Weiwei Ma, Bo Zhang, Fulong Cai, Baoyou Huang, Lei Zhang
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We present the microstructures, fabrics, calculated seismic velocities, and seismic anisotropies of mylonitic amphibolite from a horizontal shear layer preserved within the Ailao Shan-Red River shear zone, southwestern Yunnan, China. The investigated sample is characterized by pronounced centimeter-scale compositional banding. The microstructures and fabrics suggest that migmatitic amphibolite rocks within deep crust may delineate regions of deformation-assisted, channelized, reactive, porous melt flow. The origin of compositional banding in the studied migmatitic amphibolite is attributed primarily to partial melting together with some horizontal shearing deformation. The microfabrics and structures investigated in this study are considered to be typical for the base of active horizontal shear layers in the deep crust of southeastern Tibet. Seismic responses are modeled by using crystal preferred orientations for minerals of the migmatitic amphibolite by applying the Voigt-Reuss-Hill homogenization method. Calculated P-wave and S-wave velocities are largely consistent in the various layers of the migmatite. However, seismic anisotropies of P-wave (<i>AV</i><sub><i>p</i></sub>) and S-wave (<i>AV</i><sub><i>s</i></sub>) are higher in the melanosomes (<i>AV</i><sub><i>p</i></sub> = 5.6%, <i>AV</i><sub><i>s</i></sub> = 6.83%) than those in the leucosomes and the whole rock (<i>AV</i><sub><i>p</i></sub> = 4.2%–4.6%, <i>AV</i><sub><i>s</i></sub> = 3.1%–3.2%). In addition, there is pronounced, S-wave splitting oblique to the foliation plane in the migmatitic amphibolite. The multiple parallel compositional layers generate marked variation in the geometry of the seismic anisotropy (<i>V</i><sub><i>s1</i></sub> polarization) in the whole rock. Combined with the macroscale geographical orientation of fabrics in the Ailao Shan-Red River shear zone, these compositional banding effects are inferred to generate significant variations in the magnitude and orientation of seismic anisotropy, especially for shear-wave anisotropy (<i>AV</i><sub><i>s</i></sub>) in the deep crust. Hence, our data suggest that layering of various origins (e.g., shear layers, partial-melting layers, and compositional layers) represents a new potential source of anisotropy within the deep crust.</p>","PeriodicalId":15607,"journal":{"name":"Journal of Earth Science","volume":"18 1","pages":""},"PeriodicalIF":4.1000,"publicationDate":"2024-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Microstructures, Deformation Mechanisms and Seismic Properties of Synkinematic Migmatite from Southeastern Tibet: Insights from the Migmatitic Core of the Ailao Shan-Red River Shear Zone, Western Yunnan, China\",\"authors\":\"Weiwei Ma, Bo Zhang, Fulong Cai, Baoyou Huang, Lei Zhang\",\"doi\":\"10.1007/s12583-022-1678-1\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<p>Seismic anisotropy originating within the continental crust is commonly used to determine the deformation and kinematic flow within active orogens and is attributed to regionally oriented mica or hornblende grains. However, naturally deformed rocks usually contain compositional layers (e. g., parallel compositional banding). It is necessary to understand how both varying mineral contents and differing intensities of compositional layering influence the seismic properties of the deep crust. In this study, we analyzed the seismic response of migmatitic amphibolite with compositional banding structures. We present the microstructures, fabrics, calculated seismic velocities, and seismic anisotropies of mylonitic amphibolite from a horizontal shear layer preserved within the Ailao Shan-Red River shear zone, southwestern Yunnan, China. The investigated sample is characterized by pronounced centimeter-scale compositional banding. The microstructures and fabrics suggest that migmatitic amphibolite rocks within deep crust may delineate regions of deformation-assisted, channelized, reactive, porous melt flow. The origin of compositional banding in the studied migmatitic amphibolite is attributed primarily to partial melting together with some horizontal shearing deformation. The microfabrics and structures investigated in this study are considered to be typical for the base of active horizontal shear layers in the deep crust of southeastern Tibet. Seismic responses are modeled by using crystal preferred orientations for minerals of the migmatitic amphibolite by applying the Voigt-Reuss-Hill homogenization method. Calculated P-wave and S-wave velocities are largely consistent in the various layers of the migmatite. However, seismic anisotropies of P-wave (<i>AV</i><sub><i>p</i></sub>) and S-wave (<i>AV</i><sub><i>s</i></sub>) are higher in the melanosomes (<i>AV</i><sub><i>p</i></sub> = 5.6%, <i>AV</i><sub><i>s</i></sub> = 6.83%) than those in the leucosomes and the whole rock (<i>AV</i><sub><i>p</i></sub> = 4.2%–4.6%, <i>AV</i><sub><i>s</i></sub> = 3.1%–3.2%). In addition, there is pronounced, S-wave splitting oblique to the foliation plane in the migmatitic amphibolite. The multiple parallel compositional layers generate marked variation in the geometry of the seismic anisotropy (<i>V</i><sub><i>s1</i></sub> polarization) in the whole rock. Combined with the macroscale geographical orientation of fabrics in the Ailao Shan-Red River shear zone, these compositional banding effects are inferred to generate significant variations in the magnitude and orientation of seismic anisotropy, especially for shear-wave anisotropy (<i>AV</i><sub><i>s</i></sub>) in the deep crust. 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引用次数: 0
