The Analvsis Of Development Of Fractures In The Folding Progress

As a natural phenomenon, fold has the features of continous deformation. Base on field observations folds, the authors have found lots of fractures formed during the evolution of stratum deformation. Fractures related fold are paid more and more concern at present time, but also procokes hot debates throughout the geosciences world because its complex tectonic features. In this thesis, many typical compresional clastic rocks fold are selected to study the mechanism and control factors of fractures. The conclusions are summarized as follows:1. The early structural fractures are formed in the horizontal rocks by the comressional process in the horizontal rocks before folding. The early fracture development has a close relationship with bed thickness.Two sets of shear conjugate fractures were chiefly develop in thin layers (<95cm) during the horizontal compression, which are perpendicular to layer surfaces, whereas three sets fracture were formed in the thicker layers (>95cm), one of three sets fractures which are nearly parallel to the layer surface are extension fractures, and two other sets fractures are shear conjugate fractures which are also perpendicular to layer surfaces.I found that the shear conjugate fractures density is proportional to the layer thickness in early stage of folds; the early extension fractures are formed during the early compression. The maxium compression stress is parallel to layer surface, so as the products of the maxium compression stress, the extension fractures are parallel to layer surface, after the formation of extension fractures, the thicker layers were cut into thin ones. The early fracture will limit the development of later fracturs. The later fractures which were oblied to layer surfaces may result from the interlayer slip.2. After the folds were formed, the fractures density is not proportional to the layer thickness, it shows the polynomial function in the hinge of folds; and statistics shows that there is positive correlation between fracture fractal dimension and layer thickness in the limbs, frature fractal dimension increases with increasing fracture density.3. In the simple monocline, I adopted statistics regression to predict the relationships between the changes tend of fractures density and dips, layer thickness. The results show that dips and layer thickness influenced the fractures density and dips played a more important role than layer thickness.In the complex monocline, two main special type fractures which are secondary structures to the fold are formed. Fractures data from outcrops in the upper Jurassic strata of Lin’an are combined with Geometry modeling to understand the relationship between the formation of fractures and the deformation of strata. Two main type fractures are secondary structures to the fold. These fractures are referred to as fold accommodation fractures, and named as D and D’fractures respectively, directing the motion of local layer is D’fracture, and directing the motion of adjacent layer is D fracture. They form primarily because of an increase in bed curvature within fold cores. According to the geometrical analysis of the fold, the relationship between fracture density and layer thickness was established, and I found that fractures density is proportional to the layer thickness.Based on analysis of the distribution of strain, I found the limb of fold are mainly suffered force of shears and the hinge of fold are mainly suffered force of tension. In the hinge, besides the early conjugate fracture, some extension fractures are formed due to tension force. Both of them are orthogonal to beding.4. The rock characteristics affect the fractures development in the stacking pattern of sandstone and mudstone. In the sandstone, the fractures which are extension fractures are mainly perpendicular to the layer surfaces, while in the mudstone, the mineral particles are directional alignment due to the compressional force, and the cleavages which are oblied to the layer surfaces are well developed. The cleavages may be the products of simple shears.5. The evolution of stratum deformation can be classified structurally as early stage, middle stage and late stage. The purpose of this thesis is to research the different mechanism and control factors for different stages of fold.(1) In the early stage:the deformation degree of fold is very little, the stratum is about horizontal, the early fracture is formed during the compression, the angle bisector of the conjugate fractures is parallel to the maxium the compression forces, and the early extension fractures are parallel to the maxium the compression forces. The later shear fractures are limited by the early fractures.(2) In the middle stage, the stratums of fold have a clear curved deformation. The fractures have differential development in differen positions, in the limb, some frature that related to interlayer-silp are developed, and in the hinge of fold, extension fractures are developed. The strains are distributed in belts which are parallel to anticline hinge.(3) In the late stage, the stratums of fold have distorted and well-developed and faults widely developed in the outcrops. The cleavages will be developed in the mudstones. All thickness layers develop a lot of fractures and the fractures of this stage are high-angle obligque to layer surface.