北京房山地区太平山褶皱的变形特征和形成时代:华北克拉通早白垩世挤压构造的意义

Deformation Characteristics and Time of Taipingshan Folds in Fangshan Area, Beijing: Implications for Early Cretaceous Compressional Tectonics of North China Craton

北京西山地区广泛发育轴向近E-W向的褶皱,对理解华北克拉通东部构造演化至关重要,但目前对其形成时代和构造动力学背景却没有明确的认识.北京房山地区的太平山褶皱是轴向近E-W向褶皱的典型代表.通过精细的野外构造观察和构造年代学研究,确定太平山褶皱的空间展布特征和构造样式,厘定构造变形的时代,为该区乃至华北克拉通东部的构造演化提供重要依据.通过详细的野外构造观察和系统的β图解研究,表明太平山褶皱由直立倾伏背斜和斜歪倾伏向斜构成.通过对早期卷入褶皱变形的煌斑岩席和晚期切层侵入褶皱的闪长玢岩脉进行锆石U-Pb定年,得到煌斑岩席和闪长玢岩脉的形成时代分别为147.2±2.4Ma和129.0±3.2Ma,表明太平山褶皱的形成时代为燕山运动B幕.另外,北京西山地区还存在NW-SE向的伸展构造样式.代表本区NW-SE向伸展的房山穹窿(约136Ma)晚于N-S向挤压构造,侵入切割太平山褶皱,塑造了研究区现今主体构造格架,进一步限定太平山褶皱的形成时代为147～136 Ma.因此,研究区轴向近E-W向的褶皱为早白垩世近N-S向挤压构造的产物;N-S向挤压和NW-SE向伸展的构造样式,为华北克拉通中生代构造体制的转换和动力学背景提供关键依据.

The folds with near E-W-trending fold axis are widely developed in the Western Hills of Beijing, which are very important for understanding the tectonic evolution of the eastern part of North China craton (NCC). However, their deformation age and tectonic dynamic background are poorly understood. Taipingshan folds in the Fangshan area, Beijing, are typical folds with near E-W axial orientation. The detailed field structure observation and tectonic geochronology were carried out to determine the spatial distribution characteristics and tectonic style of Taipingshan folds and their deformation age. The detailed field structure observation and systematic β-diagrams study show that the Taipingshan folds consist of an upright capsized anticline and an inclined capsized syncline. A deformed lamprophyre sheet which is earlier than the folds structure gives a weighted zircon U-Pb age of 147.2±2.4 Ma, and the later porphyrite dyke that cuts strata of the folds gives a weighted zircon U-Pb age of 129.0±3.2 Ma, suggesting that the deformation time of the Taipingshan folds is between ca. 147 and 129 Ma, belonging to phase B of "Yanshanian movement". In addition, the extensional structure with NW-SE extensional direction is also developed in the Western Hills of Beijing. Fangshan dome (about 136 Ma), which is a typical product of the extensional tectonic, intruded into and cut the Taipingshan folds, further limiting the deformation age of the Taipingshan folds ranges from ca. 147 to 136 Ma. Thus, the folds with E-W-trending fold axis in the studied area are products of nearly N-S compressional tectonics in Early Cretaceous. The tectonic regime transformation from N-S compressional to NW-SE extensional tectonics provides a key understanding on tectonic dynamics of the eastern part of NCC in Late Mesozoic.