西藏东巧方辉橄榄岩的显微构造特征及其流变学意义

西藏东巧地区蛇绿岩套中橄榄岩是青藏高原出露的为数不多的地幔岩体之一, 对于揭示该地区的上地幔流变学特征具有重要意义.报道了该地区方辉橄榄岩中橄榄石的位错显微构造特征, 估算了上地幔流变学参数.显微构造研究表明, 东巧方辉橄榄岩发育残斑结构, 橄榄石中位错组态类型比较丰富, 包括自由位错、位错壁(包括宽阔型和紧密型)、位错弓弯和位错网, 表明橄榄石的主导变形机制可能为位错蠕变.该区地幔岩变形大致分为2个阶段: (1) 地幔缓慢塑性流动变形, 形成宽阔型位错壁; (2) 蛇绿岩侵位过程中的变形, 产生紧密型位错壁.根据2种位错壁估算的流动应力平均值分别为3 9.3MPa (宽阔型)和113.9MPa (紧密型), 计算结果获得东巧地区上地幔流动速率为1.13× 10-12 ~ 2.95× 10-11s-1, 有效粘度为4.44×1017~ 1.16×1019Pa°s, 这些参数为东巧地区上地幔物理特征提供了流变学的约束条件      

橄榄石中H的结合机制及扩散行为

名义上无水矿物(nominally anhydrous minerals,NAMs)中以点缺陷形式存在的结构水,因其对于矿物物理化学性质的显著影响而受到越来越广泛的关注。橄榄石是上地幔中含量最丰富的矿物,水在橄榄石中的存在形式、扩散机制和速率对上地幔的流变学和电导性质有着重要的影响。因此,对于橄榄石中H的结合机制及其扩散机制和速率的了解有助于理解地球深部的水循环,也有助于构建上地幔流变结构以及解释电导测量结果。本文总结了近几十年来学界对于H在橄榄石中存在的缺陷类型与OH红外光谱吸收峰之间的对应关系,以及H在矿物晶格中的扩散机制、扩散速率等重要问题的研究成果,并探讨了现有研究中依然存在的问题。     

江苏六合方山幔源包体橄榄石的位错构造研究

本研究采用光学显微镜(氧化缀饰)法和透射电子显微镜法,首次查明江苏六合方山玄武岩的包体橄榄石中存在自由位错、位错壁、位错弓弯、位错环、亚晶粒和位错扭折条带等显微、超显微构造,它们是包体橄榄石在上地幔高温、高压条件下发生塑性蠕变的产物,不同位错形态反映主矿物塑性变形的不同状态。根据橄榄石中自由位错密度和包体中辉石的化学成分,估算江苏六合方山地区上地幔某些流变学参数为:温度(t)为1052℃,压力(P)为2060MPa;深度(Z)为66.6km;流动应力(σ_1—σ_3)为51.3MPa;流动速率(ε)为3.6×10~(-14)/s;粘稠应(η)为5.7×10~(21),这些参数值与中国东部其它地区上地幔的同类参数值相接近。     

根据实验室蠕变资料对地幔流变的推断

板块构造理论应基于上地幔的对流。研究蛇绿岩以及碱性玄武岩和金伯利岩中的地幔包体表明,上地幔主要是由橄榄岩组成。橄榄石既是橄榄岩中最多的矿物(约占60~90%),也是最容易发生塑性变形的矿物。因此,大量实验工作的目的在于了解橄榄石的流变学问题。尽管橄榄石的流动性质已在实验室中进行了广泛的研究,但是,以往的实验压力都局限在1.8GPa以下。橄榄石蠕变象大部分高温条件下物质一样,是可以用幂律蠕变方程来描述: :%#::式中:ε—应变速率,А—常数,σ—流动应力,n—常数(通常3—4),     

上地幔的岩石组构和各向异性

着重介绍了上地幔各向异性及其成因,研究内容和方法,各向异性研究的地球动力学意义,并强调指出橄榄石和辉石在塑性固态流动过程中产生的晶格优选方位或组构是导致上地幔各向异性最主要原因。     

