班公湖—双湖—怒江—昌宁—孟连对接带时空结构——特提斯大洋地质及演化问题

班公湖—双湖—怒江（中北段）—昌宁—孟连对接带广泛出露特提斯大洋岩石圈俯冲消减过程中产生的不同时代、不同构造环境、不同变质程度、不同变形样式的洋板块构造地层系统、增生混杂的构造—岩石组合体,可识别出增生的远洋沉积岩、海沟浊积岩、古生代—中生代蛇绿岩、蛇绿混杂岩、洋岛-海山消减增生楔、洋底沉积增生杂岩,基底残块以及以蓝片岩、榴辉岩为代表的高压—超高压变质岩带,记录了青藏高原原古特提斯大洋形成演化的地质信息。班公湖—双湖—怒江—昌宁—孟连对接带是青藏高原中部一条重要的原古特提斯大洋自北向南后退式俯冲消亡的巨型增生杂岩带,构筑了冈瓦纳大陆与劳亚-泛华夏大陆分界带。     

Tectonic setting required for the preservation of sedimentary mélanges in Palaeozoic and Mesozoic accretionary complexes of southwest Japan

The Palaeozoic to Mesozoic accretionary complexes of southwest Japan include various types of mélange. Most mélanges are polygenetic in origin, being sedimentary or diapiric mélanges that were overprinted by tectonic deformation during subduction. Sedimentary mélanges, without a tectonic overprint, are present in the Permian accretionary complexes of the Akiyoshi and Kurosegawa belts and in the Early Cretaceous accretionary complex of the Chichibu Belt. These mélanges are characterized by dominant basalt and limestone clasts, within a mudstone matrix. The basalt and limestone clasts within the sedimentary mélanges were derived from ancient seamounts. Subduction of a seamount results in deformation of the pre-existing accretionary wedge, and it is difficult to incorporate a seamount into an accretionary wedge; therefore, preservation of seamount fragments requires a special tectonic setting. Oceanic plateau accretion might play an important role in interrupting the processes of subduction and accretion during the formation of accretionary complexes. Especially the Mikabu oceanic plateau might have caused the cessation of accretion during the Early Cretaceous. The subduction and accretion of volcanic arcs and oceanic plateaux helps to preserve sedimentary mélanges from tectonic overprinting by preventing further subduction.     

Accreted Terranes in the Paleo‐Asian Ocean

Terrane accretion is considered to be one of the main contributors to the growth of continental crust (Stern and Scholl, 2010). Allochthonous terranes can be categorized into three general groups, including island arcs, seamounts (oceanic plateaus, submarine ridges), and continental fragments. The Central Asian Orogenic Belt (CAOB) is one of the largest and long‐lived accretionary collages on the globe, and is composed of numerous island arcs, and accretionary wedges, seamounts, and microcontinents associated with the growth and consumption of the Paleo‐Asian Ocean (Fig.1; ?engör et al., 1993; Windley et al., 2007; Xiao et al., 2015; Yang et al., 2015).     

Effect of fluid pressure distribution on the structural evolution of accretionary wedges

Numerical experiments on evolving accretionary wedges usually implement predefined weak basal décollements and constant strength parameters for overlying compressed sequences, although fluid pressure ratio, and therefore brittle strength, can vary strongly in sedimentary basins. A two‐dimensional finite difference model with a visco‐elasto‐plastic rheology is used to investigate the influence of different simplified fluid pressure ratio distributions on the structural evolution of accretionary wedge systems. Results show that a linear increase in fluid pressure ratio towards the base leads to toeward‐verging thrust sheets and underplating of strata, while simulations with a predefined décollement form conjugate shear zones supporting box‐fold‐type frontal accretion. Surface tapers are in agreement with the critical wedge theory, which here is modified for cases of varying fluid pressure ratio. Furthermore, the numerical results resemble findings from natural examples of accretionary wedges.     

