The Tibetan Plateau is the highest, largest, youngest plateau in the world. Its uplift in Cenozoic has important effects on global climatic change. However, different viewpoints existed on the studies of uplift of the Tibetan Plateau. M. Coleman et al. suggested that the Tibetan Plateau got maximum altitude 14 Ma ago. T. Harrison and P. Molnar et al. proposed that the plateau arrived the present altitude 8Ma ago. But Li Jijun et al. thought that intense uplift resulting in the Tibetan Plateau happened in the last 3.6 Ma. M. Searle indicated that the way to resolve difference of viewpoints is dating faults. River terraces in the Tibetan Plateau and its circumference?mainly resulted from tectonic uplift, recorded uplift history of the Tibetan Plateau. Therefore, we think that the best way is dating surfaces such as terraces.
Qilianshan Mountains is located in the margin of the Tibetan Plateau. There are many terraces along the rivers rising in Qilianshan Mountains. Here we discuss terrace development in two rivers. One is from the Shiyang River in East Qilianshan Mountains, another one from The Oil River from the middle Qilianshan Mountains.
The Shiyang River, one of inland rivers in Northwest China, developed a pediment and five terraces. Loess on terraces and pediment is beneficial to dating terraces and pediment. The pediment, 300 m above river, is distributed on the upper part of valley. About 100 m loess deposited on it. The study on magnetic stratigraphy of loess indicated that the pediment is formed 1.4 Ma ago. The fifth terrace, 100 m above river level, covered with 200 m thick loess. The magnetic stratigraphy and paleosol sequence of loess showed that the age of the fifth terrace is about 840 ka. The fourth terrace, 75m above river level, was overlaid with 80m thick loess. S4 at the bottom of the loess profile indicated that the fourth terrace was formed 400 ka ago. The third terrace is 40m above river level and 55m thick loess deposited on it. S2 and a 225±17 ka TL age at the bottom of the loess profile demonstrated that the third terrace was formed 220 ka ago. The second terrace, 30 m above river level, was covered with 30 m thick loess. S1 and a 78±7 ka TL age at the bottom of the loess profile indicated that the age of the second terrace is about 150 ka. The lowest terrace, 15 m above river level, was overlaid with 2 m thick loess. A 14C age of alluvial sediment is 12800±100 a BP, which showed that the first terrace was formed 10 ka ago.
The Oil River, a distributary of The Black River, developed seven terraces. The lower three terraces (T1, T2 and T3) are 25 m, 50 m and 70 m above river level respectively. The upper four terraces (T4, T5, T6 and T7) are 110 m, 180 m, 270 m and 310 m above river level respectively. According to the ESR date of terrace sediment and magnetic stratigraphy of the relative deposition of terraces, T4, T5, T6 and T7 were formed 0.15 Ma, 0.8 Ma, 1.1 Ma and 1.8 Ma ago.
Distribution, characteristics, age and formation environment of terraces showed that tectonic uplift is an important factor controlling formation of river terrace in Qilianshan Mountains. Therefore, development history of river terraces in Qilianshan Mountain recorded uplift processes of the Tibetan Plateau, and formation of each terrace represents an event of plateau uplift. Based on age of terraces mentioned above, it is deduced that there were at least uplift events in last 2 Ma in the Tibetan Plateau. They happened 0.01, 0.15, 0.22, 0.4, 0.8, 1.1 and 1.8 Ma ago respectively.