2) The Paleogene extensional fault blocks in North China Basin are active, with many faults, large subsidence range and high sedimentation rate. The fault activity is deep, and multi-stage rock basalt erupts. It reflects the characteristics of Paleogene asthenosphere uplift, lithospheric thinning and fault subsidence. At the end of Paleogene, after regional compression, the whole area rose to land with slight folds, forming regional unconformity between Paleogene and Neogene. Neogene and Quaternary entered a new period of structural development, characterized by large-scale deposition in the depression, and weak fault and fold activities. The Neogene and Quaternary in the North China Plain are widely superimposed on the Paleogene group and the old strata in a state close to the horizontal layer, forming a unified North China basin, with the main subsidence center in Bozhong Depression. The settlement analysis shows that there are two obvious settlement periods in Paleogene and Neogene. The above phenomenon shows that the Neogene in North China is not simply in the thermal subsidence stage after the Paleogene asthenosphere uplift and lithosphere thinning. In the early Neogene, it seems that the extension and thinning of the lithosphere occurred, which are two stages of episodic development of the basin.
3) West of Taihang Mountain, Neogene and Quaternary are different from North China Basin, and rifting activities are still dominant, such as a series of ne-trending rifted basins formed in Jinzhong in Neogene and Quaternary. Taiyuan basin is a Neogene basin with a thickness of 4600 meters. The thickness of N+Q in He Lin fault depression is more than 3000 m, that in Yinchuan fault depression is 3600 m, that in Weihe fault depression is 4,000 m, and that in Quaternary system is 1400 m ... These phenomena show that the Neogene fault depression activity in the western part of North China platform is still very common. Its origin should be related to the long-distance compression of the Indian plate on the Eurasian plate, which led to the northeast compression of Liupanshan area in this area. This shows that Ordos and its surrounding basins west of Taihang Mountain are in different geodynamic backgrounds from North China Basin. Therefore, the eastern and western parts of North China should be divided into two tectonic units with Taihang Mountain as the boundary.
4) The development time of the fault basin around Ordos is from Neogene to Quaternary, which indicates that there may be asthenosphere uplift and lithosphere thinning around Ordos at this time, rather than lithosphere thermal subsidence stage. The nearly symmetrical development characteristics (Figure 10.27) of the Neogene-Quaternary fault basins around Ordos support that the development of the fault basins around Ordos may belong to a simple shear model, which is different from the pure shear model of North China Basin. This further shows that they are in different geodynamic backgrounds and should belong to two different tectonic units.
Fig. 10.27 schematic diagram of symmetrical development of fault basins at the north and south ends of Ordos basin.
(1) Nanjixian sag; (2) Sangzhen sag; (3) shallow depression in Balahai; (4) Wuyuan is deeply involved.