琉球岛弧地区的地震分布、Benioff带形态及应力状态h

利用ISC及中国台网的资料,研究了琉球岛弧及冲绳海槽的地震分布及震源机制解,讨论了Benioff带的形态及应力状态.mb4.0的地震主要分布于琉球海沟西侧的弧形带,并形成明显的Benioff带.吐噶喇海峡以北俯冲带弯曲明显,深部倾角大,约92,70km以下张应力轴沿俯冲方向;吐噶喇海峡以南,俯冲带较平直,深部倾角较小,约55,压应力轴基本沿俯冲方向。冲绳海槽内处于NNW向近水平的拉伸,华北应力场与之类似,没有受到菲律宾海块挤压作用的影响.      

冲绳海槽北部基底构造特征

利用最新的重磁数据对冲绳海槽北部的基底构造进行了推断解释.研究表明,在海槽内部主要发育两条火山带,一条为著名的吐噶喇火山链,由一系列活动的或休眠的串珠状展布的活火山岛构成;另一条沿海槽中央张裂轴分布,主要由孤立的海底火山构成.冲绳海槽的基底具有沿东西向凹－凸－凹相间的构造格局,深度在1 km~8 km之间变化,受吐葛喇断裂带的北部断裂F1的作用,第三系基底构造发生了显著变化,说明吐葛喇断裂带的北部断裂F1是一条构造转换带.莫霍面为一北北东向的向东倾伏的幔坡,地壳厚度由21 km减至18 km.     

冲绳板块应力场数值模拟及其动力学特征div>

冲绳板块位于菲律宾海板块向欧亚板块俯冲形成的西太平洋边缘活动带上,构造应力场图像及其动力学机制表现得相当复杂．采用伪三维有限元方法,以WSM2008 观测应力场数据的应力取向和应力型两方面指标作为主要约束,对冲绳板块构造应力场进行了数值模拟．通过对计算结果的分析,对模型涉及的各种作用力作出了估计．在此基础上,对冲绳板块岩石层的状态,以及该地区的板块动力学特征进行了探讨,并得到了以下一些初步认识:① 软流层静压推力控制着该地区构造应力场的基本形态;② 冲绳海槽的演化过程,例如该地区的岩石层减薄与其下地幔流的上升等,也在很大程度上影响了该地区的板内应力场空间分布特征;③ 琉球俯冲带边界力的作用是分段的,不同区段作用力对板内应力场的影响有所不同．      

琉球海沟的构造和运动特征

琉球海沟是欧亚板块与菲律宾海板块之间的边界，海沟西坡是大陆性质的琉球岛弧，东坡是大洋性质的菲律宾海地壳．海底地震反射探测和地震震源定位表明，菲律宾海板块沿海沟向琉球岛弧下俯冲，俯冲角度与深度沿海沟走向变化．有证据显示，由于俯冲板前缘的横向移动，海沟和岛弧正朝大洋方向后退，弧后盆地-冲绳海槽发生拉张变形．最近一次的海沟后退与冲绳海槽扩张可能是从上新世末(2Ma前)开始的，岛弧的后退移动和弧后拉张在南部与海沟走向垂直，在中部和北部与海沟走向斜交，总体上向南的运动分量占优势、与海沟后退相关的弧后拉张集中在冲绳海槽，没有证据表明对其西侧的中国东海陆架盆地产生影响、海沟后退的原因可能与俯冲板的动力不平衡以及它与周围地幔的相互作用有关．     

东海及琉球沟弧盆系的海底热流测量与热流分布

利用钻井资料获得了东海陆架地区15个热流值，分析了海底热流测量的误差来源及数据精度，对东海及琉球沟弧盆系的热流值进行了分类整理，将海底热流分为可信热流值、较可信热流值和参考热流值，本文使用了前两类热流数据，共得305个，对研究区的热流站位进行了分析，发现热流测站分布很不均匀，冲绳海槽几个高热流异常区的热流测站总数占全部东海热流测站的一半多，仍有相当一些构造单元热流站住很少或者几乎没有热流值，总体上，研究区的热流分布明显地和沟弧盆系的构造特征相吻合，呈现北东向条带状分布，东海陆架为正常热流值区，冲绳海槽为高热流异常区，琉球群岛为正常热流值区，琉球海沟和菲律宾海都为低热流异常值区，根据热流推测冲绳海槽下存在深部热物质上拱，琉球海沟垭口之下存在海洋板块的俯冲，本研究区自东向西初步表现出长波长热流振荡现象，但由于测量数据稀少，目前还不能对此作更进一步的研究。     

