哥斯达黎加地震起源计划——IODP334航次介绍

2011年3月13日至4月13日哥斯达黎加地震起源计划IODP334航次在中美洲哥斯达黎加俯冲带大陆边缘斜坡带实施。该航次的主要科学目标是研究俯冲剥蚀控制的俯冲带大地震的聚集和破裂过程。在1个月的钻探时间内,总共在4个站位、8个钻孔中获得1492．88m的岩芯,获得许多中新世到现代的火山灰层,初步确定上覆板块内沉积物和...     

剥蚀型汇聚板块边缘大地震成因机理研究:来自国际综合大洋钻探344航次的报告

旨在探究剥蚀型汇聚板块边缘大地震成因机理的国际综合大洋钻探(IODP)344航次于2012年10月23日至12月11日在中美洲地震频发的哥斯达黎加西部海域实施钻探。介绍了钻探区域的大地构造特征、该航次的主要科学目标、执行情况、所取得的初步成果以及对航次后研究工作的展望等。航次后更多深入细致的研究工作正在进行中,所取得的研究成果将集中在2014年南京召开的航次后学术研讨会上汇报、交流、集成、总结,从而提升对剥蚀型汇聚板块边缘大地震起源机理的认识。     

关于发展洋板块地质学的思考

为揭示造山带物质组成和结构构造,发展洋板块地质学,阐明大陆形成演化过程和动力来源,应用板块构造理论和地质学方法,对造山带俯冲增生杂岩带、蛇绿岩带等大洋岩石圈板块地质建造、结构构造进行系统研究,寻找俯冲带岛弧前弧火成岩组合;研究洋板块初始俯冲过程中,从前弧玄武岩到玻安岩、高镁安山岩,再到弧拉斑玄武岩和钙碱性熔岩的岩浆作用分阶段递进演变历史,以揭示洋盆向大陆转化的原始弧性质和前弧火成岩组合及洋陆转换过程,为建立和发展洋板块地质学奠定科学基础.     

南海东部马尼拉俯冲带的地壳结构和俯冲过程

全球汇聚板块边缘是产生8级以上大地震和破坏性海啸的地方,一直以来是全球科学家关注的焦点和热点区域。马尼拉俯冲带位于南海东部,也是许多地震、海啸和活火山活跃的区域。本文依据以往穿过马尼拉俯冲带的多条多道反射地震测线和海底地震仪剖面数据,分析了马尼拉俯冲带海沟沉积物充填厚度变化、增生楔宽度变化、海底变形特征以及地壳速度结构变化,提出马尼拉俯冲带具有明显的分段特征,分为北吕宋区段、海山链区段和南部西吕宋区段。不同区段的俯冲过程明显不同,提出俯冲增生和俯冲剥蚀(构造剥蚀)两种机制分别控制了该俯冲带的南、北区段。北段主要受到俯冲增生机制的控制,在海沟和弧前盆地之间形成巨大的增生楔构造,在南海北部大陆边缘10~15km厚的减薄陆壳不断俯冲作用下,引起许多与俯冲有关的地震活动和构造变形。南段海山链区段海底地形复杂和粗糙,在俯冲增生、剥蚀或构造剥蚀的联合控制下,5~6km厚大洋板块不断俯冲形成较小的增生楔结构,部分沉积物可能随着板块的俯冲被拖曳到板块边界的深部。     

