Activity of the Mianzishan-Qingshuiying fault in the southeast margin of the Yinchuan basin

Studying the tectonic activity characteristics of the Yinchuan basin is of great significance for understanding the interaction among the three major blocks of Qinghai Tibet, Ordos and Alxa. The terrain elevation difference in the Mianzishan-Zhutouling area on the southeastern margin of the Yinchuan basin is significant, and the linear trace on satellite image is obvious, indicating the development of an unknown active fault. This article conducted field geological surveys, trench excavation and stratigraphic dating along the Mianzishan-Qingshuiying line. The results showed that the linear trace along the Mianzishan-Qingshuiying line had been dominated by fault, which was developed on the southeastern margin of the Yinchuan basin with a total length of 50 km. It is divided into two segments, the Mianzishan fault and the Qingshuiying fault. The Mianzishan fault is mainly characterized by high angle normal faults, the scratches on the fault surface show a sinistral strike slip motion, heading northeast, several gullies across the fault layer display left-handed twisting characteristics. The Qingshuiying fault, on the other hand, exhibits a reverse fault trending NEE, developing steep slope landforms. The fault has displaced sedimentary strata deposited at (23.65 ± 1.17) ka BP and (33.16 ± 1.82) ka BP, indicating its latest activity was postdated by Late Pleistocene. The discovery of this fault indicates that the boundary of the northwestern Ordos block is a complex zone associated with a wide structural deformation zone. This zone is not strictly controlled by the boundary faults of Yinchuan basin, but expanded to interior of the Ordos block, which may be related to the continuous northeast pushing and expansion of the Qinghai Tibet Plateau.