Complex slab structure and arc magmatism beneath the Japanese Islands

A dense nationwide seismic network recently constructed in Japan has resulted in the production of a large amount of high-quality data that have enabled the high-resolution imaging of deep seismic structures in the Japanese subduction zone. Seismic tomography, precise locations of earthquakes, and focal mechanism research have allowed the identification of the complex structure of subducting slabs beneath Japan, revealing that the subducting Philippine Sea slab underneath southwestern Japan has an undulatory configuration down to a depth of 60–200 km, and is continuous from Kanto to Kyushu without disruption or splitting, even within areas north of the Izu Peninsula. Analysis of the geometry of the Pacific and Philippine Sea slabs identified a broad contact zone beneath the Kanto Plain that causes anomalously deep interplate and intraslab earthquake activity. Seismic tomographic inversions using both teleseismic and local events provide a clear image of the deep aseismic portion of the Philippine Sea slab beneath the Japan Sea north of Chugoku and Kyushu, and beneath the East China Sea west of Kyushu down to a depth of ∼450 km. Seismic tomography also allowed the identification of an inclined sheet-like seismic low-velocity zone in the mantle wedge beneath Tohoku. A recent seismic tomography work further revealed clear images of similar inclined low-velocity zones in the mantle wedge for almost all other areas of Japan. The presence of the inclined low-velocity zones in the mantle wedge across the entirety of Japan suggests that it is a common feature to all subduction zones. These low-velocity zones may correspond to the upwelling flow portion of subduction-induced convection systems. These upwelling flows reach the Moho directly beneath active volcanic areas, suggesting a link between volcanism and upwelling.     

Distribution of fluorite deposits in Japanese Islands

The occurrence of fluorite deposits in Japan is limited in the provinces characterized by tin and tungsten mineralization within Southwest Japan. The deposits were formed near acidic igneous rocks of Cretaceous to Tertiary age. The ores in limestone are generally associated with skarn and metallic ore minerals such as cassiterite, scheelite and chalcopyrite. Granitic rocks in the provinces are the ilmenite-series (Ishihara, 1977) having enhanced fluorine contents and high initial strontium ratios. A reducing condition of the ilmenite-series granitic magma may have been more favorable for the concentration of fluorine in the residual magma because of the crystallization of relatively Fe-rich mafic minerals. Presence of thick crust bearing carbonaceous matter at the site of magmatism could have involved in the enrichment of fluorine as well as the reducing condition.     

日本油气发展战略及其对中国油气行业的启示

总结了日本在不同时期根据国际油气市场结构的变化而采取的相应的油气政策,重点对日本的石油天然气和金属矿物资源机构的职能、石油和液化石油气的储备体系、油气自主开发和能源外交政策,以及最新颁布的《新国家能源战略》作了介绍和分析。最后结合中国油气发展的现状,提出了一些针对性的建议。     

日本油气发展战略及其对中国油气行业的启示

总结了日本在不同时期根据国际油气市场结构的变化而采取的相应的油气政策,重点对日本的石油天然气和金属矿物资源机构的职能、石油和液化石油气的储备体系、油气自主开发和能源外交政策,以及最新颁布的《新国家能源战略》作了介绍和分析。最后结合中国油气发展的现状,提出了一些针对性的建议。     

日本油气发展战略及其对中国油气行业的启示

总结了日本在不同时期根据国际油气市场结构的变化而采取的相应的油气政策,重点对日本的石油天然气和金属矿物资源机构的职能、石油和液化石油气的储备体系、油气自主开发和能源外交政策,以及最新颁布的<新国家能源战略>作了介绍和分析.最后结合中国油气发展的现状,提出了一些针对性的建议.     

Global tectonic significance of the Solomon Islands and Ontong Java Plateau convergent zone