Asymmetric post-spreading magmatism in the South China Sea: based on the quantification of the volume and its spatiotemporal distribution of the seamounts

ABSTRACT

The volume of seamounts is an essential indicator of the intensity of mantle convection during the evolution of the oceanic lithosphere. Drilling and dredging samples suggest volcanic seamounts are widely distributed in the oceanic basin of the South China Sea, and most of them were formed after cessation of seafloor spreading. By using an SRTM15_PLUS Digital Elevation Model with a 15-arc-second grid, we developed a spatial filtering method based on the Top Hat Transform to extract seamounts. With a combined analysis of basalt dating results from previous studies and gravity anomaly data, an accurate estimation of seamount volume and its spatiotemporal distribution have been obtained. In addition to an asymmetric distribution of the seamounts with a larger volume in the northern flank, clusters of seamounts can be observed at certain locations in the abyssal plain. Due to the consistency between the distribution of the seamount volume and the seafloor spreading features in the South China Sea basin, we propose the ridge jump may induce additional partial melting zones which account for the larger number of seamounts developed in the northern flank while the re-orientation of the extension during seafloor spreading dominated the distribution of post-spreading magmatism. Similar to other marginal basins, magmas formed by spontaneous partial melting would migrate through weak lithosphere where the extension direction changed, resulting in post-spreading magmatism in those lithospheric weak zones.     

Comparative Study of Magmatism in East Pacific Rise Versus Nearby Seamounts: Constraints on Magma Supply and Thermal Structure Beneath Mid‐ocean Ridge

Major elements of 2202 basalts from the East Pacific Rise (EPR) and 888 basalts from near-EPR seamounts are used to investigate their differences in magma crystallization pressures and mantle melting conditions. Crystallization pressure calculation from basalts with 5.0wt%相似文献   

新疆西准噶尔达拉布特蛇绿岩E-MORB型镁铁质岩的地球化学、年代学及其地质意义

新疆西准噶尔地区是古生代经过俯冲-增生形成的复合造山带,该地区分布有多条蛇绿岩带,其中之一的西准噶尔达拉布特蛇绿岩被认为是最大的一条蛇绿岩带,可能代表了古亚洲洋壳的残余。本文的资料显示蛇绿岩带内的镁铁质岩呈现出N-MORB、E-MORB和似OIB的地球化学特征,通过对阿克巴斯套岩体中的浅色辉长岩LA-ICP-MS锆石年龄测定,获得达拉布特蛇绿岩E-MORB型镁铁质岩的年龄为302±1.7Ma。鉴于达拉布特蛇绿岩中E-MORB和似OIB型镁铁质岩成因的复杂性,结合前人研究成果,对辉长岩锆石U-Pb年龄所代表的意义存在两种可能性：（1）E-MORB型和似OIB型镁铁质岩可能是弧后盆地扩张后期的产物,代表蛇绿岩的年龄,其表明西准噶尔地区可能晚石炭纪还有洋盆存在;（2）E-MORB型镁铁质岩是蛇绿岩消亡阶段由于扩张脊和俯冲带碰撞作用而形成的弧前海山,形成时代晚于达拉布特主体蛇绿岩,但其成因与蛇绿岩的演化密切相关。本文侵向于第二种可能性,认为新疆北部晚石炭-早二叠可能仍存在活动陆缘,俯冲作用仍然存在,扩张脊俯冲形成的板片窗效应导致地幔楔、俯冲板片和沉积物等熔融促使基性岩浆向长英质酸性岩浆转变,从而引发了二叠纪大规模玄武质岩浆底侵,导致了该时期的构造-岩浆-成矿-造山作用的发生。     

Current-topography interactions at mid-ocean seamounts and the impact on pelagic ecosystems

