Three-dimensional Crustal Structure In The Northern South China Sea And Its Geological Significances

The South China Sea is located at the intersection of the two global tectonic systems of the Pacific and Tethys domains,with strong geological processes and complex tectonic deformation.The northern continental margin of the South China Sea experienced the transition from Late Mesozoic active continental margin to the Cenozoic passive continental margin,making it as an important area to investigate the tectonic inversion of continental margin.The characterization of the crustal structure in the northern part of the South China Sea could provide clues for solving a series of outstanding scientific problems,such as the Cenozoic lithospheric rupture,the tectonic inversion of continental margin in the northern South China Sea and the genesis of the lower crustal high-velocity layer.In this thesis,33 seismic profile data collected from the northern South China Sea were analyzed,with a length of 9,540 km in total.More than 30,000 stratigraphic structure data points are obtained by digitization,to construct a three-dimensional crustal structure model of the northern area of the South China Sea.The crust in the northern part of the South China Sea can be divided into four interfaces: the sea floor,bottom boundary of the sedimentary layer,bottom boundary of the upper crust and the Moho surface.Among them,the depth of the sea floor in the north of the South China Sea is within 0-4.5 km,and it is shallower near the South China continent.The depth of the sea floor in the Central Basin is more than 4 km,showing the trend of shallower in the northwest and deeper in the southeast.The depth of the bottom boundary of the sedimentary layer is within 1-12 km,which is still shallow in the northwest and deep in the southeast on the whole.However,the depth of the bottom boundary of the sedimentary layer at the Baiyun slope on the west side is deeper than that around.The depth of the upper crust is 4-18 km;the deepest areas occur in the east of Taiwan Island and Qiongdongnan Basin,the shallowest one is the basin in the southwest of Taiwan.Most of the upper crust depth is in the range of 8-10 km,showing deeper upper crust in the northeast and southwest,and shallower in the middle area,but in the north part of Baiyun slope,the upper crust is deeper.The Moho depth in the northern part of the South China Sea ranges from 8 to 35 km,and the deepest is in the northern shelf area and the shallowest in the eastern basin area,showing a trend of deep in the northwest and shallow in the southeast.The thickness of the sedimentary layer in the northern South China Sea ranges from 0 to 12 km.The thickness of the sedimentary layer in most areas is less than 2km.The offshore in the east of Taiwan Island is the thickest and reaches 12 km.The thickness in the Baiyun slope varies from 4 to 8 km.The sediments from the South China continent transported to the Baiyun slope through the Pearl River Mouth account for the anomalous thickness.The thickness of the upper crust is between 0 and 15 km,and the thickness of the northwest and the west is relatively large,mostly 8-15 km,gradually reduces to less than 4 km to the Central Basin.The thickness of the lower crust is large in the northwest,generally 14-25 km,gradually thinning to less than 10 km in the Central Basin along the southeast direction,and the thinnest part is even less than 5km.Generally,the crustal thickness in the northern South China Sea ranges from 4 to 35 km,and it still gradually thins from the northwest(> 20 km)to the southeast,until it is less than 10 km in the Central Basin.The crustal thickness of the northern South China Sea is larger than the typical oceanic crust(5-7 km)and less than that of the continents,which reflects the characteristics of the crustal structure of the marginal seas.In addition,the possible areas of lower crustal high-velocity layer are the south side of Southwest Taiwan Basin,the east side of the Pearl River Mouth Basin,the southwest side of the Pearl River Mouth Basin,the southwest side of the Northwest Sub-basin and the south side of the Northwest Sub-basin.The crustal thickness of the two high-velocity layers in the Pearl River Mouth Basin and southwest Taiwan Basin are relatively thick,which is not conducive to the formation of the high-velocity layers by serpentinization.It is speculated that the magma underplating during the spreading of the South China Sea results in these two high-velocity layers.