Velocity Structures At Typical Areas In South China And Their Tectonic Implications

The South China Block is one of the most important blocks in Asia,which composes of two major blocks,the Yangtze to the northwest and the Cathaysia to the southeast,and amalgamated along the Jiangnan Orogen in the early Neoproterozoic.In the Neoproterozoic,the formation of unified South block by the Yangtze block collision with the west Cathaysia block,following west-directed subduction at ca.820 Ma,with the widespread magmatic activities.Subsequently,the Yangtze block rapidly subsided and middle Neoproterozoic rift basins were formed in the South china block.After that,this region underwent series of tectonic activities.Especial,during the Early Permian to the Late Cretaceous,the Paleo-Pacific plate subducts under the east of the South China block,and this period produced considerable amounts of granite.Finally,at Jurassic,the South China ending of orogeny and beginning of slab break-off.Due to the complicated tectonic movement,the South China block has a complex structure.On a large scale,the current research on the structure and evolution of the South China block,and the boundary between the Yangyze block and the Cathaysia block are weak.On a small scale,the Xinfengjiang Reservoir in southeastern China is one of the typical reservoir-induced earthquake sites in the world.However,the research on its velocity structure and seismic mechanism is relatively weak.Therefore,this paper studies South China from these two aspects.The Xinfengjiang Water Reservoir(XWR)in Guangdong,China,is one of the reservoirs that have triggered earthquakes of magnitudes greater than 6.Numerous earthquakes have occurred since the impoundment of the reservoir,making it one of the most active seismic zones in Guangdong.However,due to the lack of seismic stations,the detailed seismic structures and earthquake mechanisms within XWR have not been resolved,and the significance of XWR as a typical protracted earthquake location is not well judged.In this study,by collecting waveform data from both permanent and temporary stations from 2012 to 2015,we relocated 1,528 earthquakes and inverted both Vp and Vs structures from traveltimes of these earthquakes.Using waveform data,we also investigated focal mechanisms of earthquakes with magnitude greater than 1.5 in this region.Our results reveal fine crustal structure that has never been shown before and show complicated crust structure with several low-velocity zones extending to 5–10 km depth under the major faults.Earthquake focal mechanisms show more dip-slip faults than strike-slip faults,and the two types of earthquakes are roughly divided by the reservoir boundary.The direction of principle stress of the earthquakes is northwest southeast,consistent with the direction of tectonic principal stress.Combining the above results,and investigation of historical earthquakes and water level change,we suggest that water loading cycle and diffusion play important role in XWR seismicity.They increase the pore pressure,make the earthquakes migrate to deeper depth,and change the type of earthquakes.It is generally agreed that the boundary between the Yangtze and Cathaysia block is extensional settings,however,the genesis of granulite remains controversial,and the properties of the suture between the two blocks are not entirely clear.In this study,we obtained high resolution lithospheric structure and Moho depth distribution of the Jiuyishan and its surrounding area by joint inversion of receiver functions and ambient noise data.Our results show relatively thick crust beneath the Yangtze block(~37 km)and thin crust beneath the Cathaysia block(~27 km).The boundary of thin and thick Moho lies roughly along the Chenzhou-Linwu fault(CLF),with a transition of about 5 km in Moho depth across it.Moreover,the three-dimension shear wave velocity structure shows prominent low velocity blocks beneath Jiuyishan and Cathaysia block near the CLF at depth of 60 km to at least 80 km.Meanwhile,the upper and middle crust beneath Jiuyishan are thicker and have lower velocity than surrounding area.Our results suggest that the CLF is the boundary of Yangtze and Cathaysia block in the south part.Near this suture,there is an upwelling from asthenosphere beneath the Jiuyishan and its surrounding area,which may melt the lithosphere and further modify the lithosphere and crustal structure.This melting may explain the low oxygen fugacity and low water activity of granite in Jiuyishan.