地幔对流及其对地壳表层拉张盆地的影响

回顾近十年来在地幔对流方面的最新研究进展,阐述了板块俯冲后在地幔中的演化特征,以及地幔上升流即地幔柱的驱动机制和地质效应。特别介绍了地幔柱对地壳表层拉张盆地形成和充填过程的影响。进一步探讨了地幔柱活动在济阳坳陷的地质表现及其引发的火山岩浆活动、碎屑充填特征。认为盆地演化的阶段性间接反映了地幔对流的阶段性。     

伸展作用序列及其深部背景: 以晚中生代以来鲁西隆起和济阳坳陷为例

露头区野外地质调查、隐伏区地质与地球物理资料研究表明,晚中生代-新生代鲁西隆起区和济阳坳陷区正断层发育,包括陡倾斜的控凹边界断层和缓倾斜的滑脱断层两类,两者构成伸展滑脱半地堑,且滑脱构造在隆起北部和坳陷南部最发育。K Ar和FT测年结果指示伸展断层的发育时间为176~103 Ma、67～49 Ma和42～25 Ma 3个时期。隆起区、坳陷区陡断层分别在中地壳22 km 和15 km左右变平,成为拆离滑脱断层。构造物理模拟表明,在伸展＋塑性物质上涌机制下隆起区和坳陷区正断层均具有由南向北的发育极性,大致对应中侏罗世-早白垩世、古新世-早始新世、中始新世-渐新世3个发育阶段,且伴随控凹断层发育的同时,断块掀斜引起滑脱断层同步发育。层析成像表明中生代早期扬子板块沿作为转换断层的郯庐断裂以近EW向与华北板块俯冲的残留体可能导致晚中生代地幔物质上涌,新生代地幔上涌则可能与太平洋板块与欧亚板块俯冲及印欧板块碰撞的远程效应有关。研究区正断层受控于地幔物质上涌+伸展作用,以齐河-广饶断层为界呈前展式分别由南向北发育,并控制着坳陷区油气的形成、运聚和分布向北迁移。     

济阳坳陷新生代盆地结构差异性及成因机制

通过断层活动性分析、典型测线平衡剖面恢复、伸展率统计等方法, 对济阳坳陷新生代盆地结构的静态特征和演化过程进行了详细分析, 从凹陷内部结构差异、不同凹陷间结构差异、区域结构差异3个层次上对济阳坳陷盆地进行了对比分析并探讨了成因机制.研究结果表明: 凹陷内部及不同凹陷间结构的差异主要受控于北西向控盆断裂以及不同演化阶段控盆断裂体系的发育, 而区域结构差异主要受控于板块俯冲方式转变下的郯庐断裂带由左旋走滑到右旋走滑的转型以及深部地幔物质由上涌到下沉的转变.     

济阳坳陷地幔热流和深部温度

济阳坳陷深部地热状况对于分析岩石圈深部结构特征、探索该盆地形成和演化的地球动力学过程具有重要意义.依据济阳坳陷最新的钻探资料和深部地球物理探测结果, 按沉积盖层、上、中、下地壳4层结构, 建立了分别代表该区凹陷部位和凸起及斜坡带上的2种地壳结构模型.通过多道能谱分析, 测试了区内4 3块岩心样品的放射性元素U、Th、40K含量, 统计得出了济阳坳陷沉积盖层的平均生热率为(1.40±0.26) μW/m3.在研究大地热流分布的基础上, 结合济阳坳陷地壳各岩层放射性生热率, 采用“剥层”法, 从地表开始, 由浅到深逐步扣除各层段所提供的热量, 得到了济阳坳陷的地幔热流.并且采用相似的方法, 利用一维稳态热传导方程, 分析了地壳上地幔顶部的温度状况.结果表明, 济阳坳陷的地幔热流约为38.4~39.2 mW/m2, 占整个地表总热流量的5 8%;地幔顶部温度约为602~636℃.与世界上其他各类地质构造单元相比, 济阳坳陷无论是地幔热流值或其与地表热流之比值都是比较高的, 其深部地热状态具有介于稳定地区和构造活动区之间的特点.      

