Back-arc rifting in the Okinawa Trough

Geological and geophysical data reveal that the Okinawa Trough shows incipient continental rifting, and crustal separation started from about 2 Ma. The early extensional movements in the trough are probably of Miocene age. In addition to the Miocene phase, two main periods of extension are recognized: a Pleistocene phase between 1.9 and 0.5 Ma and the present day phase. During the stage short central rifts (Central Grabens) were formed. The opening however, may have occurred only in the southern part of the trough basin having an average half spreading rate of 2 cm yr^?1 since Early Pleistocene time, producing its present width of several tens of kilometres. These activities were well represented by igneous intrusions, sedimentary facies and sedimentary structures in and around the Okinawa Trough. The width of the zone affected by back-arc extension (defined as Greater Okinawa Trough) is larger than the present Okinawa Trough, whose width is 200–250 km. The present form of the Greater Okinawa Trough started to form at the same time as that of the Okinawa Trough.     

Holocene tephra deposits in the northern Okinawa Trough

1 Introduction Volcanic activities have played an important role in the process of global climatic changes. Being good stratigraphic markers, widespread tephra layers in ma- rine sediments can provide reliable evidences for the regional paleoceanographic …     

冲绳海槽北部的全新世火山碎屑沉积

分析了冲绳海槽北部的CSH1岩芯沉积物中矿物组成、化学元素质量分数和粒度的变化规律,研究结果表明,在全新世期间,日本九州岛发生了2次大规模火山喷发活动,造成大量的火山物质沉积在冲绳海槽北部。从岩芯最上部的沉积物中可识别出2层火山碎屑沉积,它们都具有沉积物粒径突然变粗、SiO2和Na2O等化合物的质量分数突然增加以及粘土矿物、挥发份、Fe2O3、MgO、CaO和Sr的质量分数迅速下降等特征。沉积物粒度数据的端员分析结果表明,冲绳海槽北部全新世沉积物中的硅酸盐碎屑是由陆源碎屑物质和火山灰按不同的比例混合而成的。有火山物质混入的沉积物的粒度分布多呈现双峰模式,粒度众数值分别为74.3和7.81μm。浮游有孔虫壳体的AMS14C测年结果表明,2层火山碎屑所对应的火山喷发年代分别为7 210和10 870 cal.aBP。其中前者与K-Ah火山的喷发时间一致,后者可能与Kuju火山在12~10 cal.kaBP期间的喷发有关。     

冲绳海槽中段磁异常特征

磁异常显示在东海陆架坳陷带以降低负磁异常为主 ,而陆架边缘隆褶带异常区则以跳跃升高的正磁异常为主 ,冲绳海槽异常区表现出 2种磁异常面貌 :在西部陆坡表现为较平缓的负磁异常 ,而冲绳海槽底部则表现为高频跳跃变化升高异常。琉球岛弧异常区则表现为降低平缓负磁异常 ,只有在靠近冲绳海槽一带表现为高频升高正磁异常。磁力异常在这几个构造带显示出明显的条带状分布特征。在测区的中南部 (2 7°4 0′ N ,12 7°0 0′ E附近 ) ,磁异常呈现出明显的磁条带特征 ,与海底地形吻合很好 ,说明海槽中段已经进入了扩张阶段     

论冲绳海槽的张裂运动

琉球岛弧系向海沟方向的旋张掀斜运动和岛弧-弧后地区地幔流的波动运动是冲绳海槽张裂运动的动力源.在张应力场作用下,自早第三纪以来,冲绳海槽长期为一负向构造.现代海槽是老海槽的继承性、阶段性发展的产物,晚中新世以来冲绳海槽的张裂运动增强与菲律宾海板块的俯冲作用增强密切相关,但它不可能形成大规模的扩张.     

