The Study Of Upper Mantle Anisotropy In Lower Yangtze Region And Its Adjacent Area

The Lower Yangzi region is located in the eastern part of the Yangtze Plate and geographically contains parts of eastern Jiangxi,Anhui,Jiangsu,Shanghai,and Zhejiang.It is one of China's important coastal economic zones and one of China's important ore clusters.The metallogenic belt in the middle and lower reaches of the Yangtze River is located in the study area.There have been many tectonic movements in the Lower Yangtze region and it is of great significance to study the deep part of the area and to deepen understanding of the orogenic movement and metallogenic mechanisms.We have found that four earthquakes with magnitude 7 or higher have found travel anisotropy in different directions,through the three-component continuous seismic waveform events recorded by 92 broad-banded temporary stations devolped by the Chinese Academy of Geological Sciences and China University of Geosciences(Beijing),30-135° teleseismic waveform events were selected to perform shear wave splitting studies.This thesis calculates the splitting parameters by calculating the direct S-wave and SKS phases.The direct S-wave uses the rotation correlation method and the minimum eigenvalue method to calculate the splitting parameters.The SKS seismic phase adopts the rotation correlation method and the minimum energy method.Anisotropic distribution.Two hundred and fifty-five effective S-wave splitting results and 80 effective splitting SKS results were calculated,and the results of direct splitting of S-wave and SKS average splits were calculated.The results of the study show that the splitting results of the direct S-wave and SKS seismic calculations in this paper are basically the same,which is basically consistent with the previous results in this region;the fast wave delay time in the study area is between 0.6 and 1.6 s,and the average delay is about 1.17.S,presumably originating from the upper mantle asthenosphere in the area,there are some differences between the anisotropy direction in the study area and the absolute motion direction(APM)of the plate in the area.According to the results of tomographic imaging,there is an obvious low-speed anomaly in the upper 100-200 km range of the upper mantle in the Yangtze region.Therefore,it is speculated that the subduction of the Pacific Plate in the Lower Mesozoic Lower Yangtze region caused the thermal mass of the asthenosphere to move from the west to the west.The east extrusion results in the orientation of the hot material crystals of the upper mantle asthenosphere during this period,resulting in an anisotropy of the lattice preferred direction(LPO)type.