Lithospheric Deformation Characteristics And Related Mechanisms In SE Tibetan Plateau

The southeastern Tibetan plateau(SETP),located in the frontal margin of the India-Eurasia continental-continental collision zone,is widely characterized by the large-scale strike-slip active faults and generates many devastating earthquakes,which is mainly controlled by the Indian-Eurasian collision and subducting of the IndianBurma and Pacific-Philippine slabs.However,the lack of geophysical observations hinders our understanding of the lithospheric deformation and related dynamic mechanisms.In this study,we firstly construct a 3D accurate crustal deformation model based on dense GPS observations,and analyze the deformation characteristics and corresponding thermal dynamics of the SETP by combining relocated small earthquakes,deep geophysical observations and active tectonics.Secondly,to study the middle-to-lower crust and lithospheric mantle deformation,we collect and reanalyze the published seismic anisotropy observations(i.e.,XKS and Pms splitting orientations,and surface wave anisotropy),and argue the caveats in explaining these seismic anisotropy observations in previous studies.Thirdly,we quantify the large-scale mantle flow field underneath East Asia based on assumption that the athenospheric seismic anisotropy comes from the shear between lithosphere and sub-asthenospheric mantle with the reality of the deviating of GPS velocities from rigid plate motion in East Asia.Fourthly,by combining the deep geophysical detection data,including seismic tomography and magnetotelluric sounding,with 3D crustal movement model,we discuss the middle-to-lower crustal structures and possibility of the existence of the crustal flow and its influence to lithospheric deformation in SETP.Finally,we discuss the dynamic mechanisms of the complex lithospheric deformation in SETP based on surface wave anisotropy,crustal deformation model and XKS splitting orientations and mantle flow.Our results are as follows:(1)A set of dextral NW/NNW-striking secondary dextral slip faults in SETP yield similar slip rates of 1-3 mm/yr with the Red River and Lancangjiang faults,and is characterized by distributed deep structures.Together with thin brittle upper crust of 8-11 km and lower middle-to-lower crust viscosities,we argue that the contemporary crustal deformation in SE Tibetan Plateau is characterized by on-going diffuse deformation between the Sagaing and Xianshuihe-Xiaojiang faults,which was established in the mid-late Miocene.(2)The crust of the southern Sichuan-Yunnan block is coupling and rotates along the eastern Himalayan Syntaxis and wholly extrude outward driven by the northward advance of the Indian plate.With the block of the Indochina,the strain transformation occurs in the north of the Red River fault,which decreases the sinistral slip of the Xiaojiang fault and increases the seismicity of the Jianshan and Qujiang faults with the generation of many major earthquakes including 1970 M7.7 Tonghai event.(3)Two middle-to-lower crustal low seismic velocity passages exist and is divided by the high seismic anomaly zone,which owes to the Emeishan basalts and is modified by the Emeishan mantle plume erupting in the Permian.Meanwhile,the existence of the high seismic velocity anomaly zone indicates that the geometry of the crustal flow should be channelized if there is middle-to-lower crustal flow in SETP.Moreover,two low seismic velocity channels are highly related with shear deformation along two surface shear zones.(4)There is two-layer anisotropy in SETP even up to East Asia,including lithospheric and athenospheric anisotropy.The explanation of the athenospheric anisotropy requires to consider the shear between the lithospheric plate motions with respect to intermediate APM reference frame,between HS3 and NNR APM model,and related sub-asthenospheric mantle flow with magnitude of 1-2 cm/yr.The depth of this mantle flow is from the Transition zone(at 410-660 km continuation to 700 km)to lower mantle.This mantle flow drives East Asia plate motion and connect IndianEurasian continent-continent collision with subduction of the Pacific plate.(5)The contribution of the lithospheric anisotropy to XKS splitting orientations may be related with lithospheric thickness and magnitude of the crustal deformation.In SETP,thin lithosphere(50 to 60 km)and low crustal deformation indicate little contribution of lithospheric anisotropy,and deformation of the lithospheric mantle is manily resulting from deep mantle.Therefore,seismic anisotropy mainly comes from differential motion between lithosphere and sub-asthenospheric mantle flow.Additionally,there may be two kinds of mantle flow,including lithospheric/asthenospheric mantle flow(at depth of ?100 km)driven by Indian and Sunda subduction and sub-asthenospheric mantle flow(at depth of 410 km to lower mantle)attributed to subduction of the Pacific slab and/or mantle upwelling in South Africa.(6)The middle-to-lower crustal flow no matter if it has been formed by the escaping of the crustal materials and the whole lateral extrusion of the Sichuan-Yunnan block are both the shallow lithospheric dynamic process of the lithospheric deformation in SETP.The temporal and spatial evolution and transition of the complex mantle dynamic systems such as Indian-Eurasian collision,subductions of the Indian-Burma and Sunda and Pacific Slabs,and upwelling of the South Africa seem to be significant to induce the complex lithospheric deformation in SETP.