Sedimentary processes caused by felsic caldera-forming volcanism in the Late Miocene to Early Pliocene intra-arc Aizu basin, NE Japan arc

Six large Late Miocene to Quaternary calderas, > 10 km in diameter, cluster together with several medium to small calderas and stratovolcanoes in a 60 × 30 km area of the Aizu volcanic field, southern NE Japan arc. These caldera volcanoes were built on a WNW–ESE trending highland coincident with a local uplifted swell since Late Miocene. The flare-up of felsic volcanism occurred synchronously along the NE Japan arc. Pyroclastic flow sheets from the calderas spread over the surrounding intra-arc basins and are interstratified with various sediments. Geochronological data indicates that the large-caldera eruptions have occurred six times since 8 Ma, at intervals of 1 to 2 million years. Late Miocene to Early Pliocene extra-caldera successions in the basin consist of nine sedimentary facies associations: (1) primary pyroclastics, (2) lahars, (3) gravelly fluvial channels, (4) sandy fluvial channels, (5) floodplains, (6) tidal flats, (7) delta fronts, (8) pro-delta slopes, and (9) pro-delta turbidites. The distribution of facies associations show westward prograding of volcaniclastic aprons, made up of braid delta, braidplain, pyroclastic flow sheet, and incised braided river deposits. The extra-caldera successions record: 1) an increase in felsic volcanism with an associated high rate of volcaniclastic sediment supply at about 10 Ma, prior to catastrophic caldera-forming eruptions; and 2) progradation of volcaniclastic aprons toward the back-arc side in response to the succeeding caldera-forming eruptions and sea-level changes, until about 3 Ma.     

The making and breaking of supercontinents: Some speculations based on superplumes, super downwelling and the role of tectosphere

The mechanisms of formation and disruption of supercontinents have been topics of debate. Based on the Y-shaped topology, we identify two major types of subduction zones on the globe: the Circum-Pacific subduction zone and the Tethyan subduction zone. We propose that the process of formation of supercontinents is controlled by super downwelling that develops through double-sided subduction zones as seen in the present day western Pacific, and also as endorsed by both geologic history and P-wave whole mantle tomography. The super-downwelling swallows all material like a black hole in the outer space, pulling together continents into a tight assembly. The fate of supercontinents is dictated by superplumes (super-upwelling) which break apart the continental assemblies. We evaluate the configuration of major supercontinents through Earth history and propose the tectonic framework leading to the future supercontinent Amasia 250 million years from present, with the present day Western Pacific region as its frontier. We propose that the tectosphere which functions as the buoyant keel of continental crust plays a crucial role in the supercontinental cycle, including continental fragmentation, dispersion and amalgamation. The continental crust is generally very thin, only about one tenth of the thickness of the tectosphere. If the rigidity and buoyancy is derived from the tectosphere, with the granitic upper crust playing only a negligible role, then supercontinent cycle may reflect the dispersion and amalgamation of the tectosphere. Therefore, supercontinent cycle may correspond to super-tectosphere cycle.     

Izu-Bonin arc subduction under the Honshu island, Japan: Evidence from geological and seismological aspect

The Izu-Bonin intra-oceanic arc with 20–35 km thick continental crust is being subducted under the Honshu, presumably since 17 Ma. Tomographic image clearly demonstrates that the whole Izu-Bonin arc is subducting under the Honshu arc. Geologic cross section and the thickness of continental crust do not support the accretion of thick crust in spite of the continued subduction over 17 Ma.     

Early Paleozoic subduction initiation volcanism of the Iwatsubodani Formation,Hida Gaien belt,Southwest Japan

International Journal of Earth Sciences - In placing Japanese tectonics in an Asian context, variation in the Paleozoic geological environment is a significant issue. This paper investigates the...     

A model for the hydrogen coma of a comet

A model for the hydrogen coma of a comet on the basis of the Monte Carlo method is presented. In this model isotropic ejections of H atoms produced by photodissociation of H₂O and OH, thermalization of the H atoms due to collisions with ambient H₂O molecules, and the solar radiation pressure have been taken into account. A production spectrum of H atoms from OH is evaluated by using the predissociation rates and the level populations of OH, confirming that the spectrum has a sharp peak around 8.0 km sec^?1 with the standard deviation of 0.1 km sec^?1. Including the above effects, velocity distribution functions of the H atoms at various positions in the coma for the first time, as well as their density and outflow velocity profiles, have been calculated. It is pointed out that the collisional thermalization process in the inner coma is an important factor at small heliocentric distances in determining the density profiles and the velocity distributions. It is shown that thermalization leads to an increase in the H density in the inner coma larger than those expected from other models such as the vectorial model, in which collision is not taken into account. Lyman α isophotes and its line profiles in the optically thin region are computed by using the velocity distribution function.     

