Observation of oceanic structure around Tosa-Bae southeast of Shikoku     

Yoshihiko Sekine  Haruki Ohwaki  Motoya Nakagawa 《Journal of Oceanography》1994,50(5):543-558

The hydrographic observations in the vicinity of a seamount, the Tosa-Bae, southeast of Shikoku have been carried out two times in summer of 1991 and 1992. The temperature, salinity fields are observed by CTD and velocity fields are measured by ADCP. Results of these observation are presented in this paper. It is shown that salinity maximum water at a depth of 100 m is confined to a southeastern are of the Tosa-Bae, however, salinity minimum water is found in northern side of the Tosa-Bae. This indicates the westward intrusion of less saline water over northern slope. A positive correlation is detected between the estimated Rossby height (fL/N) and the observed height of Taylor Column estimated from the vertical change in the isotherms and isohalines. Almost both heights give smaller value than representative depth of bottom topography of the Tosa-Bae, it is indicated that the topographic effect of the Tosa-Bae is not fully reached to the surface. From the correlations between the vertical difference of geostrophic flow and that of ADCP velocity, ageostrophic flow component is detected.  相似文献   

Effects of soft x-ray flux on the lower solar atmosphere in flares     

J. C. Henoux  Y. Nakagawa 《Solar physics》1977,53(1):279-280

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Stratigraphy and optically stimulated luminescence (OSL) dating of a Quaternary sequence along the Dong Nai River, southern Vietnam   总被引：1，自引：0，他引：1  

Toshiyuki Kitazawa  Takahiro Nakagawa  Tetsuo Hashimoto  Masaaki Tateishi 《Journal of Asian Earth Sciences》2006,27(6):788-804

The pre-Holocene Cenozoic sequence outcrops in the terrestrial part of the eastern margin of the Mekong Basin. However, the stratigraphy of the sequence is still unclear. Its detailed stratigraphy and chronology were therefore studied along the Dong Nai River, southern Vietnam, and the lithofacies and the relations among the formations were investigated from the outcrops. The ages of the deposits were determined by using optically stimulated luminescence (OSL) dating.The Ba Mieu Formation was deposited about 176±52 ka during marine isotope stage (MIS) 7–6. The Thu Duc Formation was deposited about 97±27 ka during MIS 5. Both the Ba Mieu and Thu Duc formations are composed of fluvial and tidally influenced coastal deposits. The newly proposed Nhon Trach Formation was originally an eolian (blanket) deposit, but it has been partly reworked by fluvial processes. The Nhon Trach Formation was deposited about 10.9±4.7 ka, in the last part of the Pleistocene to the beginning of the Holocene. The OSL ages for the Ba Mieu, Thu Duc, and Nhon Trach formations are younger than the ages previously assigned to these formations.  相似文献   

Oxygen isotope speedometry in granulite facies garnet recording fluid/melt–rock interaction (Sør Rondane Mountains,East Antarctica)     

Fumiko Higashino  Daniela Rubatto  Tetsuo Kawakami  Anne‐Sophie Bouvier  Lukas P. Baumgartner 《Journal of Metamorphic Geology》2019,37(7):1037-1048

In situ analysis of a garnet porphyroblast from a granulite facies gneiss from Sør Rondane Mountains, East Antarctica, reveals discontinuous step‐wise zoning in phosphorus and large δ¹⁸O variations from the phosphorus‐rich core to the phosphorus‐poor rim. The gradually decreasing profile of oxygen isotope from the core (δ¹⁸O = ~15‰) to the rim (δ¹⁸O = ~11‰) suggests that the ¹⁸O/¹⁶O zoning was originally step‐wise, and modified by diffusion after the garnet rim formation at ~800°C and 0.8 GPa. Fitting of the ¹⁸O/¹⁶O data to the diffusion equation constrains a duration of the high‐T event (~800°C) to c. 0.5–40 Ma after the garnet rim formation. The low δ¹⁸O value of the garnet rim, together with the previously reported low δ¹⁸O values in metacarbonates, indicates regional infiltration, probably along a detachment fault, of low δ¹⁸O fluid/melt possibly derived from meta‐mafic to ultramafic rocks.  相似文献   

The multiple chronological techniques applied to the Lake Suigetsu SG06 sediment core,central Japan     

