The Characteristics Of River Terraces Of Heihe And Liyuan River In Hexi Corridor And The Response Of Its Neotectonic Movement

The study area of this paper is located in the Yumu Shan area in the northeast of Tibetan Plateau where the tectonic moves strongly.The evidence of the stage of neotectonic activity and the uplift process of Yumu Shan can be provided through studying the development of the terrace and the character of the deformation in this region.In this paper,we carried out thorough measurement and description of the profile of each terrace,altitude and crosssection of the valley of the Heihe and Liyuan River.A lot of work,for example,getting the OSL dating sample,measuring the microgeomorphology and processing the data has also been done.The conclusion can be drawn as follows:Through a series of indoor and field work on the river terraces of Heihe and Liyuan River,we get the following conclusions :In the study area,the terrace development highest series is 6 level.and Liyuan River is the highest 5 terraces.It shows that there are at least 6 different degrees of uplift in this area.River terraces are well developed with flat terraces and most of them are cultivated areas.The terraces T1~T5 of Heihe formed respectively at:5.54 ±0.61,12.7±0.9ka,19.69±1.32 ka.,47.0±3.4ka,and 54.7±4.6ka.T6 formed in the late pleistocene.According to the rank phase diagram,the tectonic uplift of the Heihe in the Shang longwang Temple wre strongest,T3 and T4 are respectively faulted 2.564 m and 5.913 m in the vertical direction.The third terrace of Heihe thrust rate is 1.0mm/a,the fourth terrace thrust rate is 1.25mm/a,from the late Pleistocene to late Pleistocene,thrust rate is 0.12mm/a T4,reflecting the tectonic activity of late Pleistocene in Heihe.According to the analysis,we divided into five new tectonic stages of thetectonic movement in this area : the new tectonic movement of Pliocene to early Pleistocene,Yumu Shan experienced a slow uplift in the late Oligocene,Miocene about about 3.58 Ma to the Yumen conglomerate deposit,northeastern margin of the Tibetan Plateau began a recent tectonic uplift.The rapid uplift of the northern Qilian Mountains and the formation of the Jiuquan foreland basin have gradually formed.In the Middle Pleistocene,the river begined to form terraces cuting,and T5 of Liyuan River and T6 of Heihe were formed.From earlier of late Pleistoceneto middle,especially in the early stage of late Pleistocene,Yumu Shan had a tectonic event in 0.1ka.The strong tectonic activity in this region,in the left bank of Heihe,we can see a steep cliffs where the gray green Ordovician tuff thrust to the late Pleistocene loess and gravel layer.Affected by faults,the Pliocene is nearly upright,and the dip angle of the lower Pleistocene series is larger than that of the Pliocene.This phenomenon reflects that in the late Pleistocene tectonic moved strongly in Heihe.During the middle Holocene to now,the plain rivers were incised and developed low terraces,mainly T1 and T2 terraces of Heihe and Liyuan river.