Strain localisation in two-phase materials: Insights from centimetre-scale numerical models and laboratory experiments with ice mixtures

Abstract:

Most numerical models of lithosphere deformation approximate the rheological behavior of polymineralic crust and mantle via single-phase flow laws assuming that the weakest or most abundant material controls the bulk rheology. However, previous work showed that in two phase aggregates the bulk viscosity of the dominant phase is significantly affected by second phase particles. Here we combine two unconventional approaches to quantify the relative impact of such particles on strain localisation and bulk response: (1) We run centimetre-scale numerical models of a matrix with inclusions using the elasto-visco-plastic FEM software Slim3D. Recrystallization-induced weakening processes in the matrix, i.e. grain boundary migration and nucleation, are approximated using strain-dependent viscous softening. (2) We conduct high T, constant strain rate deformation experiments with a matrix of deuterated ice (D₂O) containing rigid or soft particles, i.e. calcite and graphite, respectively. Ice is a valuable rock analogue, as it replicates the microstructural and fabric changes as well as the non-Newtonian response of other anisotropic minerals, such as olivine and quartz.

The laboratory experiments exhibit two types of rheological behaviour: stress partitioning between ice and particles and strain localization in rheologically softer material. To quantify the contribution of both response types, we calibrate numerical simulations with data derived from laboratory experiments. The strain rate, stress, and viscosity evolution of the numerical experiment provides insight to non-linear strain localization processes, particle motion and time-dependent stress concentrations during the deformation. We fit the parameters of the viscous softening function and thereby quantify the amount of additional weakening in the matrix of ice mixtures in comparison to pure ice, which allows to constrain softening parameters used in large-scale simulations of glacial flow and lithosphere deformation.