冲绳海槽南部基底构造特征

利用最新的重磁数据对冲绳海槽南部的基底构造进行了推断解释,研究表明,在冲绳海槽南部,莫霍面构造为一南北两端高,中间低的鞍状构造,地壳厚度在15.5~21 km之间变化.基底深度一般在4~6 km之间变化,表现为南段深度大,北段深度小,在北西向断裂系的作用下,基底局部构造大都成北西向展布,说明冲绳海槽早期的北东向带状构造受到了后期的北西向构造活动的强烈改造作用.在冲绳海槽南部发育有两条沿北北东向展布的火成岩带,一条分布在冲绳海槽扩张轴以东及琉球岛弧西侧下坡;另一条分布在冲绳海槽与琉球岛弧隆褶带的结合部位,火成岩主要是由北东向构造活动产生的,而与北西向断裂关系不密切.     

中国东海及邻近海域一条剖面的地壳速度结构研究

1982年以来，中国科学院海洋研究所在东海海域进行了二十多个站位的遥测浮标折射地震测量，1991年又在东海陆架区进行了OBS测量，本文在我国东海域选择了横穿东海陆架，冲绳海槽，琉球岛弧，琉球海沟和菲律宾海盆的一条剖面，利用上述折射地震资料及其它该剖面附近的折射地震资料，对该剖面的地壳速度结构进行了研究，并进行了速度年代对比。研究表明，剖面的速度结构在纵向上和横向上都表现出明显的差异，横向上可分为三隆三盆，纵向上大致可划分为1．8－2．2km/s,2.4-2.8km/s,3.0-3.6km/s,4.2-5.1km/s和5.75-6.0km/s的速度层，从地壳的速度结构否则 本海区至少有如下的沉积旋回：降冲 槽的中抽外，上新世纪至第四纪本海区沉积环境稳定，而冲第槽中轴可能一直处于构造活跃的状态；始新世为本区沉积的全盛渐新世该区域处于抬升的时期,钓鱼岛隆起区、琉球岛弧隆起区在此期的沉积被剥蚀殆，东海陆架和冲 槽此时斯 沉积也受到相当程度的剥蚀，东海陆架盆地和冲绳槽此时期的沉积也受到相当沉的剥蚀。，东海陆架盆地和冲槽盆地的出现可能在5．75－6．0km/s的速度层沉积之后，菲律宾海盆为典型的大洋地壳结构。     

冲绳海槽地区地壳结构与岩石层性质研究

用冲绳海槽地区地质、地球物理、地球化学数据与资料，对横跨大陆边缘—大洋板块的中国东部—冲绳海槽—琉球岛弧地区进行综合研究. 根据地震体波、面波成像结果，空间与布格重力异常及磁力异常的综合解释等结果，利用调和系数法对该研究区的地壳厚度进行了计算，并对地壳属性进行分析. 同时，选择中国东部东北地区的宽甸、华北地区的汉诺坝、华南的闵溪和台湾海峡的澎湖列岛与冲绳海槽和日本岛弧的新生代玄武岩开展元素—同位素对比研究. 结果认为冲绳海槽岩石层明显减薄，上地幔低速层已达到莫霍面，在地幔中已形成交代软流层. 元素—同位素研究表明：它不同于中国东部，与琉球岛弧地区的特点也不相同. 综合分析认为：冲绳海槽地壳属于过渡型地壳，与大洋地壳的许多特点颇为相似.     

Strain segmentation and lateral membrane deformation rate of the subducted Ryukyu slab