古巴推覆构造带周边盆地充填序列及其构造演化

根据古巴群岛地层特征,将古巴推覆构造带及其周边划分为尤卡坦、巴哈马、中部火山岛弧及南部火山岛弧四个区域。尤卡坦构造单元为古陆残留盆地,以侏罗纪陆相沉积为特征,巴哈马构造单元属于被动陆缘,以侏罗纪碳酸盐岩台地沉积为特征,中部火山岛弧为白垩纪火山岛弧,由拉斑玄武岩–钙碱性玄武岩、喷出岩及火山碎屑岩组成,南部火山岛弧为古近纪火山岛弧,以古近系火山岩地层为主。通过与周缘地区地层进行对比、分析,认为侏罗纪–早白垩世时期组成古巴的四个地层分区起源于不同的古板块位置,分别位于北美板块边缘及古加勒比弧,其中尤卡坦构造单元起源于尤卡坦台地,巴哈马构造单元起源于巴哈马台地,中部火山岛弧构造单元起源于古加勒比弧白垩纪岛弧部分,南部火山岛弧构造单元起源于古加勒比弧古近纪岛弧部分。在K-T界线时期,加勒比板块与北美板块的碰撞作用导致了古加勒比海槽的逐步消亡,推动了古加勒比弧与北美大陆边缘的碰撞拼合。此次碰撞作用将古巴不同地区拼合在了一起,控制了古巴群岛褶皱推覆带和各个盆地的演化与沉积充填过程。     

日本的综合大洋钻探计划(IODP)

对日本的IODP活动以及科学目标进行简要介绍。IODP核心技术支撑之一的立管钻探船“地球号”可在水深4 000 m的海底钻进7 000 m,钻达发震带和上地幔,实现日本的IODP科学目标。日本的IODP科学规划主要有三大科学主题和八项研究目标,三大科学主题包括地幔过程和地球系统演化,地壳作用过程和地球系统演化,俯冲带和地球系统演化过程中的动力学及物质循环。其八项研究目标为：①钻探西太平洋洋底高原,认识核—幔作用过程;②钻探太平洋白垩纪—新生代沉积物,详细研究地球温室期间的物质循环及从温室环境到冰室环境的转化过程;③钻探大洋岛弧,认识大陆地壳形成过程;④钻探扩张的弧后,认识洋壳岩石圈形成过程;⑤钻探亚洲边缘海及陆坡,认识陆壳—洋壳—大气圈关系;⑥调查增生楔中的碳循环及深部生物圈;⑦调查汇聚板块边缘大地震周期及形成机制、构造及物质循环;⑧研究生活于增生楔环境中极端微生物生物学。     

扬子板块西缘早元古代俯冲体系的地球化学证据—有关变基性岩的微量元素地球化学研究

本文通过对云南早元古代哀牢山群、底巴都组与大红山群的地质地球化学研究，从地球化学角度证明早元古代扬子板块西缘存在板块运动，前特提斯域的元古大洋板块在其东北方向的俯冲作用中，与古扬子板块发生碰撞而在俯冲带大陆边缘弧与弧后盆地分别形成哀牢山群、底巴都组与大红山群的基性火山岩。     

小兴安岭西北部奥陶系火山岩形成环境

本文依据火山岩岩石化学、微量元素和稀土元素地球化学特征，论述了小兴安岭西北部中奥陶统火山岩形成于陆缘弧环境。并且，用板块俯冲机制对其形成机制进行了解释。     

俯冲带形成机制的可检验假说和检验方案

五十年前板块构造理论的诞生是地球科学领域的一场革命,它为理解地球如何运作构建了基本框架。过去五十年对该理论的进一步研究告诉我们地质过程最终都是地球热损失的结果。例如,大洋岩石圈板块在洋中脊形成,其运动和增生以及最终通过俯冲带进入地幔导致地幔冷却降温,从而导致大规模的地幔对流。亦即,板块构造的直接驱动力是俯冲大洋岩石圈板块的下沉力。因此,没有俯冲带就没有板块构造,但是俯冲带如何开始仍然有争议。对俯冲起始的研究从未中断,有数值模拟也有地质推断。2014年在西太平洋用三个IODP航次(350、351和352)来检验“自发”和“诱发”俯冲开始的想法。所有这些努力都值得肯定,但这些是无法检验的想法。无法检验意味着没有结果。本文介绍至今唯一可用地质学方法检验的假说,亦即“岩石圈内横向物质组成差异导致的浮力差是俯冲带形成的起因”。这种浮力差位于海底高原的边部和被动大陆边缘,因此这些部位是未来俯冲带起始的必然轨迹。在远离这些部位的正常洋盆内因缺乏浮力差而俯冲带不可能起始。换句话说,“所有岛弧一定有大陆(或海底高原)基底”,这可以通过采集和研究岛弧基底岩石来验证。     

