排序方式： 共有12条查询结果，搜索用时 46 毫秒
1.
Abstract The Nankai Trough, off southwest Japan, is one of the best sites for the study of geomorphic characteristics of a clastic accretionary prism. A recent multibeam survey over the central and eastern parts of the Nankai accretionary prism has revealed a large variation of the topography along the trough axis. Analysis of the bathymetric data suggests the existence of prism deformational features of different scales, such as depressions, embayment structures and cusps. These structures are the results of slope instability caused by basement relief of subducted oceanic plate. Unstable slopes recover by new accretion and development of a low angle thrust. Small-scale deformation due to the subduction of a small isolated seamount is then adjusted to the regional trend. By contrast, a 30 km indentation of the wedge observed in the eastern part of the Nankai Trough, the Tenryu Cusp, has seemed to retain its geometry. The subducted Philippine Sea plate has deformed greatly near the eastern end of the Nankai Trough, because of the collision between the Izu-Ogasawara (Bonin) arc and central Japan. Therefore, the indentation may be the result of the continuous subduction of a basement high, such as the Zenisu Ridge, which has been formed under north-south compression due to the arc-arc collision. 相似文献
2.
Lake Suwa, located in the center of the largest Japanese island of Honshu was a typical hypertrophic lake with a dense scum Microcystis species in summer during the 1970's. However, due to the introduction of a sewage treatment plant and the awareness of environmental pollution by local residents, a decrease in the concentration of nutrients in lake water since 1981 has been observed (from 1600 g l–1 and 160 g l–1 in 1977 to 990 g l–1 and 110 g l–1 in 1984 for total nitrogen and phosphorus respectively). 相似文献
3.
Abstract In the present study the seismic structure of oceanic core complexes (OCC) in the Parece Vela Basin, Philippine Sea have been imaged. Together with recent work on the Atlantis Massif OCC on the Mid-Atlantic Ridge, including deep drilling, this work provides an unprecedented opportunity to advance our understanding of OCC internal structure. A continuous, strong and relatively smooth reflection that was ca 0.15 s (two way time) below the sea floor of an OCC in the Chaotic Terrain of the Parece Vela Basin was identified. This reflection, termed the D-reflector, is similar to that observed beneath Atlantis Massif. A faster P-wave velocity (>6 km/s) is observed very shallow beneath the Chaotic Terrain OCC, suggesting that the core of these OCC is dominantly gabbroic. The D-reflector might be common beneath OCC, owing to localized alteration along fractured zones within gabbro. We further observed a series of three detachment events in the Chaotic Terrain. The first and second detachments exhumed shallow basaltic crust to deeper gabbroic core, whereas the last one only exhumed shallow basaltic crust. 相似文献
4.
Ken-ichi Hirauchi Izumi Segawa Yui Kouketsu Yumiko Harigane Yasuhiko Ohara Jonathan Snow Atlanta Sen Masakazu Fujii Kyoko Okino 《Island Arc》2021,30(1):e12419
We determined the mineralogical and petrological characteristics of ultramafic rocks dredged from two oceanic core complexes: the Mado Megamullion and 23°30′N non-transform offset massif, which are located within the Shikoku back-arc basin in the Philippine Sea. The ultramafic rocks are strongly serpentinized, but can be classified as harzburgite/lherzolite or dunite, based on relict primary minerals and their pseudomorphs. Strongly elongated pyroxene porphyroclasts with undulatory extinction indicate high-temperature (≥700 °C) strain localization on a detachment fault within the upper mantle at depths below the brittle–viscous transition. During exhumation, the peridotites underwent impregnation by magmatic or hydrothermal fluids, lizardite/chrysotile serpentinization at ≤300 °C, antigorite crystallization, and silica metasomatism that formed talc. These features indicate that the detachment fault zones formed a fluid pathway and facilitated a range of fluid–peridotite interactions. 相似文献
5.