Richard A. Staff  Takeshi Nakagawa  Gordon Schlolaut  Michael H. Marshall  Achim Brauer  Henry F. Lamb  Christopher Bronk Ramsey  Charlotte L. Bryant  Fiona Brock  Hiroyuki Kitagawa  Johannes van der Plicht  Rebecca L. Payne  Victoria C. Smith  Darren F. Mark  Alison Macleod  Simon P. E. Blockley  Jean‐Luc Schwenninger  Pavel E. Tarasov  Tsuyoshi Haraguchi  Katsuya Gotanda  Hitoshi Yonenobu  Yusuke Yokoyama  Suigetsu Project Members 《Boreas: An International Journal of Quaternary Research》2013,42(2):259-266

The varved sediment of Lake Suigetsu (central Japan) provides a valuable opportunity to obtain high‐resolution, multi‐proxy palaeoenvironmental data across the last glacial/interglacial cycle. In order to maximize the potential of this archive, a well‐constrained chronology is required. This paper outlines the multiple geochronological techniques being applied – namely varve counting, radiocarbon dating, tephrochronology (including argon–argon dating) and optically stimulated luminescence (OSL) – and the approaches by which these techniques are being integrated to form a single, coherent, robust chronology. Importantly, we also describe here the linkage of the floating Lake Suigetsu (SG06) varve chronology and the absolute (IntCal09 tree‐ring) time scale, as derived using radiocarbon data from the uppermost (non‐varved) portion of the core. This tie‐point, defined as a distinct (flood) marker horizon in SG06 (event layer B‐07–08 at 1397.4 cm composite depth), is thus derived to be 11 255 to 11 222 IntCal09 cal. years BP (68.2% probability range).  相似文献   

An unusual zone of seismic coupling in the Bonin arc: The 1972 Hachijo-Oki earthquakes and related seismicity     

Tetsuji Moriyama  Fumiko Tajima  Tetsuzo Seno 《Pure and Applied Geophysics》1989,129(1-2):233-261

The 1972 February and December Hachijo-Oki earthquakes (M _s=7.3 and 7.4), in the northernmost part of the Izu-Bonin subduction zone, are the only major events (M _s>7.0) in the Bonin arc for the past 80 years. Relocation of the hypocenters, using one smaller event having a wellconstrained focal depth as a master event, shows that the depth of the February event is 10 km shallower than that of the December event. We have determined the rupture process for both events by minimizing the error in waveform between observed and synthetic seismograms. Although the number of available stations are limited, the depth range of the major energy release for the December event extends deeper than for the February one. The rupture propagated up-dip for both events. It is likely that the rupture zone of the two events overlapped, and that the December event ruptured the deeper part. This suggestion is consistent with the observation that the aftershock zones of both events overlap with that of the December event shifted landward. The waveforms of the December event have a smaller high frequency component than those of the February event, suggesting that the stress at the thrust zone became more uniform or reduced after the February event.No thrust type smaller event occurred near the rupture zone. Instead, theP-axes of smaller events are parallel to the dip of the slab and theirT-axes dip to the southwest. Focal depths of these events estimated byP-wave forward modeling are generally between 40–50 km and located beneath the thrust zone. We thus interpret them as the events within the Pacific slab near the zone ruptured by the two major events. The stress concentration around the rupture zone of the major events is suggested to have triggered these slab events. After the occurrence of the large events, the slab events are concentrated near the deeper portion of the rupture zone. These events may have been caused by the loading of the down-dip compressional stress near the down-dip end of the rupture zone due to the rupture. The occurrence of the doublet of large earthquakes and a number of down-dip compressional events beneath their rupture zones in a shallow portion of the subducting slab indicates an unusual zone of seismic coupling in the Bonin arc, most of which is seismically quiescent.  相似文献   

Composition and origin of serpentinites from the Mid-Atlantic Ridge near 24° and 30° North Latitude     

Akiho Miyashiro  Fumiko Shido  Maurice Ewing 《Contributions to Mineralogy and Petrology》1969,23(2):117-127

