Abstract: The gold content in siliceous deposits formed from geothermal waters at the Hatchobaru geothermal power station in central Kyushu, Japan, was determined by inductively coupled plasma quadrupole mass spectrometry (ICP–MS). It ranges from 1.3 to 4.6 mg/kg. The highest gold content was obtained from the siliceous deposit formed from acidic waters. In this siliceous deposit, hydrous iron(III) oxide of amorphous state is a major constituent, suggesting that hydrous iron(III) oxide might play an important role when gold is concentrated into siliceous deposits formed from acidic and iron-rich geothermal waters. 相似文献
Heavy rainfall on the south side of the Rokko Mountains has often caused severe landslides and debris flows. Analysis of the annual summation of rainfall in excess of 100 mm/day shows that the rainfall in this area has dominant periodicities of about 25–30, 10–13, and 5–7 years. The period of about 25–30 years corresponds to that of occurrence of the natural disasters produced by heavy rainfall; years when the maximum rainfall correspond to years when there have been severe landslides and debris flows in the area. Temporal change in this rainfall may provide a first approximation for erosional force. Analyses of tree ring width from these mountains indicate that the sequences have a dominant periodicity of about 25–30 years. Cross-spectral analyses for rainfall and ring width in this area show high coherency for the periods of about 25–30 years; evidence that variations in the ring width may be used as proxy data for erosional force. 相似文献
We discuss several models of the evolution of the trench-trench-trench triple junction off central Honshu during the past 1 m.y. on the basis of plate kinematics, morphology, gravity and seismic reflection profile data available for the area. The study area is characterized by large basins, 7–8 km deep on the inner lower trench slope on the Philippine Sea side and the deep (9 km) Izu-Bonin Trench to the east. Between the basins and the trench, there are 6–7 km-deep basement highs. The triple junction is unstable due to the movement of the Philippine Sea plate at a velocity of 3 cm/yr in WNW direction with respect to Eurasia (Northeast Japan), subparallel to the strike of the Sagami Trough. Generally we can expect the boundary area between the Philippine Sea and Pacific plates to be extended because the Pacific plate is unlikely to follow the retreating Philippine Sea plate due to the obstruction of the southeastern corner of Eurasia. The above peculiar morphology of the junction area could have resulted from this lack of stability. However, there are several possible ways to explain the above morphology.
Our gravity model across the trench-basement high-basin area shows that the basement highs are made of low-density materials (1.8–2 g/cm3). Thus we reject the mantle diapir model which proposes that the basement highs have been formed by diapiric injection of serpentinites between the retreating Philippine Sea plate and the Pacific plate.
The stretched basin model proposes that the basins have been formed by stretching of the Philippine Sea plate wedge. We estimated the extension to be about 10 km at the largest basin. We reconstructed the morphology at 1 Ma by moving the Philippine Sea plate 20 km farther to the east after closing the basins, and thus obtained 8 km depth of the 1 Ma trench, which is similar to that of the present Japan Trench to the north. Although this stretched basin model can explain the formation of the basins and the deep trench, other models are equally possible. For instance, the eduction model explains the origin of the basin by the eduction of the Philippine Sea basement from beneath the basement high, while the accretion model explains the basement highs by the accretion of the Izu-Bonin trench wedge sediments. In both of these models we can reconstruct the 1 Ma trench depth as about 8 km, similar to that of the stretched basin model.
