褶皱相关断裂构造形成条件与发育机制：燕山中部露头尺度变形研究为例

Deformation mechanisms for the development of fold-accommodation faults: Examples of outcrop-scale structures from central intraplate Yanshan Orogenic Belt,North China

总结了褶皱相关断裂发育机制的3个构造几何学模型:同心圆褶皱模型、膝折带褶皱模型和弯流褶皱模型。基于燕山中部中、新元古界地层中发育的5个露头尺度褶皱及其中、小型断裂构造的实例剖析,探讨了收缩变形过程中褶皱与断裂构造发育时序与褶皱相关断裂构造的产生机制。研究指出,规模与所在褶皱构造相当或略小的断裂构造当中,既有形成时间早于褶皱变形的断层,也有在褶皱变形过程中调节褶皱不同部位应变差异的褶皱相关断裂构造,而且卷入后期变形的早期断裂可能成为制约褶皱成核位置的影响因素,以及成为枢纽叠覆楔构造的形成方式之一。断层位移-距离曲线特征和断层与褶皱变形几何学、运动学关系分析,可用来判断断层、褶皱变形发生相对时序。认为影响褶皱相关断裂构造发育的机制主要有3种:(1)纵弯滑褶皱作用中,翼部顺层滑动受到限制而无法持续时,将通过断层向上切层的方式予以调节,从而形成翼部或转折端揳入逆冲断裂以及背离向斜和指向背斜逆冲断层;(2)各种因素导致的褶皱曲率变化是褶皱相关断裂产生的重要机制之一,褶皱曲率变化可由褶皱轴面的合并和新生直观反映,轴面合并引起褶皱曲率变化的层位,可能是诱发褶皱相关断裂,如背离向斜和指向背斜逆冲构造开始产生的重要部位;(3)能干性差异和强硬层之间距离较大的岩层组合发生纵弯褶皱变形时,软弱岩系在褶皱核部的聚集和逃逸,是迫使递进收缩的强硬层产生褶皱相关断裂构造的重要机制。

Three geometric models concerning the development mechanisms of fold-accommodation faults,namely concentric folding,kink-band folding,buckle flexural-slip folding,are summarized.Five well-exposed metre-scale folds with minor faults are analyzed in detail.They developed within Meso-and Neoproterozoic sequence in central intraplate Yanshan Orogenic Belt,northern North China Craton.These observations provide us opportunities to a better understanding of factors controlling the development of fold-accommodation faults and time sequence between scale compatible folds and faults formed within contraction regime.The distance-displacement diagrams are employed to define the sequential development of faults relative to their host-folds together with the geometric and kinematic relations between them.It is found that faults spatially close-related folds in contraction deformation could form previous to,synchronous with or after the folding deformation.Only those developed synchronous with folding in response to the strain contrast related to structural position and stratigraphic mechanics could be treated as fold-accommodation faults.Some preexisting faults may serve as nucleation sites during followed folding deformation and finally lead to the formation of hinge wedges.Development of fold-accommodation faults may be effectively influenced by buckling flexural-slip folding,tightness change of folds and escape of incompetent units from cores of folds.（1）Accumulated layer-parallel shearing in limbs associated with buckling flexural-slip folding may give way to faults of cut-up-section when layer-parallel movement was blocked.Traceable faults formed in limbs or/and hinge zones,such as wedge thrusts,out-of-syncline and into-anticline thrusts,could form as a result of this mechanism.（2）Curvature change of folds derived from various factors served as another important factor in the development of fold-accommodation faults.Stratigraphic levels where convergence of axial planes occurred are expected to be potential position for the initiation of fold-accommodation faults.（3）During progressive contraction deformation of inter-bedded competent and incompetent units with higher competency contrast and larger distance between competent units,escape of incompetent units from the core of fold following the previously intensive concentration would result in the formation of fold-accommodation faults.