纵弯横跨叠加褶皱中被强烈改造的早期褶皱之识别:以内蒙古白云鄂博群为例

Identification of Early Folds in First Mode of Superposed Buckling:A Case Study of the Bayan Obo Group,Inner Mongolia

在纵弯横跨叠加褶皱变形中,早期相对宽缓褶皱被晚期紧闭褶皱强烈改造后的正确识别,对于重建叠加褶皱变形地区的构造演化过程具有重要意义。本文基于前人模拟研究所建立的定性模型,利用可视地质Visible Geology网站在线软件进行正演与反演分析,提出了纵弯横跨叠加褶皱模型(Mode 1)中被强烈改造的早期褶皱的识别方法,即根据晚期紧闭褶皱转折端产状的空间变化规律以及相应枢纽产状的数据统计,确定早期褶皱的形态和位态。基于大比例尺详细构造填图,在内蒙古中部白云鄂博群中运用该方法首次识别出了先于E-W至NEE向主期褶皱而形成的NNW向褶皱构造。该早期区域性褶皱的识别,合理地解释了白云鄂博群主期褶皱的形态和位态特征,为重新厘定该套地层的构造变形序列,进而全面认识和评价白云鄂博矿区的成矿和赋矿构造特征,提供了全新视角和重要证据。同时,为极强烈后期变形可能产生的纵弯横跨叠加褶皱干涉样式,提供了一个良好的天然变形实例。

In the First Mode of superposed buckling(Ghosh et al.,1992),identification of the early folds which were strongly modified by the late large-scale tight folds is critical for a better reconstructing of the tectonic evolution in regions with complicated superposed folding.With implement of the online software provided by the“Visual Geology”(http://app.visiblegeology.com/profile.html),a quantitative method for identifying the early folds in this circumstance is proposed and tested through forward and backward analyses of superposed folding.Using this method,the spatial distribution of the late fold hinge zones and the stereonet plot of the hinge lines are exploited to determine the shapes and orientations of the early folds.A case study of the Bayan Obo Group in Inner Mongolia,North China Craton,is conducted based on detailed mapping and voluminous measurements of structural elements.An early generation of NNW-trending regional scale folds predated the dominated E-W to NEE trending folds was recognized.The identification of these regional scale early folds not only provides a much better interpretation for the shapes and orientations of the dominated folds in the Bayan Obo Group,but also establishes a renewed deformation sequence in this area.In addition,current results might also shed lights on the evaluation of the metallogenic and ore-bearing structures.This case study on the Bayan Obo Group provides an excellent natural example for the First Mode of superposed buckling with extreme contraction of late folding.