The deformation of the basement of the basins constitutes the best criterion to differentiate between these models. The multi-channel seismic reflection profiles show that the basement of the largest basin is cut by normal faults, in particular at its eastern edge. This suggests that the stretched basin model is most likely. However, the upper part of the sediments shows that the basement high to the east has been recently uplifted. This uplift is probably due to the recent (0.5 Ma) start of accretion of the trench wedge sediments beneath this basement high. 相似文献
Abstract Quartz c-axis fabrics of the Sambagawa schists produced along a late Mesozoic convergent plate margin were analysed so that their tectono-metamorphic history could be clarified. It has been noted by many authors that quartz fabrics produced by earlier phase deformation are easily modified by strain increment during later phase deformation. This paper attempts to elucidate the high-temperature phases of prograde metamorphism (Sim-Bim phase) and of retrograde metamorphism (Sb1 phase and Sb2−1 phase) from quartz grains included in garnet and plagioclase porphyroblasts. Quartz c-axis fabrics for all these phases are explained in terms of a type I crossed girdle, without (only rarely with) higher concentration in the principal axis of strain Y (X>Y>Z), that must have been produced by the activity of a dominant slip system such as rhomb and basal. As a result, the plastic deformation of quartz, which was responsible for the formation of the type I crossed girdle, occurred even under temperatures greater than 500°C and pressures a little greater than 10–11 kb, which correspond to the physical condition of the Sim-Bim phase. It has been assumed that a high strain rate (and/or low H2O content) caused rhomb and basal to be active as dominant slip systems in the subduction zone related to the formation of the Sambagawa schists even under high temperatures (> 500°C). 相似文献
Abstract The significance of timing and formation of mélange in accretionary prisms, particularly concerning basaltic and related rocks and pelagic sediments, is exemplified in the Sawadani area of the Jurassic Chichibu accretionary complex in Shikoku, southwest Japan. Major and trace element geochemistry of the basaltic and related rocks indicates that all are of a hot-spot origin which produced a seamount. Most of the rocks have a trend of differentiation from an alkalic parental magma. The time relationship between the blocks and matrices of the mélange deduced from radiolarian fossil evidence and macro- to microscopic characteristics of contacts between different lithologies indicates two stages of mixing of materials in the seafloor. The first mixing occurred on the flank of the seamount in the pelagic environments in the Late Permian, and the second occurred on the trench floor or in the accretionary prism after the Early Jurassic. These two stages show respectively the geological phenomena of a seamount within the Izanagi-Kula plate and its incorporation into the Asian continental margin. 相似文献
Aoki, K. and Shiogama, Y., 1993. Geoengineering techniques used in the construction of underground openings in jointed rocks. In: M. Langer, K. Hoshino and K. Aoki (Editors), Engineering Geology in the Utilization of Underground Space.Eng. Geol., 35: 167–173.
Examples of underground openings used for pumped storage hydroelectric power stations and storage facilities for crude oil in Japan are described. In these examples the observational construction technique is important to compensate for missing data in the preliminary surveys. Some new drilling and assessment techniques that have been developed are also introduced. 相似文献
A three-dimensional formulation based on Green's functions of cylindrical loads in layered semi-infinite media is employed to investigate the dynamic behaviour of piles in homogeneous and non-homogeneous half spaces. The pile-soil-pile interaction taking place in pile groups is incorporated in the model. The results presented in this paper include the dynamic stiffnesses and dampings of single piles as well as those of representative 2 × 2 and 4 × 4 square pile groups in the soil media considered in this study. In addition, the distribution of forces applied on the pile cap among the individual piles in a group is investigated. 相似文献
The relationship between the slip activity and occurrence of historical earthquakes along the Median Tectonic Line (MTL), together with that of the fault systems extending eastward has been examined. The MTL is divided into three segments, each containing diagnostic active faults. No historical earthquakes have been recorded along the central segment, although the segment has faster Quaternary slip rates compared with the other segments that have generated historical earthquakes. This discrepancy between earthquake generation and slip rate can be explained by a microplate model of southwest Japan. The microplate model also provides spatial and temporal coupling of slip on adjacent fault systems. In the context of this model, slip on adjacent faults reduces the normal stress on the MTL. Historical data and paleoseismic evidence indicate that slip on this segment occurs without significant strong ground motion. We interpret this as indicating anomalously slow seismic slip or aseismic slip. Slip on the central segment of the MTL creates transpressional regions at the eastern and western segments where historical earthquakes were recorded. Alternatively, the earthquakes at the eastern and western segments were triggered and concentrated shear stress at the edge of the segments resulted in postseismic slip along the central segment. The sequence of historical events suggests that the MTL characteristically does not produce great earthquakes. The microplate model also provides a tectonic framework for coupling of events among the MTL, the adjacent fault systems and the Nankai trough. 相似文献
Ray tracing of seismic surface waves is applied to tsunami propagation to examine bathymetric effect along its propagation path. Computations are made for trans-Pacific tsunamis and for near-field tsunamis in the Japan Sea. For tsunamis across the Pacific Ocean, the comparison to a uniform ocean shows that focusing and defocusing, due to bathymetry, are significant for some combinations of source and receiver. For example, the refraction of rays is predominant at the East Pacific Rise for the tsunami from Chile. The tsunamis in the Japan Sea are strongly affected by the shallow Yamato Rise. The predicted arrival time and amplitude distribution generally agree with the observations from an actual tsunami. Since the computation can be made very quickly, the method is useful for preliminary analysis of tsunami propagation, such as in an operational warning system or in the determination of computational area for finite-difference computation. 相似文献