三角剪切断层传播褶皱应变分布

Strain distribution of tri-shear fault propagation folding

三角剪切断层传播褶皱是褶皱-冲断带重要的构造样式和圈闭类型,通过研究构造变形几何特征与应变分布规律可探讨其成因机制。采用修正的满足速度连续性和应变相容性的三角剪切带速度分布模型,使用柯西方程计算三角剪切带内瞬时应变速率,分析其应变差异分布;使用MATLAB软件对三角剪切断层传播褶皱进行正演模拟,获得不同参数影响下褶皱的几何形态与累积应变分布特点,根据应变分布特征探讨可能的断层传播模式。结果表明:三角剪切带在断层端点前方区域存在应变集中,断层端点邻近区域变形强烈,将导致断层端点向前传播,应变随远离断层端点迅速衰减,岩层以褶皱形式协调变形;影响三角剪切断层传播褶皱应变分布的参数为三角剪切角与P/S比值。三角剪切角决定了褶皱宽度,剪切角越小,褶皱愈紧闭,应变也愈强烈;P/S比值影响着卷入变形的地层范围与累计应变的时间。高P/S比值时,断层迅速传播,地层变形微弱;低P/S比值时,地层累计变形时间长,变形强烈,断层下盘向斜构造明显发育,出现地层增厚的现象。

The tri-shear fault propagation fold is an important type of structural pattern and trap of fold-thrust belts.Its genetic mechanism can be explored by studying the characteristics of structural deformation geometry and strain distribution patterns.Based on improved tri-shear velocity distribution model which meets the requirements of divergence-free velocity field and strain compatibility,the instantaneous strain rate within the tri-shear zone was worked out,and in turn its strain difference distribution was analyzed.The forward modeling of the tri-shear fault propagation folds was performed with MATLAB software to obtain the geometry and cumulative strain distribution of folds under the influence of various para-meters.Subsequently,the possible fault propagation modes were discussed according to the strain distribution.The results show that(1)the predicted strain concentrates directly in the region near the fault tip and serious deformation occurs in the adjacent areas of the fault tip,causing the forward propagation of the tip,so the strain attenuates sharply with increa-sing distance from the tip,and rocks fold accordingly;(2)The parameters affecting the strain distribution of tri-shear fault propagation folds are the apical angle of the tri-shear model and the ratio of P/S.The width of the fault propagation fold is directly controlled by the apical angle:with the decrease of the apical angle,the deformation zone becomes tighter and the strain grows stronger;while the P/S ratio affects the extent of strata involved in deformation and the cumulative strain time.When the ratio is high,the fault propagates fast and there is no significant internal deformation within the tri-shear zone,but when the ratio is low,beds have sufficient time to accumulate strain and deform before being cut by fault and result in pronounced synclines on the downthrown side of the fault and bed thickening.