Studies On Tectonic Deformation Of The Yellow River Terrace In Lanzhou Based On High-resolution GPS Surveying

Lanzhou Active Faults Surveying and Earthquake Risk Assessment Project is one of most important projects on city active faults exploration, which carried out by Chinese Seismology Bureau (CSB) in the 10th five-year plan period of China. There developed typical and multilevel terraces because of Lanzhou basin's geographical environment. Terrace was produced by Cenozoic tectonics movement and climate changes, which recorded the detailed information of climate changes and terrain uplifting. Therefore, it deserves to do further research on the tectonic deformation of Yellow River terraces. Meanwhile, by high-resolution surveying and analysis on the characteristics of different-level terrace, we can get the geomorphological evidence for the tectonic activities.Firstly, we carried out the high-resolution GPS surveying for the Yellow River terraces of Lanzhou basin. The area covers from east to 50km and from south to 4-15km or so. Then we analyzed the characteristics of their distribution and development. For instance, we can get the geomorphological map and vertical section based on DEM (1:10,000) and the elevation data got from high-resolution GPS surveying individually. By on-site measurement and laboratory analysis, we studied the characteristics of the tectonic deformation in Lanzhou basin, such as deformation belts and magnitude.Along the Yellow River in Lanzhou basin, there are 7-level terraces. But in some parts of area we can also see 9-level terraces. Anyway, the distribution of terraces is much different. The study result shows the tectonic deformation mainly distributes in Baitashan and Moujiatai area, which is corresponding to Gaolanshan-Jiuzhoutai and Chaijiatai promontory individually that previous research mentioned. This region has a deformation belt in the range of 9–12m, which certifies that folding deformation is the most important tectonic activities in Lanzhou basin. And the higher the terrace is, the larger the deformation magnitude is. As for nouthern riverside, the deformation magnitude of 2nd-level terrace is 20m, 3rd-leve1 is 30m, 3rd-level2 is 45m, 4th–level is 120m. And as for sorthern riverside, 2nd-level terrace is 10-15m, 3rd-level1 is 18m, 3rd-level2 is 35m, 4th–level is 70m. As far as the same level terrace is concerned, we find that the elevation of southern side of Yellow River in Lanzhou basin mostly is higher that of northern riverside, which reflects that the elevation increase higher in the south of Lanzhou basin. This phenomenon explains that the deformation force of Lanzhou basin came from the southern force, which may be the result from the activities of Maxianshan fault.In conclusion, our study indicates that folding effect plays major role in tectonic deformation of Lanzhou.while late-Quaternary tectonic activities plays minor role.