济阳坳陷中、新生代构造沉降与板块聚敛速率关系探讨

根据回剥模型,将钻井资料与区域地震剖面相结合,对济阳坳陷内东营、沾化和惠民3个次一级凹陷进行沉降史模拟。6121钻井的沉降曲线表明济阳坳陷中、新生代的构造沉降分为6个阶段：（1）中生代残余盆地发展阶段;（2）孔店期初始裂谷阶段;（3）沙四-沙三期断陷一期;（4）沙二-沙-期断陷二期;（5）东营期断陷三期;（6）新近纪热沉降阶段。将以上构造沉降速率同太平洋板块相对欧亚板块的聚敛速率进行比较,两者相吻合,表明太平洋板块的俯冲作用是济阳坳陷从“初始裂谷-快速沉降-裂谷逐渐萎缩-热沉降”构造演化过程的主要控制因素。     

济阳坳陷构造演化特征

盆地的形成和演化与板块运动有密不可分的关系，可以说中国陆相盆地的形成和演化是古生代以来板块运动的必然产物。济阳坳陷属于渤海湾盆地的一部分，所以研究其演化特征必须要了解渤海湾盆地的形成。济阳坳陷所处的区域在不同时代有不同的特征。古生代华北板块总体上以升降运动为主，但由于北缘长期处于活动大陆边缘状态，东南缘又经历了多阶段的板块构造演化，其开合作用造成了华北内部地壳较弱的挤压变形；中生代的裂前拱起和初始断陷阶段，济阳坳陷已初具雏形；新生代是济阳坳陷的强烈构造运动阶段，此时板块运动形成了盆地大体框架后，经过地幔波状运动的内部改造便形成了当今四凹六凸“北断南超”的箕状断陷盆地。     

渤海海域侏罗—白垩纪沉积与构造演化

覆盖区侏罗—白垩系分布、变形特征及构造演化对理解华北克拉通破坏过程具有重要意义。根据编制的地层分布图和地震资料解释,研究了渤海海域侏罗—白垩纪时期沉积、构造变形及演化特征。渤海海域燕山期构造变形与板块俯冲引起的地幔上拱有关。早-中侏罗世,库拉—伊泽奈崎板块北西向俯冲,地层展布继承了印支期古构造格局,呈近东西向,属于坳陷成盆期。晚侏罗—早白垩世,库拉—伊泽奈崎板块北北西向俯冲,火山活动强烈,为热拱断陷期。受郯庐断裂左行活动影响,地层展布具有明显分带性,多呈北东—南西向和北西西—南东东向。晚白垩世,太平洋板块北西向俯冲挤压,岩浆冷凝,进入萎缩隆褶期。     

中国东北地区太平洋板块精细俯冲特征

中国东北地区的火山较为活跃。一些学者认为这些火山的起源与西太平洋板块的俯冲和软流圈物质的上涌存在密切联系。尽管天然地震层析成像结果明显地显示出火山区下方存在着太平洋板块,但板块的厚度、存在范围等俯冲特征仍然不是特别清晰。为此,采用远震层析成像和走时拟合相结合的方法深入研究了中国东北地区的壳幔速度结构,特别是太平洋俯冲板块的精细结构。研究结果表明:(1)板块的平均厚度和速度异常分别为85 km和1%,而且与日本海地区结果进行对比可推测板块并未增厚;(2)板块以低角度俯冲进入地幔过渡带,到达过渡带底部后弯曲成水平状,向西延伸至东经127°而停止;(3)水平延伸的板块恰好位于火山区的下方。这些俯冲特征有助于人们更好地理解陆内火山的起源及地球深部动力学过程。     