Oceanic plateaus, areas of anomalously thick oceanic crust, cover about 3% of the Earth's seafloor and are thought to mark the surface location of mantle plume “heads”. Hotspot tracks represent continuing magmatism associated with the remaining plume conduit or “tail”. It is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones, and, therefore: (1) influences the eventual composition of continental crust and; (2) is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs. The Ontong Java Plateau (OJP) of the southwestern Pacific Ocean is the largest and thickest oceanic plateau on Earth and the largest plateau currently converging on an island arc (Solomon Islands). For this reason, this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones.This volume consists of a series of four papers that summarize the results of joint US–Japan marine geophysical studies in 1995 and 1998 of the Solomon Islands–Ontong Java Plateau convergent zone. Marine geophysical data include single and multi-channel seismic reflection, ocean-bottom seismometer (OBS) refraction, gravity, magnetic, sidescan sonar, and earthquake studies. Objectives of this introductory paper include: (1) review of the significance of oceanic plateaus as potential contributors to continental crust; (2) review of the current theories on the fate of oceanic plateaus at subduction zones; (3) establish the present-day and Neogene tectonic setting of the Solomon Islands–Ontong Java Plateau convergent zone; (4) discuss the controversial sequence and timing of tectonic events surrounding Ontong Java Plateau–Solomon arc convergence; (5) present a series of tectonic reconstructions for the period 20 Ma (early Miocene) to the present-day in support of our proposed timing of major tectonic events affecting the Ontong Java Plateau–Solomon Islands convergent zone; and (6) compare the structural and deformational pattern observed in the Solomon Islands to ancient oceanic plateaus preserved in Precambrian and Phanerozoic orogenic belts. Our main conclusion of this study is that 80% of the crustal thickness of the Ontong Java Plateau is subducted beneath the Solomon island arc; only the uppermost basaltic and sedimentary part of the crust (7 km) is preserved on the overriding plate by subduction–accretion processes. This observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both Precambrian and Phanerozoic orogenic belts.     

The Bangong Lake ophiolite (NW Tibet) and its bearing on the tectonic evolution of the Bangong–Nujiang suture zone

The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong–Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20–25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69–73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO₂ (53.2–57.9 wt%), MgO (6.5–12.5 wt%), Cr (166–752 ppm) and Ni (63–213 ppm) and low TiO₂ (0.22–0.37 wt%) and Y (5.34–8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong–Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia.     

Characteristics of the stream network composition of drainage basins in the Japanese Islands

This research is to analyze the composition of stream networks of drainage basins in Japan and to discuss the characteristics of drainage basins in the Japanese Islands. Study areas were selected for 180 drainage basins in the entire country. In this study, a topographic map with a scale of 1:50,000 was used and inserted into the valley line by V-shaped-contour methods in each drainage basin. The streams are ordered according to Strahler's system and measured by the stream number, stream length, stream slope, drainage area, and stream fall in each drainage basin. Horton's law of drainage network composition holds good in most drainage basins in Japan. The average values of the bifurcation ratio, stream length ratio, stream slope ratio, drainage area ratio, and stream fall ratio are 4.24, 1.91, 1.98, 4.52, and 1.04, respectively. The mean bifurcation ratio of drainage basins in Japan is 4.24, and this ratio is slightly greater than in other countries. It is also greater than the theoretical value of 4.0. This implies that the slope of drainage basins in Japan is so steep and the relief energy so large that excess streams have developed to a rather marked degree. The average stream fall ratio is 1.04 for drainage basins in Japan. It suggests that drainage basins are generally considered to be in a stage of maturity in stream channel development for the Japanese Islands. The stream fall ratio, however, is slightly greater than 1.0. Although, the stream beds are nearly in dynamic equilibrium in most cases, it tends to be in a slightly aggrading state.     

Geology of the Talaud Islands, molucca sea collision zone, northeast Indonesia

The Talaud Islands lie at the northern margin of the collision zone between the Sangihe and Halmahera island arc systems. Rock units on Talaud are Neogene marine strata, basalt and andesite, tectonic mélange, and ophiolite. The units are exposed in N–S trending belts that are commonly separated by faults. The marine strata consist of tuffaceous siltstone, sandstone, shale and marl. They are strongly deformed by west-verging folds with wavelengths of 20–500 m. Volcanic rocks of island arc affinity are exposed on the east coast of Karakelang Island and appear to be interbedded with the lowermost marine strata. Tectonic mélanges contain blocks of serpentinite, gabbro, basalt, red middle Eocene chert and limestone, and greywacke turbidites. The blocks range in length from a few millimetres to hundreds of metres, and are enclosed in a scaly clay matrix. Several mappable slabs of ophiolite are separated by Tertiary strata or mélange. The dismembered ophiolites consist of serpentized peridotite, gabbro, spilites and cherts. Locally, the mélanges and ophiolites are thrust over the younger sedimentary rocks along east-dipping faults. The dominant eastward dips of mélange foliation, the westward vergence of structures in the Neogene strata, the Eocene ages of the cherts, and the Miocene age of the strata overlying the ophiolite slabs suggest that the ophiolites are pieces of Eocene or older oceanic crust (derived from a mid-ocean ridge or back-arc basin) and upper mantle that were emplaced as thrust slices into the lower slope of a west-facing arc during the Miocene and have been uplifted during arc—arc collision.     

