Study On The Lithospheric Structure Of Mainland China Using Broadband Ambient Noise Tomography

In this study,a high-resolution S-wave velocity model of the lithosphere in mainland China is determined from only ambient noise data.We use 2-year continuous waveforms recorded by 1031 broadband stations of the China Regional Seismic Network and NECESSArray.By applying the procedures of noise cross-correlation and time-frequency domain phase-weighted stack,we obtain the interstation cross-correlation functions with high signal-to-noise ratios.We extract a large amount of group and phase velocity dispersions at periods of 5–125 s from the cross-correlation functions.We acquire group and phase velocities of each period from surface wave tomography,on this basis,we further determine the 3-D S-wave velocity model of the lithosphere(< 150 km depth)in mainland China.Our result is generally similar to the previous models,and reveals some more detailed structural features of the lithosphere.The main findings and understandings are as follows:The Sichuan,Ordos and Sichuan basins in eastern China are characterized by deep(> 150 km)lithospheric roots.However,the lithosphere in the eastern part of the Ordos and Songliao basins may have been significantly modified.In addition,the mantle lithosphere beneath the southern part of the Songliao basin shows vertical discontinuities,which may indicate that the lithosphere is delaminated in this area.The mantle lithosphere beneath the Changbai,Jingpohu and Datong volcanos is almost absent,and it is very thin beneath the vicinities of these volcanoes.The lithosphere in the eastern North China craton,the eastern Yangtze craton and Cathaysia block is ~70-100 km,and the lithosphere in the eastern Northeast China block is only ~ 60-80 km.In general,the lithosphere in eastern China is thin,and shows a thinning trend from west to east,which may be due to a series of deep processes caused by the subduction of the Pacific plate to the Eurasian plate,resulting in the strong lithospheric thinning in eastern China.Beneath the basin-range junction belts,suture zones between blocks,as well as deep faults,the mantle lithosphere is thin and its S-wave velocity is low,and these features are particularly obvious beneath the Tanlu fault belt,indicating that the fault belt may have cut through the lithosphere.We also observed differences in the structural features of different segments of the Tanlu fault belt.The mantle lithosphere beneath the two branches of the northern segment of the fault belt is significantly thin.In the middle segment of the Tanlu fault belt,the profile across the northern Bohai bay basin shows that the lithosphere–asthenosphere boundary beneath the west side of the fault belt gradually becomes shallow from west to east,while the lithosphere–asthenosphere boundary beneath the east side is almost flat,the profile to the south corresponds to an intense uplift of the mantle lithosphere beneath the fault belt.For the southern segment of the Tanlu fault belt,the mantle lithosphere on the two sides differs in thickness,and beneath the fault belt,the mantle lithosphere is obviously thin with reduced velocities.The velocity of the upper mantle(< 150 km depth)beneath the Songpan-Ganzi terrane in the eastern Tibet Plateau is lower than that beneath surrounding tectonic units.By combining the results of various geophysical observations and studies onwidely distributed magmatic rocks in the Songpan-Ganzi terrane formed in the post-collisional tectonic background,we infer that this phenomenon may be related to the upwelling of hot asthenospheric material caused by lithospheric delamination.