南海复蘑菇状地幔低速柱结构及其地幔动力学

运用高分辨率天然地震面波层析成像和体波层析成像技术,研究东亚西太平洋地区及全球地幔三维速度结构时发现南海地区地幔存在深达2000km以上的巨型复蘑菇状地幔低速柱,结合地质、地球化学和地球物理相关标志,将复蘑菇状地幔低速柱称为南海复蘑菇状地幔柱。本文在论述南海复蘑菇状地幔柱的地质地球物理特征基础上,将地幔柱划分为柱头、柱体、柱尾、幔枝和热点等部分,建立起地幔柱三维几何结构模型,探讨了复蘑菇状地幔柱在南海海盆扩张过程中的主导作用以及欧亚板块、菲律宾海板块和印度洋板块相互作用对南海演化过程的影响。     

东北地区地幔热柱构造与成矿成藏作用

东北地区处于古亚洲构造域和西太平洋构造域的叠合部位.该区地幔热柱存在8个地质标志:①南北以西伯利亚板块和华北板块太古代基底(断裂)为界,东西以变质核杂岩体分布为界,基本与日本海毗邻的范围为亚地幔柱的规模;②在佳木斯-牡丹江缝合线内见有科马提岩、苦橄岩,可能为地幔热柱的中心;③水系分布呈放射状及莫霍面分布为松辽盆地幔隆起一致;④典型的热穹隆状态;⑤以花岗岩为主的火成岩省;⑥中生代基性玄武岩基本落入Nb/Ta-Nb图解的地幔柱范围;⑦形成坍塌裂谷特征;⑧岩石圈地幔呈蘑菇云状上涌,地震层析资料证实与太平洋板块俯冲具有相辅相成关系.中生代地幔热柱成矿成藏作用的时空分布具有同时代、同火成岩构造控制,山岭成矿,盆地成藏特征.从He及Ar同位素组成阐明金属矿床与油气田的同源性,分析了幔枝构造是在地表的表现形式,无疑对当今深部找矿及海相油气田的勘查具有理论指导意义.     

The dynamics of big mantle wedge,magma factory,and metamorphic–metasomatic factory in subduction zones

The East Asian continental margin is underlain by stagnant slabs resulting from subduction of the Pacific plate from the east and the Philippine Sea plate from the south. We classify the upper mantle in this region into three major domains: (a) metasomatic–metamorphic factory (MMF), subduction zone magma factory (SZMF), and the ‘big mantle wedge’ (BMW). Whereas the convection pattern is anticlockwise in the MMF domain, it is predominantly clockwise in the SZMF and BMW, along a cross section from the south. Here we define the MMF as a small wedge corner which is driven by the subducting Pacific plate and dominated by H₂O-rich fluids derived by dehydration reactions, and enriched in large ion lithophile elements (LILE) which cause the metasomatism. The SZMF is a zone intermediate between MMF and BMW domains and constitutes the main region of continental crust production by partial melting through wedge counter-corner flow. Large hydrous plume generated at about 200 km depth causes extensive reduction in viscosity and the smaller scale hydrous plumes between 60 km and 200 km also bring about an overall reduction in the viscosity of SZMF. More fertile and high temperature peridotites are supplied from the entrance to this domain. The domain extends obliquely to the volcanic front and then swings back to the deep mantle together with the subducting slab. The BMW occupies the major portion of upper mantle in the western Pacific and convects largely with a clockwise sense removing the eastern trench oceanward. Sporadic formation of hydrous plume at the depth of around 410 km and the curtain flow adjacent to the trench cause back arc spreading. We envisage that the heat source in BMW could be the accumulated TTG (tonalite–trondhjemite–granodiorite) crust on the bottom of the mantle transition zone. The ongoing process of transportation of granitic crust into the mantle transition zone is evident from the deep subduction of five intra-oceanic arcs on the subducting Philippine Sea plate from the south, in addition to the sediment trapped subduction by the Pacific plate and Philippine Sea plate. The dynamics of MMF, SZMF and BMW domains are controlled by the angle of subduction; a wide zone of MMF in SW Japan is caused by shallow angle subduction of the Philippine Sea plate and the markedly small MMF domain in the Mariana trench is due to the high angle subduction of Pacific plate. The domains in NE Japan and Kyushu region are intermediate between these two. During the Tertiary, a series of marginal basins were formed because of the nearly 2000 km northward shift of the subduction zone along the southern margin of Tethyan Asia, which may be related to the collision of India with Asia and the indentation. The volume of upper mantle under Asia was reduced extensively on the southern margin with a resultant oceanward trench retreat along the eastern margin of Asia, leading to the formation of a series of marginal basins. The western Pacific domain in general is characterized by double-sided subduction; from the east by the oldest Pacific plate and from the south by the oldest Indo-Australian plate. The old plates are hence hydrated extensively even in their central domains and therefore of low temperature. The cracks have allowed the transport of water into the deeper portions of the slab and these domains supply hydrous fluids even to the bottom of the upper mantle. Thus, a fluid dominated upper mantle in the western Pacific drives a number of microplates and promote the plate boundary processes.     