摘要
源于大陆地壳的地震各向异性通常用于确定活动造山运动中的变形和运动流,并归因于区域定向的云母或角闪石晶粒。然而,自然变形岩石通常包含成分层(如平行成分带)。有必要了解不同的矿物含量和不同强度的成分层如何影响深部地壳的地震特性。在本研究中,我们分析了具有成分带状结构的辉长岩的地震响应。我们展示了中国云南西南部隘老山-红河剪切带中保存的水平剪切层中的辉长岩的微结构、构造、计算地震速度和地震各向异性。调查样本具有明显的厘米级成分带状特征。这些微观结构和构造表明,深部地壳中的辉长岩可能划分出变形辅助、通道化、反应性、多孔性熔流区域。所研究的辉长岩中成分带的形成主要归因于部分熔融和一些水平剪切变形。本研究调查的微织物和结构被认为是西藏东南部深部地壳中活跃水平剪切层底部的典型结构。采用 Voigt-Reuss-Hill 均质化方法,利用辉长岩矿物的晶体优选取向建立地震响应模型。计算得出的 P 波和 S 波速度在辉绿岩各层基本一致。然而,黑云母的 P 波(AVp)和 S 波(AVs)的地震各向异性(AVp = 5.6%,AVs = 6.83%)高于白云母和整个岩石(AVp = 4.2%-4.6%,AVs = 3.1%-3.2%)。此外,在偏闪闪长岩中,存在明显的斜向褶皱面的 S 波分裂。多个平行的成分层使整个岩石的地震各向异性(Vs1 极化)的几何形状产生了明显的变化。结合隘老山-红河剪切带中织构的宏观地理方位,可以推断这些成分带效应会导致地震各向异性的幅度和方位发生显著变化,尤其是深部地壳中的剪切波各向异性(AVs)。因此,我们的数据表明,各种来源的分层(如剪切层、部分熔融层和成分层)是深地壳内各向异性的一个新的潜在来源。
Microstructures, Deformation Mechanisms and Seismic Properties of Synkinematic Migmatite from Southeastern Tibet: Insights from the Migmatitic Core of the Ailao Shan-Red River Shear Zone, Western Yunnan, China
Seismic anisotropy originating within the continental crust is commonly used to determine the deformation and kinematic flow within active orogens and is attributed to regionally oriented mica or hornblende grains. However, naturally deformed rocks usually contain compositional layers (e. g., parallel compositional banding). It is necessary to understand how both varying mineral contents and differing intensities of compositional layering influence the seismic properties of the deep crust. In this study, we analyzed the seismic response of migmatitic amphibolite with compositional banding structures. We present the microstructures, fabrics, calculated seismic velocities, and seismic anisotropies of mylonitic amphibolite from a horizontal shear layer preserved within the Ailao Shan-Red River shear zone, southwestern Yunnan, China. The investigated sample is characterized by pronounced centimeter-scale compositional banding. The microstructures and fabrics suggest that migmatitic amphibolite rocks within deep crust may delineate regions of deformation-assisted, channelized, reactive, porous melt flow. The origin of compositional banding in the studied migmatitic amphibolite is attributed primarily to partial melting together with some horizontal shearing deformation. The microfabrics and structures investigated in this study are considered to be typical for the base of active horizontal shear layers in the deep crust of southeastern Tibet. Seismic responses are modeled by using crystal preferred orientations for minerals of the migmatitic amphibolite by applying the Voigt-Reuss-Hill homogenization method. Calculated P-wave and S-wave velocities are largely consistent in the various layers of the migmatite. However, seismic anisotropies of P-wave (AVp) and S-wave (AVs) are higher in the melanosomes (AVp = 5.6%, AVs = 6.83%) than those in the leucosomes and the whole rock (AVp = 4.2%–4.6%, AVs = 3.1%–3.2%). In addition, there is pronounced, S-wave splitting oblique to the foliation plane in the migmatitic amphibolite. The multiple parallel compositional layers generate marked variation in the geometry of the seismic anisotropy (Vs1 polarization) in the whole rock. Combined with the macroscale geographical orientation of fabrics in the Ailao Shan-Red River shear zone, these compositional banding effects are inferred to generate significant variations in the magnitude and orientation of seismic anisotropy, especially for shear-wave anisotropy (AVs) in the deep crust. Hence, our data suggest that layering of various origins (e.g., shear layers, partial-melting layers, and compositional layers) represents a new potential source of anisotropy within the deep crust.
期刊介绍:
Journal of Earth Science (previously known as Journal of China University of Geosciences), issued bimonthly through China University of Geosciences, covers all branches of geology and related technology in the exploration and utilization of earth resources. Founded in 1990 as the Journal of China University of Geosciences, this publication is expanding its breadth of coverage to an international scope. Coverage includes such topics as geology, petrology, mineralogy, ore deposit geology, tectonics, paleontology, stratigraphy, sedimentology, geochemistry, geophysics and environmental sciences.
Articles published in recent issues include Tectonics in the Northwestern West Philippine Basin; Creep Damage Characteristics of Soft Rock under Disturbance Loads; Simplicial Indicator Kriging; Tephra Discovered in High Resolution Peat Sediment and Its Indication to Climatic Event.
The journal offers discussion of new theories, methods and discoveries; reports on recent achievements in the geosciences; and timely reviews of selected subjects.