祁连山玉石沟橄榄岩显微构造特征及演化

橄榄岩作为上地幔的主要成分,影响着上地幔的流变学行为,其显微构造记录了岩石形成发展过程中所经受的构造事件.通过对祁连山玉石沟橄榄岩样品的镜下显微构造观察、岩石组构测定及TEM位错分析,探讨了与变形相关的温度、围压、含水性和应变速率等因素,总结该岩石的变形机制和变形历史,并推断其形成发展时大地构造环境.研究表明,玉石沟橄榄岩产自上地幔,其变形改造经历了上地幔演化和脆-韧性变形2个阶段.上地幔演化阶段橄榄石发育明显的A型原生组构,电子背散射衍射技术（EBSD）测定的橄榄石显微组构表明,该地区橄榄岩形成于高温（＞ 1200℃）、低应力（＜350 MPa）、低应变速率、低含水量的地幔浅部环境条件下;脆-韧性变形阶段叠加改造了原生A型组构,而发育明显的D型次生组构.橄榄石变形主控因素为动态恢复作用,普遍发育亚晶粒、消光带和扭折显微构造等相关组构,并与透射电镜下所观察到的位错排对应,同时还发育以微破裂为主的一套脆性变形组构,表现出两组共轭剪破裂和另一组张性破裂等现象,揭示出与韧性动态恢复现象一致的应力场方向指示.     

夏威夷橄榄岩中橄榄石的水含量存在多尺度的不均一性

<正>橄榄石是上地幔的主要组成矿物,尽管它的理想化学式中不含H,但是晶格缺陷中仍然能够含有10-6量级的"水"。虽然含量少,但是这些结构水对于矿物(以至于岩石系统)的熔融行为、流变学特征以及电导率和热导率都有显著的影响。因此,研究橄榄石中H的分布和活动性是理解地球动力学过程的重要内容,也是近年来学界的热点之一。美国NASA的研究员Anne Peslier博士及其合作者最近对来自夏威夷     

雅鲁藏布江蛇绿岩山幔型韧性剪切带的发现及其意义

本文论述了在雅鲁藏布江蛇绿岩中发现的大型幔型韧性剪切带的几何学和运动学特征，通过研究超镁铁质糜棱岩中变形橄榄石的显微构造，分析了上地的流变状态，探讨了幔型韧性剪切带生成的构造环境和阿尔卑斯型铬铁矿的地幔剪切动力成因机制。     

从单晶褶皱看斜方辉石流变学的特征

迄今为止,上地幔流变学的研究主要集中于橄榄石单晶与多晶集合体的实验和野外观察,但对辉石的流变学特征知之甚少,虽然辉石亦是上地幔岩石的主要组成矿物.因此,查清斜方辉石的流变学行为、塑性变形机制、重结晶作用以及与其共生的橄榄石之间的流变强度差等,现已成为国际地学界研究的新的热点课题.新疆中天山南缘库米什地区的榆树沟高压变质地体中出现地幔超糜棱岩,其中带状拉伸的斜方辉石(En90)碎斑晶发生了强烈地弯滑褶皱,周围橄榄石(Fo90)重结晶成细粒(～10 μm)多晶基质.显微构造研究表明,单晶斜方辉石的褶皱主要通过单一的(100)[001]滑移进行的,而细粒重结晶橄榄石基质的变形机制则最可能以超塑性(颗粒边界滑移与扩散)为主.单晶斜方辉石发生弯滑褶皱,而不形成常见的膝折(Kinks),说明所处物理化学条件下斜方辉石晶体位错攀移与原子扩散并不足够活跃.此外,即使其晶格发生高达140.的旋转,斜方辉石依然没有发生光学显微镜下足以识别的重结晶结构,说明启动重结晶作用的临界剪切应变应不少于5.5.根据现有的层状材料的褶皱理论,推测榆树沟高压变质地体中地幔岩发生塑性变形时位错蠕变的斜方辉石的流动强度至少比相同条件下超塑性变形的细粒橄榄石多晶基质高近2个数量级.     