Ocean currents impinging on topographic obstacles such as seamounts create a high level of variability in mesoscale physical oceanography. In the N Pacific, for example, the structure of the Kuroshio and its extension differ significantly E and W of the Emperor Seamount chain, and eddy fields detected downstream may be attributed to seamount effects. Nearfield effects of seamounts have been theoretically predicted for several decades but only recently has theory been confirmed by observation. Taylor columns, quasi-stationary eddies over seamounts, alter flow patterns and thus have impacts on both benthos on seamounts and on the biota in water overlying the seamount. SE Hancock Seamount, located at the N end of the Hawaiian Ridge (29°47′N; 179°04′E), has a summit depth of 265 m. This seamount is located near the subtropical front and is at the southerly extent of productive seamounts where trawl fisheries have existed in the past. The pelagic ecosystem in the upper 200 m over the seamount clearly differs from waters at control stations at distances of 10's of kilometers away as shown by plankton and midwater trawl hauls and hydroacoustic transects conducted during 1984 and 1985. Over the seamount, hydroacoustic transects show a significantly higher biomass of scatterers as compared to control stations. Sampling these scattering layers with small midwater trawls demonstrates high densities of a resident micronekton fauna dominated by the sternoptychid fish “Maurolicus muelleri” and the mysid “Gnathophausia longispina”; these taxa were virtually absent from the control stations, were oceanic micronekton, particularly larger forms, were generally in higher abundance than at the seamount stations. Similarly, ichthyoplankton abundance differs above the seamount and at reference stations. In summer sampling, larval fishes were less abundant over the seamount whereas in winter the abundance was greater there. The differences in distribution and abundance of both micronekton and ichthyoplankton are significant and consistently observed, suggesting that physical or biological processes at the seamount have important effects on the pelagic ecosystem. Hypotheses concerning current — topography interactions, exclusion of vertical migrators, and predation by resident micronekton and fishes can be used to explain the observed effects. Seamounts and other areas of complex topography are frequently sites of highly productive ecosystems; the S Emperor and N Hawaiian Ridge seamounts provide a good example, with a catch of approximately one million tons of boarfish in ten years. The interaction of ocean currents and complex topography may play an important role in this high productivity, as demonstrated in the high biomass of lower trophic levels in the seamount ecosystem. Interannual variability in the latitudinal position of the subtropical front and the strength of current flow over these seamounts may result in significant differences in mesoscale physical oceanography and therefore in the productivity of these ecosystems.     

西准噶尔洋中海山及其俯冲带处地质效应

随着研究的不断深入,在中亚造山带(CAOB)不断有不同时代的洋岛玄武岩(OIB)被识别出来。在中亚造山带西南缘的西准噶尔地区的多条蛇绿混杂岩带中,也存在具有OIB特征的玄武岩。这些玄武岩呈枕状,与超基性岩、辉长岩、块状玄武岩、灰岩及紫红色硅质岩等紧密共生。地球化学研究表明枕状玄武岩均为碱性系列,具有较高的TiO2含量(大多>2.5%)、强烈富集轻稀土元素、无明显Nb、Ta负异常,与典型的OIB极为相似,认为其可能形成于大洋板内与地幔柱有关的海山环境。通过对海山的发展阶段分析认为,西准噶尔地区海山应该发展到爆炸海山阶段,因为其中发育大量的枕状熔岩。海山中火山岩或火山碎屑沉积物富集大离子亲石元素和高场强元素,海山的俯冲将对弧及弧后地区火山岩地球化学产生明显影响,而西准噶尔地区泥盆纪-石炭纪火山岩中恰恰存在海山的信号。因此,海山俯冲模式可能能更好地解释西准噶尔地区火山岩中存在OIB特征火山岩的成因。另外,海山俯冲还存在潜在的资源效应,因此应该寻找和研究古海山及火山岛链俯冲的迹象,将对进一步找金铜等矿提供可靠依据。     

甘肃北山马鬃山俯冲增生杂岩带中特殊"古海山"的发现及其沉积序列解析

古海山是缝合带的关键组成部分,中亚造山带西段的天山造山带内已发现多处古海山,而北山地区却鲜有报道,古海山的发现可以弥补该地区海山研究的不足.通过在北山中部野马泉开展地质调查、测制剖面,发现了一套原始层序完整、以玄武岩、玄武质凝灰岩和大理岩为主的地层,具有火山岩基座和碳酸盐岩顶盖的结构,二者原始接触关系为整合接触,符合海山的沉积特征.依据地层中的岩石组合和沉积构造,确定其形成于海山斜坡相.野马泉古海山残骸呈NWW-SEE向延伸,东南部更接近海山顶.该海山中的玄武岩富集大离子亲石元素、亏损高场强元素,具有岛弧玄武岩的特征.该海山为洋内弧型海山,野马泉一带位于海山斜坡,其被构造肢解后呈NWW-SEE向分布.      