大陆俯冲带壳幔相互作用

由板块俯冲引发的深部物质循环过程是地球内部的一级运行机制,主宰了地球从内到外的演化进程,是地球科学研究的重要前沿.俯冲带化学地球动力学研究不仅需要确定俯冲带地壳物质再循环的机制和形式,而且需要确定俯冲带动力来源和热体制及其随时间的变化.为了识别不同类型壳源熔/流体对地幔楔的交代作用、寻求板片-地幔界面反应的岩石学和地球化学证据、理解汇聚板块边缘地壳俯冲和拆沉对地幔不均一性的贡献,我们必须将俯冲带变质作用、交代作用和岩浆作用作为一个地球科学系统来考虑.板块俯冲带变质过程中发生一系列物理化学变化,这些变化不但是导致板块进一步俯冲的主要驱动力,同时也控制着释放的熔/流体组成和俯冲到地球深部的物质组成,对俯冲带化学地球动力学过程产生重要影响.地幔楔作为俯冲系统中连接俯冲盘和仰冲盘的关键构造单元,在地球层圈之间物质循环和能量交换等方面起着重要作用.造山带地幔楔橄榄岩直接记录了俯冲带多种性质的熔/流体交代作用,以及复杂的壳幔物质循环过程.俯冲带岩浆岩是大洋/大陆板块俯冲物质再循环的表现形式,这些岩石样品记录了俯冲带从深部地幔到浅部地壳的过程,也为认识地球深部物质循环提供了理想的天然样品.尽管国际上在俯冲带岩石学和地球化学领域针对地球深部过程的研究方面取得了多项重要进展,但由于研究工作缺乏密切的协同配合,包括俯冲带熔/流体的物理化学性质、俯冲带壳幔相互作用的机制和过程、俯冲带幔源岩浆活动的物质来源和启动机制以及深部地幔过程对地表环境的影响等许多关键科学问题尚未得到根本解决.将来的研究需要聚焦俯冲带物质循环这一核心科学问题,进一步查明俯冲带变质作用、交代作用、岩浆作用等过程的各自特征和相互联系,包括挥发性组分在地球深部的迁移过程及其资源和环境效应,着力考察研究相对薄弱的古俯冲带,阐明板块俯冲与地球深部物质循环之间的耦合机制.      

Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins

The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each model, representative P–T paths are displayed for selected markers. Both the intensity of subduction erosion and the size of the frontal accretionary wedge are strongly dependent on the rheology chosen for the overriding continental crust. Tectonically eroded upper and lower continental crust is carried down to form a broad orogenic wedge, intermingling with detached oceanic crust and sediments from the subducted plate and hydrated mantle material from the overriding plate. A small portion of the continental crust and trench sediments is carried further down into a narrow subduction channel, intermingling with oceanic crust and hydrated mantle material, and to some extent extruded to the rear of the orogenic wedge underplating the overriding continental crust. The exhumation rates for (ultra)high pressure rocks can exceed subduction and burial rates by a factor of 1.5–3, when forced return flow in the hanging wall portion of the self-organizing subduction channel is focused. The simulations suggest that a minimum rate of subduction is required for the formation of a subduction channel, because buoyancy forces may outweigh drag forces for slow subduction. For a weak upper continental crust, simulated by a high pore pressure coefficient in the brittle regime, the orogenic wedge and megascale melange reach a mid- to upper-crustal position within 10–20 Myr (after 400–600 km of subduction). For a strong upper crust, a continental lid persists over the entire time span covered by the simulation. The structural pattern is similar in all cases, with four zones from trench toward arc: (a) an accretionary complex of low-grade metamorphic sedimentary material; (b) a wedge of mainly continental crust, with medium-grade HP metamorphic overprint, wound up and stretched in a marble cake fashion to appear as nappes with alternating upper and lower crustal provenance, and minor oceanic or hydrated mantle interleaved material; (c) a megascale melange composed of high-pressure and ultrahigh-pressure metamorphic oceanic and continental crust, and hydrated mantle, all extruded from the subduction channel; (d) zone represents the upward tilted frontal part of the remaining upper plate lid in the case of a weak upper crust. The shape of the P–T paths and the time scales correspond to those typically recorded in orogenic belts. Comparison of the numerical results with the European Alps reveals some similarities in their gross structural and metamorphic pattern exposed after collision. A similar structure may be developed at depth beneath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable lid.     

