A fundamental problem with the kinematic interpretation of geological structures

Non-steady flows are ubiquitous in nature. Not only do imposed boundary conditions vary with time and rock rheology change during the course of deformation but also deformation is generally heterogeneous and all of these conditions lead to non-steady flow histories.In modern kinematic analysis, flow apophyses, the instantaneous stretching axes and the vorticity vector, collectively referred to as the `eigen directions' of deformation, are used in place of Bruno Sander's kinematic axes. For deformation with a steady flow history, this practice is well justified and has led to great advances in structural geology. But for non-steady flow histories, the geometrical relationships among eigen directions (flow pattern) vary with time. This makes it inappropriate to correlate an association of structures and fabrics with certain `time-invariant' flow patterns and hence raises the question: How should we interpret geological structures and fabrics kinematically, without the assumption of homogeneous and steady deformation? We suggest that the answer lies primarily in forward-modeling of deformation, based on a knowledge of rock properties.