水对上地幔电性结构的影响

上地幔可以存储大量的水,水能够极大地增强地幔矿物的电导率。在实验室高温高压条件下获得了含水和不含水矿物电导率不同的结论,有必要广泛开展含水矿物电导率实验研究。基于最新实验结果,充分考虑上地幔热力学条件,矿物体积随深度变化和矿物间水分配情况,对2个研究小组上地幔4种主要矿物实验室电导率结果采用平均方法建立随水含量变化的上地幔电导率-深度曲线,并与实测地球物理模型进行了对比。研究表明,水能够极大地影响上地幔的电性结构,推测上地幔平均水质量分数为0.01%,位于地球化学方法推断的上地幔水质量分数范围(50×10-6~200×10-6)。海洋上地幔软流圈高电导率异常很可能是碳酸盐熔体、硅酸盐熔体及水共同作用的结果。     

华北大陆地壳─上地幔岩石学结构与演化

本文基于出露的前寒武纪变质岩系、中、新生代岩浆活动以及新生代玄武岩中上地幔包体的岩石学与地质压力计研究，结合地球物理测涤资料与高温高压下岩石中地震波传播速度的实验成果，提出了华北大陆三个地区（河北平原、太行－五台、鄂尔多斯）的地壳－上地幔岩石学结构，讨论了界面性质及其演化。在强调ｖ＿ｐ、ｖｂ、σ结构与岩石学结构共同约束的基础上，有效地识别了不同地区硅铝质陆壳在物质组成上的差异和上地幔低速层或矿物相转变等特征。本文提出壳－幔岩石学结构及其演化，密切地与陆壳主要形成时期的太古－早元古构造岩浆事件相关，又与显生宙构造岩浆事件对它的改造程度有关，壳－幔岩石学结构是我们追索大陆的构造性质及其演化的一个重要记录和科学依据。     

华北大陆地壳—上地幔岩石学结构与演化

本文基于出露的前寒武纪变质岩系、中、新生代岩浆活动以及新生代玄武岩中上地幔包体的岩石学与地质压力计研究，结合地球物理测深资料与高温高压下岩石中地震波传播速度的实验成果，提出了华北大陆三个地区(河北平原、太行一五台、鄂尔多斯)的地壳一上地幔岩石学结构，讨论了界面性质及其演化。在强调Vp、 Vs、a结构与岩石学结构共同约束的基础上，有效地识别了不同地区硅铝质陆壳在物质组成上的差异和上地幔低速层或矿物相转变等特征。本文提出壳-幔岩石学结构及其演化，密切地与陆壳主要形成时期的太古一早元古构造岩浆事件相关，又与显生宙构造岩浆事件对它的改造程度有关，壳-幔岩石学结构是我们追索大陆的构造性质及其演化的一个重要记录和科学依据。     

华南东南部上地幔远震P波速度结构及意义

本研究利用114个固定台站记录的121个远震事件,以钦杭结合带为中心,采用天然地震层析成像构建了华南东南部上地幔P波速度结构模型。研究结果表明：（1）钦杭结合带、武夷成矿带以及南岭成矿带的深部结构存在着差异,说明3个成矿带经历了不同的构造演化过程;（2）江绍断裂的上地幔中存在着低速异常,推测该低速异常为从地幔过渡带或者下地幔上涌的热物质,与钦杭结合带和武夷成矿带的成矿作用有着密切的关系;（3）下扬子地区上地幔底部的高速异常可能为拆沉的岩石圈,而华夏板块上地幔顶部的高速异常则有待进一步研究。本研究的结果为认识华南东南部的深部结构提供了新的证据。     

