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.

我们对折叠和折叠的理解建立在详细的几何分析之上。正确描述褶皱及其与其他结构（如裂缝、解理和线形）的关系，以及物理和数值建模，是将褶皱与应力、应变、运动学、力学和潜在构造过程联系起来的基础。在过去一个世纪左右的时间里积累了大量的分类方案和方法，本概述批判性地考虑了这些方案的很大一部分以及折叠形成的模型。我们发现折叠和折叠对许多不同的因素都很敏感，包括材料特性、层厚度、机械各向异性、边界条件、初始层取向、传播折叠或相邻层之间的结构相互作用，继承的断裂和断层结构、变形机制、温度和围压。然而，褶皱几何形状和地层深度之间没有很强的关系，因为在这方面微尺度变形机制的重要性有限。出于这个原因，粘土/沙箱实验中探索的几何关系直接适用于晶体塑性机制在变质条件下形成的褶皱。折叠的最基本区别可能是被动折叠与主动折叠的区别。被动折叠，其中粘度对比很小或可以忽略，很好理解并且易于建模。主动折叠，其中折叠成核和放大由粘度或强度的对比控制，更复杂，未来的工作应该集中在岩石中观察到的活动褶皱几何形状的定义明确的例子的实验和数值模拟上。此外，弯曲的概念可以包括被动和主动元素，有助于维护。主动和被动折叠反映流变学和应变，但与构造制度没有直接关系。有关构造体制的信息必须来自其他信息来源，但当已知时，可以应用褶皱分析来表征和量化该体制中的变形。未来的工作应侧重于整合基于现场的观测、地下数据集和不同模型配置（层数、层厚度、扰动类型及其在层界面中的振幅、