| The South China continent,composed of the Yangtze Craton and the Cathaysia block,together with North China continent,forms the eastern China.Ever since the Mesozoic,eastern China has undergone three major tectonic events,including the collisional orogeny,the transformation of structural system from compression to extension and large-scale thinning of the lithosphere.As a result,the eastern part of South China is almost completely covered by extensional structures and volcano-magmatic activities.Meanwhile,magmatic activities are accompanied with extensional basins,forming a unique basin-range structure system.However,due to the extreme lack of deep geophysical data,the South China continent evolution process and dynamic mechanism are still controversial.The broadband seismic array observation is an effective way to obtain the fine three-dimensional structure information inside the earth.The dense coverage and massive data formed by the combination of temporary stations array and permanent stations can substantially improve the imaging resolution of the Earth's internal structure.Receiver functions method has been a popular technique to study Earth's structure.P-wave receiver functions is effective in studying the crust and the mantle transition zone(MTZ)structures with a higher resolution and S-wave receiver functions have obvious advantages in imaging the lithosphere-asthenosphere boundary(LAB).Combining teleseismic P-wave and S-wave receiver functions has obtained the three-dimensional seismic structure of the main crust-upper mantle discontinuities(Moho surface,LAB,410 km discontinuity,660 km discontinuity)in southeastern China.We have collected seismic data recorded by 271 portable broadband stations and 204 permanent stations in the study area,and a total of 68892 P-wave receiver functions records with obvious first arrivals and high signal to noise ratios were chosen.We have obtained the crustal thickness,Poisson's ratio and image of the Moho surface by using the H-? stacking method and common conversion point(CCP)stacking method.The results show that: 1)The average crustal thickness is 32 km,with an average Poisson ratio of 0.24,which shows a relatively thin crust and low Poisson's ratio in the study area.2)East of the North-South Gravity Lineament(NSGL),the Moho surface gradually uplifts from northwest to southeast.The crust in the Qinling-Dabie orogenic Belt is thick(> 37km),the crust in southern margin of North China,the Lower Yangtze,and the Jiangnan orogenic belt are relatively thin(28?35km),besides,the crust of Cathaysia block is thinner(<30km).The Poisson's ratio in the Lower Yangtze block is relatively high(> 0.26),followed by the Cathaysia block(0.22?0.27),and the Jiangnan orogenic belt,which is the lowest(< 0.24).There is an obvious negative correlation between the crustal thickness of the Poisson's ratio in the Yangtze block and Cathaysia block.3)Three extreme crustal thinning subregions are discovered in our images and the average crustal thickness is less than 30 km.One region is located in the southern margin of the Sulu orogenic belt,roughly along the Tan-lu fault(LH1),which extends southward and ends near the Jiangnan buried fault with a larger Poisson's ratio,and most of the Middle-Lower Yangtze Metallogenic belt are distributed in it;the second region,along the Guangzhou-Shaoguan-Gangzhou-Jian-Nanchang profile shows a southwest-northeast spreading,which extends northward and ends near the south of Jiangnan buried fault(29° N);and the third is distributed along the XiangzhongJianghan series extensional basin,intersecting with LH2 to form a "V" or "Y" shape.The intersection is located in the eastern part of Nanling Metallogenic belt,around which lie the world-class tungsten deposits.We have collected the seismic data recorded by 271 portable broadband stations and 161 permanent stations in the study area,and a total of 9930 S-wave receiver functions records were chosen by using the coordinate rotation method to estimate the optimal polarization of the incoming S wave from the particle motion in the vertical plane.Based on the CCP stacking results of the 13 cross sections from 22°N to 34°N and the information of P-wave receiver functions,high-resolution three-dimensional images of the LAB are obtained.The results show that: 1)The thickness of the lithosphere in the study area is between 55 and 80 km,with characteristics of the northwest thicker and the southeast gradually thinner.On the background of overall lithospheric thinning,some local or independent thinning zones indicate that the lithospheric thinning is heterogeneous.2)The lithosphere on the east side of NSGL is more than 15 km thinner than the west side.The overall lithospheric thickness of the Yangtze block is greater than both the Cathaysia block and the Jiangnan orogenic belt.The lithosphere in the southeastern coastal area and the southern part of the study area are thinner,and some parts of lithosphere in the part of Wuyi orogenic belt in northwest Fujian are thicker.3)The thickness of both the crust and the lithosphere becomes thinner smaller from the northwest to the southeast(toward the sea),suggesting that their extensional deformation is coupled.The lithospheric thinning area on the west side of the Tan-lu fault corresponds roughly to the crustal thinning area LH1,and the Ganjiang fault lithospheric thinning area corresponds roughly to the crustal thinning area LH2,indicating that the crustal and lithospheric deformation are strongly coupled.Based on the 68892 P-wave receiver functions records and adjusted the IASPEI91 model according to the H-k stacking results of each station in the study area.We obtain a new study on the 3-D structure,temperature and water content of the MTZ by the P-wave receiver function CCP stacking.The results show that: There is a boundary of MTZ structure at 29°N in the study area to the east of the NSGL.In the northern part,the 660 km discontinuity is generally deeper than globe model and there are two independent local depressions characterized by high speed,low temperature and water shortage.We consider that both of which are all subduction plates slab stagnated in MTZ,and may be left by two subductions in the early and late stages.While in the southern part,there is only a northeast-trending narrow-band 410 km discontinuity depression belt,which is associated with the relatively high temperature and water-rich features at the bottom of MTZ(660km discontinuity).The above studies provide important evidence that there is a boundary of mantle structure at 29°N in southeastern China.The previous model of the subducting Pacific plate stagnates in the MTZ can apply to the north of the boundary.Meanwhile,the results also further limit the application area of the model,the western margin of the stagnant slab extends approximately to the NSGL and the southern margin about 29°N.The consistency of the crustal and lithospheric thickness variation trends and the corresponding relationship between the crustal thinning and the lithospheric thinning position indicate strong coupling within the lithosphere.Mainly based on the existence of the “V” or “Y” shape crustal thinning zone,we tried to use the dynamic model of the interaction between the Pacific plate and the western North America,“flat subduction-scraper wedge delamination” interaction to trigger “thermal convection in the asthenosphere”.The model can be used to explain the coupling extension and local nonlinear thinning of lithosphere,the local depression of 410 km discontinuity and the relative water-rich of the bottom of MTZ. |
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