Current theoretical modeling of ductile high-strain zones is either based on the assumption of a Newtonian fluid bounded by parallel rigid walls, for mechanical modeling, or based on the assumption that progressive deformation is steady state, for kinematic modeling. Neither assumption is well justified. First, there is little to suggest that rocks in the crust and lithospheric mantle behave like Newtonian fluids and shear zone wall rock deformation is commonly observed. Second, heterogeneous deformation, as reflected by strain gradients and variation of structures across and along shear zones, is ubiquitously observed. It has been demonstrated that heterogeneous flow generally leads to non-steady state progressive deformation.

I have recently incorporated non-steady state deformation into kinematic modeling of transpressional zones. It is found that the deformation path a volume of rock experienced will depend on: 1) the time-dependence (unsteadiness) of the flow field itself, 2) the spatial variation (heterogeneity) of the flow field, and 3) the locality and transport velocity of the volume of rock. For a simple situation where a high-strain zone has a bell-shaped shear strain rate distribution across the zone, at the center of the zone, the degree of non-steadiness of the deformation is directly the consequence of the time dependence of the flow field itself. If the flow field does not vary with time, albeit being heterogeneous, the finite strain in the center will accumulate in a steady-state manner. However, rocks in the margins of the zone have remarkable non-steady state deformation path. If the flow field itself is unsteady, then finite deformation will have to be non-steady everywhere, but rocks near the margins will be far more non-steady then those at the center. If the flow field is more complex than a bell-shaped distribution, more complex patterns of deformation history are expected.

This work explains the general observation that near a shear zone center, kinematics and strain are quite simple while in lower strain margins, more complicated kinematics are observed. Considering the fact that natural deformation in shear zones is generally heterogeneous, non-steady, and triclinic, forward modeling is likely an effective approach to interpret shear zone kinematics.