Mechanism of Formation of Special Fold Accommodation Fractures
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摘要: 浙江省临安市上侏罗统劳村组火山凝灰岩露头中发育了很多与挠曲相关的裂缝, 地层构造简单、露头良好, 为研究与挠曲相关裂缝提供了良好的条件.通过对两处野外露头的精细测量, 发现裂缝面平直、且未被充填, 均为构造裂缝, 在剖面上裂缝的组合呈扇形.裂缝面与岩层层面夹角为44°~80°, 裂缝密度随着地层厚度的增加而增加.根据裂缝面与层面的交切关系, 将两组裂缝分别命名为: 锐夹角指示邻层相对运动方向的D裂缝和锐夹角指示本层相对运动方向的D’裂缝.通过对地层挠曲变形的几何分析, 发现两组裂缝的形成是为了调节核部地层在变形过程中产生缩短量和剩余剪切, 地层厚度与地层中产生剩余剪切成正比.结果表明, 地层厚度越大, 形成的裂缝也越多.Abstract: To explore the relationship between the formation of fractures and the deformation of strata, fractures data from outcrops in the Upper Jurassic strata of Lin'an are integrated with geometry modeling in this study. It is found that two main type fractures are secondary structures to the fold, which are referred to as fold accommodation fractures in this paper, and named as D and D' fractures respectively. An angle ranging 44°-80° exists between fractures and stratum layers, with D' fracture directing the motion of local layer and D fracture directing the motion of adjacent layer. The fractures formed primarily because of an increase in bed curvature within fold cores. The relationship between fracture density and layer thickness is established on the basis of geometrical analysis of the fold and we find that fractures density is proportional to the layer thickness.
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Key words:
- fold accommodation fractures /
- outcrop /
- geometry modeling /
- mechanism /
- structural geology
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图 1 研究区位置(据朱光等，1999)
Ⅰ.华北板块；Ⅱ.扬子板块；Ⅱ1-1.大别山造山带；Ⅱ1.胶南造山带，Ⅱ2.滁全坳陷；Ⅱ3.沿江坳陷；Ⅱ4.皖南-苏南坳陷；Ⅱ5.江南隆起带；Ⅱ6.钱塘台褶带；Ⅲ.华南板块；JSF.嘉山-响水断裂；CHF.滁河断裂；JNF.江南断裂；TBS.天目山-白际山剪切带，1.断裂；2.缝合线；3.边界断层；4.研究区
Fig. 1. The geological map of study area
图 6 地层厚度与裂缝密度的关系(X轴代表地层厚度、Y轴代表裂缝密度)
Fig. 6. Map shows that the thickness to fracture density ratio
图 7 D和D'裂缝的形成机制
a.下部地层轴面为AC、BC，上部地层轴面为CD，导致上部地层曲率增加；b.上下地层夹角的变化是剩余剪切(ΔS=ΔS1+ΔS2)形成的原因；c，d剩余剪切致使D和D'裂缝的形成
Fig. 7. Mechanism of formation of D and D'fracture
图 9 左旋剪切作用下的裂缝的类型(Bartlett et al., 1981; Keller et al., 1997; Ahlgren, 2001)
SZ代表相对滑动面，R和R'代表里德尔型裂缝，T代表张裂缝
Fig. 9. A characteristic array of shear fractures of sinistral shear
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