下地壳的研究现状及展望

地球物理、地球化学及超深钻资料揭示出,下地壳无论在横向或垂向上都是极不均匀的,其中存在着众多的地震反射面、大型低角度逆掩断层及不同波速的层状构造.大陆地壳剖面由上至下显示出由绿片岩相→角闪岩相→麻粒岩相的变质分带现象.证实了Conrad面在横向上是不连续的及地球物理意义上的Moho面与岩石学和地球化学意义上的壳一幔界面之间存在着差异性.证实了下地壳中高导层的广泛存在.一般认为高导层主要是由自由的含水流体造成的.其成因的正确解释对于我们深入理解与岩石圈动力学有关的问题起着重要作用.关于下地壳的岩石类型,现已趋于认为主要由麻粒岩相岩石构成,可能还包括少量角闪岩相岩石.但关于下地壳的化学组成争议较大.已提出的成分模型范围可从基性—中性—酸性.同时还发现许多地区麻粒岩相岩石具有贫K、Th、U及其它LIL元素的特征.地球物理与同位素证据还表明,地壳的形成与演化只涉及上地幔.其结果是产生出贫化的上地幔,而下地幔仍处于原始状态.上、下地壳主要是通过壳内物质调整产生的.     

西准噶尔及邻区的岩石圈密度结构特征及其构造意义

对西准噶尔及周边地区壳幔结构的研究是揭示准噶尔盆地演化的重要基础.利用最新的卫星重力场模型, 通过计算得到西准噶尔及周边地区的布格重力异常, 进而采用三维反演技术, 对西准噶尔及周边地区的地壳与上地幔顶部进行密度成像, 得到了0~80 km深度范围的密度异常结构.地壳密度分布显示古准噶尔洋壳有可能向NE和NW分别俯冲于西伯利亚板块和西准噶尔地块之下.上地幔顶部密度变化表明: 阿尔泰褶皱带具有相对较低的密度, 可能为古大陆巨厚的硅铝层所致; 哈萨克斯坦-准噶尔盆地具有相对完整的高密度结构; 天山褶皱带区域的密度大幅度变化刻画了超岩石圈断裂对岩石圈的切割以及岩石圈形变与构造活动的痕迹.      

1976年唐山大地震的孕震环境:密集地震台阵观测得到的结果

1976-07-28唐山地区发生了震惊中外的7.8级大地震。为什么在华北古老克拉通内部的唐山地区能够发生如此的大地震一直是一个令人费解的问题。是否会在唐山地区再次出现同样的破坏性地震值得认真研究。利用流动地震观测台阵数据和接收函数反演方法,我们研究了唐山地区60 km深度范围内的三维地壳上地幔速度结构。结果表明:(1)由活动断裂切割的唐山断块与周围介质存在明显差异,围限唐山断块的断裂均为超壳的活动断裂;(2)唐山大震区中上地壳具有明显的非均匀壳内低速体;(3)该地区壳幔界面表现为明显的断块式隆升,与两侧相比,唐山菱形地块下方的上地幔顶部异常隆起的高度达到10 km左右,下伏的上地幔具有异常的非均匀结构;(4)唐山大震区可能有幔源物质较大规模的侵入,形成了中、上地壳内的低速体。由于较已往的研究结果有更高的空间分辨率,我们得到了一些以往尚未发现的有关唐山地区深部结构的异常特征;(5)首都圈地区内破坏性地震发生的地点绝非偶然,它们均与其相应的深部构造背景密切有关,这为强震发生地点的预测提供了可能。根据本文结果,我们认为,1976年唐山大地震的主因源于上地幔的垂向运动变形及壳幔之间物质及能量的交换,区域水平向应力场为次要作用。这有助于解释为什么能够在我国华北古老克拉通地区发生7级以上强震,在唐山地区再次发生7级以上大地震的可能性值得给予进一步的研究和关注。     

基于重力大数据的中国大陆区域地质构造解析

本文从重力异常角度对中国大陆区域地质构造的深部结构进行了研究。基于卫星重力大数据,利用改进后的位场分离方法获取了从地表到上地幔顶部不同深度的重力异常场。分析了各个碰撞-俯冲带、造山带及盆地等地区上地幔顶部（参考深度约为72～76 km）和中地壳（参考深度约为12～16 km）深度的异常特征。结合地质构造、地球化学和地球物理学证据,重点讨论了重力异常产生的原因及其与中国大陆主要构造之间的关系,获得了中国大陆在环太平洋动力学体系和特提斯动力学体系这两大动力作用下不同地区差异性的壳幔响应。     

科洛新生代火山群富钾火山岩中的幔源包体特征及上地幔热状态

黑龙江省科洛火山群富钾火山岩主要岩石组合为碱玄岩、响岩等岩石,岩石学、岩石化学、地球化学、同位素等特征研究表明,其岩浆来自上地幔并为板内裂谷环境,是上地幔岩部分熔融形成的岩浆喷至地袁冷凝形成的.岩石中幔源金云母、尖晶石二辉橄榄岩包体研究表明,科洛地区的上地幔是"过热"的,这种"过热"的状态,正是诱发大陆裂谷构造形成的根本原因.     

