DISCUSSION ON DEVELOPMENT MODELS OF THE SHEARING FRACTURES IN FAULT BEND FOLDS
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摘要:
围绕库车前陆盆地内某典型断层转折褶皱剖面，实测统计该剖面不同构造部位的剪切裂缝，分析断层转折褶皱中剪切裂缝的发育规律，进而建立断层转折褶皱剪切裂缝的发育模式。统计结果表明，研究区剪切裂缝有2种类型，一类是与岩层面高角度相交的高角度缝，广泛发育在构造各个部位；另一类是与层面近于平行的顺层裂缝，主要发育在断层面附近，并随岩层的变形，产状发生改变。远离断层面，剪切裂缝发育程度整体上呈减小趋势，下盘裂缝发育强度的减小大于上盘，上盘裂缝发育强度整体大于下盘。褶皱内层剪切裂缝较外层发育，褶皱两翼裂缝发育强度大于褶皱枢纽。断层产状的改变，会引起上盘运动状态的改变，使得在上盘活动轴面附近出现应力集中，局部剪切裂缝发育强度出现增大现象。断层转折褶皱剪切裂缝的发育模式可以分为3个阶段，断层形成前主要发育与层面高角度相交的区域性剪切裂缝；断层形成初期，断层面附近两盘牵引变形不明显，上盘穿过断坡下部破折点处活动轴面的部分会产生新的剪切裂缝，即与岩层高角度相交剪切缝和与断层面低角度相交剪切缝；牵引构造明显时，断层面附近两盘弯曲部位会产生顺层剪切裂缝，两翼的裂缝发育强度大于枢纽。
Abstract:This paper focuses on a typical section of fault-bend fold in Kuqa foreland basin, by use of the shear fractures data measured in different structural position of the outcrop, and the fractures development laws are analyzed and then development models of the shearing fractures in fault-bend fold in Kuqa foreland basin are established. Statistical result shows that the shear fractures in the outcrop can be divided into two types: shear fractures at a high angle to layers extensively develops in each position in the outcrop, and bed-parallel fractures presenting near the fault, mainly develops near the fault with attitudes changing with the deformation of the layers. The intensity of shear fractures development decreases with the distance away from the fault as a whole. The gradient of shear fractures development in the footwall has a greater descending rate than that in the hanging wall. The density of shear fractures in the hanging wall is greater than that in the footwall. The density of shear fractures in the hinge of the fold is less than that in the forelimb and backlimb, besides higher density appears in the internal surface with respect to that in the external surface. The change of the dip angle in the fault bendfold causing the alteration of the movement for the hanging wall makes the stresses concentrating on the axial faces. So the phenomenon of the increase of intensity of shear fractures near this position can be appeared. The development model of shear fractures in the fault-bend fold shall be separated into 3 stages: regional fractures are mainly developed before the generation of fault; after the formation of the fault with dragging deformations near not obvious, new shear fractures will be formed in the position where the axial surface pinned at the lower ramp inflection point passes through, one at a high angle to the layer and the other at a low angle to the fault; the bed-parallel fractures will be presented in the curved sections and the intensity of fractures development near fold hinge is higher than that in two limbs, when the dragging deformations are obvious. The study on the model of shearing fractures development has a guiding significance for exploring the law of fractures development and prospecting the fractured reservoir in the foreland basin, especially in the deep layers with complex tectonic style.
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Key words:
- fault-bend fold /
- shear fractures /
- distribution law /
- development model
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表 1 裂缝发育强度与距断层距离关系统计
Table 1. Statistics of relationship between fracture development intensity and distance to the fault
点号 密度(条/m) 位置 1 33.3 下盘紧靠断层面 2 11.1 下盘距层面1.5 m 3 5.0 下盘距层面3.5 m 4 3.7 下盘距断面6.2 m 5 23.0 上盘距断面1.0 m 6 33.3 上盘距断面1.7 m 7 19.7 上盘距断面3.5 m 8 14.1 上盘距断面4.2 m 9 15.6 上盘距断面6.0 m 10 8.2 上盘距断面9.6 m 11 6.3 上盘距断层面12.0 m 
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表 2 卡普沙良河剖面上断层相关褶皱不同部位的裂缝密度
Table 2. Fracture densities in different positions of fault-related fold in Kapushaliang River
构造部位 前翼(a处) 枢纽(b处) 后翼(c处) 裂缝密度/(条·m-1) 33.3 25 58.5 25 20 33.3 20 12.72 25 9.1 10 16.7 注:每列数据自上而下反映的是从褶皱核部至外部的裂缝发育情况
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