冲绳海槽断裂、岩浆构造活动和洋壳化进程

讨论了冲绳海槽断裂、岩浆构造活动特征和它们之间的关系,认为雁行排列的地堑斜交于陆架外缘隆起带,海槽北段断块隆脊、龙王构造带和海槽南段"棉花构造带"可能保留了海槽各幕断陷前的火山岩浆活动特征,而现在活动的吐噶喇火山岛弧可沿海槽南段岛坡追踪到台湾。吕宋岛向台湾的碰撞挤压引起的旋张活动加强了海槽南段的地壳拉张,诱发了地堑内火山岩浆活动,在洋壳化进程中起关键作用,其中最典型的八重山地堑已经形成洋壳。断裂和岩浆活动主要是单向地向岛弧侧迁移,由洋中脊扩张产生的对称条带状磁异常模式难以解释冲绳海槽的洋壳化进程。     

Faulting, magmatism and crustal oceanization of the Okinawa Trough

The paper focuses on the characteristics of faulting and magmatism of the Okinawa Trough and the relation between them.En-echelon grabens are ranked oblique to the continental shelf edge uplift,and the Longwang uplift,the rifting block ridge in the northern segment and the“Mianhua uplift”in the southern segment have possibly preserved characteristics of volcanism and magmatism occurring with those rifting phases.The clockwise rotation of the southern Ryukyu Islands,driven by collision between Luzonand Taiwan,has played a key role in the crustal oceanization,enhancing the crustal extension of the southern segment and inducing volcanic magmatism in those grabens,among which the Yaeyama graben is a typical example of the presence of oceanic crust.Faulting and magmatism were mainly migrating towards the island are asymmetrically.The crustal oceanization of the Okinawa Trough is difficultly interpreted by the linear magnetic anomaly model,which is fit for the symmetric spreading of the mid-oceanic ridges.     

Particulate materials and microbial assemblages around the Izena black smoking vent in the Okinawa trough

The plume of particulate DNA (0.7–1g/l) was extended from a black smoker vent (c.a. 1340 m depth) to the west-southwest directions in the Izena bottom-water region. High concentrations of particulate materials (70–110 g C/l; 300–570 g S/l) were also detected in the bottom-waters. Microscopic observation showed that the bottom-waters were rich in alcian blue-stainable large amorphous particles which contained coccoid and rod-shaped microbial cells mostly smaller than 1m. These microbial matrix compounds appeared to contribute to low P-DNA/P-C ratio (0.011±0.008;n=27) in the vent environment. Sulfur was detected in various kinds of particles in the waters, while the content varied with calcium. Microbial population in the P-DNA plume water was in the order of 10⁵ cells/ml and the most (>99.9%) were non-culturable. The composition of culturable heterotrophs differed between the bottom-waters and surface sediments surrounding the vent; contributions of low temperature (4°C)-culturable bacteria and manganese-oxidizing bacteria to the total heterotrophs were higher in the sediments than in the waters. In contrast, percentages of orange-pigmented heterotrophs and microorganisms capable of growing in thiosulfate- and ammonia-based media to the total heterotrophs were higher in the waters than in the sediments. These results suggest that the culturable bacterial community in the bottom-waters was nutritionally versatile. Izena hydrothermal activities seemed to have a great influence in concentrations and compositions of particulate mateirals and in biomass and compositions of microbial community in the vicinity of this aphotic deep-sea environment.     

冲绳海槽沉积物干酪根特征

本文采用元素分析、镜下观察、红外光谱、扫描电镜和热解分析等技术,分析了冲绳海槽柱状样中腐植酸、富里酸和干酪根的化学、物理特征,并探讨了它们之间的成因关系和干酪根类型。沉积物的产烃能力甚差,每吨沉积物仅能生成0.49～1.18kg烃类物质。     

冲绳海槽沉积物混合作用的研究

介绍了利用方向性同位素＾２１０Ｐａ活度在岩芯中的垂直分布，根据沉积作用的数学模式，结合有关资料对冲绳海槽海水－海底界面沉积物的混合作用强度进行了了探讨。＾２１０Ｐｂ资料表明冲绳海槽表层沉积物中，＾２１０Ｐｂ放射性活度存在空间差异，介于２６．２６－９２．４０ｄｐｍ／ｇ，沉积物混合系数个于０．５１－６．８８ｃｍ＾２．ａ，这反映了研究海区不同的混合作用强度和沉积环境。     

