青藏高原东南缘中-下地壳地震波各向异性成因——来自云南六合深源包体组构学的约束

本文通过对出露于青藏高原东南缘云南六合地区的新生代深源岩石包体(斜长角闪岩、角闪石岩和石榴石透辉岩)的显微组构和地震波各向异性的研究来约束新生代青藏高原东南缘的地壳各向异性.通过角闪石地质压力计计算得知斜长角闪岩、角闪石岩和石榴石透辉岩包体来源于地壳28～36km,为中-下地壳岩石包体.EBSD测量结果显示包体中角闪石的CPO (晶格优选定向)为Type-IV型和(100)001]滑移,单斜辉石的CPO为SL型和(100)001]滑移,暗示中-下地壳为高温强变形的特征.通过CPO数据计算获得斜长角闪岩、角闪石岩和石榴石透辉岩包体全岩VP各向异性为1.9%～13.3%,最大分裂的剪切波各向异性(AVS)为1.17%～8.01%.结合前人的研究结果,该地区的地壳岩石能够解释利用Pms震相测量获得的分裂延迟时间,表明云南西北地区的壳内各向异性源于中-下地壳矿物的定向排列.云南西北地区的Pms快波方向近NW-SE向分布并与SKS的快波方向相近,暗示岩石圈变形是耦合的,受控于青藏高原向东南挤出的构造背景.

Origin of the middle-lower crust seismic anisotropy in SE Tibetan Plateau: Constrained from the EBSD data on deep-derived xenoliths in Liuhe,Yunnan Province

Pms (Moho converted Ps phase) measurements revealed complex seismic anisotropy of the crust beneath the southeastern Tibetan Plateau, while the interpretation of it is still lack of effective constraints from the deep crust rocks. In this study, we present data of the petrological, microstructures and seismic properties from Cenozoic deep-derived xenoliths (amphibolite, hornblendite and garnet diopside) and use them to derive constrains on the crustal seismic anisotropy in southeastern Tibetan Plateau. Based on the calculation of hornblende geobarometer, the amphibolite, hornblendite and garnet diopside xenoliths were middle-lower crust-derived xenoltihs, corresponding to a depth of 28~36 km depth within the crust. From Electron Back-Scatted Diffraction (EBSD) data, the dominant crystallographic preferred oreintation (CPO) of hornblende in xenoliths is Type-IV with (100)001] slip system. Clinopyroxene shows SL-type CPO with dominant (100)001] slip system. The CPO of hornblende and clinopyroxene recorded the deformation processes of middle-lower crust beneath the southeastern Tibetan Plateau, which was affected by intense deformation at high-temperature during Eocene-Oligocene. By using major phases fabric and their respective modal content to calculate, seismic anisotropy of xenoliths show that the V_P anisotropy range from 1.9% to 13.3%, and the maximum V_S anisotropy (AV_S) range from 1.17% to 8.01%. According to the previous studies, the delay time of Pms wave splitting around Liuhe in Yunnan Province mainly originates from the middle-lower crust. Thus, we propose that the crustal seismic anisotropy is attributed to the mineral orientation of amphibole-and mica-rich metamorphic rocks. Large-scale strike-slip shear zone on southeastern Tibetan Plateau, which causes aligned anisotropic minerals such as mica and amphibole in the middle-lower crust, can explain the nearly NW-SE fast polarization direction observed by Pms. The predominant fast wave polarization direction as observed by the Pms and SKS splitting is rather consistent in northwestern Yunnan, implied that the crust and lithospheric mantle are mechanically coupled and controlled by the southeastward extrusion of Tibetan Plateau.