估算上地幔温压条件与氧化状态的最新热力学方法

上地幔的温压条件与氧化状态,对于了解上地幔的热结构、物态与相关系、深部地质作用、特别是岩浆起源及超镁铁岩的成因具有重要的意义。本文概述了近年来新提出的估算尖晶石、石榴石二辉橄榄岩相矿物平衡的温度、压力和氧逸度的热力学方法,对于从事金伯利岩、蛇绿岩和上地幔橄榄岩包体的研究具有重要的实用价值。     

青藏高原的形成和演化机制

青藏高原是地球上最高大和最雄伟的年青隆起区。对于它的形成和演化机制,一直是国内外地质和地球物理学者注意的问题之一。 近十年来,人们认为青藏高原的形成是由于相距千里之遥的印度板块向北漂移并与欧亚板块碰撞的结果。然而,根据作者对喀喇昆仑和喜马拉雅等地的野外考察及其深部地球物理资料的研究,提出青藏高原原来是一个统一的前震旦纪陆壳区,后经震旦纪以来多次的拉开和挤压碰撞而形成的新观点。这种拉开和挤压的运动方式,是深部鳗隆和慢拗的分异作用引起的。     

Geophysical constraints on geodynamic processes at convergent margins: A global perspective

Convergent margins, being the boundaries between colliding lithospheric plates, form the most disastrous areas in the world due to intensive, strong seismicity and volcanism. We review global geophysical data in order to illustrate the effects of the plate tectonic processes at convergent margins on the crustal and upper mantle structure, seismicity, and geometry of subducting slab. We present global maps of free-air and Bouguer gravity anomalies, heat flow, seismicity, seismic Vs anomalies in the upper mantle, and plate convergence rate, as well as 20 profiles across different convergent margins. A global analysis of these data for three types of convergent margins, formed by ocean–ocean, ocean–continent, and continent–continent collisions, allows us to recognize the following patterns. (1) Plate convergence rate depends on the type of convergent margins and it is significantly larger when, at least, one of the plates is oceanic. However, the oldest oceanic plate in the Pacific ocean has the smallest convergence rate. (2) The presence of an oceanic plate is, in general, required for generation of high-magnitude (M > 8.0) earthquakes and for generating intermediate and deep seismicity along the convergent margins. When oceanic slabs subduct beneath a continent, a gap in the seismogenic zone exists at depths between ca. 250 km and 500 km. Given that the seismogenic zone terminates at ca. 200 km depth in case of continent–continent collision, we propose oceanic origin of subducting slabs beneath the Zagros, the Pamir, and the Vrancea zone. (3) Dip angle of the subducting slab in continent–ocean collision does not correlate neither with the age of subducting oceanic slab, nor with the convergence rate. For ocean–ocean subduction, clear trends are recognized: steeply dipping slabs are characteristic of young subducting plates and of oceanic plates with high convergence rate, with slab rotation towards a near-vertical dip angle at depths below ca. 500 km at very high convergence rate. (4) Local isostasy is not satisfied at the convergent margins as evidenced by strong free air gravity anomalies of positive and negative signs. However, near-isostatic equilibrium may exist in broad zones of distributed deformation such as Tibet. (5) No systematic patterns are recognized in heat flow data due to strong heterogeneity of measured values which are strongly affected by hydrothermal circulation, magmatic activity, crustal faulting, horizontal heat transfer, and also due to low number of heat flow measurements across many margins. (6) Low upper mantle Vs seismic velocities beneath the convergent margins are restricted to the upper 150 km and may be related to mantle wedge melting which is confined to shallow mantle levels.     

Geoelectric section of the crust and upper mantle of the northern Sikhote-Alin from magnetotelluric sounding data

Magnetotelluric soundings (MTS) were conducted in a broad frequency range of 10 kHz to 0.001 Hz at a total of fifty-seven sounding sites of the profile spaced 5 km apart and intersecting the northern Sikhote-Alin across the strike. The analysis of the obtained magnetotelluric parameters has been made which shows three-dimensional geoelectric nonuniformities in the lower crust and upper mantle. The MTS curve interpretation was carried out in the framework of a three-dimensional model. As a result of the inverse problem solution, the geoelectric section has been constructed down to 150 km depth. The section distinguishes the crust with a resistivity higher than 1000 Ohm m and variable thickness between 30 and 40 km which is consistent with deep seismic sounding (DSS) data. The crust is subdivided into four blocks by deep faults, and each block is characterized by a set of parameters. The data support the existence of the Vostochny deep fault in the study area, whereas, on the contrary, the deep roots for the Central Sikhote-Alin fault have not been established. The upper mantle structure is nonuniform; three low-resistivity zones are identified that coincide with the boundaries of crustal blocks. In the revealed zones, an increase in the resistivity is noted from the continent to the Tatar Strait coast. A high-resistivity layer of 300–400 Ohm m was observed in the coastal area, which was steeply dipping from the crustal base down to 120 km depth and extended beneath the continent. Based on a set of geological and geophysical data, the ancient subducting plate is suggested in this area, and the evolutionary model of the region is proposed starting from the Late Cretaceous. The most probable mechanism of conductivity within the upper mantle is determined from petrological and petrophysical data. The low resistivity values are linked to dry peridotite mantle melting.     