The dynamics of big mantle wedge, magma factory, and metamorphic–metasomatic factory in subduction zones

The East Asian continental margin is underlain by stagnant slabs resulting from subduction of the Pacific plate from the east and the Philippine Sea plate from the south. We classify the upper mantle in this region into three major domains: (a) metasomatic–metamorphic factory (MMF), subduction zone magma factory (SZMF), and the ‘big mantle wedge’ (BMW). Whereas the convection pattern is anticlockwise in the MMF domain, it is predominantly clockwise in the SZMF and BMW, along a cross section from the south. Here we define the MMF as a small wedge corner which is driven by the subducting Pacific plate and dominated by H₂O-rich fluids derived by dehydration reactions, and enriched in large ion lithophile elements (LILE) which cause the metasomatism. The SZMF is a zone intermediate between MMF and BMW domains and constitutes the main region of continental crust production by partial melting through wedge counter-corner flow. Large hydrous plume generated at about 200 km depth causes extensive reduction in viscosity and the smaller scale hydrous plumes between 60 km and 200 km also bring about an overall reduction in the viscosity of SZMF. More fertile and high temperature peridotites are supplied from the entrance to this domain. The domain extends obliquely to the volcanic front and then swings back to the deep mantle together with the subducting slab. The BMW occupies the major portion of upper mantle in the western Pacific and convects largely with a clockwise sense removing the eastern trench oceanward. Sporadic formation of hydrous plume at the depth of around 410 km and the curtain flow adjacent to the trench cause back arc spreading. We envisage that the heat source in BMW could be the accumulated TTG (tonalite–trondhjemite–granodiorite) crust on the bottom of the mantle transition zone. The ongoing process of transportation of granitic crust into the mantle transition zone is evident from the deep subduction of five intra-oceanic arcs on the subducting Philippine Sea plate from the south, in addition to the sediment trapped subduction by the Pacific plate and Philippine Sea plate. The dynamics of MMF, SZMF and BMW domains are controlled by the angle of subduction; a wide zone of MMF in SW Japan is caused by shallow angle subduction of the Philippine Sea plate and the markedly small MMF domain in the Mariana trench is due to the high angle subduction of Pacific plate. The domains in NE Japan and Kyushu region are intermediate between these two. During the Tertiary, a series of marginal basins were formed because of the nearly 2000 km northward shift of the subduction zone along the southern margin of Tethyan Asia, which may be related to the collision of India with Asia and the indentation. The volume of upper mantle under Asia was reduced extensively on the southern margin with a resultant oceanward trench retreat along the eastern margin of Asia, leading to the formation of a series of marginal basins. The western Pacific domain in general is characterized by double-sided subduction; from the east by the oldest Pacific plate and from the south by the oldest Indo-Australian plate. The old plates are hence hydrated extensively even in their central domains and therefore of low temperature. The cracks have allowed the transport of water into the deeper portions of the slab and these domains supply hydrous fluids even to the bottom of the upper mantle. Thus, a fluid dominated upper mantle in the western Pacific drives a number of microplates and promote the plate boundary processes.     

