Kinematics of rock flow and the interpretation of geological structures, with particular reference to shear zones

Current interpretations of structures are generally based on homogeneous and steady deformation models, despite the fact that both the heterogeneity of rocks (materially, rheologically and geometrically) and the time dependence of imposed geological conditions give rise to significant heterogeneous and non-steady flow. In concert with field observations, we emphasize that the expectation of heterogeneity and non-steadiness is the key to understanding natural deformation and that in order to carry out successful structural analysis and tectonic interpretation, it is necessary to recognize the first-order distinction between imposed boundary conditions typically used to define the tectonic regime (e.g. transcurrent, transpression) and the response recorded by rocks within the zone (structures and fabrics). Using SC fabric as an example, it is demonstrated how flow with a non-zero spinning component resulting from the rheological contrasts and/or geologically realistic time-dependent boundary displacement can drastically change the ‘ideal’ geometric and kinematic relations between the fabric and the host zone. In agreement with both theoretical analysis and field observation, it is shown that natural flow regimes range from pure shear to pure rotation, including super-simple shear. In consideration of the heterogeneity and non-steadiness of natural deformation, kinematic analysis is justifiable only within a homogeneous domain and steady period. Flow kinematics and mechanisms are interrelated in that, firstly, mechanisms provide internal constraints on kinematics, ensuring that only certain flows are possible and, secondly, flow kinematics will favour development of certain mechanisms.