塔里木盆地北缘幔源岩石包体特征及岩石圈上地幔流变规律

岩石圈上地幔岩石包体保存着上地幔物质组成和岩石学的特征以及岩石圈上地幔构造变形特性、岩石变形机制和流变性质的丰富信息。塔里木盆地西北缘的巴楚地区的幔源包体主要由橄榄岩组成,次为单辉橄榄岩,与中国东部以及西部其他地区发育的尖晶石、石榴石二辉橄榄岩幔源包体有所区别。从莫霍面至岩石圈上地幔下部,随着深度、温度和压力的增高,幔源包体橄榄石中出现直线状位错、位错缠结、网络—曲线状、环状位错—直线状位错、位错缠结、网络—曲线状位错,环状位错的变化规律;其微观变形机制有从碎裂—位错滑移—位错蠕变—扩散蠕变的变化规律;差异应力值有从低—高—低—高的正弦波状变化趋势,在莫霍面附近以及上地幔韧性软层与脆性层附近差异应力值一般较高;应变速率值则是逐渐减慢的总体变化趋势,整体反映出岩石和矿物从脆—韧性的流变性质。     

大陆地区构造活动、地热及地壳-上地幔结构之间关系的探讨

随着板块构造学说的兴起和发展,对大洋地区构造活动、地热及地壳-上地幔结构之间的关系已有了较为明确的认识。在大陆地区,由于其构造发展历史的复杂性,对这种关系的认识还远没有大洋区那样系统和清晰。但近二十年来,大陆地区各种地球物理资料的大量积累已为进行这种研究提供了较为坚实的基础。     

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。     

Non-transparent uppermost mantle in the island-arc region of Japan

In Japan, the crust and uppermost mantle seismic character is yet unimaged although many refraction surveys have been recorded. The longest seismic profiles are analyzed. A remarkable feature, a long-duration coda wave after the PmP wave (reflected wave at the Moho boundary), is observed on the record sections. Several possible models are considered to explain the long-duration coda wave. The model with many scatterers located in the uppermost mantle explains the observed data well while the undulating Moho and continuous layering models do not account for some aspects of the observed data. The scatterer distributed uppermost mantle is not consistent with that of continental region which is often characterized as transparent. We estimate the scattering coefficient of the uppermost mantle and crust using simulations. The scattering coefficients obtained for upper crust, lower crust, and uppermost mantle are 0.01, 0.02, and 0.025, respectively. The scattering coefficient of the uppermost mantle is slightly larger than that of lower crust, which is characterized as being reflective. The many scatterers in the uppermost mantle might be related to magmatism in Japan. This will be one of the important observations for understanding formation processes of the Moho boundary and uppermost mantle in the island-arc environment.     

Imaging of Vp, Vs, and Poisson’s ratio anomalies beneath Kyushu, southwest Japan: Implications for volcanism and forearc mantle wedge serpentinization

We determine detailed 3-D V_p and V_s structures of the crust and uppermost mantle beneath the Kyushu Island, southwest Japan, using a large number of arrival times from local earthquakes. From the obtained V_p and V_s models, we further calculate Poisson’s ratio images beneath the study area. By using this large data set, we successfully image the 3-D seismic velocity and Poisson’s ratio structures beneath Kyushu down to a depth of 150 km with a more reliable spatial resolution than previous studies. Our results show very clear low V_p and low V_s anomalies in the crust and uppermost mantle beneath the northern volcanoes, such as Abu, Kujyu and Unzen. Low-velocity anomalies are seen in the mantle beneath most other volcanoes. In contrast, there are no significant low-velocity anomalies in the crust or in the upper mantle between Aso and Kirishima. The subducting Philippine Sea slab is imaged generally as a high-velocity anomaly down to a depth of 150 km with some patches of normal to low seismic wave velocities. The Poisson’s ratio is almost normal beneath most volcanoes. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the low Poisson’s ratio zone. A high Poisson’s ratio region is found in the forearc crustal wedge above the slab in the junction area with Shikoku and Honshu; this high Poisson’s ratio could be caused by fluid-filled cracks induced by dehydration from the Philippine Sea slab. The Poisson’s ratio is normal to low in the forearc mantle in middle-south Kyushu. This is consistent with the absence of low-frequency tremors, and may indicate that dehydration from the subducting crust is not vigorous in this region.     

