中国东部的含科石英榴辉岩带的发现——郯庐断裂平移作用的证据

根据近年来在中朝板块与扬子板块碰撞带上,所发现的科石英或含科石英假象的榴辉岩,推测中国东部的郯庐断裂的左行平移幅度可达550km,活动时代在中生代之后。     

Ultrahigh-pressure metamorphism and decompressional P-T paths of eclogites and country rocks from Weihai, eastern China

Abstract Altered quartz-rich and nearly quartz-free eclogitic rocks and completely retrograde quartz-rich garnet amphibolites occur as blocks or lenses in gneisses at Weihai, northeastern tip of the Sulu ultrahigh-P belt. Eclogitic rocks with assemblage garnet ± clinopyroxene ± coesite + rutile have experienced three-stage metamorphic events including ultrahigh-pressure eclogite, granulite and amphibolite facies. Granulite metamorphic event is characterized by formation of the hypersthene + salite + plagioclase ± hornblende corona between garnet and quartz + clinopyroxene. P-T conditions for the three-stage recrystallization sequence are 840 ± 50°C, >28 kbar, about 760±50°C, 9 kbar, and ~650°C, <8 kbar respectively. Most country rock gneisses contain dominant amphibolite-facies assemblages; some garnet-bearing clinopyroxene gneisses recrystallized under granulite-facies conditions at about 740±50°C and 8.5 kbar; similar to granulite-facies retrograde metamorphism of the enclosed eclogitic blocks. Minor cale-silicate lenses within gneisses containing an assemblage grossular + salite + titanite + quartz with secondary zoisite and plagioclase may have formed within a large pressure range of 14-35 kbar. Eclogitic boudins and quartzo-feldspathic country rocks may have experienced coeval in situ UHP and subsequent retrograde metamorphism. The established nearly isothermal decompression P-T path suggests that this area may represent the interior portion of a relatively large subducted sialic block. The recognized UHP terrane may extend eastward across the Yellow Sea to the Korean Peninsula.     

Seafloor hydrothermal alteration at an Archaean mid-ocean ridge

A hydrothermally metamorphosed/altered greenstone complex capped by bedded cherts exposed in the North Pole, Pilbara Carton, Western Australia, is interpreted as an accretionary complex. It is distinctive in being characterised by both duplex structure and an oceanic crust stratigraphy. This complex is shown to represent an Archaean upper oceanic crust with a mid‐ocean ridge hydrothermal metamorphism that increases in grade stratigraphically downward. Three mineral zones have been defined; Zone A of the zeolite facies, the prehnite‐pumpellyite facies or the lower‐greenschist facies at high‐X_CO2 condition, Zone B of the greenschist facies, and Zone C of the greenschist/amphibolite transition facies. In Zone A metabasites, Ca‐Al silicates including Ca‐zeolites, prehnite and pumpellyite are absent and epidote/clinozoisite is extremely rare. Instead, abundant carbonates are present with chlorite suggesting high‐X_CO2 composition in the fluid. On the other hand, in Zones B and C metabasites, where Ca‐amphibole + epidote/clinozoisite + chlorite + Ca‐Na plagioclase are the dominant assemblages, carbonate is not identified. The metamorphic conditions boundary of Zones B/C were estimated to be about 350 °C at a pressure of <0.5 kbar. Fluid compositions coexisting with Archaean greenstones at the transition between Zones B and C were estimated by thermodynamic calculation in the CaFMASCH system (T = 350–370 °C, P = 150–1000 bar) at X_CO2 of 0.012–0.140, such values are higher than present‐day vent fluids collected near mid‐ocean ridges with low‐X_CO2 values, up to 0.005. The Archaean seawater depth at the mid‐ocean ridge was estimated to be 1600 m at X_CO2 = 0.06 using a depth‐to‐boiling point curve for a fluid. The carbonation due to high‐X_CO2 hydrothermal fluids occurred near the ridge‐axis before or was coincident with ridge metamorphism.     

