Analysis Of Deformation And Anisotropy Of The Lithosphere In SE Tibetan Plateau

The collision between Indian plate and Eurasia plate in Cenozoic caused intense deformation and uplift of the Tibetan Plateau.As an important site for the plateau material to expand to southeast under collision,the structure and dynamics process of SE Tibetan Plateau are crucial for understanding the tectonic evolution of Tibetan Plateau.Seismic anisotropy is an important means to understand the internal deformation of the Earth.In recent years,a large number of seismic anisotropy measurements have been carried out on SE Tibetan Plateau,including Pms(Moho converted Ps phase),SKS(shear wave splitting),etc.Researchers have proposed different tectonic models corresponding to the geophysical measurements.However,these models lack the constraints from deep petrology to determine the rationality and reliability of geophysical measurements.Studies have shown that seismic anisotropy is mainly attributed to the crystallographic preferred orientation(CPO)of anisotropic mineral in rocks.The seismic anisotropy of middle–lower crust is mainly formed by the CPO of major minerals(mica and hornblende)in the rock.Seismic anisotropy can be mainly explained by the CPO of olivine and pyroxene,which are dominant minerals in upper mantle.The comparison between seismic anisotropy calculated from mineral CPO and regional anisotropy deduced from geophysical measurements is therefore particularly useful for interpreting geodynamic and geochemical process which affect the lithosphere.Therefore,researches on deformation of the rocks in deep earth beneath SE Tibetan Plateau are needed to constrain the seismic anisotropy,and deformation characteristics of the lithosphere beneath SE Tibetan Plateau.In this paper,we performed detailed studies of petrology,geochemistry,microstructure and seismic anisotropy of the deep–derived xenoliths captured by Cenozoic potassic–ultrapotassic rocks on SE Tibetan Plateau.Combining with the geophysical measurements carried out in SE Tibetan Plateau,tectonic model of the lithosphere around SE Tibetan Plateau was established,and the lithosphere deformation characteristics of the SE Tibetan Plateau in Cenozoic were confined.The deep–derived xenoliths hosted by early–stage ultrapotassic rocks in northwestern Ailaoshan–Red River shear zone on SE Tibetan Plateau are mainly garnet diopsides,amphibolites and hornblendites.Composition analysis results show that the source depths of amphibolite and hornblendite xenoliths range from 28 to 36 km,indicating the middle–lower crust origin.However,the source depth of garnet diopsidite xenoliths range approximately from 46 to 104 km,which were from the bottom of the lower crust to the upper mantle.EBSD data show that the dominant deformation fabric of hornblende in crustal xenoliths is mainly Type–IV and(100)[001] slip system,suggesting the high temperature and intense deformation in middle–lower crust.While the characteristics of Type–I hornblende CPO are rare.The dominant CPO of plagioclase is Axial–B type and(100)[001] slip system.Clinopyroxene shows(100)[001] slip system and SL–type CPO.Garnets do not show significant CPO type.All these results show that deformation of the middle–lower crust on SE Tibetan Plateau was characterized by high temperature and intense deformation in Eocene–Oligocene.Seismic anisotropy calculated from the EBSD measurements shows that VP anisotropy(AVP)of the whole crustal xenoliths range from 1.9 % to 13.3 %,and the maximum anisotropy of shear wave(AVS)range from 1.17 % to 8.01 %.The 0.54 s delay time observed by Pms measurement around Liuhe in Yunnan Province could be explained by 15 km thick middle–lower crust metamorphic rocks with approximately vertical foliation.The deep–derived xenoliths enclosed by late–stage ultrapotassic rocks at southeastern Ailaoshan–Red River shear zone are mainly spinel lherzolites and spinel harzburgites.Calculated equilibrium temperatures of the peridotite xenoliths predominantly range from 1016 °C to 1148 °C.The calculated equilibrium pressure(source depth)range from 15.6 kbar to 22.2 kbar(51–71 km),indicating that the deep–derived xenoliths are originated from lithospheric mantle.Water content calculated from unpolarized FTIR analyses show that olivine,orthopyroxene and clinopyroxene contain 5–21 ppm,66–207 ppm,136–357 ppm wt H2 O,respectively.Major mineral compositions show that the peridotite xenoliths underwent different degree of partial melting.Microstructure characteristics demonstrate that the peridotite in lithospheric mantle underwent dislocation creep followed by effective static recrystallization and Si-rich fluid percolation.EBSD data show that the dominant fabric of olivine is AG–type and quasi–A–type CPO.CPO of orthopyroxene and clinopyroxene is characterized by their [001] sub–parallel to the [100] of olivine.The dominant CPO of orthopyroxene is AC–type and ABC–type.Clinopyroxene shows S–type and SL–type CPO.Seismic anisotropy calculated based on olivine and pyroxene CPO data displays that these peridotite xenoliths have moderate to strong polarization anisotropy(AVs)ranging from 2.91 % to 7.03 %,and the VP anisotropy of the whole mantle xenoliths range from 4.41% to 8.13%.Olivine and pyroxene CPO show that the lithospheric mantle peridotites deformed by dislocation creep under high temperature,dry and low stress conditions.AG–type olivine CPO is related to transpression and melt assisted deformation during the Eocene–Miocene in lithospheric mantle.Compared with the seismic anisotropy measured by SKS,our results imply that the seismic anisotropy on SE Tibetan Plateau is attributed to the CPO of olivine and pyroxene coupled with oreinted melt pockets(MPO)in lithospheric mantle with approximately vertical foliation.Based on the researches above,we can infer that the intense shear in large–scale strike–slip fault zone on SE Tibetan Plateau aligns anisotropic minerals such as mica and hornblende in the middle–lower crust and can explain the N–S nearly NW–SE fast polarization direction observed by Pms.The deformation of lithospheric mantle is controlled by the uplift and extrusion flow of asthenosphere.The flow direction of the asthenosphere change from NW–SE to E–W at 26°N.Thus on SE Tibetan Plateau,the crust and lithospheric mantle are vertically coherently deformed in the north of 26°N.While in the south of 26°N,the crust and lithospheric mantle are decoupled.