KINEMATICS OF ROCK FLOW AND THE INTERPRETATION OF GEOLOGICAL STRUCTURES, WITH PARTICULAR REFERENCE TO SHEAR ZONES

Current interpretations of structures are generally based on homogeneo us and steady deformation models, despite the fact that both the heter ogeneity of rocks (materially, rheologically and geometrically) and th e time dependence of imposed geological conditions give rise to signif icant heterogeneous and non-steady flow. In concert with held observat ions, we emphasize that the expectation of heterogeneity and non-stead iness is the key to understanding natural deformation and that in orde r to carry out successful structural analysis and tectonic interpretat ion, it is necessary to recognize the first-order distinction between imposed boundary conditions typically used to define the tectonic regi me (e.g. transcurrent, transpression) and the response recorded by roc ks within the zone (structures and fabrics). Using S-C fabric as an ex ample, it is demonstrated how how with a nonzero spinning component re sulting from the rheological contrasts and/or geologically realistic t ime-dependent boundary displacement can drastically change the 'ideal' geometric and kinematic relations between the fabric and the host zon e. In agreement with both theoretical analysis and field observation, it is shown that natural flow regimes range from pure shear to pure ro tation, including super-simple shear. In consideration of the heteroge neity 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, mechanism s provide internal constraints on kinematics, ensuring that only certa in flows are possible and, secondly, how kinematics will favour develo pment of certain mechanisms.