Numerical shear experiments of quartz-biotite aggregates: Insights on strain weakening and two-phase flow laws

For progressively deforming multi-phase aggregates, it is unclear to what extent the change in geometry and orientation of the involved phases leads to textural strain weakening and thus may control strain localization. Consequently, the question arises how the ductile flow of multi-phase rocks can be described or determined. To contribute to the understanding of these knowledge gaps, two-dimensional numerical shear experiments of quartz-biotite aggregates were conducted at variable conditions. Textural variations after a shear strain of γ ≈ 10 appear to be dependent on the viscosity contrast between the minerals involved. To estimate whether a numerical experiment is undergoing strain weakening or hardening, the temporal evolution of the mean second invariant of the deviatoric stress tensor was tracked. The results suggest that strain weakening occurs if biotite is distinctly isolated in form of strong or weak inclusions and that it is more effective under conditions with larger viscosity contrasts between matrix and inclusions. However, observed stress drops in experiments purely based on textural strain weakening are low compared to other strain weakening processes. Numerical results from experiments with variable strain rate, temperature and biotite content were combined to determine two-phase flow law parameters for quartz-biotite aggregates, which are in broad agreement with existing analytical mixed-aggregate flow laws. •Numerical shear experiments are conducted for varying temperatures and strain rates.•Textural strain weakening is not restricted to weak inclusions in a strong matrix.•Textural rearrangement is of minor importance for strain weakening in shear zones.•The experiments are further used to determine two-phase flow law parameters.•Flow-law parameters are in broad agreement with flow laws based on existing mixing laws.