Timing of dextral oblique subduction along the eastern margin of the Asian continent in the Late Cretaceous: Evidence from the accretionary complex of the Shimanto Belt in the Kii Peninsula, Southwest Japan   总被引：1，自引：1，他引：0  

Tetsuya  Tokiwa 《Island Arc》2009,18(2):306-319

Paleomagnetic studies and hotspot track analyses show that the Kula Plate was subducted dextrally with respect to the Eurasian Plate from the Coniacian to Campanian. However, geological evidence for dextral subduction of the Kula Plate has not been reported from Southwest Japan. Studies of the Coniacian to lower Campanian Miyama Formation of the Shimanto Belt reveal that the mélange fabrics show a dextral sense of shear both at outcrop and microscopic scales. In addition, thrust systems at map-scale also show dextral shearing. Restored shear directions in the mélange indicate dextral oblique subduction of an oceanic plate. This indicates that the Kula Plate subducted dextrally along the eastern margin of Asia during the Coniacian to early Campanian. Combinations with other published kinematic and age constraints suggest that Southwest Japan experienced a change from sinistral to dextral and back to sinistral shear between 89–76 Ma. This history is compatible with global-scale plate reconstructions and places good constraints on the timing of plate boundary interaction with the Cretaceous East Asian margin.  相似文献   

Characteristic features in Venus’ nightside cloud-top temperature obtained by Akatsuki/LIR     

Makoto Taguchi  Tetsuya Fukuhara  Masahiko Futaguchi  Mitsuteru Sato  Takeshi Imamura  Kazuaki Mitsuyama  Masato Nakamura  Munetaka Ueno  Makoto Suzuki  Naomoto Iwagami  George L. Hashimoto 《Icarus》2012,219(1):502-504

Mid-infrared images of almost the entire Venus nightside hemisphere obtained by the Longwave Infrared Camera (LIR) onboard Akatsuki on December 9 and 10, 2010 reveal that the brightness temperature of the cloud-top ranges from 237 K in the cold polar collars to 243 K in the equatorial region, significantly higher than the values obtained by Venera 15. Other characteristic features of the temperature distributions observed are zonal belt structures seen in the middle and low latitudes and patchy temperature structures or quasi-periodic streaks extending in a north–south direction in the northern middle latitudes and southern low latitudes.  相似文献   

Upper Silurian and Devonian pelagic deep-water radiolarian chert from the Khangai–Khentei belt of Central Mongolia: Evidence for Middle Paleozoic subduction–accretion activity in the Central Asian Orogenic Belt     

Toshiyuki Kurihara  Kazuhiro Tsukada  Shigeru Otoh  Kenji Kashiwagi  Minjin Chuluun  Dorjsuren Byambadash  Bujinlkham Boijir  Sersmaa Gonchigdorj  Manchuk Nuramkhan  Masakazu Niwa  Tetsuya Tokiwa  Gen’ya Hikichi  Takafumi Kozuka 《Journal of Asian Earth Sciences》2009,34(2):209-225

Recent mapping projects undertaken in Central Mongolia have revealed the widespread occurrence of radiolarian chert within a Paleozoic accretionary complex. We present the results of the first detailed tectonostratigraphic and radiolarian biostratigraphic investigations of the Gorkhi Formation in the Khangai–Khentei belt of the Central Asian Orogenic Belt.The Gorkhi Formation consists of sandstone shale, alternating sandstone and shale of turbidite affinity and chert with small amounts of siliceous shale, basalt, limestone, and clast-bearing mudstone. Radiolarian chert that is completely devoid of terrigenous clastic material is commonly associated with underlying basalt (sedimentary contact) and with conformably overlying siliceous shale and turbidite deposits. The tectonic stacking of basalt–chert and chert–turbidite successions is the most remarkable structural feature of the formation.The recovery of moderately well-preserved radiolarians and conodonts from red chert led to the recognition of four radiolarian assemblages that have a combined age range from the latest Silurian (Pridolian) to the Late Devonian (Frasnian). No age control exists for the siliceous shale, shale, and sandstone, although they are considered to be latest Devonian or slightly younger on the basis of stratigraphic relationships with underlying chert.The Gorkhi Formation has previously been interpreted as a thick sedimentary basin deposit overlying an unexposed Archean–Neoproterozoic basement; however, the stratigraphy within individual tectonic slices clearly corresponds to that of an ocean plate stratigraphy of an accretionary complex generated by the trenchward movement of an oceanic plate. From the lowermost to uppermost units, the stratigraphy comprises ocean floor basalt, pelagic deep-water radiolarian chert, hemipelagic siliceous shale, and terrigenous turbidite deposits. The biostratigraphic data obtained in the present study provide corroborating evidence for the existence of an extensive deep-water ocean that enabled the continuous sedimentation of pelagic chert over a period of nearly 50 million years. These data, together with structural data characterized by tectonic repetition of the stratigraphy, indicate that these rocks formed as an accretionary wedge along an active continental margin, possibly that of the Angara Craton. The mid-oceanic chert was probably deposited in the Northern Hemisphere portion of the Paleo–Pacific Ocean that faced the Angara Craton and the North China–Tarim blocks. Thus, we propose that subduction–accretion processes along the Paleo–Pacific rim played an important role in the accretionary growth of the active continental margin of the Angara Craton, directly influencing the evolution of the Central Asian Orogenic Belt.  相似文献   