Abstract Focal mechanisms of intermediate-depth earthquakes within the subducted Ryukyu slab indicate a systematic pattern of down-dip extension in the northern part under Kyushu. These mechanisms switch rapidly around the Tokara channel to down-dip compression in the southern part toward Taiwan. This intriguing pattern of strain segmentation, together with the differences between the slab geometry of the northern and southern parts, has raised the question of whether stress segmentation, as defined possibly by a tear fault, might exist in the slab. However the Ryukyu trench has a concave oceanward shape in the northern segment while it is convex in the southern oceanward part toward Taiwan. The inflection zone is located around the Tokara channel. The concept of Gaussian curvature of a curved surface suggests that the along-arc variation of the geometric configuration of a subducted slab is related to the shape of the trench. This is in order to accommodate the lateral membrane deformation of the slab as the oceanic lithosphere subducts from a spherical shell to the geometry delineated by the Wadati-Benioff zone. The membrane deformation regime of the subducted Ryukyu slab and its relation with the trench geometry was examined by assuming that the subduction be modelled by the flow field of a thin viscous sheet. A projection operator was utilized to compute the membrane strain-rate tensor of an arbitrary non-Euclidean surface. Numerical experiments indicated that the northern segment of the slab was dominated by lateral compression and down-dip extension and the southern part by lateral extension and down-dip compression. This transition is sharply located near the Tokara channel. These patterns were compatible with what had been observed from studies of focal mechanisms, suggesting that the strain segmentation might be controlled, at least in part, by the lateral membrane deformation within the slab due to the trench shape in this subduction zone. The slab geometry was predicted by minimizing the integrated total dissipation power; this revealed distinct features that were consistent with observations. This implies that the slab geometry may also be affected by the membrane deformation in a systematic fashion.     

Shimajiri Group equivalent sedimentary rocks dredged from sea knolls off Kume Island,central Ryukyus: Implications for timing and mode of rifting of the middle Okinawa Trough back-arc basin

The late Cenozoic geohistory of the Ryukyu arc is closely related to the rifting history of the Okinawa Trough. The submarine geology and stratigraphy of areas around Kume Island, which is situated near the eastern rifted margin of the middle Okinawa Trough, provide key constraints to understand the timing and mode of Okinawa Trough rifting. Here we report the lithology of sedimentary rocks dredged along slopes of ~1000-m-deep sea knolls located north and northwest off Kume Island, and their depositional ages determined by calcareous nannofossils and strontium (Sr) isotope analyses. Various types of sedimentary rocks, such as siltstone, very fine-grained sandstone, medium-grained sandstone, fossiliferous coarse-grained sandstone, and tuffaceous sandstone, were recovered at two dredge sites. These sedimentary rocks are lithologically similar to those in the Aka Formation and a part of the Maja Formation of the Shimajiri Group in nearby Kume Island. Calcareous nannofossils and Sr isotope analyses indicate their depositional ages from the early Pliocene to the early Pleistocene, which are generally consistent with those of the Aka Formation. The finding of the dredged rocks similar in lithology and ages to the Aka Formation indicates that marine deltaic area continued toward north and northwest from Kume Island during these periods. The presence of the Shimajiri Group equivalent sedimentary rocks at the dredge sites are likely related to the main rifting of the Okinawa Trough after ca. 2 Ma in the central Ryukyus.     

Assessment of long-term variation in displacement for a GPS site adjacent to a transition zone between collision and subduction

A transition and subduction zone adjacent to the Ryukyu Arc, Ryukyu Trench, and Okinawa Trough, extends between southern Japan and northeastern Taiwan. It is generated during the northwestward subduction of the Philippine Sea Plate, which lies the Eurasian Plate along the Ryukyu Trench. The movement of the Philippine Sea Plate is hindered at the northeastern corner of Taiwan, which causes complicated structure of the Philippine Sea Plate at the western end of the Ryukyu subduction zone. Development of the active subduction and transition boundary near the western Ryukyu Arc is evaluated statistically by using displacements derived from GPS site data. The statistical model shows that the absolute displacement derived from GPS measurements of nearly 8 years indicates a maximum spatial variation of 0.625 m. Three trends are observed for such long-term progress, and use of linear regression also reveals quite good consistency between the data and statistic models. Such rate is also elevated following the trend development. Southeastern and nearly horizontal movement is suggested to the main development of for the site movements, it is likely related to the tensional activity adjacent to this boundary.     

Structural control of the Gagua “Wedge” Zone east of Taiwan Island on the southern Okinawa Trough

The sea area east of Taiwan Island is a unique ac-tive arc-continental collision margin in China and even in the world with the NE-nearly EW extending Ryu-kyu trench-arc-backarc basin system on the east and the nearly N-S extending Manila trough and Luzon arc on the west. The sea area east of Taiwan Island differs from the Ryukyu trench-arc-basin system in structural property, but there is a certain genetic relation between the two (Fig. 1)[1-3]. Since the 1990s, many investigations and …     

Some features of the arc-continent collision zone in the Ryukyu subduction system, Taiwan Junction area