西太平洋海域海山地形分形特征研究

对太平洋麦哲伦海山区、威克—马尔库斯海山区、马绍尔群岛、中太平洋海山区及莱恩群岛5个海山区的平顶海山与尖顶海山地形进行分形研究,结果表明两种类型海山地形具有不同的分形特征。海山形态投影覆盖法揭示平顶海山具有单分形的大尺度构造分形,尖顶海山具有大尺度构造分形和小尺度结构分形的双分形结构。海山等高线尺度法分形结果显示,同一海山区平顶海山分维值小于尖顶海山的;同一海山不同高程段等高线分维值基本保持稳定,垂向上具有明显的分段性,可以参考并使用它进行地貌垂向分带。     

南海盆海山火山碎屑岩的发现及其地质意义

海山火山碎屑岩是水下爆发性火山作用的产物.南海盆两座海山上发现火山碎屑岩表明海山顶部曾经一度高于PCL(压力补偿深度).岩相学特征显示这些火山碎屑岩的胶结物主要为一混合相,包括粘土矿物和黑色铁质矿物等,这从一个侧面反映海山的正地形限制了粗粒外生碎屑到达海山顶部.岩石的单矿物组分、主量元素化学特征与同海山玄武质熔岩具有可比性,属碱性岩浆系列.计算获得海山平均最小下沉速率为0.06mm/年,最大可能达0.30mm/年.海山岩石的机械风化产物对周围海盆的沉积作用作出重要贡献.     

New data on the structure of the Vitoria-Trindade seamount chain (western Brazil basin,South Atlantic)

In 2008, during cruise 24 of the R/V Akademik Vavilov, much of our research work was focused on the central segment (Jaseur and Davis seamounts, Dogaressa Bank) of the Vitoria-Trindade seamount chain (west of the Brazil basin) extending along 20.5° S. Work was conducted to survey the upper part of the sedimentary cover and to perform subbottom profiling. The samples dredged on the seamount slopes are represented by volcanites and Fe-Mn crusts.     

造山带中古海山残片的识别 ——以拉脊山缝合带青沙山和东沟地质填图为例

造山带内海山/洋岛残片的识别是确定古缝合带和古洋盆存在的直接地质证据.祁连造山带被普遍认为是原特提斯洋盆俯冲和闭合的产物,然而南祁连地区是否存在古洋盆长期存在争议.通过对拉脊山关键地段早古生代岩石开展大比例尺地质填图和室内综合研究,在青沙山和东沟地区识别出连续的由洋岛型枕状玄武岩、火山碎屑岩、泥岩、凝灰岩和灰岩组成的火...     

New data on the structure of the Bahia seamounts (Western Portion of the Brazil Basin,South Atlantic)

During the 28th trip of the R/V Akademik Sergei Vavilov undertaken in 2009, exploration was carried out using a Parasound acoustic profilograph that permitted examining the upper portion of the sedimentary cover and getting transverse bathymetric profiles through a number of seamounts within the northern range of the Bahia group of seamounts, which were also sampled by dredging. The studies performed indicated that this linear structure consists of paleovolcanoes and formed under impulse conditions of volcanic activity. A number of seamount types have been distinguished, the morphostructural settings and the positions of which reflect the intensity of volcanic impulses and the association with deep permeable zones. The strongest impulse led to the formation of a northwest striking arched uplift, on which there are several central type paleovolcanoes of different heights. The largest of these paleovolcanoes, including the Stocks and Groll Seamounts, have flattened tops formed as a result of marine abrasion. Isolated seamounts have formed during shorter and lower power impulses of volcanic activity. The northern range of the Bahia seamounts, which have a major northwesterly strike, is an ensemble of structures with northwesterly, nearly east-west, and nearly north-south strikes, which indicates the presence of relevant deep permeable zones for the mantle matter; that mantle matter is the source of the melts, eruptions of which led to the formation of the volcanic edifices. The prevailing zone of weakness has a northwesterly strike. Volcanic activity in the Bahia group of seamounts ended in the Paleocene, but tectonic movements continued there up until the Holocene. Neotectonic structures are represented by horsts, diapirlike piercement structures, and folded zones. The paleovolcanoes are areas where slope processes of transport of sedimentary material occurred.     

