库车坳陷东西段盐下构造变形差异演化数值模拟分析

Numerical simulation of different subsalt structural features and their evolution in the eastern and western segments of the Kuqa depression

库车坳陷东、西段盐下构造在北部构造带以南出现明显的差异构造变形特征,在西段的克拉苏构造带盐下发育由一系列逆冲断裂组成的叠瓦构造,而在东段的东秋构造带盐下发育一个大型的断背斜构造。本文基于地震资料构造解析,采用离散元数值模拟方法,设计了5组数值模拟实验来探究先存断层、同构造沉积和滑脱层强度对褶皱冲断带构造形态及演化的影响,进而分析库车坳陷东、西段构造特征差异的成因机制。模拟结果表明含盐褶皱冲断带往往发育垂向分层构造变形。先存断裂会影响褶皱冲断带构造传播的方式,导致盐下楔体形态、断裂数量和盐上构造样式的差异。同构造沉积会增强盐上和盐下的分层差异变形,滑脱层强度增强会减弱盐上和盐下的分层变形。通过将模拟结果与库车坳陷东、西段构造特征进行对比分析,我们认为先存断层可能是影响库车坳陷东、西段盐下构造差异演化的重要因素。库车坳陷东段的东秋构造带受先存断裂影响,在先存断裂的位置优先变形,形成东秋断裂和上覆东秋背斜,同时先存断裂既有助于应力释放,又阻拦应力向前传递,形成乱序式变形传播。库车坳陷西段的克拉苏构造带,没有先存断裂的影响,盐下主要发育前展式构造变形,形成由一系列逆冲断层组成的叠瓦构造楔体。

The subsalt structural geometry in front of the north structural belt is quite different in eastern and western segments of the Kuqa depression.The subsalt structure is composed of an imbricate thrusting belt in the Kelasu structure of the western segment,while it is a faulted anticline in the Dongqiu structure of the eastern segment.In this paper,based on seismic interpretation,five discrete element models are designed to explore the influence of pre-existing faults,syntectonic sedimentation and properties of salt layer on the structural geometry and evolution of fold-and-thrust belts,as well as the causes behind the different structural features of the eastern and western segments in the Kuqa depression.The simulation results show that the deformations of units above and below the salt layer are decoupled.A pre-existing fault can affect the deformation propagation,causing the differences in subsalt wedge shape,fault numbers and suprasalt structural geometry.Syntectonic sedimentation could enhance the deformation decorrelation above and below the salt layer,while a stronger salt property decreases it.Comparing the simulation results with the structural features of the eastern and western segments in the Kuqa depression,we suggest that a pre-existing fault could be the controlling factor of the structural variations between eastern and western segments・In the eastern Kuqa depression,the location of pre-existing fault first deforms,forming the Dongqiu fault and the Dongqiu anticline above it.Besides,the pre-existing fault absorbs most of the shortening,and prevents the deformation from propagating forward,thus causing an out of sequence deformation propagation.In the western Kuqa depression,with no interruption from pre-existing fault,the deformation of subsalt units sequentially propagates forward and forms an imbricate thrusting belt.