The Chemical Composition of Global Subducting Sediments and Its Geological Significance

Subducted sediments play an important role in crust-mantle interaction and deep mantle processes, especially for subduction zone magmatism and mantle geochemistry. The current rate of Global Subducting Sediments (GLOSS) is 0.5~0.7 km³/a. The GLOSS are composed of terrigenous material(76 wt.%), calcium carbonate(7 wt.%), opal(10 wt.%) and mineral-bound H₂O⁺(7 wt.%). The chemical compositions of GLOSS are similar to those of upper continental crust which is mainly controlled by the terrigenous materials, and yet the materials formed by marine processes will dilute the terrigenous materials. The components of subducted sediments are different among trenches. In the accretionary margin, the components of subducted sediments are similar to those of the upper crust, while in the non-accretionary margin the components are terrigenous materials plus those produced by marine processes. During subduction, subducted sediments will released fluids, melt or supercritical fluid to affect island arc/back-arc basin magmatism by means of aqueous fluid or sediment melt. In addition, a part of subducted sediments, together with underlying altered oceanic crust/lithosphere, recycle into the mantle and contribute to the mantle heterogeneity. Geochemical tracers indicate that subducted sediments play variable contributions to the magmatic processes in different tectonic setting. Thus, subducted sediments play an important role in two relatively independent dynamics systems (plate tectonics and mantle plume), as well as related mantle evolution models. As a result, by accurately calculating the compositions of subduction sediments and using various geochemical indicators, we can further limit the input and output fluxes of various elements or isotopes, and then obtain more accurately residual subducted components, which can provide us some important clues for geodynamic process.     

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.     

Characteristics of paleoproterozoic Subduction System in Western Margin of Yangtze Plate

INTRODUCTIONDebate centered on Proterozoic tectonic style and crustalevolution has existed for a long time.The customary viewbelieved that the Proterozoic undeveloped solid plate could notput into effect on the subduction because of the high heat flowin the earth,and the continental crustal growth was dominatedby mafic magm a vertical underplating (Wyborn,1988;Etheridge etal.,1987) .However,many observations recentlyobtained from Proterozoic mobile zones in the world suggeststhat Proter…     

Sediment subduction versus accretion around the pacific

Subducting oceanic plates are typically broken by normal faults as they bend downward into subduction zones, usually forming regular patterns of grabens. The faults strike parallel or subparallel to the trench axes and are most commonly 5–10 km in spacing and width. Rupture occurs initially near the outer topographic high and vertical displacement or graben depth increases as the plate descends, the 400 m or more at many trench axes. It is suggested that the grabens provide void spaces within the surface of the subducting plate, below the plane of subduction, into which the trench sediments are tectonically displaced and thus subducted. Around the Pacific, the only regions of apparent fore-arc sediment accretion are where the graben structures are missing or masked by thick sediment deposits. Even in these cases sediment subduction, by inclusion in subducting plate grabens or by other mechanisms, must be invoked to explain the relatively small fore-arc sediment volumes compared to calculated accretion volumes based on historical convergence. Where trench sediment volumes are small compared to the graben volumes the grabens may abrade the leading edge and underside of the overriding plate and subduct the eroded material. It is concluded that sediment subduction is dominant around the Circum-Pacific and that the bending-induced graben structures of the subducting plates are a major factor for sediment subduction and tectonic erosion.     

