Surface-wave Azimuthal Anisotropy And S-Velocity Structure Of North China Craton And Its Surrounding Regions

The North China Craton has undergone intensely tectonic reactivation since the Mesozoic,Which resulted in lithospheric modification,thinning and destruction,and accompanied by voluminous magmatism.The neotectonic movement is strong and destructive earthquakes occur frequently in this region.It is of great significance to obtain the medium deformation and structure information in the crust and upper mantle for understanding the related dynamic problems.We have obtained high-resolution Rayleigh surface-wave phase velocity and azimuthal anisotropy at 10-120s from Eikonal surface wave tomography in North China Craton and its surrounding regions.A high-resolution 3D S-velocity model into the depth of 200 km is obtained using a linear iterative inversion method.Additionally,we have estimated the lithosphere thickness based on this 3D S-velocity model.The surface-wave data comes from the portable stations of ChinArray Phase Ⅱ and Ⅲ,and some permanent stations of China National Seismic Network(CSN).The high-resolution imaging results provide vital basis for understanding these scientific issues such as the characteristic of tectonic deformation and deep structure in the crust and upper mantle,the interactions between tectonic blocks,the seismogenic environment of strong earthquakes,the spatial distribution range of lithospheric thinning or destruction,and their dynamic mechanism.The main results and conclusions of this study are as follows:(1)The lithosphere is thick in the west and thin in the east in North China Craton,in addition to this foremost characteristic there are also some smaller scale variations of lithospheric thickness.It includes,(ⅰ)within the Ordos block,the lithosphere is thinner in the north than that in the south,(ⅱ)within the peripheral rift zone around Ordos block is characterized by significantly heterogeneous thinned lithosphere,and(ⅲ)there is obvious difference between Yanshan Orogenic Belt and the northern region of North China plain.(2)It exhibits a wide range of high-velocity and weak anisotropy in the Ordos block,but the anisotropy is strong in the southwest of this block.The direction of the fast wave maintains a good continuity with the northeast margin of the Tibetan Plateau.The results indicate that the lithosphere has undergone significant deformation in the southwestern Ordos block under the influence of the Qinghai-Tibet Plateau.The influence of the plateau expansion on the Ordos block is obviously beyond the Liupanshan thrust belt.The anisotropy at long period(>80s)and S-wave velocity of the corresponding depth(>100km)showed that the high-velocity anomaly shrinks to the central and southeastern regions in this block,and the anisotropy is significantly stronger around the high-velocity region,which reveals that the boundary zones of Ordos block have been subjected to different degrees of deformation and modification,and resulting in non-uniform lithospheric thinning.(3)The northeastern margin of the Qinghai-Tibet Plateau exhibits a consistent N WW-SEE fast wave direction at whole periods.Its lithospheric velocity and thickness are significantly lower(smaller)than that of the surrounding stable blocks.In this area,the low-velocity and low-mechanical-strength lithosphere has undergone significant tectonic deformation under the strong action of uplift and expansion,and the crustmantle medium deformation is continuous and consistent.The result supports that the northeastern margin of the Qinghai-Tibet Plateau has undergone the vertical coherent deformation.(4)In Shanxi rift zone,both the northern and southern regions exhibit varying degrees of low velocity anomalies in the upper mantle(<100km),which are separated by a high velocity anomaly zone in the central area.At depth of more than 150km,a remarkable low-velocity anomaly belt oriented NNE is observed from the southern edge of Taihang Mountain to the northern edge of Shanxi rift zone,indicating that the shallow upper mantle low-velocity anomalies are connected in the deep.Combined with some other research findings,we speculate that these low-velocity anomalies may stem from a greater depth(>200km),potentially linked with the stagnant dehydration of the subducted Pacific plate and consequent upwelling of thermal material in the upper mantle,as well as small-scale mantle convection.(5)Datong volcanic filed in the north of Shanxi Rift zone is characterized by low velocity anomaly at periods of greater than 20s,and the anisotropy shows significant difference at different periods.We speculate that this phenomenon is related to the geometry of lithosphere in the northeastern Ordos block and the long-range compression effect from Qinghai-Tibet Plateau.(6)The lithospheric structures of the Yanshan Orogenic belt are obviously different from the northern region of North China Plain.The thin lithosphere and significant low-velocity anomalies beneath the North China Plain indicate a hot upper mantle in the region.We consider that the significant differences in crust-mantle velocity and thermal structure are related to the distribution characteristics of the western Pacific lagging plate in the upper mantle transition zone.The ZhangjiakouBohai seismic zone locates in a transitional zone with significant differences in lithospheric and soft fluvial structures.The combined effect of significant differences in lithospheric structure and deep thermal action,as well as the far-field extrusion effect on the Tibetan Plateau,may be the main factors contributing to the higher seismicity in this zone.In the Yanshan tectonic zone,northern North China Plain,and nearby Bohai Bay,the high-speed anomalies near 200 km depth may be related to local delamination of the lithosphere.(7)The results of this study,combined with the previous research results of geological structures evolution,ground motion from GPS observations,and medium deformation from seismic anisotropy,reveal that the uplift and expansion of the Tibetan Plateau has significantly affected the eastern North China Craton.We believe that the tectonic deformation of the crust-mantle medium in the North China Craton,the deep environment of strong earthquakes and the evolution process of neotectonic movement are controlled by the subduction of the Pacific Plate and the uplift and expansion of the Tibetan Plateau.