Teleseismic P-wave Velocity And Anisotropy Of Upper Mantle Beneath Lower Yangtze Region And Adjacent Areas

The Lower Yangtze River Region in the eastern part of the Yangtze plate has experienced strong continental orogenic movement in the Mesozoic,forming an important polymetallic deposits in the Middle-Lower Yangtze River Valley Metallogenic Belt..We have collected 32728 teleseismic P-wave arrival times from 479 earthquakes through the improved muli-channel cross correlation method(MMCC).Applying seismic tomography and harmonic decomposition analysis of P velocity perturbation under the condition of weak anisotropy,velocity and anisotropy anomalies are obtained from the P-wave traveltime data recorded by 321 sesimic stations.1°x1° and 2°x2° were selected by using the CRT as suitable velocity and anisotropic parameter mesh.The three-dimensional P wave velocity structure of lower Yangzi and the adjacent region over 800 km was obtained by using TOMOG3 D program.On the basis of obtaining high precision velocity structure,four anisotropic parameters are introduced to obtain the anisotropic distribution in the study area by using the harmonic decomposition analysis of P velocity perturbation under the condition of weak anisotropy.(1)Velocity isotropy results show that there is a high-speed anomaly in the deep part of eastern China.In combination with geological data constraints,it is considered that the shallow high-speed anomaly is a reflection of the devolution lithosphere,while the deep high-speed anomaly reflects the subduction of the Pacific plate.The apparent low-velocity anomaly in the upper mantle is interpreted as the upwelling of hot material in the mantle.The results of velocity anomaly further indicate that deep phenomena such as lithospheric devolution thinning and plate subduction do exist in the lower Yangtze region.(2)Velocity anisotropy results show that the direction of P wave in the shallow part below the Yangtze plate in the eastern part of the Chinese mainland is consistent with the direction of the absolute plate movement,which is the SE direction.It is believed that the anisotropy of the upper mantle is caused by the movement difference between the lithosphere and the asthenosphere.In the deep part,the anisotropic direction of the mantle transition zone tends to turn to the northeast,and this depth velocity structure shows a high-speed anomaly.It is speculated that the anisotropic direction of the deep part is related to the westward subducted Pacific plate,and it may be that the deep Pacific plate changes the direction of mantle flow.The direction of the anisotropic fast wave in the central part of south China is parallel to the tectonic strike,which is roughly in the direction of northeast to southwest.Therefore,it is speculated that the direction of the anisotropic fast wave here reflects the result of the pressure of the lithosphere in the direction of northwest to southeast.The cause of the anisotropy in north China may be related to the cracking of the north China craton.The anisotropy in the middle part of the dabie orogenic belt is nearly horizontal,indicating that the mantle material flows eastward below.Combined with the seismological results obtained and based on the geological and geophysical data,the Middle and Lower Reaches of the Yangtze River has experienced a total of three tectonic events: during Triassic Period:(1)during the Triassic period,the sedimentary strata were squeezed due to the proximity of the Yangtze and north China plate,which caused the fold deformation and mantle uplift of the ore gathering area;(2)at the junction of Jurassic and cretaceous,tectonic transformation and mineralization occurred in the area;(3)in the late Eocene,the bottom intrusion of asthenosphere and the re-superposition of magma may be the reason for the development of deeper deposits.