Lithospheric Structure Of South China From Autocorrelation Of Teleseismic P-Wave Coda

The evolution of the South China block can be traced back to the Neoproterozoic collision and amalgamation of the Yangtze Craton and the Cathaysia block.Since early Mesozoic(~250 Ma),the lithosphere of South China block experience intensive alteration due to orogeny and magmatic activities triggered by the subduction of the paleo-Pacific plate.In past decades,Earth scientists have adopted various geophysical and geochemical techniques to study lithospheric structure of the South China and rich information have been accumulated.Based on analysis of these information,various tectonic models have been proposed to explain the geodynamic evolution of South China block,but for the specific process and geodynamic mechanism related to the lithospheric alteration of South China block are still in hot debate.Detailed information about lithospheric structure is essential to understand the tectonic evolution of the South China block.In this paper,we adopt a novel seismic technique to image the lithospheric structure of the South China block.The obtained results will provide new seismic evidence to support the study of geodynamic evolution of the South China block.Seismic Daylight Imaging(SDI)is a technique used to extract reflected seismic wave signals from the transmitted wave by using autocorrelation.As Seismic Daylight Imaging mainly uses the 0.5-2 Hz high-frequency teleseismic P-wave and coda,the resolution of the obtained results may up to 4 km.Previous and our study demonstrate that the Seismic Daylight Imaging has the potential to detect detailed lithospheric structure of the Earth.In this study,we firstly carry out numerical experiment to verify the feasibility of the Seismic Daylight Imaging,and influence of multiples and conversions on constructed P-wave reflectivity is discussed,too.Due to complicated waveform pattern of constructed P-wave reflectivity,we combined analysis of P-wave reflectivity envelope,P receiver function and P-and S-wave velocities model from previous tomography study,to extract the lithospheric structure information from constructed P-wave reflectivity.In this work,the topography of Moho and lithosphere–asthenosphere boundary(LAB)of South China block have been retrieved from analysis P-wave reflectivity of349 permanent seismic stations deployed in South China.The topography of Moho show that Moho depth varies from the ~30 km in coastal region of Cathaysia to the more than ~50 km in Sichuan Basin of Yangtze Craton.This lateral Moho depthvariation show good agreement with the previous studies.In the western Yangtze Craton,a region with thick crust(~50 km),may represent nucleus of the Yangtze craton formed in the Archean and acted as a present-day obstacle to the eastward extrusion of Tibetan Plateau.Due to the rigid basement of the Sichuan Basin,the crustal channel flows turn to a rheological weak crustal corridor in the east.The Moho depth variation from the Yangtze Craton to the Cathaysia block may suggest Mesozoic paleo-Pacific plate subduction induced erosion of South China lithosphere.The lithosphere–asthenosphere boundary(LAB)topography shows that the Yangtze Craton has a relative thick LAB(~200 km),while Cathaysia has a relative thin LAB(~90 km).These observations may imply that the lithospheric erosion and thinning of eastern and southeastern parts of South China block may terminate beneath the Jiangnan Orogen.Six profiles trending NW and NE directions are constructed to further analyze the lateral spatial variation of the South China block lithospheric structure.The three NW striking profiles which perpendicular to the Jiangshan-Shaoxing fault show that the crustal thickness is gradually thinner from ~50 km in western Yangtze Craton to~30 km in the coastal area of Cathaysia.Our results are in good agreement with previous studies,which may suggest the subduction and roll back of the Mesozoic paleo-Pacific plate.In addition,the P-wave reflectivity profile also shows the strong heterogeneity of the South China block lithosphere.Numerous reflectors were observed at various depths in the lithospheric mantle,combined analysis with tomographic shear wave velocity model,suggesting that the apparent discontinuities can be identified as aggregates of fine-scale velocity structures.For the reflections that can be associated with the(mid-lithosperic discontinuity)MLD and LAB are not as distinctive as the reflections within the crust.However,our results exhibit a highly complex lithospheric mantle structure,which may be better described as a volume of subtle velocity and reflectivity gradients,rather than assuming prominent seismic velocity discontinuities.