscosities (ηmatrix/ηweak = 2.6–5.5), regardless of the relative abundance and spatial distribution of weak domains. Kinematic decoupling of porphyroblasts occurs at low viscosity contrasts when weak domains are proximal (ηmatrix/ηweak = 2–5), but for all other spatial distributions and modal abundances investigated, kinematic decoupling occurs at viscosity contrasts of ηmatrix/ηweak = 15–20. These data indicate that bulk weakening due to rheological heterogeneity is not necessarily coincident with kinematic decoupling.

► We document boudinage and rotation of staurolite porphyroblasts, which both require shear coupling.
► SEM-EBSD measurements indicate an average misorientation of 35° between formerly continuous grain fragments.
► Large magnitudes of observed rotation occur out of the shear plane.
► Numerical models indicate that bulk strength and clast kinematics are sensitive to rheological heterogeneity.
► Bulk weakening and kinematic decoupling of clasts occur at discrete thresholds, which do not coincide.