The formation of micafish: A modeling investigation based on micromechanics

• Micafish are simulated based on micromechanics of non-Newtonian viscous materials.
• Micafish are regarded as anisotropic ellipsoids dispersed in an isotropic matrix.
• Slip along cleavage and the presence of anisotropy lead to natural micafish geometries.
• Mica is weaker than matrix for slip along cleavage and stronger without cleavage slip.
• Final micafish geometries at large finite strains are insensitive to their initial states.

Micafish porphyroclasts are common and excellent shear-sense indicators in mylonites. Although their geometries are well characterized by recent studies, they are still poorly understood. Here we simulate the behavior in mylonites of micafish by regarding them as anisotropic power-law ellipsoidal inhomogeneities embedded in a power-law viscous material subjected to general plane-straining non-coaxial flows. We apply the Eshelby's solutions extended for general non-Newtonian power-law materials. The anisotropy of mica is defined by two different shear viscosities: a weak shear viscosity associated with slip on cleavage and a strong shear viscosity without activating the cleavage slip. Our modeling reproduces naturally observed geometries of micafish. Natural micafish geometries require that the weak shear viscosity of mica must be lower and the strong shear viscosity must be greater than the matrix viscosity. We demonstrate that regardless of the initial condition of micafish, the shape long axis and the cleavage trace observed on the vorticity-normal section will most commonly end up at a small antithetic angle relative to the shear plane or parallel to it. Our modeling results are consistent with the mica rheological properties determined by rock deformation experiments.