DISCUSSIONS ON STRUCTURAL COUPLING

The structural coupling is a common geological phenomenon. The structural differences between eastern and western active continental margins of modern Pacific and between paleo-Pacific and modern-Pacific continental margins are related to the characteristics and status of the subducting oceanic plate, namely, 1. subducting angle; 2. change in subducting angle; 3. subducting velocity; 4. change in subducting velocity; 5. subduction depth; 6. horizontal distance between the leading edge of the subducting plate and the trench; 7. the structural form of the subducting plate at the 670kin boundary between the upper and lower mantle; 8. the displacement and the direction of displacement of subducting plate. The control and influence toward the shallow-level structures by the deep-level structural activities is a detailed representation of the structural coupling on active continental margin. The basin-maintain coupling phenomenon is an intracontinental structural coupling. The far field effect of collision be     

On melting of the subducted oceanic crust: Effects of subduction induced mantle flow

Based on petrological and geochemical arguments, it is possible that arc magma is derived from subducted oceanic crust. In this paper, regional thermal models have been constructed to study the feasibility of melting cold subducted oceanic crusts at shallow depth (i.e. at depths of about 100 km) by a dynamic mantle. Calculated results suggest that plate subduction will generate an induced flow in the wedge above the subducting slab. This current continuously feeds hot mantle material into the corner and onto the slab surface. A high temperature thermal environment can be maintained in the vicinity of the wedge corner, immediately beneath the over-riding plate. Our regional models further demonstrate quantitatively that production of local melting is possible just about 30 km down dip from the asthenosphere wedge corner. Additional geological processes such as reasonable amounts of shear heating and minor dehydration (which will lower the local melting temperature) will further increase the probability of melting a cold subducted oceanic crust at shallow depth.     

Deep slab subduction and dehydration and their geodynamic consequences: Evidence from seismology and mineral physics

We present new pieces of evidence from seismology and mineral physics for the existence of low-velocity zones in the deep part of the upper mantle wedge and the mantle transition zone that are caused by fluids from the deep subduction and deep dehydration of the Pacific and Philippine Sea slabs under western Pacific and East Asia. The Pacific slab is subducting beneath the Japan Islands and Japan Sea with intermediate-depth and deep earthquakes down to 600 km depth under the East Asia margin, and the slab becomes stagnant in the mantle transition zone under East China. The western edge of the stagnant Pacific slab is roughly coincident with the NE–SW Daxing'Anling-Taihangshan gravity lineament located west of Beijing, approximately 2000 km away from the Japan Trench. The upper mantle above the stagnant slab under East Asia forms a big mantle wedge (BMW). Corner flow in the BMW and deep slab dehydration may have caused asthenospheric upwelling, lithospheric thinning, continental rift systems, and intraplate volcanism in Northeast Asia. The Philippine Sea slab has subducted down to the mantle transition zone depth under Western Japan and Ryukyu back-arc, though the seismicity within the slab occurs only down to 200–300 km depths. Combining with the corner flow in the mantle wedge, deep dehydration of the subducting Pacific slab has affected the morphology of the subducting Philippine Sea slab and its seismicity under Southwest Japan. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent small mantle plumes, or hot upwelling associated with the deep slab subduction. Slab dehydration may also take place after a continental plate subducts into the mantle.     

