| The continental China is located at the convergent region where the Paleo-Asian Ocean,Tethyan and Western Pacific domains meet with each other.In the east,the Pacific plate and the Philippine Sea plate are subducting beneath the eastern and southeastern China.In the southwest,the India plate is subducting beneath the southwestern China.Under the complex interactions of the surrounding oceanic and continental plates,the present continental China is characterized by the unique framework of cratons and basins assembling along orogenic belts.From Archean to Cenozoic,the continental China has gone through a complex tectonic evolution,and there are many intraplate volcanoes and strong earthquakes widely distributed in Chinese continent and surrounding regions.In order to better understand the tectonic evolution and the geodynamic process of the continental China,a robust velocity model for the China region is essential.In this way,we first combined SOLA BackusGilbert method and double-difference seismic tomography method together to evaluate the inverted velocity model.Then we conducted seismic tomography in South China and northeastern China,respectively,and finally,in the whole continental China.The main contents include the following 4 aspects:(1)The application of SOLA Backus-Gilbert method in double-difference seismic tomographyIt is important to obtain a robust and reliable model in geophysical inversion.In addition to the inverted model,the model evaluation,including model resolution and uncertainty,is also very essential to geological explanation.Due to the regularization term in the damped least-squares method,the inverted model,model resolution and covariance may be biased.What’s more,it is generally very difficult to obtain the model resolution and covariance matrices in the large-scale geophysical inversion problem.Compared to the damped least-squares method,Backus-Gilbert method can obtain the inverted model,model resolution and covariance simultaneously,but with a high computational cost.The SOLA(Subtractive Optimally Localized Averages)Backus-Gilbert method,an updated Backus-Gilbert method,retains all of its advantages and is more computationally efficient.As a consequence,it can be applied to large-scale geophysical inversion problems.In this study,we combined the SOLA Backus-Gilbert method and double-difference seismic tomography method in order to obtain the model resolution and covariance at the same time.We tested our new method by a synthetic test,and compared our inversion result with the inversion result from damped least-squares method.These two inversion results are consistent with each other,which can resolve well in the region with good data coverage,and produce artifacts in the region with poor data coverage.Compared with checkerboard result and ray-density distribution,we propose that the model evaluation from SOLA Backus-Gilbert method is most reliable.(2)High-resolution Vs tomography of South China by joint inversion of body wave and surface wave dataTo take advantage of complementary strengths of earthquake body wave data and surface wave data from both earthquake and ambient noise sources,we have jointly used them to determine a high-resolution 3D Vs model of the lithosphere in South China.Compared to the separately inverted models,the resolution of the jointly inverted Vs model has improved,and the joint inversion model can fit both data types at a similar level compared to separate inversions.In comparison with previous models,the new velocity model better delineates the sedimentary basins in the shallow crust and high velocity root in the lithosphere beneath the Sichuan basin,as well as better resolves the velocity variations between western and eastern Yangtze craton,thin lithosphere beneath eastern Yangtze craton and Cathaysia block,and helps us to better understand the mechanism of the lithosphere thinning.From the correlation between high velocity anomalies in the middle crust and the distribution of Neoproterozoic basalt samples,we propose the amalgamation suture between the Yangtze craton and Cathaysia block is along the Jiangshan-Shaoxing fault and the western margin of the Jiangnan orogen volcanic rocks.Our new joint inversion model is useful for better understanding the orogenic processes and complex tectonics of the South China block.(3)The mantle velocity model of northeastern China by teleseismic doubledifference tomographyIn the late Permian to early Triassic,with the closure of the Paleo-Asian Ocean,northeastern China collided with the North China craton along its southern edge and the Solonker suture was thus formed.In the late Jurassic to early Cretaceous,after the closure of the Mongol-Okhotsk Ocean,it amalgamated with the Siberia craton along its northern boundary and the Mongol-Okhotsk suture was thus formed.In the late Mesozoic,northeastern China experienced intensive extensional deformation and volcanism,and there are many Cenozoic volcanoes distributed surrounding the Songliao basin,including Changbaishan,Jingpo,Wudalianchi,Halaha and Abaga volcanoes.Among these,Changbaishan is the largest active volcano in northeastern China,and many deep earthquakes occurred一 300 km to the east of Changbaishan volcano.It is important to study the mechanisms of intraplate volcanism and deep earthquakes for better understanding the tectonics of northeastern China.In this study,we conducted teleseismic double-difference tomography in northeastern China and obtained a reliable high-resolution mantle velocity model.Our imaged high-velocity subducting slab has a trend of stagnancy-penetration-stagnancy in the mantle transition zone from north to south,and the Changbaishan volcano and deep earthquakes are distributed to the west of and east of the southern edge of the penetrated slab,respectively.We propose that it is the vertical tearing of the slab at the transitional stage from penetration to stagnancy that causes the formation of the Changbaishan volcano and deep earthquakes.(4)The mantle velocity model of Chinese continent by teleseismic doubledifference tomographyBased on the seismic tomography in South China and northeastern China,we presented a robust mantle velocity model of continental China by using teleseismic double-difference tomography.In order to validate the reliability of our model,we have conducted checkerboard test,restoration test,and tested our inversion result with different initial models and crust corrections.We find that the velocity model around Tibetan plateau is not well resolved at depth of 40 km,and the structure below 80 km is well resolved in the whole continental China,with a horizontal resolution of～2°and depth resolution of～100 km.We speculated that there is a possible link between the upwelling hot materials of asthenosphere and the formation of carbonatiteassociated rare-earth-element deposits in southeastern Tibetan plateau,and we analyzed the velocity structure beneath 4 intraplate volcanoes,including Wudalianchi,Datong,Tengchong and Kunlun volcanoes,and speculated their formation mechanisms.The velocity structure beneath Changbaishan volcano from this study is consistent with that of the tomography model of northeastern China,which implies the reliability of our velocity model. |
PDF Full Text Request