Thermal Structure and Rheology of the Upper Mantle beneath Guangdong and Hainan Provinces, China

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Seismic investigation of crust-mantle transition in continental rift systems - Jordan-Dead Sea rift and Rhinegraben

The recent acquisition of high-quality seismic refraction data in the Jordan—Dead Sea rift and adjacent areas has made possible the investigation of the dynamic properties of seismic P-waves refracted and reflected at the crust—upper mantle boundary.

These waves cause high-amplitude arrivals near the outer cusp of the travel-time curve which are followed by an abrupt decrease in amplitudes at increasing distances beyond the cusp.

It has been shown that such amplitude distributions can only be the result of a smooth rapid increase of velocity with depth. In the case of the Jordan—Dead Sea rift the amplitude distribution indicates the presence of a transition zone between the lower crust and upper mantle in which the velocity increases smoothly. The interpretation of seismic refraction data in the Rhinegraben indicates the existence of a similar transition zone. In both rifts the crust—mantle boundary outside the rift is represented by sharp velocity discontinuity.

The comparison of the velocity structure of the crust—upper mantle boundary suggests that a smooth transition zone at the base of the lower crust is a characteristic property of continental rifts which could be interpreted in terms of crust—mantle interaction.     

从地壳上地幔构造看大陆岩石圈伸展与裂解

本篇讨论大陆岩石圈拆沉、伸展与裂解作用过程。由于大陆岩石圈厚度大而且很不均匀,产生裂谷的机制比较复杂。大陆碰撞远程效应的触发,岩石圈拆沉,以及板块运动的不规则性和地球应力场方向转折,都可能产生岩石圈断裂和大陆裂谷。岩石圈拆沉为在重力作用下"去陆根"的作用过程,演化过程可分为大陆根拆离、地壳伸展和岩石圈地幔整体破裂三个阶段。大陆碰撞带、俯冲的大陆和大洋板块、克拉通区域岩石圈,都可能产生岩石圈拆沉。大陆岩石圈调查表明,拉张区可见地壳伸展、岩石圈拆离、软流圈上拱和热沉降;它们是大陆岩石圈伸展与裂解早期的主要表现。从初始拉张的盆岭省到成熟的张裂省,拆离后地壳伸展成复式地堑,下地壳幔源玄武岩浆侵位,断裂带贯通并切穿整个岩石圈,表明地壳伸展进入成熟阶段。中国东北松辽盆地和西欧北海盆地曾处于成熟的张裂省。岩石圈破裂为岩浆侵位提供了阻力很小的通道网。岩浆侵位作用伴随岩石圈破裂和热流体上涌,成熟的张裂省可发展成大陆裂谷。多数的大陆裂谷带并没有发展成威尔逊裂谷带和洋中脊,普通的大陆裂谷要演化为威尔逊裂谷带,必须有来自软流圈的长期和持续的热流和玄武质岩浆的供应。威尔逊裂谷带岩石圈地幔和软流圈为地震低速带,其根源可能与来自地幔底部的地幔热羽流有关。     

3D thermal structure of the continental lithosphere beneath China and adjacent regions