Seismic Features of the Crust-Upper Mantle Beneath Karamay-Kuqa, Xinjiang

1. IntoductionSponsored by the National 305 Program, China, a teleseismic experiment was performed jointly by the Chinese Academy of Geological Sciences and the French Scientific Research Center in Tian Shan area from June 1997 to February 1998. The array was mostly deployed along the main road, starting from Karamay in the north to Kuqa in the south. Geologically, the 700 km-long profile covers the Junggar basin, the Tian Shan Mts., the Bo-A Fault and the Korla Fault, ending in nort…     

Process of Rifting and Collision along Plate Margins of the Qinling Orogenic Belt and Its Geodynamics

The Qinling erogenic belt underwent complicated processes of rifting and collision, as shown by the coexistence of (1) ocean extension and plate margin rifting and (2) subduction of the frontal oceanic crust and extension of the rear plate margin. These resulted in a basin-mountain framework characterized by the coexistence of plates separated by the ocean basin and continental blocks demarcated by the rifting sea trough in the marginal region and the coexistence of subduction orogeny and ocean extension. Generally speaking, the plate marginal area between the North China plate and Yangtze plate continually rifted from north to south and the rifted micro-plates continually accreted northwards. This especial orogenic process was probably controlled by two events of deep mantle geody-namic adjustment and mantle plume activities, which occurred in the Shangdan suture belt and Mianlue suture belt from north to south respectively.     

俯冲板块的动力学性质对平板俯冲的影响：数值模拟

平板俯冲是地球上一种独特的俯冲模式,主要发生在南美洲地区,与该地区的地震、火山等构造地质现象有着密切联系。平板俯冲的形成机制和影响因素仍然需要进一步地研究。文章通过数值模拟的方法,研究了俯冲板块的动力学性质对于平俯冲板片形态的影响。模拟实验结果表明,俯冲板块的厚度和密度差（与地幔）对平板俯冲的形成有着决定性的影响。合适的俯冲板块厚度（70 km 左右）有利于在俯冲过程中形成平板片。厚度较大的板片难以发生弯曲,阻碍了平板片的形成。俯冲板块与地幔的密度差越小,越容易形成平板俯冲,平板片的长度也越长。俯冲板片的密度差太大也不利于形成平板片。此外,高粘度的俯冲板块容易形成平板俯冲,俯冲板块的粘度与形成的平板片的长度也成正比。研究还发现,平板俯冲的形成伴随着海沟后撤速率的减小。参考模型重现了智利中部平板俯冲的形态,为研究该地区的平板俯冲机制提供了新认识。     

Deep slab subduction and dehydration and their geodynamic consequences: Evidence from seismology and mineral physics

We present new pieces of evidence from seismology and mineral physics for the existence of low-velocity zones in the deep part of the upper mantle wedge and the mantle transition zone that are caused by fluids from the deep subduction and deep dehydration of the Pacific and Philippine Sea slabs under western Pacific and East Asia. The Pacific slab is subducting beneath the Japan Islands and Japan Sea with intermediate-depth and deep earthquakes down to 600 km depth under the East Asia margin, and the slab becomes stagnant in the mantle transition zone under East China. The western edge of the stagnant Pacific slab is roughly coincident with the NE–SW Daxing'Anling-Taihangshan gravity lineament located west of Beijing, approximately 2000 km away from the Japan Trench. The upper mantle above the stagnant slab under East Asia forms a big mantle wedge (BMW). Corner flow in the BMW and deep slab dehydration may have caused asthenospheric upwelling, lithospheric thinning, continental rift systems, and intraplate volcanism in Northeast Asia. The Philippine Sea slab has subducted down to the mantle transition zone depth under Western Japan and Ryukyu back-arc, though the seismicity within the slab occurs only down to 200–300 km depths. Combining with the corner flow in the mantle wedge, deep dehydration of the subducting Pacific slab has affected the morphology of the subducting Philippine Sea slab and its seismicity under Southwest Japan. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent small mantle plumes, or hot upwelling associated with the deep slab subduction. Slab dehydration may also take place after a continental plate subducts into the mantle.