Sulfur isotope study of Quaternary volcanic rocks from the Japanese Islands Arc

The sulfide and sulfate contents and their δ³⁴S values were determined in Quaternary volcanic rocks from the Japanese Islands Arc. The total sulfur contents are much lower (less than 40 ppm) and the δ³⁴S values are higher (+4.4 ± 2.1) than those of ocean-floor basalts (800 ± 100 ppm and +0.8 ± 0.5, respectively; Moore and Fabbi, 1971; Sakaiet al., 1982). Lateral variations of both sulfur content and δ³⁴S values were observed in the four volcanic belts in Japan. In the Northeast Japan belt, the sulfur content (30 ± 10 ppm) of the rocks in the inner zone (the Japan Sea side) is 3 to 5 times that in the outer zone (the Pacific side), although the δ³⁴S values of the two zones are almost the same (+4.3 ± 1.0). The δ³⁴S values for the two belts in West Japan are on the average 2%. higher than those of East Japan.This study suggests that the primary magmas that formed the island arc volcanic rocks are initially depleted in sulfur (<120 ppm) and enriched in ³⁴S ($\text{δ}{}^{34}\text{S: +5 ～ +7}$) compared to ocean-floor tholeiitic basalts which formed at mantle under oceanic region. This indicates that the upper-mantle is heterogeneous in sulfur content and isotope composition.     

Long-term earthquake prediction along the seismic zone of the Solomon Islands and New Hebrides based on the time- and magnitude-predictable model

Instrumental and historical information on strong main-shocks for 13 seismogenic sources along the seismic zone of the Solomon Islands and New Hebrides has been used to show that the interevent time,T _t (in years), between two strong earthquakes and the magnitude,M _f, of the following mainshock are given by the relations $$\begin{gathered} \log T_t = 0.17M_{\min } + 0.31M_p - 0.33 \log \dot M_O + 6.36, \hfill \\ M_f = 0.51M_{\min } - 0.12M_p + 0.541 \log \dot M_O + 9.44, \hfill \\ \end{gathered} $$ whereM _min is the surface wave magnitude of the smallest main-shock considered,M _p is the magnitude of the preceding mainshock, andM _o is the moment rate per year in each source. On the basis of these relations, the probability for the occurrence of a mainshock during the decade 1993–2002 as well as the magnitude of this expected mainshock in each seismogenic source has been calculated. The highest probability value (P ₁₀ = 0.79) was estimated for the seismogenic sources of Santa Cruz-Ndeni Islands (H ₁) and Tana Island (H ₅) for the occurrence of a large or great earthquake with expected magnitudesM _f = 7.7 and 7.5, respectively.     

Tourism resources and their development in Maldive Islands

Maldive Islands are characterized by a unique coral nature and thus they posses unique tourism resources though in a one-sided way, namely in the submarine and littoral environment of the islands, lagoons and reefs, associated with the year-round tropical climate. Therefore, not only divers and snorklers enjoy the profuse underwater resources, but also beach tourists are attracted by the nature of the beaches and the climatic conditions as well. This environment really guarantees relaxation and recreation. Tourism in Maldives which was introduced in 1972 only, has successfully developed so far. Proof is given by the rapidly growing number of tourists and tourist resorts (islands) over the past 10 year period. On the European market, Maldives today rank among the most attractive tourist destinations in the tropics. Maldives offer large natural resources for tourism to be further developed in the future. The sensitive environment does however call for most considerate measures.     

准噶尔永进地区深部储层的保存与发育机理

随着油气勘探向深部发展,在深部寻找优质储层是一个亟待深入研究的问题。永进地区位于准噶尔腹部中石化中部3区块,勘探目的层埋藏深,大于5000m,储层多为低孔-特低孔低渗-特低渗储层,因此,在该地区深部寻找相对优质储层具有重要的理论意义和勘探意义。该地区发育区域性不整合,即J/K不整合。在不整合形成期间,大气淡水具有高的流速和相对开放的流体环境,可以溶蚀易溶矿物并将溶蚀产物带离出溶蚀区;颗粒包壳可以抑制石英加大和碳酸盐矿物沉淀,保存原生孔隙和形成的次生孔隙;永进地区油气充注早,早期的油气充注改善了流体成岩环境,抑制胶结作用,同时,其携带的有机酸溶蚀易溶矿物,产生次生孔隙,油气早期充注从两个方面改善储层。     

Pressure–temperature conditions of ongoing regional metamorphism beneath the Japanese Islands

