Northward cooling of the Kuncha nappe and downward heating of the Lesser Himalayan autochthon distributed to the south of Mt. Annapurna,western central Nepal     

Toru Nakajima  Harutaka Sakai  Hideki Iwano  Tohru Danhara  Takafumi Hirata 《Island Arc》2020,29(1)

We investigated the tectonothermal history of the Lesser Himalayan sediments (LHS), which are tectonically overlain by the Higher Himalayan Crystalline. Fission‐track dating and the track length measurement of detrital zircons obtained from the Kuncha nappe and the Lesser Himalayan autochthonous sediments in western central Nepal revealed northward cooling of the nappe and possible downward heating of the autochthon by the overlying hot nappe. Nine zircon fission‐track (ZFT) ages of the nappe showed northward‐younging linear distribution from 11.6 Ma in the front at Tamghas, 6 Ma in the central at Naudanda, and 1.6 Ma in the northernmost point at Tatopani. Thermochronological invert calculation of the ZFT length elucidated that the Kuncha nappe gradually cooled down (30 °C/Myr) at the front and rapidly cooled down (120 °C/Myr) at the root zone. In contrast, the ZFT age of the Chappani Formation, located just beneath the Kuncha nappe in the central part, demonstrated a totally reset age of 6.8 Ma, whereas the Virkot Formation, structurally far from the nappe, yielded a partially reset age of 457.3 Ma. This suggests that the LHS underwent downward heating, resulting in a thermal print on the upper part of the LHS; however, the thermal effect was not sufficient to anneal ZFT totally in the deeper part. Presently, the nappe cover is eroded and denuded from this area. Detrital zircons from the Chappani Formation in Tansen area to the south of the Bari Gad Fault did not show any evidence of annealing, suggesting that nappe never covered the LHS distributed to the south of the fault.  相似文献   

Northward younging zircon fission‐track ages from 13 to 2 Ma in the eastern extension of the Kathmandu nappe and underlying Lesser Himalayan sediments distributed to the south of Mt. Everest     

Toru Nakajima  Harutaka Sakai  Hideki Iwano  Tohru Danhara  Takafumi Hirata 《Island Arc》2020,29(1)

This study is concerned with the tectono‐thermal history of the Kathmandu nappe and the underlying Lesser Himalayan sediments (LHS) that are distributed in eastern Nepal. We carried out zircon fission‐track(ZFT) dating and obtained 16 ZFT ages from the eastern extension of the Kathmandu nappe, the Higher Himalayan Crystalline, Kuncha nappe, and the Main Central Thrust (MCT) zone. The ZFT ages of the frontal part of the Kathmandu nappe range from 13.0 ±0.8 Ma to 10.7 ±0.7 Ma and exhibit a northward‐younging tendency. These Middle Miocene ZFT ages indicate that the frontal part of the Kathmandu nappe remained at a temperature above 240 °C until the termination of its southward emplacement at 12–11 Ma. The ZFT ages of the LHS range from 11.1 ±0.9 Ma in the southern part of the Okhaldhunga Window to 2.4 ±0.3 Ma of the augen gneiss in the northern margin and also exhibit a northward‐younging age distribution. The ZFT ages show the northward‐younging linear distribution pattern (?0.16 Ma/km) along the across‐strikesection from the frontal part of the Kathmandu nappe to the root zone, without a significant age gap. This distribution pattern indicates that the Kathmandu nappe, the underlying MCT zone, and the Kuncha nappe cooled from the frontal zone to the root zone as a thermally united geologic body at a temperature below 240 °C. An older ZFT age (456.3 ±24.3 Ma), which was partially reset at the axial part of the Midland anticlinorium in the central part of the Okhaldhunga Window, was explained by downward heating from the “hot” Kathmandu nappe. The above evidence supported a model that southward emplacement of the hot Kathmandu nappe resulted in a thermal imprint on the upper part of the LHS; however, the lower part did not reach 240 °C.  相似文献   

Regionality of deep low-frequency earthquakes associated with subduction of the Philippine Sea plate along the Nankai Trough, southwest Japan     

Shoichi Yoshioka  Mamiko Toda  Junichi Nakajima 《Earth and Planetary Science Letters》2008,272(1-2):189-198