THE ROLE OF CLOUDE/RADIATION IN LONG-RANGE NUMERICAL WEATHER PREDICTION*

We investigate the role of clouds and radiation in the general circulation of the atmosphere using a model designed for 30-day predictions.Comprehensive verifications of 30-day predictions for the 500 hPa geopotential height field have been carried out,using the data from ECMWF objective analyses that cover the period from May 5 to June 3,1982.We perform three model simulations,including experiments with interactive cloud formation,without clouds,and without radiative heating.The latter two experiments allow us to study the effects of cloud/radiation interactions and feedbacks on the predicted vertical velocity,and the meridional and zonal wind profiles,averaged over a 30-day period.We demonstrate that the Hadley circulation is maintained by the presence of clouds.The radiative cooling in the atmosphere intensifies the vertical motion in low latitudes and,to some extent,also strengthens the overall meridional circulation.The meridional winds are correctly reproduced in the model if clouds are incorporated.The zonal winds are significantly affected by clouds and radiative cooling.Without an appropriate incorporation of these physical elements,the model results would deviate significantly from observations.The presence of clouds strengthens the westerlies in middle and high levels.In May,the northerly movemen of the jet stream over eastern Asia is,in part,associated with the presence of clouds.     

Dynamic stiffness matrices for two circular foundations

A systematic procedure is presented for generating dynamic stiffness matrices for two independent circular foundations on an elastic half-space medium. With the technique reported in References 1–3, the analytic solution of three-dimensional (3D) wave equations satisfying the prescribed traction due to the vibration of one circular foundation can be found. Since there are two analytic solutions for two prescribed tractions due to the vibrations of two circular foundations, the principle of superposition must be used to obtain the total solution. The interaction stresses (prescribed tractions) are assumed to be piecewise linear in the r-directions of both cylindrical co-ordinates for the two circular foundations. Then, the variational principle and the reciprocal theorem are employed to generate the dynamic stiffness matrices for the two foundations. In the process of employing the variational principle, a co-ordinate transformation matrix between two cylindrical co-ordinate systems is introduced. Some numerical results of dynamic stiffness matrices for the interaction of two identical rigid circular foundations are presented in order to show the effectiveness and efficiency of the present method, and some elaborations for its future extensions are also discussed.     

Impact of absorbing aerosol deposition on snow albedo reduction over the southern Tibetan plateau based on satellite observations

We investigate the snow albedo variation in spring over the southern Tibetan Plateau induced by the deposition of light-absorbing aerosols using remote sensing data from moderate resolution imaging spectroradiometer (MODIS) aboard Terra satellite during 2001–2012. We have selected pixels with 100 % snow cover for the entire period in March and April to avoid albedo contamination by other types of land surfaces. A model simulation using GEOS-Chem shows that aerosol optical depth (AOD) is a good indicator for black carbon and dust deposition on snow over the southern Tibetan Plateau. The monthly means of satellite-retrieved land surface temperature (LST) and AOD over 100 % snow-covered pixels during the 12 years are used in multiple linear regression analysis to derive the empirical relationship between snow albedo and these variables. Along with the LST effect, AOD is shown to be an important factor contributing to snow albedo reduction. We illustrate through statistical analysis that a 1-K increase in LST and a 0.1 increase in AOD indicate decreases in snow albedo by 0.75 and 2.1 % in the southern Tibetan Plateau, corresponding to local shortwave radiative forcing of 1.5 and 4.2 W m⁻², respectively.

