Three-dimensional Velocity Structure Model Of The Middle And Lower Yangtze River Metallogenic Belt And The Study Of The Deep Dynamic Process In The Region

The Middle and Lower Yangtze River region is one of the most importantmetallogenic belt in East China. In this thesis, we have applied the teleseismictomography method to determine a3-D P-wave velocity structure model down to500km depth beneath the Middle and Lower Yangtze River region by using15,826highquality P-wave arrival times collected from678teleseismic events recorded by100seismic stations located in this area. According to this model, we have discussed thedeep dynamic process happened in this belt.During our process to create the velocity structure model, we first proposed a newirregular-grid method according to the distribution of rays. In the region where raysdistribute better, grids are dense; however, in the region where rays distribute worse,grids are sparse. Compared with the former regular-grid model, this new irregular-gridcan ensure both reliability and resolution in the region where rays distribute better.This model has plenty of advantages.We second did crust-correction study in the Middle and Lower Yangtze River region.Although teleseismic tomography has a good ability to distinguish deep velocitystructure, suitable for the study of the upwelling of hot asthenosphere, the shape ofplume, the subduction of slab and the delamination of lithosphere, it cannotdistinguish the swallow velocity well, especially the crust. However, the velocity incrust has some influence on the quality of final tomography. In order to eliminate orreduce the influence of crust velocity structure, we have conducted crust-correction inthe Middle and Lower Yangtze River region. By reprocess the travel-time data, wereduced the travel-time of seismic rays in crust, and the new data helped to improvethe quality and reliability of final tomography.From the velocity structure model obtained from our study, we have found that thevelocity structure beneath the Middle and Lower Yangtze River region is extremelycomplex. But from the profiles, we have found that beneath the Middle and Lower Yangtze River Metallogenic belt, there occurs an obviously low velocity anomaly in100~200km depth and an obviously high velocity anomaly in300~400km depth.The trend of both the low and the high velocity anomaly are paralleled the trend of theMiddle and Lower Yangtze River Metallogenic belt and the depth of the high velocityanomaly are gradually shallower from the south to the north. Based on presentmaterial, we interpreted the low velocity anomaly is the upwelling of hotasthenosphere and the high velocity anomaly is the delamination of lithosphere. At thesame time, we have discussed the deep dynamic process happened in the Middle andLower Yangtze River Metallogenic belt combined with previous studies, and we tendto support the delamination of lithosphere evolution model, which means theformation of the Middle and Lower Yangtze River Metallogenic belt was caused bythe process of collision and extrusion of slabs, delamination of lithosphere andunderplate of hot asthenosphere after Permian.