Continental rifting as a function of lithosphere mantle strength

The role of the uppermost mantle strength in the pattern of lithosphere rifting is investigated using a thermo-mechanical finite-element code. In the lithosphere, the mantle/crust strength ratio (S_M/S_C) that decreases with increasing Moho temperature T_M allows two strength regimes to be defined: mantle dominated (S_M > S_C) and crust dominated (S_M < S_C). The transition between the two regimes corresponds to the disappearance of a high strength uppermost mantle for T_M > 700 °C. 2D numerical simulations for different values of S_M/S_C show how the uppermost mantle strength controls the style of continental rifting. A high strength mantle leads to strain localisation at lithosphere scale, with two main patterns of narrow rifting: “coupled crust–mantle” at the lowest T_M values and “deep crustal décollement” for increasing T_M values, typical of some continental rifts and non-volcanic passive margins. The absence of a high strength mantle leads to distributed deformations and wide rifting in the upper crust. These numerical results are compared and discussed in relation with series of classical rift examples.     

Rheologic properties of the lithosphere and applications to passive continental margins

Thermal and petrologic models of the crust and upper mantle are used for calculating effective viscosities on the basis of constant creep rates. Viscosity—depth models together with pressure—depth models are calculated for continental and oceanic blocks facing each other at continental margins. It is found from these “static models” that the overburden pressure in the lower crust and uppermost mantle causes a stress which is directed from the ocean to the continent. The generally low viscosity of 10²⁰–10²³ poise in this region should permit a creep process which could finally lead to a “silent” subduction. In the upper crust static stresses act in the opposite direction, i.e. from the continent to the ocean, favouring tension which could produce normal faulting in the continent. Differences between observations and the results obtained from the static models are attributed to dynamical forces.     

国外深地震反射研究之现状

本文概括地总结和回顾了深地震反射研究成果,并针对存在的一系列问题进行了讨论。许多国家利用深地震反射法来研究大陆地壳和上地幔的内部结构。研究成果表明,大陆地壳和上地幔的反射特征具有显著的差异,这些差异反映了不同的构造单元和状态。     

Fossil flat-slab subduction beneath the Illinois basin, USA

The Illinois basin is one of several well-studied intracratonic sedimentary basins within the North American craton whose formational mechanisms and subcrustal structure are not well understood. We study the S-velocity structure of the upper mantle beneath the Illinois basin and its surrounding area through seismic tomography. We utilize continental scale waveform data of seismic S and surface waves, enhanced by regional earthquakes located near the Illinois basin. Our 3D tomographic model, IL05, confirms the existence of a slow S-velocity structure in the uppermost mantle beneath the Illinois basin region. This anomalously slow region exists from the base of the crust to depths of  90 km, and is slower than the North American cratonic average by about 200 m/s. This anomalous uppermost mantle beneath the Illinois basin is underlain by a faster lithosphere, typical of the surrounding craton, to depths of  200 km. Excluding the formation of the Reelfoot Rift, this area of North American has been stable for over 1.0 Gy. Thus, we do not expect thermal anomalies from before that time to persist into present day S-velocity anomalies and we consider a delamination origin as an explanation of Illinois basin subsidence unlikely. We cannot rule out that the slow mid-lithosphere beneath the Illinois basin is caused by an uppermost mantle enriched by a deep, but weak plume. We attribute the slow mid-lithosphere to the presence of either oceanic, hydrous crust, or, a relatively cool mantle wedge with preserved hydrous minerals in the Illinois basin's uppermost mantle, related to a fossilized flat subduction zone.     