Abstract To the northeast of Taiwan, northwestward subduction of the Philippine Sea plate is occurring beneath the Eurasian plate along the Ryukyu Trench. The Ryukyu Trench, which is well defined along the northeastern part of the Ryukyu arc, cannot be easily defined west of 123° east. This is an area where the Gagua Ridge (whose origin is controversial) enters the trench from the south. On the basis of the marine geophysical survey data the following results have been obtained. The structural elements associated with the Ryukyu subduction system deform and partially disappear west of 123° east. Among other things the Ryukyu Trench terminates close to the western slope of the Gagua Ridge. The Gagua Ridge is the result of tectonic heaping and is likely to be an uplifted sliver of oceanic crust. The interaction between the Ryukyu subduction system and the Taiwan collision zone encompasses a wide region from Taiwan to the longitude 124.5° east. The Gagua Ridge is a boundary between the active deformation zone related to the collision in Taiwan and the West Philippine Basin. It is proposed that there is a tectonic zone that can be traced from the Okinawa Trough on the north to the southern termination of the Gagua Ridge on the south.     

Structural control of the Gagua "Wedge" Zone east of Taiwan Island on the southern Okinawa Trough

Based on compositive analysis and interpretation of the observed and historical data, the geophysical field characters and structural properties of the Gagua "Wedge" Zone of the sea area east of Taiwan Island and the primary tectonic stress direction and its variabilities of backarc spreading in the southern Okinawa Trough are studied. It is concluded from the study results that the Gagua "Wedge" Zone is structurally consistent with the Gagua ridge and two fault basins on both sides of the Gagua ridge, and adjusts the moving direction and distance of the western Philippine Sea plate to make the northwestward motion of the plate on its east side change to the northward subduction of the plate on its west side so that the primary tectonic stress direction of the Okinawa Trough changed from NW-SE to nearly N-S, which provided the stress source for the Okinawa Trough to enter the second spreading stage.     

Radiometric ages from Ryukyu arc region and an40Ar/39Ar age from biotite dacite on Okinawa

A biotite dacite that intrudes metamorphic rocks on Okinawa in the Ryukyu island arc has been dated at 12 m.y. by the⁴⁰Ar/³⁹Ar method. The details of this age measurement and a compilation of radiometric ages for the Ryukyu island arc and adjacent regions are presented. These data suggest that from 65 to 12 m.y. ago the magmatic axis of the Ryukyu arc was confined to a very narrow zone along the arc. In Kyushu and Shikoku, the southern Japanese islands, intrusive and volcanic igneous rocks dated as 21?12 m.y. occur over a much wider zone than in the Ryukyu arc. The apparent difference in width of the magmatic zones may be due to different absolute motions of the overthrust plates of those two regions of subduction. The dissimilarity of available radiometric ages for the Ryukyu arc and for Taiwan suggest different histories for the development of these two features. The occurrence of active volcanoes in association with the Okinawa Trough, northwest of the Ryukyu island arc, may indicate that the trough itself developed in the last 12 m.y.     

俯冲带深部应力场的二维粘弹性有限元数值模拟

基于粘弹性平面应变有限元数值模拟方法，研究了俯冲带深部的应力场特征．当根据Karato等的研究结果给出俯冲板块的粘度结构时，橄榄石——尖晶石相变界面以下中心低粘区两侧出现应力集中区，而且其主压应力方向沿俯冲方向走向．其特征与已有的深源地震主压应力方向沿俯冲方向及深源地震有两个条带的地震观测结果相吻合．相变过渡区因矿物更小的颗粒粒度而导致的更小的等效粘度对俯冲带深部应力场的影响不大；存在和不存在亚稳态橄榄石楔的情况下，橄榄石——尖晶石相变过渡区附近都有最大剪应力的最大值出现，而其主压应力方向有垂直于橄榄石——尖晶石相变过渡区走向的趋势，尚与地震观测资料不符．      

Recent Dasycladales (Chlorophyta) in Okinawa Jima in the Central Ryukyus,southwestern Japan: Paleontological implications

Recent dasycladalean algae (Dasycladales, Chlorophyta) are classified into two families, 11 genera, and 44 species, and inhabit the tropical to warm‐temperate waters of the world. Twelve species of dasycladalean algae occur in Okinawa Jima, Ryukyu Islands. In this paper, the present‐day dasycladalean species and their distributions in Okinawa Jima are reported and their paleontological implications are discussed. Dasycladalean algae grow mainly on coral rubble and rocks, often forming large meadows in calm shallow lagoons. However, Dasycladus vermicularis occurs in deeper waters, at depths of 15–89 m, around Okinawa Jima. Neomeris annulata is found at water depths of 0–30 m. These data throw doubt on the interpretation that the occurrence of dasycladalean algae is linked to warm shallow habitats.     

