Study On P-wave Anisotropy Beneath The Metallogenic Belt In The Middle And Lower Reaches Of Yangtze River By Teleseismic Tomography

In the middle and lower reaches of the Yangtze River, there are many important polymetallic metallogenic belts in China. 190?230 Ma years ago, the mineralization and mineralization of the metallogenic belt in the middle and lower reaches of the Yangtze River was strong, which forms an important endogenetic metallic mineral deposits.The scope of the study area is Longitude of 115 degrees ?122 degrees,latitude of 28 degrees ?36 degrees,mainly located in Anhui, Jiangsu, Jiangxi,Zhejiang etc., the structure is located in the middle and lower part of Yangtze block and the deep fault zone - the Tan Lu fault zone on the west side of the study area is considered as a channel of magma upwelling. Previous studies have shown that there are obvious low velocity anomalies in the upper mantle beneath the middle and lower Yangtze metallogenic belt, which seems to be related to Mesozoic lithospheric delamination. In order to further discuss the deep dynamic process of the region,teleseismic P wave anisotropy tomography method was used in this paper. 300 teleseismic event data magnitude greater than 5.5 were selected from 103 fixed stations and 72 mobile stations in China Seismic Network and 22800 P wave travel time relative residuals were calculated from the teleseismic waveform data by using the improved multi-channel cross-correlation technique (MMCC). We inversed upper mantle 3D velocity structure and anisotropy distribution of the study area with the data above; attempted to give direct evidence for the flow direction of asthenosphere in deep mantle upwelling and explain the causes of anisotropy at different depths.Research on the anisotropy of upper mantle has great significance on the development of such theories as earthquake prediction, asthenosphere convection dynamics and lithospheric delamination In the mesh generation, 1°×1° horizontal grid was used above 300 km and 2°×2° horizontal grid below 400 km for the reason of resolution in the horizontal direction; the resolution ranged from 50 to 100 km in the vertical direction.The isotropic velocity study showed that the upper mantle above 100 km in the study area showed a low velocity anomaly on the west side and a high velocity anomaly on the East side. However, a high velocity anomaly was appeared on the west side in 200?400 km. This change is thought to have been caused by partial delamination of the lithosphere. The results of anisotropic inversion show that the fast axis direction of seismic wave velocity is consistent with the tectonic background above 100 km - NE,NNE, NW; the fastest speed direction gradually changed to nearly horizontal direction with the depth below 100 km and there are signs of turning anisotropic azimuth when the seismic waves encounter the high velocity anomaly, which may indicate the specific flow direction of the deep mantle fluid during upwelling. This paper argues that in the interpretation of geology and construction of deep dynamics model, with depth more than 300 km, the seismic wave in fast axis direction is mainly controlled by the low velocity anomalous body movements or upwelling, the migration direction of hot mantle is north-south; below 400 km depth, the subduction of Pacific plate will play a certain effect.