Geochemistry of subducted metabasites exhumed from the Mariana forearc: Implications for Pacific seamount subduction

Seamounts on the drifting oceanic crust are inevitably carried by plate motions and eventually accreted or subducted. However, the geochemical signatures of the subducted seamounts and the significance of seamount subduction are not well constrained. Hundreds of seamounts have subducted beneath the Philippine Sea Plate following the westward subduction of the Pacific Plate since the Eocene (~52 Ma). The subducted oceanic crust and seamount materials can be exhumed from the mantle depth to the seafloor in the Mariana forearc region by serpentinite mud volcanoes, providing exceptional opportunities to directly study the subducted oceanic crust and seamounts. The International Ocean Discovery Program (IODP) expedition 366 has recovered a few metamorphosed mafic clasts exhumed from the Mariana forearc serpentinite mud volcanoes, e.g., the Fantangisña and Asùt Tesoru seamounts. These mafic clasts have tholeiitic to alkaline affinities with distinct trace elements and Nd-Hf isotopes characteristics, suggesting different provenances and mantle sources. The tholeiites from the Fantangisña Seamount have trace element characteristics typical of mid-ocean ridge basalt. The Pacific-type Hf-Nd isotopic compositions, combined with the greenschist metamorphism of these tholeiites further suggest that they came from the subducted Pacific oceanic crust. The alkali basalts-dolerites from the Fantangisña and Asùt Tesoru seamounts show ocean island basalt (OIB)-like geochemical characteristics. The OIB-like geochemical signatures and the low-grade metamorphism of these alkali basalts-dolerites suggest they came from subducted seamounts that originally formed in an intraplate setting on the Pacific Plate. The Pacific Plate origin of these metabasites suggests they were formed in the Early Cretaceous or earlier.Two types of OIBs have been recognized from alkali metabasites, one of which is geochemically similar to the HIMU-EMI-type OIBs from the West Pacific Seamount Province, and another is similar to the EMII-type OIBs from the Samoa Island in southern Pacific, with negative Nb-Ta-Ti anomalies and enriched Nd-Hf isotopes. Generally, these alkali metabasites are sourced from the heterogeneous mantle sources that are similar to the present South Pacific Isotopic and Thermal Anomaly. This study provides direct evidence for seamount subduction in the Mariana convergent margins. We suggest seamount subduction is significant to element cycling, mantle heterogeneity, and mantle oxidation in subduction zones.     

俯冲增生杂岩带变形特征、成因机制及与后期变形的区别

俯冲增生杂岩带是造山带重要的组成单元,它记录了从俯冲到碰撞以及碰撞后陆内的演化历史,具有重要的研究价值。由于增生楔形成过程复杂,而后期的碰撞以及陆内变形又会强烈改造俯冲期的变形,因此如何区分增生杂岩中俯冲期间和碰撞阶段的变形就非常重要,但明确的区分两者又是非常困难的工作。我国几乎所有地区发育的俯冲-增生杂岩都经历了后期强烈的改造,因而正确合理地筛分俯冲阶段和碰撞阶段的变形,在我国的造山带研究中日益突出。本文在详细介绍俯冲期间相关变形及其机制的基础上,从不同构造要素的分布、发育特征、形成环境、成因机制等方面综合对比了俯冲阶段和碰撞阶段以及之后构造变形的异同,提出了区别不同阶段变形的主要原则。相比碰撞阶段变形,俯冲阶段的变形主要集中在俯冲隧道中,以简单剪切或一般剪切为主(逆冲断层多见),底板垫托以及双冲构造是变形的重要特征,变形呈弥散性,断层和面理以及褶皱等具有优势的构造极性,但缺少区域尺度的大型褶皱;纯剪变形少见,主要发育在俯冲隧道上方的增生楔中。流体作用以及水岩反应强烈,直接控制变形行为,发育有从显微尺度到区域尺度的变形分解现象。而碰撞阶段主要是在陆上环境进行,主要变形集中在接触带以及大型断裂/剪切带附近。断层和面理的构造极性不明显,增生楔整体变形,出现区域尺度的大型褶皱;流体作用虽有,但不如俯冲阶段明显和强烈,以逆冲和走滑断层多见。然而很多指标和依据并不是某种环境下唯一的,因此在实际工作中需要综合各方面信息和要素进行判断,合理区分不同阶段的变形。     