Subsidence of the Miyako‐Sone submarine carbonate platform,east of Miyako‐jima Island,northwestern Pacific Ocean

Bathymetric mapping and observations of the seafloor using a remotely operated vehicle (ROV, Hyper‐Dolphin 3K) were carried out on the slopes of the Miyako‐Sone submarine platform, east of Miyako‐jima in the Ryukyu Islands, northwestern Pacific Ocean. The bathymetric map indicates that terraces are present at water depths of approximately 140 m, 330 m, 400 m, and 680 m on the northwestern slope of the platform. A number of NW–SE trending lineaments, probably faults, extend perpendicular to the axis of the Ryukyu Island Arc. Two ROV surveys were conducted at water depths ranging from 519 m (on the slope) to 121 m (shallowest part of the platform). The surveys revealed that well‐indurated carbonate rocks are exposed at terrace margins and on upper slopes, and that the lower slopes are covered with modern sediments consisting of unconsolidated, coarse‐sand‐sized bioclastic carbonates. Calcareous nannofossils from the well‐indurated carbonate rocks indicate a Middle–Late Pleistocene age, which suggests that the rocks correlate with the Quaternary reef and fore‐reef deposits of the Ryukyu Group (Ryukyu Limestone) on the Ryukyu Islands. No siliciclastic deposits corresponding to the upper Miocene–lower Pleistocene Shimajiri Group (as exposed on Okinawa‐jima and Miyako‐jima islands) were recovered during the surveys. Coeval well‐indurated carbonate rocks, all of which formed in a similar sedimentary environment, have been downthrown towards the west due to displacements on the western sides of normal faults. Subsidence of the Miyako‐Sone submarine platform was the result of large vertical displacements on such normal faults. The timing of initial subsidence cannot be tightly constrained, but the presence of the youngest limestone at progressively lower levels towards the west suggests the subsidence continued until after 0.265 Ma.     

伽师强震群的深部动力学条件

根据天山造山带及其两侧的塔里木盆地和准噶尔盆地的岩石圈二维速度结构、二维密度结构、二维电性结构、壳幔过渡带的详细结构以及大地热流和震源深度的分布,再结合对新疆西北部的蛇绿岩带、高压变质带和岩浆岩分布的综合分析,建立了天山造山带的地球动力学“层间插入消减”模型。该模型认为,塔里木板块的中上地壳在库尔勒断裂附近向天山造山带的中下地壳层间插入;而下地壳连同岩石圈地幔向天山造山带的上地幔俯冲消减。在天山的西段（哈萨克斯坦境内）,费尔干纳地块由北向南插入到南天山之下约180km的深处,在其东段（中国境内）,塔里木盆地由南向北插入南天山之下。这两个具有不同方向的下降板片的接触部位为费尔干纳走滑断裂。我国的伽师、喀什、乌恰地震区均落在这两个具有不同俯冲方向板块的结合部位附近,有着特殊的深部构造背景。南、北两大板块的双向挤压必定产生强大的应力,在地震区附近,这种应力的积累与释放具有4个显著的特点：（1）板片的俯冲消减速度约高达每年22mm左右;（2）应力的积累与释放速率加快;（3）应力释放较容易;（4）应力释放较为集中。这4个特点可能是伽师地区在较短的时间内连续发生数次强震的构造因素。     

Heat flow in the southwestern Okinawa Trough

Lying to the southeast of the East China Sea continental shelf, the Okinawa Trough is believed to have been formed by crustal spreading behind the Ryukyu island arc. It has the highest mean heat flow among the Pacific marginal basins. The mean of 18 reliable measurements is3.34±2.75HFU. The next highest mean heat flow for a marginal basin is2.53±1.60HFU for the Fiji Plateau. The high mean value for the Okinawa Trough may be partially attributable to a sampling bias because most measurements were made in or near the well-sedimented central “rift” or graben. The reported values range from 0.36 to 10.49 HFU, with low values probably resulting from hydrothermal circulation in an area of large relief. The high heat flow suggests that the age of the Okinawa Trough may be as young as 2 Ma. Such an estimate is in gross agreement with the inferred Plio-Pleistocene age for the drifting in the Okinawa Trough. The opening of the Okinawa Trough may continue onto Taiwan but the relation between the opening of the trough and the occurrence of a nearby, onshore geothermal resource area remains unsolved.