We evaluate the pressure–temperature (P–T) conditions of ongoing regional metamorphism at the top of the oceanic crust of the subducted Pacific and Philippine Sea plates through a combination of phase diagrams and hypocenter distribution and based on the dehydration-induced earthquake hypothesis. The brute-force method was employed to find the best match thermal structure to link the hypocenter distribution and dehydration. The estimated thermal structure varies far from the values obtained from numerical simulation. Our estimates are consistent with the qualitative physical prediction for the variation of temperature in different subduction zones and provide quantitative constraints for the models.In northeastern Japan, the P–T path for the top of the oceanic crust turns to the high-T side at a depth of around 90 km. The depth corresponds to the location of the volcanic front and an active convection of the wedge mantle below this depth is suggested. Our computations also reveal the effect of an exceptional scenario beneath the Kanto region. The temperature in the Kanto region, where the cold lid of the Philippine Sea plate prevents heating by the return-flow of mantle wedge above, is much lower than that of northeastern Japan. The subduction of younger Philippine Sea plate leads to a higher-temperature in the oceanic crust. In the central Shikoku region, the thermal structure exhibits high-T/P nature. Heating by shear deformation can explain the high-T/P path in the depth range from 20 to 35 km. The Kyushu area shows moderate type T/P path reaching up to eclogite facies conditions. In the Kii and central Shikoku region, the thermal structure exhibits high-T/P nature. However, the absolute values for the areas seem to have problem in physical context. Our results has risen the significance of sediment subduction in the southwest Japan and requirement for further improvements in this technique including the aspect of variation of the bulk composition of the subducted material.     

K-Ar dating on volcanic rocks in the Bonin Islands and its tectonic implication

The K-Ar dating method was applied to the volcanic rocks, the boninite and related rocks, from the Bonin Islands. Four fresh samples gave K-Ar ages of about 40 Ma. These ages suggest that the main volcanism of the Bonin Islands occurred about 40 Ma ago, as a result of the initiation of subduction of the Pacific plate at its western margin. Results of the K-Ar dating on other rocks suggest that some volcanism or thermal disturbance may have occurred 5 ~ 10 Ma ago, at least, in Chichijima.     

克拉通地幔形成与演化及对大陆命运制约因素的思考

岩石圈地幔属性主要由熔体抽取和交代作用程度共同决定,与岩石圈起源、大陆稳定性、宜居环境演变等密切相关。全球古老大陆的命运差别很大,有的长期稳定、有的遭受破坏,其控制因素可能包括大陆根地幔形成时熔融条件和后续交代改造过程等。本文总结前人研究资料发现,南非克拉通长期处于板块离散背景下,并经历了多期地幔柱活动,岩石圈地幔主体保持难熔属性、局部发生富集;华北克拉通长期处于板块汇聚背景下,并在显生宙发生强烈岩石圈减薄、改造和地幔置换作用,仅局部有难熔地幔残留。我们认为,亟需对不同命运克拉通之下的岩石圈地幔开展早期熔融条件和交代演化历史对比研究,寻找控制大陆命运的先天条件和后天因素。研究结果可以为大陆起源、宜居星球演化等关键科学问题提供制约。

     

Lateral bearing performance and mechanism of piles in the transition zone due to pit-in-pit excavation

Acta Geotechnica - Excavation types of foundation pits for large-scale or special-purpose urban construction have been more and more complex, and the environmental safety issues caused by pit...     

Seismological evidence for compositional variations at the base of the mantle transition zone under Japan Islands

Temperature and water content anomalies just above the 660-km discontinuity under the Japan Islands are estimated from seismological observations. Two sets of seismological observations of P-wave velocity perturbations and depth variations of the 660-km discontinuity are used, which are (1) long-wavelength (~ 500 km) variations from seismic tomography based on a grid parameterization and waveform analysis of ScS reverberations and (2) moderate-wavelength (~ 150 km) variations from seismic tomography with a block parameterization and receiver-function analysis. To estimate temperature and water content anomalies, partial derivatives of velocity and depth variations with respect to temperature and water content determined by mineral physics studies are used. Under Southwest Japan, low temperature and high water-content anomalies are obtained from both sets of seismological observations, which have already been found by a previous study. Under Northeast Japan, however, there are discrepancies between the results estimated from the two data sets, possibly due to the different resolution scales. This discrepancy is dismissed when examining distributions of the temperature and water content anomalies. These anomalies can be grouped into two geographical locations, one group under Southwest Japan, the other under Northeast Japan. The two groups are clearly separated and exhibit common features in both sets of the seismological observations. The grouping is interpreted by a compositional difference between the subducting Pacific slab under Southwest Japan and a normal mantle under Northeast Japan.     

灌注桩后注浆技术应用研究及承载力影响分析

对后注浆技术未能大幅提高灌注桩单桩承载力的工程个案展开分析研判,得出注浆过程控制不力是导致单桩承载力不足的主要原因。严格注浆管、射浆管、逆止阀制作质量管控与安装及注浆过程控制是能否成功注浆进而提高承载力的关键因素。工程案例中对承载力欠缺的区域进行了补桩处理,补桩时规范注浆装置制作、加强注浆过程管控,在补桩达到龄期后进行了静载试验,结果表明其单桩竖向极限承载力满足设计要求。