Folding and boudinage of oblique single layers under non-plane conditions

褶皱和边界提供了岩石运动学、应变、流变学和变形史的信息。这就是为什么这两种结构形成的条件应该是众所周知的。由于大多数折叠和束缚的理论和建模都是基于同轴平面应变，目前在期刊和教科书上对折叠和束缚的评论很大程度上局限于二维。这是不幸的，因为新的三维测量技术允许对三维褶皱和边界进行详细的几何记录，从而导致从自然构造岩中描述的此类结构越来越多。如果主管层的初始取向相对于主应变轴是倾斜的，也可能形成三维褶皱和束缚。正交的单层会形成对称的圆柱形结构，而斜向的单层虽然体应变是平面的，但会转化为不对称的褶皱和束缚，甚至转化为斜周褶皱。如果层序一开始是倾斜的，而体应变不是平面的，情况就更复杂了。在这种三维应变场情况下，主管层最大缩短速率的方向随时间连续变化，因此会叠加屈曲或颈缩失稳的基应变速率也在变化。因此，褶皱和束缚趋向于非圆柱形和不对称，两种结构可能同时发育，甚至顺序发育。由于缺乏针对非平面条件下斜向单层共轴变形的数值和物理实验，限制了我们对相关三维结构演变的理解。因此，我们将使用非线性粘性岩石模拟物进行比例实验，以显示层与基质之间的黏度比和初始层倾角对体扁平和体收缩条件下变形几何形状的影响。本建议的主要目的是表明，当层的初始姿态相对于主应变轴发生变化时，在块体收缩和块体变平的情况下，一方面是圆顶-盆地褶皱，另一方面是片式褶皱，将如何转变为同期褶皱和褶皱。除了不间断的研究，这意味着最终应变为60%，我们将进行增量研究，以显示单个结构随时间的增长速度和方式。由于倾斜单层的褶皱和褶皱在各种地质环境中是常见的，岩石是由平坦或收缩应变施加的（例如片麻质圆顶，盐丘），本项目将对大量构造地质学家感兴趣，他们可以利用新的结果来约束自然非平面环境中的褶皱和褶皱条件。

Folds and boudins provide information on kinematics, strain, rheology and deformation history of rocks. That’s why the conditions during which both structures develop should be well known. As most of the theory and modelling of folds and boudins are based on coaxial plane-strain, current reviews of folding and boudinage in journals and textbooks are largely restricted to two dimensions. This is unfortunate as new 3D survey techniques allow detailed geometrical recording of 3D folds and boudins resulting in an increasing number of such structures described from natural tectonites. 3D folds and boudins may also develop if the initial orientation of the competent layering is oblique with respect to the principal strain axes. While orthogonal single layers result in symmetric, cylindrical structures, oblique single layers will be converted into asymmetric folds and boudins and even into periclinal folds, although the bulk strain is plane. The situation is still more complex if the layering is initially oblique on the one hand, and bulk strain is not plane, on the other hand. In such cases of 3D strain fields, the directions of maximum shortening rate in the competent layer varies continuously with time and the base strain rate on which a buckling or necking instability would be superimposed is therefore also changing. Consequently, folds and boudins are tending to be non-cylindrical and asymmetric, and both structures may develop simultaneously or even in sequence. As numerical and physical experiments focusing on coaxial deformation of oblique single layers under non-plane conditions are lacking, our understanding of the evolution of related 3D structures is restricted. For this reason we will carry out scaled experiments using non-linear viscous rock analogues to show the impact of viscosity ratio between layer and matrix and of the initial layer inclination on the deformation geometry under bulk flattening and bulk constrictional conditions. The main aim of the present proposal is to show how, under bulk constriction and bulk flattening, dome-and-basin folding on the one hand, and tablet boudinage on the other hand, will change into coeval folding and boudinage when the initial attitude of the layer varies with respect to the principal strain axes. Apart from uninterrupted studies, which imply a final strain of 60%, we will carry out incremental studies to show how fast and in which way the individual structures are growing with time. As folding and boudinage of oblique single layers is common in various geological settings where rocks are imposed by flattening or constrictional strain (e.g. gneiss domes, salt domes), the present project will be of interest for a large number of structural geologists, who can use the new results to constrain the conditions of folding and boudinage in natural non-plane environments.