Comparison of gravity anomaly between mature and immature intra-oceanic subduction zones in the western Pacific 总被引：1，自引：0，他引：1
The western Pacific hosts major subduction systems such as Izu–Bonin–Mariana and Tonga–Kermadec, but also less conspicuous systems such as Yap, Mussau and Hjort trenches which constitute the young, incomplete, or ultraslow-member in the evolutionary spectrum of subduction zones. We used satellite-derived gravity data to compare well-developed and immature subduction systems. It is shown that at spatial resolution > 10–20 km or so, the satellite data have accuracy comparable to ship-board gravity measurements over intra-oceanic subduction zones. In the isostatic residual gravity anomaly map, the width of non-isostatically-compensated region of the mature subduction zones is much wider than that of immature ones. More importantly, when the gravitational attraction due to seafloor is removed, a large difference exists between the mature and immature subduction zones in the overriding plate side. Mature subduction zones exhibit broad low gravity anomalies of ~ 200–250 mGal centered at distances of 150–200 km from the trench which are not found over immature subduction zones. The cause of the broad low gravity anomalies over mature subduction zones is debatable due to lack of information on the deep crust and upper mantle structure and property. We discuss the following four causes: (1) serpentinization of the upper mantle beneath the forearc; (2) presence of partial melt in the mantle wedge caused by release of volatiles from the slab, frictional heating and distributed by mantle circulation; (3) difference in density structure between the overriding and subducting plates caused by difference in age and thermal structures with and without compositional stratification between crust and mantle; and (4) anomalous thickness of the arc not explained by isostasy. Our analysis suggests that serpentinization cannot explain the observed gravity anomaly which appears ~ 150–200 km from the trench. Although the extent and distribution of partial melt within the mantle wedge remain in question, to our best estimate, partial melting contributes little (< 50 mGal) to the total negative gravity anomaly. The difference in density structure reflecting temperature difference can only explain less than half of the low gravity anomaly. The sinking of lighter crustal material produces a large negative anomaly in the forearc but its location does not match the observed gravity anomaly. It appears that one cannot explain the total difference in gravity anomaly without invoking anomalous thickness of the arc. Although we could not identify the sole or combination of factors that give rise to the low gravity anomaly in mature subduction zones, the comparison of gravity anomalies between mature and immature subduction zones is likely to provide an important constraint for understanding the evolution and structure of subduction zones as more complementary evidences become available. 相似文献
6.
Crustal structure of the ultra-slow spreading Knipovich Ridge,North Atlantic,along a presumed ridge segment center 总被引：1，自引：1，他引：0
Aleksandre Kandilarov Hildegunn Landa Rolf Mjelde Rolf B. Pedersen Kyoko Okino Yoshio Murai 《Marine Geophysical Researches》2010,31(3):173-195
A combined ocean bottom seismometer, multichannel seismic reflection and gravity study has been carried out along the spreading
direction of the Knipovich Ridge over a topographic high that defines a segment center. The youngest parts of the crust in
the immediate vicinity of the ridge reveal fractured Oceanic Layer 2 and thermally expanded and possibly serpentinized Oceanic
Layer 3. The mature part of the crust has normal thickness and seismic velocities with no significant crustal thickness and
seismic velocity variations. Mature Oceanic Layer 2 is in addition broken into several rotated fault blocks. Comparison with
a profile acquired ~40 km north of the segment center reveals significant differences. Along this profile, reported earlier,
periods of slower spreading led to generation of thin crust with a high P-wave velocity (Vp), composed of a mixture of gabbro and serpentinized mantle, while periods of faster spreading led to generation of more
normal gabbroic crust. For the profile across the segment center no clear relation exists between spreading rate and crustal
thickness and seismic velocity. In this study we have found that higher magmatism may lead to generation of oceanic crust
with normal thickness even at ultra-slow spreading rates. 相似文献
7.
Crustal structure of the ultra-slow spreading Knipovich Ridge,North Atlantic,along a presumed amagmatic portion of oceanic crustal formation 总被引：4，自引：4，他引：0
Aleksandre Kandilarov Rolf Mjelde Kyoko Okino Yoshio Murai 《Marine Geophysical Researches》2008,29(2):109-134
The ultra-slow, asymmetrically-spreading Knipovich Ridge is the northernmost part of the Mid Atlantic ridge system. In the
autumn of 2002 a combined ocean-bottom seismometer multichannel seismic (OBS/MCS) and gravity survey along the spreading direction
of the Knipovich Ridge was carried out. The main objective of the study was to gain an insight into the crustal structure
and composition of what is assumed to be an amagmatic segment of oceanic crust. P-wave velocity and Vp/Vs models were built
and complemented by a gravity model. The 190 km long transect reveals a much more complex crustal structure than anticipated.