A review of Titan’s atmospheric phenomena     

Mathieu Hirtzig  Tetsuya Tokano  Sébastien Rodriguez  Stéphane le Mouélic  Christophe Sotin 《Astronomy and Astrophysics Review》2009,17(2):105-147

Saturn’s satellite Titan is a particularly interesting body in our solar system. It is the only satellite with a dense atmosphere, which is primarily made of nitrogen and methane. It harbours an intricate photochemistry, that populates the atmosphere with aerosols, but that should deplete irreversibly the methane. The observation that methane is not depleted led to the study of Titan’s methane cycle, starting with its atmospheric part. The features that inhabit Titan’s atmosphere can last for timescales varying from year to day. For instance, the reversal of the north–south asymmetry is linked to the 16-year seasonal cycle. Diurnal phenomena have also been observed, like a stratospheric haze enhancement or a possible tropospheric drizzle. Furthermore, clouds have been reported on Titan since 1993. From these first detections and up to now, with the recent inputs from the Cassini–Huygens mission, clouds have displayed a large range of shapes, altitudes, and natures, from the flocky tropospheric clouds at the south pole to the stratiform ones in the northern stratosphere. It is still difficult to compose a clear picture of the physical processes governing these phenomena, even though of lot of different means of observation (spectroscopy, imaging) are available now. We propose here an overview of the phenomena reported between 1993 and 2008 in the low atmosphere of Titan, with indications on the location, altitude, and their characteristics in order to give a perspective of our up-to-date understanding of Titan’s meteorological manifestations. We shall focus mainly on direct imaging observations, from both space- and ground-based facilities. All of these observations, published in more than 30 different refereed papers to date, allow us to build a precise chronology of Titan’s atmospheric changes (including the north–south asymmetry, diurnal and seasonal effects, etc). Since the interpretation is at an early stage, we only briefly mention some of the current theories regarding the features’ nature.  相似文献   

A generic formulation for microtremor exploration methods using three-component records from a circular array   总被引：1，自引：0，他引：1  

Ikuo Cho  Taku Tada  Yuzo Shinozaki 《Geophysical Journal International》2006,165(1):236-258

  相似文献   

Geochemical evolution of a shallow magma plumbing system during the last 500 years, Miyakejima volcano, Japan: Constraints from U-Th-Ra systematics   总被引：1，自引：0，他引：1  

Tetsuya Yokoyama  Takeshi Kuritani  Eizo Nakamura 《Geochimica et cosmochimica acta》2006,70(11):2885-2901

In order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes, we investigated U-series disequilibria of rocks erupted over the past 500 years (1469-2000 AD) from Miyakejima volcano, Izu arc, Japan. Miyakejima volcanic rocks show ²³⁸U-²³⁰Th-²²⁶Ra disequilibria with excess ²³⁸U and ²²⁶Ra, due to the addition of slab-derived fluids to the mantle wedge. Basaltic bombs of the 2000 AD eruption have the lowest (²³⁰Th/²³²Th) ratio compared to older Miyakejima eruptives, yielding the youngest ²³⁸U-²³⁰Th model age of 2 kyr. This reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath Miyakejima occur episodically on a several-kyr timescale. In the last 500 years, Miyakejima eruptives show: (1) a vertical trend in a (²³⁰Th/²³²Th)-(²³⁸U/²³²Th) diagram and (2) a positive linear correlation in a (²²⁶Ra/²³⁰Th)₀ − 1/²³⁰Th diagram, which is also observed in lavas from some of the single eruptions (e.g., 1940, 1962, and 1983 AD). The variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of ²³⁰Th and ²²⁶Ra, but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab-derived fluids. A much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust, subsequently modifying the original magma composition and producing variations in (²³⁰Th/²³²Th) and (²²⁶Ra/²³⁰Th)₀ ratios via assimilation and fractional crystallization (AFC). The assimilant of the AFC process would be a volcanic edifice of previous Miyakejima magmatism. Due to the relatively short timescales involved, the interaction between the assimilant and recent Miyakejima magmatism has not been recorded by the Sr-Nd-Pb isotopic systems. In such cases, Th isotopes and (²²⁶Ra/²³⁰Th) ratio are excellent geochemical tracers of magmatic evolution.  相似文献   

Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa: Constraints from U-Th-Ra and Sr-Nd-Pb isotope systematics     

Tetsuya Yokoyama  Festus T. Aka  Minoru Kusakabe 《Geochimica et cosmochimica acta》2007,71(7):1835-1854