Serpentinites occur in transverse fracture zones and adjacent areas in the Mid-Atlantic Ridge near 24° and 30° N. In two fracture zones, about 700 km distant from each other, serpentinites show practically the same trend and range of variation in chemical composition. Their CaO content ranges from 2.05 to 0.07% by weight. Serpentinites relatively high in CaO content contain pargasite, whereas those relatively low in CaO do not. Serpentinites relatively high in CaO are chemically similar to high-temperature peridotites which are widely believed to have been derived from the upper mantle. With a decrease in CaO, the Al₂O₃, TiO₂, K₂O and FeO contents and the Fe/Mg ratio tend to decrease, whereas the H₂O⁺ content tends to increase. This compositional variation is probably due partly to heterogeneity of uper mantle peridotite from which the serpentinites were derived, and partly to chemical migration during serpentinization. The interior of the Mid-Atlantic Ridge may be mainly made up of serpentinites. Alternatively, the Mid-Atlantic Ridge Serpentinites may have been formed by serpentinization of peridotites that were intruded into fracture zones from a great depth.Lamont-Doherty Geological Observatory Contribution No. 1354.  相似文献   

Diversity and origin of abyssal tholeiite from the Mid-Atlantic Ridge near 24° and 30° North latitude     

Akiho Miyashiro  Fumiko Shido  Maurice Ewing 《Contributions to Mineralogy and Petrology》1969,23(1):38-52

On cursory examination of hand specimens and thin sections, the abyssal tholeiite in a dredge haul may appear to be uniform in composition. Chemical analyses of a considerable number of fragments, however, have always revealed the existence of regular compositional variation in them. The MgO content decreases with increasing SiO₂. In abyssal tholeiites with relatively low Al₂O₃ contents, the SiO₂, total iron, Na₂O and P₂O₅ contents tend to increase and the MgO content tends to decrease with increasing iron/magnesia ratio, probably owing to crystallization differentiation.In a certain dredge haul, high-alumina abyssal tholeiites (with Al₂O₃ contents near or over 17%) occur in association with low-alumina abyssal tholeiites. The magma of high-alumina abyssal tholeiites would be generated from that of low-alumina abyssal tholeiites by differentiation at a depth around 30 km.In pillow lavas of abyssal tholeiite free from weathering and metamorphism, the chilled rim of the pillow usually has virtually the same chemical composition as the more crystalline core except for a decrease of K₂O content toward the rim. On the other hand, the weathered rim of pillow lavas shows marked compositional change. The Fe₂O₃/FeO ratio of unweathered abyssal tholeiite is in the range of 0.1 to 0.3. This ratio and the H₂O^– and H₂O⁺ contents increase with advancing weathering.Lamont-Doherty Geological Observatory Contribution No. 1339  相似文献   

Progressive compositional change of garnet in metapelite     

Akiho Miyashiro  Fumiko Shido 《Lithos》1973,6(1):13-20

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The Role of History-Dependent Rheology in Plate Boundary Lubrication for Generating One-Sided Subduction   总被引：1，自引：0，他引：1  

Michio Tagawa  Tomoeki Nakakuki  Masanori Kameyama  Fumiko Tajima 《Pure and Applied Geophysics》2007,164(5):879-907

We have developed a two-dimensional dynamical model of asymmetric subduction integrated into the mantle convection without imposed plate velocities. In this model we consider that weak oceanic crust behaves as a lubricator on the thrust fault at the plate boundary. We introduce a rheological layer that depends on the history of the past fracture to simulate the effect of the oceanic crust. The thickness of this layer is set to be as thin as the Earth's oceanic crust. To treat 1-kilometer scale structure at the plate boundary in the 1000-kilometer scale mantle convection calculation, we introduce a new numerical method to solve the hydrodynamic equations using a couple of uniform and nonuniform grids of control volumes. Using our developed models, we have systematically investigated effects of basic rheological parameters that determine the deformation strength of the lithosphere and the oceanic crust on the development of the subducted slab, with a focus on the plate motion controlling mechanism. In our model the plate subduction is produced when the friction coefficient (0.004–0.008) of the modeled oceanic crust and the maximum strength (400 MPa) of the lithosphere are in plausible range inferred from the observations on the plate driving forces and the plate deformation, and the rheology experiments. In this range of the plate strength, yielding induces the plate bending. In this case the speed of plate motion is controlled more by viscosity layering of the underlying mantle than by the plate strength. To examine the setting of the overriding plate, we also consider the two end-member cases in which the overriding plate is fixed or freely-movable. In the case of the freely-movable overriding plate, the trench motion considerably changes the dip angle of the deep slab. Especially in the case with a shallow-angle plate boundary, retrograde slab motion occurs to generate a shallow-angle deep slab.  相似文献