The Fukuoka District Meteorological Observatory recently logged three possible deep low-frequency earthquakes (LFEs) beneath eastern Kyushu, Japan, a region in which LFEs and low-frequency tremors have never before been identified. To assess these data, we analyzed band-pass filtered velocity seismograms and relocated LFEs and regular earthquakes using the double-difference method. The results strongly suggest that the three events were authentic LFEs, each at a depth of about 50 km. We also performed relocation analysis on LFEs recorded beneath the Kii Peninsula and found that these LFEs occurred near the northwest-dipping plate interface at depths of approximately 29–38 km. These results indicate that LFEs in southwest Japan occur near the upper surface of the subducting Philippine Sea (PHS) plate. To investigate the origin of regional differences in the occurrence frequency of LFEs in western Shikoku, the Kii Peninsula, and eastern Kyushu, we calculated temperature distributions associated with PHS plate subduction. Then, using the calculated thermal structures and a phase diagram of water dehydration for oceanic basalt, the water dehydration rate (wt.%/km), which was newly defined in this study, was determined to be 0.19, 0.12, and 0.08 in western Shikoku, the Kii Peninsula, and eastern Kyushu, respectively; that is, the region beneath eastern Kyushu has the lowest water dehydration rate value. Considering that the Kyushu–Palau Ridge that is subducting beneath eastern Kyushu is composed of tonalite, which is low in hydrous minerals, this finding suggests that the regionality may be related to the amount of water dehydration associated with subduction of the PHS plate and/or differences in LFE depths. Notable dehydration reactions take place beneath western Shikoku and the Kii Peninsula, where the depth ranges for dehydration estimated by thermal modeling agree well with those for the relocated LFEs. The temperature range in which LFEs occur in these regions is estimated to be 400–500 °C.  相似文献   

Erratum to “Regionality of deep low-frequency earthquakes associated with subduction of the Philippine Sea plate along the Nankai Trough,Southwest Japan” [Earth Planet. Sci. Lett. 272 (2008) 189–198]     

Shoichi Yoshioka  Mamiko Toda  Junichi Nakajima 《Earth and Planetary Science Letters》2009,277(1-2):292

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A coronagraphic search for brown dwarfs and planets around nearby stars     

T. Nakajima  J.‐I. Morino  T. Tsuji  H. Suto  M. Ishii  M. Tamura  M. Fukagawa  K. Murakawa  S. Miyama  H. Takami  N. Takato  S. Oya  S. Hayashi  T. Kudo  Y. Itoh  Y. Oasa  B. R. Oppenheimer 《Astronomische Nachrichten》2005,326(10):952-957

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Molecular ecological study of <Emphasis Type="Italic">Siganus spinus</Emphasis> and <Emphasis Type="Italic">S. guttatus</Emphasis> from Okinawan waters based on mitochondrial DNA control region sequences     

Kensuke Iwamoto  Akihiro Takemura  Tetsuo Yoshino  Hideyuki Imai 《Journal of Oceanography》2009,65(1):103-112

The mitochondrial DNA (mtDNA) control region was sequenced to examine the genetic variability and gene flow of juvenile Siganus spinus and S. guttatus. In total, 461 nucleotide sequences were obtained from 69 specimens of juvenile S. spinus from Okinawa Island and Ishigaki Island, in which 17 variable sites and 22 haplotypes were identified. Haplotype diversity (h) was high (0.9244 in Okinawa and 0.8984 in Ishigaki), whereas nucleotide diversity (π) was low (0.0063 in Okinawa and 0.0059 in Ishigaki). The two populations were not genetically distinct. Siganus guttatus, which do not form large schools at recruitment, in contrast S. spinus, were also analyzed by studying 152 individuals collected off Okinawa Island, Miyako Island, and Ishigaki Island. Of 476 nucleotide sequences, 50 were variable, and 42 haplotypes were identified. Genetic variability values were high (h = 0.8766 and π = 0.0151 in Okinawa; h = 0.9640 and π = 0.0192 in Miyako; h = 0.9161 and π = 0.0199 in Ishigaki). The Okinawa population was genetically isolated from the Miyako and Ishigaki populations. As a result, genetic diversity was high for each of these siganid populations despite their being target species for fisheries; however, the degree of inter-population gene flow was higher for S. spinus than S. guttatus, suggesting that these species exhibit different dispersal strategies.  相似文献   

