A Study On The Structures And Evolution Of The Juncture Area Of Shanxi, Gansu Province And Ningxia Autonomous Region Since Paleozoic Time

This study is based on the field work of structural geology and sedimentology and synthesizing the data of previous work. It is focused on the Paleozoic and Mesozoic deformation and sedimentology on the western edge of the Ordos Block and the eastern part of Corridor Transitional Zone. And selecting the central and southern parts of Helan Shan, Niushou Shan, Weiningbei Shan and the transitional area between the southern part and northern part of fold-and-thrust belt along the western edge of Ordos Block as objects investigated, the author has studied the geological setting and the sedimentary characteristics, deformation of Paleozoic and Mesozoic strata in these areas, put forward some new points different from those taken by early researchers for granted, and gotten main results as below.By analyzing the folds and cleavages, the author recognized three-stage cleavages at least. These cleavages are important symbols for identifying the different stages of deformation of the Miboshan formation in Niushou Shan. They are representatives of different stages of flexural-slip mechanisms producing the folds in the Niushou Shan. And these folds could be produced by the orogeny movement acting in the early Paleozoic. They were part of the foredge belt of the thrust in that time. Although the folds in the Niushou Shan belong to the fault related fold, the large faults are rare in the outcrops. Many faults are break-through faults which are the results when folds are locked and with the stress going on then the faults are produced cutting the limbs of the folds. Many of the folds in Niushou Shan belong to detachment folds.Based on the analysis of the different characteristics of deformation and geological setting, the author thinks that the northern part is different from the southern part of the western Ordos fold-and-thrust belt in narrow sense, and brings forward the mechanism resulting in thesegmentation of the this fold-and-thrust belt. The author thinks that the opinion taking the western Ordos fold-and-thrust belt as a whole with uniform mechanism by many researchers is not right. In fact these two parts have their own mechanics and processes. The northern part includes the northern and central parts of Helan Shan, Zhuozi Shan segment and Hengshan Pu segment, which was produced by the compression between the Alashan Block and the Ordos Block. The southern part includes the southern part of Helan Shan, Majiatan segment and Shajingzi segment, which was produced by the compression from southwest. It is illustrated that these two parts of the western Mesozoic Ordos fold-and-thrust belt in narrow sense were active and produced in different time. The duration of the northern one was short and that of the southern part was much longer.On the basis of the distribution and thickness of the strata along the western Ordos, a large lateral or oblique ramp is identified between the northern and southern parts of the western Ordos fold-and-thrust belt. The northern part of the western Ordos was a sub-basin that was much deeper than that of southern part during the Carboniferous and Permian. Because of different thickness of strata and lateral (oblique) ramp, even when these two parts were active simultaneously (late Jurassic period), the processes of deformation varied from each other, which led to the segmentation of the western Ordos fold-and-thrust belt, and the salient and recess of the belt shown in the plane also illustrate the segmentation. And when these two parts were active in different time, the activity to the south did not affect the deformation of the northern part basically. It is not clear whether this lateral (oblique) ramp was controlled by fault or not. But the data from the internal Ordos Block show that it was probably controlled by the fault crossing the Ordos Block from west to east.The fieldwork, seismic profiles and data of other researchers demonstrate that there were at least three thrusting activities at different times in these areas. (1) Early Paleozoic period (late Ordovician-middle Devonian)...