Abstract:

Langshan Mountain is located in the western segment of Yinshan orogenic belt and the northwestern margin of the Hetao rift zone in Inner Mongolia. Piedmont faults and tectonic uplifts in this region developed widely since the Late Cenozoic. The research on the tectonic uplift process of Langshan Mountain since the Late Pleistocene has significance in having a good knowledge of the formation and evolution mechanism of Hetao rift zone and the influence of the uplift process to the paleogeographic framework of Hetao basin. The sedimentological, geomorphological and geochronological study of the piedmont terraces in Wenggeleqige and Ulaan Ovoo, indicates that T1 terrace formed at 47.4 kaB.P. is characterized by lacustrine deposits from Hetao megalake, and T2 terrace developed at 69 kaB.P.is characterized by fluvial deposits which may be the products of the Yellow River when it flows through the pediment of Langshan Mountain. The analysis on characteristics of terraces shows that the two terraces are mainly tectonic genesis. Therefore, the two terraces recorded the rapid uplift process in Langshan Mountain after 69 kaB.P. Between 69 kaB.P. and 47.4 kaB.P. the time-averaged uplift rates in Wenggeleqige and Ulaan Ovoo were 1.34 m/ka and 1.25 m/ka, respectively, and the uplift rates were 0.81 m/ka and 1.18 m/ka during the past 47.4 kaB.P.,respectively suggesting that the uplift rate have a decreasing trend in the southwest part of Langshan Mountain (Wenggeleqige region). Since the late Late Pleistocene (Qp^3-2), the rapid uplift of the Langshan Mountain led to east moving of the Yellow River continuously by the evidence of the ancient channel on Hetao plain. The research on the lake terraces in Langshan Mountain further confirmed the existence of the Hetao megalake in Late Pleistocene.

Key words: Langshan Mountain in Inner Mongolia, piedmont terrace, tectonic uplift, migration of the Yellow River, Hetao megalake, Qp³