The Crust-Mantle Deformation Characteristics And Their Coupling Types Beneath The Qinling Orogen Revealed By SKS Shear Wave Splitting And Pms Wave Receiver Function

The Qinling Orogenic Belt(QOB),an east-west extended Proterozoic mountain range,located between the Ordos block and the Yangtze block.It is featured in complicated crustal mantle deformation structures due to its long history during geology tectonic evolution.Thus,measuring the subsurface deformation field beneath the QOB becomes an essential research for deciphering major tectonic events which shaped the belt.Seismic anisotropy is generally believed to be caused by fabric structures that are close related to the subsurface strain field.Seismic anisotropy is often measured with two parameters,the fast polarization direction ? and delay time 8t between the fast and slow directions,from splitting or birefringence of shear waves.In this study,we measured(?,?t)from SKS recorded by 41 broadband stations that cover the QOB and its surrounding areas.We employed a 2-D grid searching method which searches for an optimum(?,?t)that minimizes the transverse energy or the second eigenvalue of the covariance matrix.Stacking of SKS data from multiple earthquakes provides robust measurements of the two splitting parameters.The measured seismic anisotropy shows an overall E-W fast polarization direction across the QOB with slight variations at its western and eastern ends.The fast polarization direction is consistent with an eastward mantle flow that escapes from the NE margin of the Tibetan plateau and flows between the Ordos and Yangtze blocks.We also compared SKS splitting with GPS measurements and crustal anisotropies,and found that the NE marging of the Tibetan plateau and western part of QOB exhibit a vertically coherent deformation(VCD)across the entire lithosphere.In the eastern part of the QOB,there is a significant difference(>15°)in the fast polarization direction measured from SKS and Pms(a P-to-S converted wave at the Moho),indicating some decoupling of deformation between the crust and upper mantle.In order to further study the crust-mantle structure of the Qinling Orogen Belt,Niu and Liu(2012)'method were used to propose a comprehensive method for estimating crustal anisotropy with a horizontal axis using radial(R)and transverse(T)receiver functions.The method includes computing three individual and one joint objective function(JOF)and a reliability analysis of the estimated anisotropy.We applied this technique to the receiver function of Pms converted wave from the Qinling Orogenic Belt and 40 surrounding stations,the observed fast polarization direction is consistent with the direction of the maximum horizontal tensile stress,and interpolated the 40 measured data to generate the two-dimensional topographic Pms,which are Moho depth and Vp/Vs ratio,respectively.The study shows that the thickest area of the crust of the Qinling Orogenic Belt occurs in the northwest part of the Qinling Orogenic Belt.The thickness of the crust is concentrated between 40-50km,and the thickness of the eastern part of the QOB is 30-40km.The eastern QOB is weakly affected by the spreading of the Tibet plateau.This observation may be related to the influence of lithospheric deformation drag caused by clockwise rotation of the Yangzi block and counterclockwise rotation of the Ordos block on the upper mantle asthenosphere material in the southern and northern margins of the orogenic belt.Combined with the observation results of Pms converted wave reception function and SKS wave splitting,it is found that the splitting time of SKS is slightly longer than that of Moho Pms stages,and the crust and upper mantle of the Qinling Orogen Belt show strong azimuthal anisotropy.The receiver function is consistent with the SKS data in the direction of rapid polarization,which is roughly parallel to the direction of the maximum horizontal tensile stress.We assume this is related to the eastward escape of the upper mantle material on the Tibet plateau and the counterclockwise rotation of the Ordos block since the Cenozoic era.The material movement pattern of upper crust and upper mantle is the same.In addition,the eastward erosion of asthenosphere material in the lower mantle of the Tibet plateau may also be another influencing factor for the replacement of mountain roots by heat flow in the Qinling Orogen Belt.