| Seismic imaging is one of the most significant ways to map the structure of the Earth.With development of the seismic tomographic method and increase of dense seismic station array,it is easier for us to obtain detailed knowledge of the Earth's structure,which is fundamental to our understanding of the formation mechanisms and evolution processes of the crust as well as dynamics of the Earth's deep interior.In this study,continuous recordings of three-component seismic data were used to extract Rayleigh and Love wave dispersion curves.We then obtained vertical polarized shear(SV)and horizontal polarized shear(SH)wave velocity models of crust and lithosphere.These detailed shear velocity models provide vital information for our study region.The South China Block(SCB),one of the largest tectonic units in China,is traditionally considered to be composed of the Yangtze Block to the northwest and the Cathaysia Block to the southeast during the Early Neoproterozoic.It has been widely accepted that these two major Precambrian continental blocks were amalgamated along the Jiangnan orogen in the southeastern margin of Yangtze Block.After that,the South China underwent multiple tectonic events.The age of basement rocks of these two blocks are different.It has been commonly considered that the Shaoxing-JiangshanPingxiang fault represents the tectonic boundary between these two blocks based on geological,geochemical,and isotopic characteristics across the fault.However,the timing,location and geodynamics of this amalgamation are still controversial because of the complicated evolution history of South China.The study of detailed shear velocity models in South China is significant to reveal the regional tectonic characteristics and evolution processes as well as deep dynamics.In order to study velocity models and tectonic characteristics in the crust and lithosphere,we used three-component seismic data recorded in 71 seismic stations around the Jiuyishan area from May 2016 to June 2017.The stations include 17 temporary broadband stations and 54 permanent China Seismic Array(CEA)stations.We performed cross-correlations and stacking between all possible station pairs to retrieve both Rayleigh and Love wave EFGs.We used frequency-time analysis(FTAN)to measure group and phase velocity dispersion curves of Rayleigh and Love wave.A tomographic method based on the ray-theory was applied to produce group and phase velocities of Rayleigh and Love wave for 2-40 s periods.The surface wave velocities are sensitive to structures at different depths.We extracted the dispersion curves of Rayleigh and Love wave and inverted them into SV and SH wave velocity models(0-40km),respectively.We further provided the radial anisotropy in South China by calculating the discrepancy of velocities of SV and SH waves.Besides,we measured the interstation phase velocities of Rayleigh wave in the period range of 10-78 s using the teleseismic data.We combined 2-20 s periods surface wave data from ambient noise with 22-78 s periods surface wave data from teleseismic data and inverted them into a SV wave velocity model(0-80km).The detailed shear velocity models and crustal radial anisotropy provide significant information of this study region: 1)an obvious SW-NE trending low velocity belt in the middle crust is reveled,which may represent a possible ancient suture zone between the Yangtze and Cathaysia in South Chinablock during the Neoproterozoic;2)notable low-velocity anomalies and negative radial anisotropy are observed at 15-25 km in the middle crust,which is very likely the metamorphized sedimentary rocks;may represent metasedimentary rocks from the upper crust;3)the lithosphere thickness is about 70 km in our study region.We also observed evident asthenosphere upwelling.High mean SV velocities in the lower crust and uppermost mantle are found,and this structure is closely related to the mineralization processes in southeastern South China.Our results provide seismic supports evidence for the genesis of unique A-type granites in the Jiuyishan granite suite and offer constraints on tectonic evolution and mineral source exploration in South China. |
PDF Full Text Request