内蒙古东南部西拉木伦缝合带两侧二叠纪以来的叠加褶皱变形:对同碰撞和碰撞后变形的启示

摘要

内蒙古东南部西拉木伦断裂两侧二叠纪地层中发育有一系列叠加褶皱, 它们与侏罗纪地层内部褶皱及断裂变形记录了该区晚古生代以来的多期构造事件.研究这些变形对探索华北北部及邻区所经历的从古亚洲构造域到古太平洋构造域转换的动力学过程具有重要意义.二叠纪、侏罗纪地层变形的详细地质填图及叠加褶皱构造样式与区域演化序列的研究, 揭示出:二叠纪地层褶皱形迹具S型展布特征, 总体走向NEE, 轴面倾向NW;中生代地层褶皱走向NE, 轴面倾向SE, 伴生逆冲断层多向SE倾斜并且上盘向NW逆冲.研究厘定区内经历三期构造变形: (D1) 二叠纪末—中三叠世NNW-SSE向区域性挤压, 二叠纪地层形成NEE向褶皱; (D2) 晚三叠世区域性剪切作用将先期形成的NEE向褶皱改造成平面弧形褶皱, 表现为Simón (2004) 划分的Type2a与Type1d型叠加褶皱样式; (D3) 晚侏罗世NW-SE向挤压导致中侏罗世地层中倒向NW的褶皱构造, 并使得二叠纪地层褶皱更加紧闭.研究认为这三期变形可能分别代表: (1) 古亚洲洋闭合和伴生的碰撞造山作用; (2) 介于西拉木伦右行走滑断裂与蒙古东南部东戈壁左行走滑断裂之间块体的NEE向挤出构造; (3) 古太平洋板块向欧亚大陆之下的俯冲作用.%Multi-scale fold interference patterns developed in the Permian strata on both sides of the Xar Moron fault, southeastern Inner Mongolia.These superimposed folds and the deformation within the Jurassic strata recorded multiple tectonic events in the research area since the Permian.Research on these deformations is crucial for a better understanding of the tectonic evolution from the Paleo-Asian system to the Paleo-Pacific system along the northern margin of the North China Block.In this study, we employed detailed field mapping to decipher the geometry of these superimposed structures and their evolution.Folds distributed in the Permian strata are characterized by sinuous axial trace with southeast verging.Folds in the Mesozoic strata, however, are trending southeast with northwest verging.Three tectonic deformation phases were identified in this research: (D1) East-northeast trending folds in the Permian strata implying a north-northwest-south-southeast compression from the end of the Permian to the Middle Triassic; (D2) Regional shear events during the Late Triassic resulting in the formation of the Type2 aand Type1 dfold interference patterns proposed by simon, 2004;and (D3) Northwest-verging folds within the Middle Jurassic strata thought to be developed under a northwest-southeast compression during the Late Jurassic.Furthermore, D3 deformation led to fold tightening within the Permian strata.We interpret these three deformation phases (D1, D2, D3) as the consequences of the final closure of the Paleo-Asian Ocean and collisional orogeny, the east-northeast extrusion of blocks between the dextral strike-slip of the Xar Moron fault in northern China and the sinistral strike-slip East Gobi Fault in southeastern Mongolia, and the westward subduction of the Paleo-Pacific oceanic plate under the Eurasian plate, respectively.