Gas hydrate associated with mud diapirs in southern Okinawa Trough

Many mud diapirs have been recognized in southern Okinawa Trough by a multi-channel seismic surveying on R/V KEXUE I in 2001. Gas hydrates have been identified, by the seismic reflection characteristics, the velocity analysis and the impedance inversion. Geothermal heat flow around the central of the mud diapir has been determined theoretically by the Bottom Simulating Reflectors (BSRs). Comparing the BSR derived and the measured heat flow values, we infer that the BSR immediately at the top of the mud diapirs indicate the base of the saturated gas hydrate formation zone (BSGHFZ), but not, as we ordinarily know, the base of the gas hydrate stability zone (BGHSZ), which could be explained by the abnormal regional background heat flow and free gas flux associated with mud diapirs. As a result, it helps us to better understand the generation mechanism of the gas hydrates associated with mud diapirs and to predict the gas hydrate potential in the southern Okinawa Trough.     

Origin and model of transform faults in the Okinawa Trough

Transform faults in back-arc basins are the key to revealing the opening and development of marginal seas. The Okinawa Trough (OT) represents an incipient and active back-arc or marginal sea basin oriented in a general NE-SW direction. To determine the strikes and spatial distribution of transform faults in the OT, this paper dissects the NW- and NNE-SN-trending fault patterns on the basis of seismic profiles, gravity anomalies and region geological data. There are three main NW-trending transpressional faults in the OT, which are the seaward propagation of NW-trending faults in the East China Continent. The NNE-SN-trending faults with right-stepping distribution behave as right-lateral shearing. The strike-slip pull-apart process or transtensional faulting triggered the back-arc rifting or extension, and these faults evolved into transform faults with the emergence of oceanic crust. Thus, the transform fault patterns are inherited from pre-existing oblique transtensional faults at the offsets between rifting segments. Therefore, the OT performs the oblique spreading mechanism similar to nascent oceans such as the Red Sea and Gulf of Aden.     

冲绳海槽晚第四纪硅藻生物地层划分与对比

本文对冲绳海槽中段B孔岩芯中的沉积硅藻进行了分析。共鉴定出52属166种和变种．根据硅藻属种组成、热带远洋种的相对含量以及整个硅藻植物群的变化特征共划分出9个组合带，并对它们的特征进行了描述；结合^14C测年数据。对该岩芯的地层年代归属进行了确定，全新世与晚更新世的界线位于该孔125cm处．上部地层为全新统，下部地层为上更新统；同时与邻近DGKS9603钻孔的硅藻生物地层进行了对比．     

冲绳海槽及邻区火山作用与形成演化

冲绳海槽是新月形的弧后扩张盆地，强烈的构造活动和广泛的火山作用是海槽重要的特征。本文作者主要借助对海槽火山岩的常量元素、微量元素、Sr-Nd 同位素和年龄数据分析，结合火山的时空分布及相关构造地质条件，揭示了海槽火山作用的演化过程及其与本地区构造活动的关系，以便更好地了解海槽和沟-弧-盆体系的形成演化历史。作者通过研究得出:①海槽的火山岩石地球化学特征表明岩石属钙碱性系列，主要包括玄武岩、玄武质安山岩和流纹岩，岩石组成具双峰结构。②在海槽的不同区段内，火山作用具有一定的差异。海槽不同区段内火山带岩浆源区深度的差异，是造成海槽不同区段火山活动性质变化的原因，而岩浆源深度受俯冲带形态和地壳厚度控制。海槽北、中段火山岩以流纹岩和安山岩为主，流纹岩分布在海槽盆地内，形成时代较新( <1Ma)，安山岩大部分分布在岛弧火山带中，形成时代是中、晚中新世。海槽南段火山岩由玄武岩和玄武质安山岩组成，从老到新分布在琉球火山弧(大约13Ma)、台北火山带(3Ma～现代)和南段中央凹陷(晚第四纪)中。③琉球火山弧形成于中、晚中新世(17～6Ma)。但大约12Ma发生的吕宋弧与欧亚板块的碰撞作用改变了本地区的动力条件，而宫古断裂对此次改变起了阻断作用，使海槽南段的张裂要晚于北、中段。另外，中新世琉球岛弧南段的形成确定了海槽南段的弧后盆地性质。 冲绳海槽是年轻的弧后盆地，其岩浆、构造活动发育。对海槽岩浆作用的研究可上溯至20世纪50年代，但由于海水及恶劣气候等条件的制约，几十年来对海槽研究程度仍很有限，且在不同区段有很大差异。特别是对区内岩浆作用的研究，极大地受到样品采集偶然性的限制。作者收集前人资料综合对比海槽不同区段内火山活动的异同，了解海槽火山活动研究中的成就与问题， 为进一步开展此项研究打下基础。     