MORPHOLOGICAL CHANGES IN DEVELOPMENT OF PHARYNGEAL TEETH IN MYLOPHARYNGODON PICEUS     

乐佩琦 《中国海洋湖沼学报》1995,(3)

Adult pharyngeal teeth in Mylopharyngodon piceus are molariform. Based on SEM observations ofthe developing teeth, this paper describes the morphological diversification of pharyngeal teeth in M.piceus. The larval and juvenile teeth are changed from conical to adult molariform teeth through sevenstages. Comparisons are made between each stage and corresponding types in some species of differentsubfamilies in Cyprinidae. It is considered that the ontogenetic resemblances of the pharyngeal teehbear relationship to the phylogeny of cyprinids.  相似文献   

The circumstellar gas of sigma orionis E     

R. Nakajima 《Astrophysics and Space Science》1985,116(2):285-297

In order to explain the variable H emission and the eclipse-like light variation of Ori E, we investigated the circumstellar gas trapped by the stellar magnetic field and corotating with the star. By considering the potential along the magnetic field line, we found that the gas concentrates to a potential minimum. The circumstellar gas forms either two condensations or a disk, depending on the inclination of the magnetic dipole to the stellar rotation axis. The geometrical thickness of the circumstellar disk, of about 0.2 stellar radii, and the distance from the center of the star to the inner edge of the disk, of about 3 stellar radii, were obtained. The H emission line profile at its maximum phase and the amplitude of light variation were calculated by assuming the isothermal gas in LTE with the maximum gas density which the magnetic field can hold. The model gives good agreement with observation in the low obliquity case, and also explains the phase correlation among the H emission maximum, the light minimum, and the magnetic extreme. The model, however, failed to explain the large IR excess in theM band.  相似文献   

Small Scale Field Study of an Ocean CO<Subscript>2</Subscript> Plume     

Peter G. Brewer  Edward Peltzer  Izuo Aya  Peter Haugan  Richard Bellerby  Kenji Yamane  Ryuji Kojima  Peter Walz  Yasuharu Nakajima 《Journal of Oceanography》2004,60(4):751-758

We have carried out a small-scale (∼20 l) CO₂ sequestration experiment off northern California (684 m depth, ∼5°C, background ocean pH ∼7.7) designed as an initial investigation of the effects of physical forcing of the fluid, and the problem of sensing the formation of a low pH plume. The buoyant CO₂ was contained in a square frame 1.2 m high, exposing 0.21 m² to ocean flow. Two pH electrodes attached to the frame recorded the signal; a second frame placed 1.9 m south of the CO₂ pool was also equipped with two recording pH electrodes. An additional pH electrode was held in the ROV robotic arm to probe the fluid interface. Local water velocities of up to 40 cm sec⁻¹ were encountered, creating significant eddies within the CO₂ box, and forcing wavelets at the fluid interface. This resulted in rapid CO₂ dissolution, with all CO₂ being depleted in a little more than 2 days. The pH record from the sensor closest (∼10 cm) to the CO₂ showed many spikes of low pH water, the extreme value being ∼5.9. The sensor 1 m immediately below this showed no detectable response. The electrodes placed 1.9 m distant from the source also recorded very small perturbations. The results provide important clues for the design of future experiments for CO₂ disposal and biogeochemical impact studies. These include the need for dealing with the slow CO₂ hydration kinetics, better understanding of the fluid dynamics of the CO₂-water interface, and non-point source release designs to provide more constant, controlled local CO₂ enrichments within the experimental area. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

A cross-correlation analysis of tidal current,water temperature and salinity records     

Hideo Nakajima  Norio Hayakawa 《Journal of Oceanography》1982,38(2):52-56

Fifteen day records of current velocity, water temperature and salinity were obtained at a coastal station in the Seto Inland Sea. It was observed that water movement was characterized by an exchange of two distinctive water masses east and west of the station. Cross-correlation analysis shows that, at the semi-diurnal period, the motion of these characteristic waters causes temperature and salinity variations. At frequencies higher than 2 cycles per day where power spectra of current velocity have a slope of ?5/3, coherence between current velocity and both water temperature and salinity decreases rapidly.  相似文献