Sequences and Ages of Fluvial Terracesalong the Northern Piedmont of the Tianshan Mountains
天山北麓河流阶地序列及形成年代

The northern piedmont of the Tianshan Mountains, which is characterized by three parallel ridges of anticlines, known as Fold-and-Thrust zones I-III from the mountains towards the foreland basin, and north-flowing rivers, is a typical region containing many scientific problems attracting a lot of geologists and geographers. Under the special geological setting, the terraces developing along these rivers contain a lot of information on climatic changes and tectonic movement. However, previous studies on fluvial terraces in the northern piedmont of the Tianshan Mountains are still divergent. This work tries to solve the problem and well understand the sequences and ages of fluvial terraces along the northern piedmont of the Tianshan Mountains. The seven fluvial terraces named as T1 to T7 developed along major river channels in the northern piedmont of the Tianshan Mountains, and some main and consecutive terraces correspond to some fans. The fans corresponding to T7, T6, T5 and T2 are respectively named as F1, F2, F3 and F4. Because the center of the upheaval migrated from south to north in the process, some fans developed one by one. But the limit of the anticlines leads to the anomalistic shape of some fans between two anticlines. According to the ESR age, OSL age, and loess-paleosol series, the highest terrace (T7), corresponding to the oldest alluvial fan (F1) which is covered by thick loess and strongly dissected, is dated as 0.54 Ma BP. The higher terrace (T6) corresponds to F2, and the surface of T6 and F2 is also covered by thick loess and dissected by gullies, some of which were abandoned. The age of T6 is estimated as 0.3-0.2 Ma on the basis of the ESR age and loess-paleosol series.