北祁连榴辉岩相变沉积岩的特征及其构造意义

在北祁连造山带中,出露典型的高压/低温变质岩石,前人对其中的低温榴辉岩已做过较多的研究,但对其中的变沉积岩研究涉及很少.本文展示了榴辉岩相变质沉积岩的岩石学、地球化学、锆石U-Pb年代学和Hf同位素方面的一些新的研究结果.变沉积岩含有榴辉岩相的矿物组合,峰期温压条件为t= 450～520℃,p=1.9～2.3 GPa,与相邻榴辉岩的温压条件一致.地球化学显示这些岩石的原岩为不成熟的沉积岩,可能形成于大陆边缘或大陆岛弧环境.变沉积岩中的碎屑锆石U-Pb年龄主要集中在1800 Ma左右和540～600 Ma之间,结合锆石Hf同位素特征,表明其原岩的碎屑来源既有周缘陆块的前寒武纪变质基底物质,又有新元古代-早古生代新生洋壳或增生物质.同时,这些数据也表明北祁连早古生代洋壳俯冲过程中发生了活动大陆边缘的构造剥蚀作用,即形成于上盘的沉积物(弧前盆地或增生楔)被构造作用运移到俯冲带中,并俯冲到60～70km深处,遭受榴辉岩相变质作用,然后折返到地表.     

Mélange formation by subduction erosion: the case of the Osa mélange in southern Costa Rica

The convergent plate margin off the Osa peninsula in southern Costa Rica is characterized by the indentation of the Cocos ridge at 4–5 Ma. The indentation causes the uplift of the Osa mélange which we interpret to represent an exhumed major channel for the transport of tectonically eroded material down into the subduction zone. We present evidence that, similar to the Nicoya segment of the Costa Rica convergent margin, subduction erosion rather than accretion has been the dominant process along the plate boundary. The composition of the Osa mélange is dominated by tectonized material of the upper-plate Nicoya ophiolite complex (basalt, radiolarite, limestone). Strong deformation is concentrated in numerous discrete shear zones and produced the layered fabric of large rock volumes, which partly experienced temperatures > 200°C. We thus interpret the Osa mélange to be a product of subduction erosion at the base of the outer arc wedge structure.     

http://www.sciencedirect.com/science/article/pii/S1674987112001065

It has been thought that granitic crust,having been formed on the surface,must have survived through the Earth’s evolution because of its buoyancy.At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction,subduction erosion, and continental subduction.Granitic rocks,the major constituent of the continental crust,are lighter than the mantle at depths shallower than 270 km,but we show here,based on first principles calculations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone,and thus can be subducted in the depth range of 270-660 km.This suggests that there can be two reservoirs of granitic material in the Earth,one on the surface and the other at the base of the mantle transition zone(MTZ).The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust.Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle.This could explain the anomalous seismic-wave velocities observed around 660 km depth.The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation,chemical discontinuities between granitic material and the surrounding mantle,or a combination thereof.     

Tectonic erosion at the front of the Japan Trench convergent margin

The imaging of a multichannel seismic record was improved by reprocessing using pre-stack techniques. The reprocessed record shows structures that indicate tectonic erosion and gravity collapse at the front of the Japan Trench margin. Much of the lower slope appears to be underlain by a detached, coherent block of continental crust. The lower slope has failed by mass wasting and the resulting apron of slump debris at the base of the slope has become involved in thrust faulting at the front of the subduction zone. Slumping continues as long as debris is removed from the front of the margin by subduction, and the apron cannot build up sufficiently to stabilize the failing lower slope. Truncated beds at the base of the upper plate indicate subcrustal erosion as well, this probably being the main cause of massive subsidence of the margin. Subsidence was the cause of oversteepening, destabilization and subsequent gravity collapse of the leading edge of the upper plate.