Superimposed Fold Deformation In The Zha'ertaishan Group Of Northwest Guyang,Inner Mongolia And Its Implications

Guyang,Inner Mongolia is located in the western part of yinshan-yanshan structural zone in the northern margin of the north China craton.Zha'ertaishan group is an important stratigraphic unit in Zhaertai-Baiyunebo rift system and is considered to be the main rift sedimentary product during the evolution of the middle-late proterozoic passive continental margin in the northern margin of north China.The Baiyunebo group in the northern part of the region was covered by the early Paleozoic strata in parallel unconformity,so the preserved deformation structure may record and reflect the tectonic evolution dynamics of the northern margin of north China since the Paleozoic,and even the correlation with the of evolution the Paleo-Asian Ocean plate.Taking the folding deformation of the Zha'ertaishan group as the research object,on the basis of predecessors'regional geological survey and research work,through the route of detailed field observation and critical regional large scale lithologic-structural mapping,combined with the geological interpretation of satellite imagery,compiled a detailed lithologic-structural diagram,and then we studied and analyzed the structural features,deformation periods,superimposed transformation relations and deformation patterns of the superimposed folds in detail.The fold structural characteristics of the Zha'ertaishan group in the study area were studied in detail.There are at least two groups of different directions of fold structure,in which NW-trending fold constitute the main tectonic framework.There are two main scale similar anticline and syncline,in which fold is relatively closed,mainly for the vertical plunging fold.Different degree of thickening were at the hing zone of the folds,mainly being similar fold.And the axis plane cleavage of NW-trending fold were development strongly.The NE-trending folds are mainly two relatively open anticline and syncline,which are mainly upright horizontal folds,and the NE-trending axial plane cleavage related to the folds in this stage is relatively developed.According to the superposition transformation relationship,the formation of open folds of NE-trending was relatively early?D₁?,while that in NW-trending was relatively late?D₂?.They are approximately cross-superposition folds relationship.The stage D₁ folding were strongly deformed by stage D₂ folding and makes the hinge of stage D₁ folding take on dramatic fluctuation and relatively slight plane bending.However,the geometric patterns of interference of the two folds formed by superposition are still recognizable,including saddle and trap on the plane and basin on the local outcropping.According to the relationship of regional stratigraphic contact and the age of the intrusive rock involved in the fold deformation and the rock mass after deformation,the two phases fold occurred between the mid-late Permian and the mid-late Jurassic.From the mechanical properties of folding,the formation time,combined with the tectonic background,the formation of the first-stage NE-trending folds may be related to the southward subduction and closure of the Paleo-Asian Ocean from late Paleozoic to early Triassic,and the second NW-trending fold may be related to the scissor closure of the Mongolia-Okhotsk Ocean from the late Triassic to the middle Jurassic.