The S Velocity Structure And Seismic Anisotropy In Ordos Block And Adjacent Area

The Ordos Block(OB)is located in the western part of the North China Craton.Under the double-effect of the NE compression of the Tibetan Plateau(TP)and the westward subduction of the Pacific plate,a series of rift basins and fault zones have developed around its periphery since the Cenozoic.As an active block that plays an important role in the Cenozoic and modern tectonic activities in North China,as well as an important part of the North China Craton,the study of the deep structure of the OB has been highly valued by geophysicists.In this paper,the high-quality seismic data recorded by the Chin Array is used to explore the shear-wave velocity structure and seismic anisotropy of the crust and upper mantle,thus to discuss the dynamic processes of the OB and the surrounding area.The main results and understandings obtained are as follows:(1)Based on 2-year seismic observation data of 461 seismic stations deployed in the OB and surrounding areas by the Chin Array,we used the ambient noise tomography method to obtain the Rayleigh surface wave phase velocity(5-46s)images with resolution up to 0.3*0.3 degree.(2)The phase velocity dispersion data from(1)was applied to the MCMC method to invert the three-dimensional S-wave velocity structure ofthe crust and uppermost mantle.The results show that obvious sedimentary layers are present in the shallow part of the OB,with thicker sedimentary layer in the west than that in the east The OB presents obvious lateral inhomogeneities in the upper crust.Particularly,the relatively low velocity in the west of the OB may be related to the different degrees of destruction and deformation generated by the northeastern margin of the TP squeezing to the western boundary of the block,resulting from collision with the Indian plate and the Eurasian plate since the Cenozoic.There is an obvious NE-directed high-velocity anomaly zone in the upper crust of the OB,which may be related to the basement of the OB.High-velocity anomaly from the middle-lower crust to uppermost mantle was exhibited in the OB,which seems to connect with the high-velocity anomaly of the Linfen Basin,Lingshi uplift,and extend eastward to the south of the Taihang orogenic belt,indicating that the high-velocity zone has not been severely damaged.The S velocity of the upper-middle crust in the southern margin of the Xingmeng Orogenic Belt has obvious horizontally heterogeneity.We speculated that it is related to the assemblage of different oceanic and continental blocks,material exchange,and strong fluctuations of the interface of the middle-lower crust caused by shortening of the crust and uplifting of the orogenic process during the entire orogenic process.(3)We investigated the distribution of anisotropies at high resolution within and surrounding the OB by shear-wave splitting analyzing teleseismic data from ?710 newly deployed temporary seismic stations.The central and eastern OB is characterized by a weak anisotropy,consistent with a relatively thick and rigid lithosphere.In the western part of the OB,the fast-wave polarization direction(FPD)is dominantly NW–SE,which is in good agreement with the anisotropy in the northeastern part of the TP.We speculate that the lithosphere in the western part of the OB may have undergone significant deformation caused by expansion and compression of the TP.Near the Datong Basin to the north of the Shanxi Rift,the FPD is mainly oriented NW–SE,perpendicular to the NE-directed compression of the TP toward the OB and parallel to the extension direction of the Datong Basin.We speculate that this anisotropy is related to the long-range effect of NE compression of the northeastern TP onto the low-velocity upper mantle in this area.The weak anisotropy and E–W-trending FPD near the Taiyuan Basin are identical to those in the eastern OB and are not affected by an NE–SWtrending extension basin,indicating that the lithospheric mantle was not significantly affected by the formation of the basin.This region of weak anisotropy is consistent with the high-velocity upper mantle anomaly shown by seismic tomography,indicating that lithosphere destruction in the middle of the Shanxi Rift is still at an early stage.The anisotropy in the Xingmeng Orogenic Belt is mainly NWW–SEE-trending,parallel to the absolute plate movement direction,indicating that the anisotropy may originate from the asthenosphere.However,the FPD in the southern of the Xingmeng Orogenic Belt next to the Yinshan Orogenic Belt is consistent with the E-W trend of surface structures in the south.We speculate that the near-E–W-trending anisotropy may be a residual of an earlier deformation.(4)We investigated the characteristics of seismic anisotropy beneath 512 seismic stations based on SKS phases from ?200 earthquake.In the central Sichuan basin(SCB),the average delay time is only ?0.35 s,consistent with the relatively thick and rigid lithosphere of the stable SCB.However,the northeastern SCB has relatively stronger anisotropy with the FPD with NE-SW orientation,which is markedly different from the weak anisotropy with NW–SE orientation in the central SCB.We speculate that owing to E–W compressive stress caused by eastward escape of material from the TP,the lithosphere in this area has undergone considerable deformation along the right-lateral maximum shear direction.In the Chuandian diamond block,the FPD of several adjacent stations was approximately E-W,almost perpendicular to the FPD in the northern and southern regions,which might be related to partial melting in the lithosphere and to compressive stress.Delay time in the southern Songpan-Ganzi block is much smaller than in the north.The FPD of the southern is different from that of the north,not dominted by NW-SE direction,which might be attributable to low-velocity mantle upwelling and complex tectonic stress in this area.