Crustal growth in south-eastern Australia—evidence from lower crustal eclogitic and granulitic xenoliths

Mafic and ultramafic xenoliths in a basaltic cone at The Anakies in south-eastern Australia are geochemically equivalent to continental basaltic magmas and cumulates. The xenolith microstructures range from recognizably meta-igneous for intrusive rocks to granoblastic for garnet pyroxenites. Contact relationships between different rock types within some xenoliths suggest a complex petrogenesis of multiple intrusive, metamorphic and metasomatic events at the crust/mantle boundary during the evolution of south-eastern Australia. Unaltered spinel lher-zolite, typical of the uppermost eastern Australian mantle, is interleaved with or veined by the metamorphosed intrusive rocks of basaltic composition. Geothermobarometry calculations by a variety of methods show a concordance of equilibration temperatures ranging from 880°C to 980°C and pressures of 12 to 18 kbar (1200-1800 mPa). These physical conditions span the gabbro to granulite to eclogite transition boundaries. The water-vapour pressure during equilibration is estimated to be about 0.5% of the load pressure, using amphibole breakdown data. Large fluid inclusions of pure CO₂ are abundant in the mineral phases in the xenoliths, and it is suggested that flux of CO₂ from the mantle has been an important heat source and fluid medium during metamorphism of the mafic and ultramafic protoliths at the lower crust/upper mantle boundary. The calculated pressures and temperatures suggest that the south-eastern Australian crust has sustained a high geothermal gradient. In addition, the nature of the mineral assemblages and the contact relationships of granulitic rock with spinel lherzolite, characteristic of mantle material, suggest that the Moho is not a discrete feature in this region, but is represented by a transition zone approximately 20 km thick. These inferences are in agreement with geophysical data (including seismic, heat-flow and electrical resistivity data) determined for south-eastern Australia. Underplating at the crust/mantle boundary by continental basaltic magmas may be an important alternative or additional mechanism to the conventional andesite model for crustal accretion.     

The role of subduction erosion in the generation of Andean and other convergent plate boundary arc magmas,the continental crust and mantle

Subduction erosion, which occurs at all convergent plate boundaries associated with magmatic arcs formed on crystalline forearc basement, is an important process for chemical recycling, responsible globally for the transport of ~1.7 Armstrong Units (1 AU = 1 km³/yr) of continental crust back into the mantle. Along the central Andean convergent plate margin, where there is very little terrigenous sediment being supplied to the trench as a result of the arid conditions, the occurrence of mantle-derived olivine basalts with distinctive crustal isotopic characteristics (⁸⁷Sr/⁸⁶Sr ≥ 0.7050; ε_Nd ≤ −2; ε_Hf ≤ +2) correlates spatially and/or temporally with regions and/or episodes of high rates of subduction erosion, and a strong case can be made for the formation of these basalts to be due to incorporation into the subarc mantle wedge of tectonically eroded and subducted forearc continental crust. In other convergent plate boundary magmatic arcs, such as the South Sandwich and Aleutian Islands intra-oceanic arcs and the Central American and Trans-Mexican continental margin volcanic arcs, similar correlations have been demonstrated between regions and/or episodes of relatively rapid subduction erosion and the genesis of mafic arc magmas containing enhanced proportions of tectonically eroded and subducted crustal components that are chemically distinct from pelagic and/or terrigenous trench sediments. It has also been suggested that larger amounts of melts derived from tectonically eroded and subducted continental crust, rising as diapirs of buoyant low density subduction mélanges, react with mantle peridotite to form pyroxenite metasomatites that than melt to form andesites. The process of subduction erosion and mantle source region contamination with crustal components, which is supported by both isotopic and U-Pb zircon age data implying a fast and efficient connectivity between subduction inputs and magmatic outputs, is a powerful alternative to intra-crustal assimilation in the generation of andesites, and it negates the need for large amounts of mafic cumulates to form within and then be delaminated from the lower crust, as required by the basalt-input model of continental crustal growth. However, overall, some significant amount of subducted crust and sediment is neither underplated below the forearc wedge nor incorporated into convergent plate boundary arc magmas, but instead transported deeper into the mantle where it plays a role in the formation of isotopically enriched mantle reservoirs. To ignore or underestimate the significance of the recycling of tectonically eroded and subducted continental crust in the genesis of convergent plate boundary arc magmas, including andesites, and for the evolution of both the continental crust and mantle, is to be on the wrong side of history in the understanding of these topics.