Flow variation in layered rocks subjected to bulk flow of various kinematic vorticities: theory and geological implications

Abstract

Flow variation is studied analytically in rocks with layers of varied competence and varied orientation with respect to constant bulk flow fields of given kinematic vorticity (0 ≤ W_k ≤ 1). The results explicitly demonstrate that flow varies significantly from layer to layer and with time in any individual layer even though the bulk flow is constant. Any flow regime can occur and the instantaneous stretching axes (ISA) spin all the time. Reverse sense of non-coaxiality can occur in the same layer at different times and in different layers at the same time. This study further reveals the significance of heterogeneity and non-steadiness in natural flows. The assumption of a time-independent flow history for natural deformation is highly unrealistic. The spinning of the instantaneous stretching axes (ISA) and the variation of flow regimes have bearings on the progressive development of fabrics and structures such as folded boudins, refolded folds in shear zones and double rotation of porphyroclasts that are often interpreted as being due to multi-deformational events.