The Study Of Upper Mantle Anisotropy Beneath Southeastern Tibet Plateau

The Tibet plateau has been subjecting to the collision between the Indian plate and the Eurasian plate.This crash can result in strong deformation and fracturing in the shallow depth.Also those processes give rise to large-scale surface uplift,deformation and strike-slip or thrust fault movement.Especially in the southeastern part of the plateau,they are characterized by complex Cenozoic structures,including surface uplift and crust thickening.There are a lot of physicists have proposed a variety of earth dynamics models to study those geological observations.But there is still inconclusive on the cause of such large-scale ground motion and lithospheric deformation features resulting from collision.We use 88 permanent broadband stations plotted in Yunan,Guizhou,Sichuan provinces operated by the China Earthquake Administration(CEA)and collect the SKS phase coming from the 125 events happening in 3 years,from July 2007 to July 2010,with epicentral distance from 85° to 120°.In this paper,for calculation of the anisotropy,we use multi-seismic synthesis weighted by signal to noise ratio to calculate the polarization direction of fast wave and the delay time between fast and slow waves.Also we both use single layer and double layer anisotropic calculation methods to measure the splitting parameters of the collected data.In the double layer anisotropic calculation,we use two different measuring methods to constrain the four splitting parameters of two anisotropic layers.One of the methods is strip-off.Its theory is,on the base of well constrained crust anisotropy,crust anisotropy can be stripped away from the SKS splitting results to get the lower layer,lithospheric mantle,anisotropy.The other method is inversion.Its theory is different from the stripped-off method,the crust anisotropy is not necessary to be known in advance.The four splitting parameters of the upper and lower layers can be inversed at the same time through two dimensions grid search process.The final objective of this study is getting the characteristics of lithospheric anisotropy.Then we can compare those with the other information including the feature of crustal anisotropy.The analysis about the crust and lithospheric mantle anisotropy may shed lights on the cause of surface uplift and the crust shortening.The happening depth of such uplift is also an important aspect need to research.The weak lower crust or the whole lithosphere both can support the large-scale surface uplift.The studies can offer the firm seismic evidences to the complicated geological structure under the study region,southeastern Tibet plateau and south Yunnan province.The main study content is to calculate the lithospheric anisotropy.And according to the different structure units,we can study the deformation relationship between them.The coupling deformation between crust and lithospheric mantle can be acquired by the multiple comparisons from surface geological structure,GPS data,crust anisotropy.In order to provide the possible existence of "lower crustal flow",we focus on the stations showing vertical varied anisotropy in the lithosphere.Applying the two layer calculation methods to such stations,we can constrain their anisotropy.On that base,the horizontal distribution of vertical varied anisotropy can be figured out to make clear about the "lower crustal flow".The study result shows a comprehensive knowledge about the plateau's geological observation,surface uplift.There is a distinct anisotropy in the crust under the region of southeastern Tibet plateau and Yungui plateau.With the thinner crust in the south of Yunnan,the splitting is too weak to be considered in the SKS splitting results.The anisotropy in the lithospheric mantle beneath Yungui plateau is weak,but there is observed splitting results beneath south Yunnan region.The north latitude 26.5°is a special transition zone that separates two different anisotropic pattern in the north and south part of study area.So the SKS splitting results may largely come from the lithospheric mantle in the south Yunnan region.These illustrate that both the horizontal and vertical deformation style in the crust and mantle are different.Around the Longmenshan area,the crust show strong anisotropy,but near the middle of Sichuan basin the crust anisotropy is weak and stable.Those are consistent with the history of earth evolution that the western margin of basin goes through huge extrusion and the inner basin has not too many active geological processes.The lithospheric mantle shows a little anisotropy with small delay time.Comparing with the crust,the distribution of lithospheric mantle anisotropy is equally,account for the stable deformation.