Finite element modelling of the evolution of pressure solution cleavage

Deformation by ‘pressure solution’ (or ‘water-assisted stress-induced diffusion transfer’) is modelled by incorporating volume transfers — driven by variations in mean stress and competency contrast — into a finite element code for viscous deformation. The model assumes a two-component, quartz–mica mixture with a composition-dependent rheology in which the viscosity of an element is solely dependent upon the volume fraction of the mobile quartz component. Both quartz-rich and mica-rich compositions are given a high competency, with a minimum viscosity at some intermediate fraction. The model is able to reproduce many structures observed in natural tectonites: propagation and coalescence of pressure solution seams, formation of microlithons and anastomosing cleavage, and, more generally, tectonic segregation.