Abstract:

The N-S stretched thrus nappe belt in west Ordos Basin is 600 km long and 50-100km wide with an area of 4500km².It is composed of N-S trending struc-tural belt of the North China Platform in the east and the Liupanshan arc structural belt of Qinling-Qilianshan geocline in the southwest. The progres-sional solution of Laplace equation is used in this paper to make analytic con-tinuation on gravity data of the present area. The nature and distribution of the slip surface of the nappe belt and the property of its deeper structures are explained as follows:Zhuozishan Section:There are two slide surfaces in this section. One is the boundary surface between E-W trending high density-strong magnetic body and NNW trend high density-weak magnetic body (the Qianlishan Group),the other is the soft layer between Carboniferous-Permian coal measure and Cambrian-Ordovician limestones. Yinchuan Section, The gravity low of Yinchuan is the reflection of Yinchuan graben. The gravity high of Niushou and Helan Mountains are the reflection of shallow structures:the gravity high of Hengshanbao and Majiatan reflects structural skeleton below T₉ and autochthonous rock bodies below the nappes respectively. The South Section:The gravity positive anomaly to the west of the Qing-tongxia-Guyuan rift (that is the arc structural belt of the Liupan Mountains)shows the structural property of allochthonous rock bodies. The area to the east of the rift shows gravity high and unevident feature of overthrust. It may be dominated by thrust and horizontal displacement. The fault surfaces of N-S thrust belt of the present area dip west, the tilt angles of the shallow parts are steep and the deeper parts are gentle;the north section is gentle and the south section is steep with local parts inversed. The strike of deep-seated geological bodies streches nearly EW in the north area and nearly SN in the south area. There is most likely a huge fault along Zhongning-Zhongwei line between these tow sections.