Subducting seamounts and deformation of overriding forearc wedges around Japan

Two subducting seamounts under inner trench slopes have been identified around Japan on the basis of magnetic anomalies, morphology and geological structure. The first one is located under the foot of the inner trench slope at the junction between the Japan Trench and the Kuril Trench. Another one occurs beneath the slope slightly seaward of the Tosabae (the basement high at the trench slope break along the Nankai Trough off Shikoku). The magnetic anomalies of seamount origin are accompanied by the characteristic morphology of a forearc wedge i.e., a swell landward and a depression seaward. The seamounts beneath the inner trench slopes have preserved magnetization showing reasonably consistent directions, which suggests that the subducting seamounts have kept roughly their original shapes. The morphology of the forearc wedge can be explained by a subducting seamount on the oceanic crust pushing the forearc material forward and upward. Deformation of the forearc wedge by the subducting seamount extends to the forearc basin. The seamounts are stronger and less deformable than the inner slope material and are not offscraped onto inner trench slopes.

Two other examples of deformed inner trench slopes around Japan which can be explained by subduction of topographic highs are presented. One example is a depression on the foot of the inner trench slope northeast of the junction between the Kyushu-Palau Ridge and the Nankai Trough. Another one is an area of complex morphology of the inner trench slope along the Japan Trench around the Daiichi-Kashima Seamount.     

Deformation history and processes during accretion of seamounts in subduction zones: The example of the Durkan Complex (Makran,SE Iran)

The Durkan Complex is a tectonic element of the Makran Accretionary Prism (SE Iran) that includes fragments of Late Cretaceous seamounts. In this paper, the results of map- to micro-scale structural studies of the western Durkan Complex are presented with the aim to describe its structural and tectono-metamorphic evolution. The Durkan Complex consists of several tectonic units bordered by mainly NNW-striking thrusts. Three main deformation phases (D₁, D₂, and D₃) are distinguished and likely occurred from the Late Cretaceous to the Miocene–Pliocene. D₁ is characterized by sub-isoclinal to close and W-verging folds associated with an axial plane foliation and shear zone along the fold limbs. This phase records the accretion of fragments of the seamount within the Makran at blueschist facies metamorphic conditions (160–300 °C and 0.6 – 1.2 GPa). D₂ is characterized by open to close folds with sub-horizontal axial plane that likely developed during the exhumation of previously accreted seamount fragments. An upper Paleocene – Eocene siliciclastic succession unconformably sealed the D₁ and D₂ structures and is, in turn, deformed by W-verging thrust faults typical of D₃. The latter likely testifies for a Miocene – Pliocene tectonic reworking of the accreted seamount fragments with the activation of out of sequence thrusts. Our results shed light on the mechanism of accretion of seamount materials in the accretionary prisms, suggesting that seamount slope successions favour the localization and propagation of the basal décollement. This study further confirms that the physiography of the subducting plates plays a significant role in the tectonic evolution of the subduction complexes.     

Travel time of accreted igneous assemblages in western Pacific orogenic belts and their associated sedimentary rocks

Accreted igneous assemblages in orogenic belts maybe divided into three types depending on whether they derive from seamounts, ocean ridges or subduction-related ophiolites. Seamount type basalts are associated with shallow water sediments—mostly reefoidal limestones. Ocean ridge type basalts are generally overlain by pelagic cherts. Subduction-related ophiolitic eruptives, often underlain by gabbroic and ultramafic rocks, are associated with hemipelagic mudstones. The age of such diverse eruptive lithologic assemblages reflects the time taken for them to have traveled from their locus of generation to their place of accretion at a continental margin. This relationship has been established for each type of accretionary complex, examples being taken mostly from Japan and the western Pacific rim in order to represent evolutionary processes at a typical active plate margin. In general, the seamount types are older, ridge types are of intermediate age, and the ophiolitic types are by far the youngest, usually close to zero age. Seamount type basalts are accreted by shallower scraping of the seamount's sediment apron together with fragments of seamount basalt, ridge type, by peeling due to permeability contrast, and the ophiolitic types by deeper scraping as a consequence of an inflected temperature gradient. Accordingly, it is concluded that the ophiolitic rocks are generated close to the trench and may be accreted as a result of ridge subduction.