Spectrophotometric scan of comet 1973 XII Kohoutek

Photoelectric spectrophotometric scans of Comet 1973 XII Kohoutek were made on November 25.85 and 28.85 UT, 1973. The vibrational temperature of C₂, the total numbers of CN and of C₂, and the production rates for CN and C₂ are derived from the observed fluxes. The temperature of C₂ was 4900 K on 25.85 November and 4300 K on 28.85 November. Mean abundance ratio of C₂ to CN was about 2.7. From the variation of production rates with heliocentric distance, it is shown that there was an outburst on 25.85 November in both CN and C₂ productions. For C₂, the production rates are derived by using two coma models, i.e. the Haser's parent-daughter model and the model relevant to a proposition of Yamamoto (1981b) that C₂ is formed via two-step photodissociation of its parent molecules. By comparing the production rate derived from the two models, it can be supported that C₂ is formed via two-step photodissociation rather than one-step photodissociation. In consequence, it is shown that the variation of the production rate with heliocentric distance is largely modified compared with that derived from the Haser's model.     

In situ stress measurements in a borehole close to the Nojima Fault

Abstract In situ stress was measured close to the fault associated with the 1995 Kobe Earthquake (Hyogo-ken Nanbu earthquake; January 1995; M 7.2) using the hydraulic fracturing method. The measurements were made approximately 2 years after the earthquake. The measured points were approximately 40 m from the fault plane at depths of about 1500 m. The maximum and the minimum horizontal compressive stresses were 45 MPa and 31 MPa, respectively. The maximum compressive stress and the maximum shear stress are very small in comparison with those of other seismically active areas. The azimuth of the maximum horizontal compressive stress was estimated from the observed azimuths of well bore breakouts at depths between 1400 m and 1600 m and was found to be N135° (clockwise). The maximum stress axis is perpendicular to the fault strike, N45°. These features are interpreted in terms of a small frictional coefficient of the fault. The shear stress on the fault was released and dropped almost to zero during the earthquake and it has not yet recovered. Zero shear stress on the fault plane resulted from the perpendicular orientation of one of the principal stress to the fault plane.     

A Numerical Simulation of the Seasonal Cycle of Temperature, Salinity and Velocity Fields in Tokyo Bay

To understand when oxygen-depleted waters occur, how they develop and when they dissipate in inner Tokyo Bay, realistic simulations were attempted with fine spatial and temporal resolution by applying realistic time dependent external forcing. A 3D hydrodynamic model was driven by time-dependent external forcing factors/parameters such as solar radiation, air temperature, relative humidity, wind velocity, and fluvial discharge, under the open boundary conditions of 1995. A simulated time series of salinity and temperature agreed fairly well with observed data, except in summer. The model failed to reproduce the development of the surface mixed layer in summer. Several sensitivity analyses on the external forcing parameters such as wind velocity and vertical diffusivity were conducted to reproduce the mixed layer. However, changing these parameter values did not improve the model results.     

An interpretation of the 1883 cataclysmic eruption of Krakatau from geochemical studies on the partial melting of granite

Pumice flow from the 1883 Krakatau eruption significantly differs in both mineral and chemical compositions from any other volcanic rocks or ejecta of the Krakatau group, which belong to the tholeiitic series. Lithic fragments of granitic Rock, discovered in the pumice flow, are similar to West Malayan granitic rocks. No other granitic rock occurs throughout the Krakatau group, therefore, we consider that the granitic fragments came from the underlying complex at depths, where they were captured as foreign materials by the magma.It is possible that sialic crustal materials plunged into depths along a peculiar tectonic structure located at the Sunda Strait, which appears to be a sheared portion caused by deformation of the Sunda arc due to differential movement between the Indo-Australian oceanic plate and the Eurasian continental crust. The crustal materials were partially melted and produced a magma of granitic composition. The magma was mixed with or assimilated by an ascending basaltic magma originating probably from the upper mantle. This resulted in a dacitic magma distinctly dominant in silica, alkalis and volatile components, and the 1883 Krakatau eruption, characterized by the pumice flow of dacitic composition, took place.     

APPLICATION OF MEM SPECTRAL ESTIMATION TO MU RADAR OBSERVATION

Preliminary results of the wind velocity estimation using the Maximum Entropy Method (MEM) to MUradar observation data sets are presented. The comparison of the results from the periodogram method and theMEM shows that the MEM estimation is reliable, and has higher accuracy, resolution and detectability than theestimation from periodogram method. The high accuracy power spectrum obtained by the MEM is veryuseful to studying the atmospheric turbulence structure. However. the MEM needs the longer computingtime for obtaining the high accuracy spectrum. Particularly, the estimation of MEM will bring serious devia-tion at lower signal-to-noise ratio.