Based on the Crust2.0 model and the topography data of Chinese continent and its adjacent regions, a three-dimensional finite element model is constructed in terms of the spherical coordinate system. In our numerical model, the average annual ground temperature from 195 meteorological stations and temperature of upper mantle derived from the seismic velocities are adopted as the top and bottom boundary conditions, respectively. The observed thermal conductivity and heat production from P wave velocity based on empirical formula are employed in our numerical model as well. The comparison between the calculated and observed surface heat flow proved that our results are reliable. The temperature beneath the Precambrian cratons is lower than that of other areas for 100–300 °C also. The typical temperature rang at the Moho is estimated to be 800–1000 °C beneath the Tibetan plateau and 500–700 °C beneath the Precambrian cratons (such as Indian plate, Sichuan basin, South China, North China and Tarim), respectively. The thermal state in the eastern part of Sino-Korean craton at the depth deeper than 60 km indicates that it was destructed. The thermal structure in center of Tibetan plateau (especially beneath Qiangtang area) supports the proposed flow of lower crustal or upper mantle material to the east. Generally, the distribution of volcanoes in Chinese continent is consistent with the high temperature areas in the crust or upper mantle. There are many obvious thermal transition zones across the orogenic belts. The thermal transition zone between eastern and western parts in the crust of Chinese continent is consistent with the north–south seismic zone.     

南海东北部及其邻近地区地壳上地幔P波速度结构

利用中国地震台网和ISC台站记录的P波到时数据,采用球坐标系有限差分地震层析成像方法反演了南海东北部及其邻近地区壳幔三维P波速度结构,并分析了不同地质单元的构造差异及其深部特征。结果表明:南海东北部表现出陆架地区的岩石层特性,属于华南大陆向海区的延伸,岩石层厚度较大,现今不存在大规模的地幔热流活动,推测大陆边缘张裂作用仅限于地壳内部而没有延伸进入上地幔,具有非火山型大陆边缘的深部特点。中央海盆附近上地幔P波速度明显降低,与海盆下方地幔热流活动密切相关。不同的速度异常特征表明:华南大陆暨台湾地区属于欧亚大陆的正常地壳或是与菲律宾海板块相互作用产生的增厚型地壳,冲绳海槽则是弧后扩张产生的减薄型地壳。滨海断裂带作为华南大陆高速异常和南海北部高速异常的分界,代表了一定地质时期华南地块和南海地块的拼合边界。断裂附近的上地幔低速异常揭示了闽粤沿海岩浆作用的深层动力机制。吕宋岛弧、马尼拉海沟、东吕宋海槽的速度异常与其所处的特殊构造位置有密切的关系,清晰地反映出岛弧俯冲带的地壳结构差异;台湾南部至吕宋岛弧的上地幔低速异常揭示了两个重要火山链的深部构造特征,北吕宋海脊下方100 km深度的条带状高速异常有可能代表了俯冲下沉的岩石层板片。     

渤海地温场特点的初步研究

渤海位于华北断块区的东部,是华北古生代地台之上发育起来的中、新代断陷盆地,是陆上埕宁隆起和辽河、黄骅及济阳三大拗陷的自然延伸地带。渤海海域的地热特点,一直为人们所关注。在研究渤海的形成和演化时,有人对渤中凹陷的地热状况作了推论;有人根据少量的地热资料认为渤海及其邻近地区是一个高热流异常区。我们根据渤海海域的地温数据,和近年对华北平原地温场的研究结果进行对比,着重对本区地壳浅部的地热状况进行分析和讨论,提出一些初步的看法。     

An overview of electrical conductivity structures of the crust and upper mantle beneath the northwestern Pacific, the Japanese Islands, and continental East Asia

This article reviews the electrical conductivity structures of the oceanic upper mantle, subduction zones, and the mantle transition zone beneath the northwestern Pacific, the Japanese Islands, and continental East Asia, which have particularly large potential of water circulation in the global upper mantle. The oceanic upper mantle consists of an electrically resistive lid and a conductive layer underlying the lid. The depth of the top of the conductive layer is related to lithospheric cooling in the older mantle, whereas it is attributable to the difference in water distribution beneath the vicinity of the seafloor spreading-axis. The location of a lower crustal conductor in a subduction zone changes according to the subduction type. The difference can be explained by the characteristic dehydration from the subducting slab in each subduction zone and by advection from the backarc spreading. The latest one-dimensional electrical conductivity model of the mantle transition zone beneath the Pacific Ocean predicts values of 0.1–1.0 S/m. These values support a considerably dry oceanic mantle transition zone. However, one-dimensional electrical profiles may not be representative of the mantle transition zone there, since there exists a three-dimensional structure caused by the stagnant slab. Three-dimensional electromagnetic modeling should be made in future studies.