南冲绳海槽及其邻域的基底断裂构造

EW、NEE和NE向基底断裂沿主构造线方向展布,多属张性断裂.基底大断裂宏观上形成和控制了各主构造带的基本构造格架和各具特色的地质构造发育.它们的形成与弧后地慢流上涌和岛弧的旋张掀斜运动而出现的海槽张裂运动密切相关.NW、NNW和NW向基底断裂多属张扭性平移断层.它们对主构造带分割断错,形成和控制了次一级构造区块各具特色的地球物理和地质构造特征.宫古断裂带长期活动,作用十分强烈.它们的形成是由于受来自菲律宾海方向的水平应力作用,地壳作破坏性应力调整的结果.     

冲绳海槽天然气水合物BSR的地震研究

根据多道地震反射资料分析,在冲绳海槽南部和中部发现了拟海底反射层(BSR)现象。通过对海底异常反射层的振幅特征、速度异常和AVO属性分析,说明该BSR可能反映了天然气水合物的存在,并发现冲绳海槽断层与天然气水合物的形成有密切关系。     

冲绳海槽沉积演化的时空特征

冲绳海槽是晚中新世(9～6Ma)以来形成的与板块俯冲作用密切相关的弧后盆地。将冲绳海槽的沉积演化分成3个阶段：中新世晚期(9～6Ma)-上新世末期(2Ma)、上新世末期-晚更新世(130ka)、晚更新世-现在。研究分析表明,冲绳海槽北段发育最早(中新世晚期),但盆地演化程度最低;中段发育次之而演化程度高;南段发育晚(上新世末期)而演化程度高。现阶段的冲绳海槽,其北段处于衰退期,而中段和南段处于扩阔期。构造活动在盆地的演化中占主导作用,它首先为海槽相当长的地质时期内提供宏观的初始沉积格局,其次为盆地沉积提供部分沉积物质(火山物质和热液)。在构造活动强烈的时期,地壳拉伸对沉积层序的影响大于沉积作用,当构造活动趋于缓和时,相对海平面变化控制了盆地内部沉积结构的变化。     

冲绳海槽晚第四纪沉积硅藻及其古海洋学意义

本文描述了冲绳海槽辫中KL22＋KL18岩芯的硅藻丰度、优势硅藻种的相对含量、微型硅藻的种类和丰度、热带远洋种的丰度的分布特征,讨论了该岩芯的地层年低、古黑潮流等问题。结果表明晚更新世以黑湖主干未流过研究区;岩芯中硅藻丰度和热带远洋种丰度的突然增加系新仙女木（YD）、海因里奇（H1）等古气候变冷事件留下的记录。     

冲绳海槽北部氧化还原敏感元素的早期成岩过程

The early diagenesis processes of several redox-sensitive trace metals(RSMs)(Mo, U and V) were studied with several short sediment cores(～25 cm) collected in the northern Okinawa Trough(OT). Pore water vertical profiles indicated that the sedimentary environments in all cores were between oxic and suboxic, not yet reaching anoxic sulfidic conditions. The recycling process of Mo in sediments was clearly associated with Mn and yielded little authigenic accumulation, while U showed a downcore increase in sediment and its authigenic mass accumulation rate(MAR) was estimated to be ～23% of the Changjiang(Yangtze) and Huanghe(Yellow) riverine flux. Benthic diffusive fluxes and MAR were calculated and the comparison of them showed that U and V fluxes matched relatively well both in direction and in magnitude, implying that diffusion processes at the sedimentwater interface is the dominant process controlling the remobilization or burial of V and U in northern OT. This work provided a systematic study(both in pore water and solid phase) on the RSMs geochemical behaviors during early diagenesis process, yielding a quantitative assessment of the remobilization or burial fluxes of the RSMs in northern OT. Such studies are in general lacking in the coastal margin of Northwest Pacific Ocean.