The magmatic crust is thinner than the global average of 7.1 ± 1.0 km. The young fractured portion of Oceanic Layer 2 has
low seismic velocities while the older part has normal seismic velocities and is broken into several rotated fault blocks
seen as thickness variations of Layer 2. The youngest part of Oceanic Layer 3 is also dominated by low velocities, indicative
of fracturing, seawater circulation and thermal expansion. The remaining portion of Layer 3 exhibits inverse variations in
thickness and seismic velocity. This is explained by a sequence of periods of faster spreading (estimated to be up to 8 mm/year
from interpretation of magnetic anomalies) when more normal gabbroic crust was being generated and periods of slower spreading
(5.5 mm/year) when amagmatic stretching and serpentinization of the upper mantle occurred, and crust composed of mixed gabbro
and serpentinized mantle was generated. The volumetric changes and upward fluid migration, associated with the process of
serpentinization in this part of the crust, caused disruption to the overlying sedimentary layers. 相似文献
8.
Shiki Machida Kentaro Nakamura Tetsu Kogiso Ryo Shimomura Koichi Horinouchi Kyoko Okino Yasuhiro Kato 《Island Arc》2021,30(1):e12395
A broad area densely covered by ferromanganese nodules was recently discovered around Minamitorishima (Marcus) Island, representing a high-potential metal resource, particularly for Co, Ni, Mo, and W. We studied 16 nodule samples from nodule fields around Minamitorishima Island. To define the fine-scale chemostratigraphy of the nodules, polished cross-sections of the samples were analyzed by microfocus X-ray fluorescence. Our results show that a general pattern of compositional variation was common throughout the growth history of the nodules in all the regions we studied. Chemical mapping clarified changes in the chemical signature and proportion of five lithological components throughout the growth history: Mn represented columnar δ-MnO2; Fe represented layered amorphous FeOOH*xH2O; Ti represented TiO2*2H2O intergrown with an amorphous FeOOH phase; P, Ca and Y represented particles of biogenic calcium phosphate; and Si, Al, K, Cu, and Ni represented pelagic sediment infills. We proposed a method for a creating a multi-dimensional compositional map of the fine-scale chemostratigraphy observed in the ferromanganese oxide layers on the basis of merging the mapped Mn, Fe, Ti, P, Si and Cu intensities. Multi-dimensional compositional mapping of the sampled nodules from the western North Pacific revealed two fundamental findings: (1) previously recognized first-order Fe–Mn layers, L0, L1, and L2, were further divided into two, three, and four sublayers, respectively, and (2) a delayed supply of material to be nuclei of nodule or a growth hiatus of Fe–Mn layer(s), leading to missing sublayers in the layers L0 and L2, regulated the nodule size. In contrast, layer L1, which does not have any missing sublayers, was commonly observed in the samples for this study and has been reported in studies of other regions in the western Pacific. We propose, therefore, that the layer L1 is a key facies for examining chemostratigraphic correlations with other areas of seafloor. 相似文献
9.
Ritoko Kikunaga Ke-han Song Shun Chiyonobu Kazuhiko Fujita Ryuichi Shinjo Kyoko Okino 《Island Arc》2021,30(1):e12425
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. 相似文献
10.
A precise ocean bottom map for ocean surveying and dredging is desired. Especially in dredging, it is essential to know the seabed topography in real time without being affected by scatterers (for example floating sand and mud) in the seawater during work. To meet these requirements, the multi-narrow-beam sonar system (MBSS) has been developed. The MBSS forms beams with the use of the complex fast Fourier transform (CFFT) algorithm. In the MBSS, arithmetic mean processing is employed to eliminate echo from scatterers and the measurement error due to the oblique incident angle is reduced by peak value detection processing. By using these processes, an ocean bottom map can be accurately obtained. It is both theoretically and experimentally shown that the distribution of echo intensity from scatterers is approximated by the Rayleigh probability density function. The arithmetic mean of four to eight successively received echoes from scatterers reduces the variance of the echo intensity distribution by 6 to 12 dB. 相似文献