Precise measurements of ²³⁸U-²³⁰Th-²²⁶Ra disequilibria in lavas erupted within the last 100 yr on Mt. Cameroon are presented, together with major and trace elements, and Sr-Nd-Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess ²³⁸U-²³⁰Th-²²⁶Ra disequilibria with ²³⁰Th (18-24%) and ²²⁶Ra (9-21%) excesses, and there exists a positive correlation in a (²²⁶Ra/²³⁰Th)-(²³⁰Th/²³⁸U) diagram. The extent of ²³⁸U-²³⁰Th-²²⁶Ra disequilibria is markedly different in lavas of individual eruption ages, although the (²³⁰Th/²³²Th) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the (²³⁰Th/²³⁸U)-(²⁰⁶Pb/²⁰⁴Pb) and (²²⁶Ra/²³⁰Th)-(²⁰⁶Pb/²⁰⁴Pb) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high D_U and D_U/D_Th, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (>0.75) and Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb > 20.47) due to late Mesozoic metasomatism.  相似文献   

Grade-Tonnage, Ore Value-Tonnage, and Enrichment Ratio-Tonnage Models for Resource Assessment     

Shoji Tetsuya Department of Environment Systems  The University of Tokyo  Tokyo -  Japan 《中国地质大学学报(英文版)》2001,12(1)

I' Grade-tonnage diagrams are one of the most useful tools inthe resource assessment (e. g. St'nger, 1993). One' grade-tonnage diagram is well known as Laskys (1950) e\-7ation, whichdemonstrates the linear relation between average grade (x) andlOgarithms of cumulative ore tonnage (ac above a given gradex(0相似文献   

Small-Amplitude Disturbances In A Radiating And Scattering Grey Medium Ii. Solutions Of Given Real Wave Number <Emphasis Type="BoldItalic">k</Emphasis>     

Noboru?KanekoEmail author  Kazuhiko?Morita  Tetsuya?Satoh  Kimitake?Hayasaki 《Astrophysics and Space Science》2005,299(3):263-306

We study the fundamental modes of radiation hydrodynamic waves arising from one-dimensional small-amplitude initial fluctuations with wave number k in a radiating and scattering grey medium using the Eddington approximation. The dispersion relation analyzed is the same as that of Paper I (Kaneko et al., 2000), but is solved as a quintic in angular frequency ω while a quadratic in k ² in Paper I. Numerical results reveal that wave patterns of five solutions are distinguished into three types of the radiation-dominated and type 1 and type 2 matter-dominated cases. The following wave modes appear in our problem: radiation wave, conservative radiation wave, entropy wave, Newtonian-cooling wave, opacity-damped and cooling-damped waves, constant-volume and constant-pressure diffusion modes, adiabatic sound wave, cooling-damped and drag–force-damped isothermal sound waves, isentropic radiation-acoustic wave, and gap mode. The radiation-dominated case is characterized by the gap between the isothermal sound and isentropic radiation-acoustic speeds within which there is not any acoustic wave propagating with real phase speed. One of the differences between type 1 and type 2 matter-dominated cases is the connectivity of the constant-volume diffusion mode, which originates from the radiative mode in the former case, while from the Newtonian-cooling wave in the latter case. Analytic solutions are derived for all wave modes to discuss their physical significance. The criterion, which distinguishes between radiation-dominated and type 1 matter-dominated cases, is given by Γ₀ = 9, where Γ₀ = C _p ^(tot)/C _V ^(tot) is the ratio of total specific heats at constant pressure and constant volume. Waves in a scattering grey medium are also analyzed, which provides us some hints for the effects of energy and momentum exchange between matter and radiation.  相似文献   

Spectral Structure of Small-Scale Turbulent and Mesoscale Fluxes in the Atmospheric Boundary Layer over a Thermally Inhomogeneous Land Surface   总被引：1，自引：0，他引：1  

Michail?A.?Strunin  Tetsuya?HiyamaEmail author 《Boundary-Layer Meteorology》2005,117(3):479-510

Spectral analysis was performed on aircraft observations of a convective boundary layer (CBL) that developed over a thermally inhomogeneous, well-marked mesoscale land surface. The observations, part of the GAME-Siberia experiment, were recorded between April and June 2000 over the Lena River near Yakutsk City. A special integral parameter termed the ‘reduced depth of the CBL’ was used to scale the height of the mixed layer with variable depth. Analysis of wavelet cospectra and spectra facilitated the separation of fluxes and other variables into small-scale turbulent fluctuations (with scales less than the reduced depth of the CBL, approximately 2 km) and mesoscale fluctuations (up to 20 km). This separation approach allows for independent exploration of the scales. Analyses showed that vertical distributions obeyed different laws for small-scale fluxes and mesoscale fluxes (of sensible heat, water vapour, momentum and carbon dioxide) and for other variables (wind speed and air temperature fluctuations, coherence and degree of anisotropy). Vertical profiles of small-scale turbulent fluxes showed a strong decay that differed from generally accepted similarity models for the CBL. Vertical profiles of mesoscale fluxes and other variables clearly showed sharp inflections at the same relative (with respect to the reduced depth of the CBL) height of approximately 0.55 in the CBL. Conventional similarity models for sensible heat fluxes describe both small-scale turbulent and mesoscale flows. The present results suggest that mesoscale motions that reach up to the relative level of 0.55 could be initiated by thermal surface heterogeneity. Entrainment between the upper part of the CBL and the free atmosphere may cause mesoscale motions in that region of the CBL.  相似文献