Fold geometry and folding - a review / Геометрия складок и складчатость - обзор

Our understanding of folds and folding builds on detailed geometrical analysis. Proper description of folds and their relation to other structures such as fractures, cleavage and lineations form, together with physical and numerical modelling, the foundation for linking folds to stress, strain, kinematics, mechanics, and underlying tectonic processes. A large number of classification schemes and approaches have accumulated over the past century or so, and this overview critically considers a substantial portion of these schemes together with models for fold formation. We find folds and folding to be sensitive to many different factors, including material properties, layer thickness, mechanical anisotropy, boundary conditions, initial layer orientation, structural interaction between propagating folds or adjacent layers, inherited fracture and fault structures, deformation mechanisms, temperature, and confining pressure. However, there is no strong relationship between fold geometry and depth of formation, since microscale deformation mechanisms are of limited importance in this regard. For this reason, the geometric relations explored in clay/sandbox experiments are directly applicable to folds formed under metamorphic conditions by crystal-plastic mechanisms. The most fundamental distinction of folds is probably that of passive versus active folding. Passive folding, where viscosity contrasts are small or neglectable, is well understood and simple to model. Active folding, where fold nucleation and amplification is controlled by contrasts in viscosity or strength, is more complicated, and future work should focus on experimental and numerical modelling of well-defined examples of active fold geometries observed in rocks. In addition, the concept of bending, which can include both passive and active elements, is useful to maintain. Active and passive folding reflect rheology and strain, but do not directly relate to tectonic regime. Information about tectonic regime must come from other sources of information, but when known, fold analysis can be applied to characterize and quantify the deformation in that regime. Future work should focus on integrating field-based observations, sub-surface data sets, and 3D numerical modelling of folds in different model configurations (number of layers, layer thicknesses, type of perturbation and its amplitude in the layer interface, type of contact between interacting layers such as free-slip and or no-slip interfaces), different geological and tectonic settings (i.e., the type of applied boundary conditions and also in the form of displacement-based and strain-rate-based boundaries), and different mechanical properties or stratigraphy. <...>