Viscoplastic Rheological Modelling- A Realistic Approach to Natural Ductile Shear Zones

Crustal deformations generally undergo a brittle-ductile transition with depth, producing fault-dominated structures at shallow depths, replaced by ductile shear zones at middle and lower crustal levels. One of the keys to shear zone modelling concerns the choice of rheological approximations that can successfully reproduce the characteristic features of natural ductile shear zones in the models.  With the help of 2D FE (finite element) simulations, this study shows viscoplastic rheology as a suitable rheological approximation to predict the competing growth and orientations of multiple sets of secondary shear bands in a ductile shear zone. The viscoplastic rheology is modelled by combining bulk viscous weakening of the shear zone material and plastic yielding (Drucker-Prager criterion) to replicate strain-softening behaviour, where the instantaneous viscosity decreases nonlinearly with increasing strain. The cohesive strength of the material is also assumed to reduce with progressive plastic strain. This rheological combination allows us to replicate the various shear band networks found in crustal-level ductile shear zones. It also addresses the conditions for fluid flow into ductile shear zones, which leads to metamorphic reactions, mineralisation, etc. We validate our model results with field observations of similar shear band structures from the Eastern Indian Precambrian craton. The present study finally leads us to conclude that a pressure-dependent viscoplastic rheology is an ideal rheological approximation to model ductile shear zones extensively found in this craton.