     

US-Russian Civilian Research and Development Fund Project： An origin of microdiamonds in metamorphic rocks of the Kokchetav Massif, Northern Kazakhstan

The Kokchetav Massif of northern Kazakhstan is a typelocality of occurrence of microdiamonds in ultrahighpressure metamorphic“ rocks. It records continental subduction and exhumation from depths greater than 120 km and provides unparalleled insights into mantle dynamics, global material recycling and the processes responsible for the genesis and destruction of continent. Through the support of a US Civilian Development and Research Fund, a group of Russian, US, Taiwanese and Kazakhstan geologists visited the reconstructed 800-m underground gallery to conduct field survey and sample collections. This gallery provides a useful geologic laboratory for international community to undertake similar geotectonic or different ecological and environmental studies.     

K0 estimation in level granular soil from anisotropic wave velocities on the basis of micromechanics

This study aims to explore the possibility for estimating K₀ in a level ground of granular soil by seismic methods on the basis of micromechanics theory. The idea was to simulate in situ cross‐hole seismic method for the measurement of wave velocities along various directions of wave‐propagation. This work made use of a field simulator to control a K₀ condition (zero lateral strain condition) in specimens. A series of vertical loading containing subsequent loading and unloading were applied to the specimen prepared by pluviation. In general, the K₀ values determined experimentally in this work agreed with the exiting empirical relations. K₀ value was also calibrated from measured anisotropic wave velocities using an optimization procedure. From the comparison of the back‐calculated and measured results of K₀, it revealed the feasibility for the determination of the in situ lateral stress in granular soil by seismic methods and on the basis of micromechanics theory as long as enough wave‐velocity measurements along various directions of wave‐propagation were available. The potential for the usage of the presented methodology for the determination of the in situ lateral stress in level‐ground of granular soil by seismic methods seems encouraging. Copyright © 2004 John Wiley & Sons, Ltd.     

Rain‐gauge network evaluation and augmentation using geostatistics

Rain‐gauge networks are often used to provide estimates of area average rainfall or point rainfalls at ungauged locations. The level of accuracy a network can achieve depends on the total number and locations of gauges in the network. A geostatistical approach for evaluation and augmentation of an existing rain‐gauge network is proposed in this study. Through variogram analysis, hourly rainfalls are shown to have higher spatial variability than annual rainfalls, with hourly Mei‐Yu rainfalls having the highest spatial variability. A criterion using ordinary kriging variance is proposed to assess the accuracy of rainfall estimation using the acceptance probability defined as the probability that estimation error falls within a desired range. Based on the criterion, the percentage of the total area with acceptable accuracy A_p under certain network configuration can be calculated. A sequential algorithm is also proposed to prioritize rain‐gauges of the existing network, identify the base network, and relocate non‐base gauges. Percentage of the total area with acceptable accuracy is mostly contributed by the base network. In contrast, non‐base gauges provide little contribution to A_p and are subject to removal or relocation. Using a case study in northern Taiwan, the proposed approach demonstrates that the identified base network which comprises of approximately two‐thirds of the total rain‐gauges can achieve almost the same level of performance (expressed in terms of percentage of the total area with acceptable accuracy) as the complete network for hourly Mei‐Yu rainfall estimation. The percentage of area with acceptable accuracy can be raised from 56% to 88% using an augmented network. A threshold value for the percentage of area with acceptable accuracy is also recommended to help determine the number of non‐base gauges which need to be relocated. Copyright © 2007 John Wiley & Sons, Ltd.     

Metamorphic evolution of kyanite–staurolite-bearing epidote–amphibolite from the Early Palaeozoic Oeyama belt, SW Japan

Early Palaeozoic kyanite–staurolite‐bearing epidote–amphibolites including foliated epidote–amphibolite (FEA), and nonfoliated leucocratic or melanocratic metagabbros (LMG, MMG), occur in the Fuko Pass metacumulate unit (FPM) of the Oeyama belt, SW Japan. Microtextural relationships and mineral chemistry define three metamorphic stages: relict granulite facies metamorphism (M1), high‐P (HP) epidote–amphibolite facies metamorphism (M2), and retrogression (M3). M1 is preserved as relict Al‐rich diopside (up to 8.5 wt.% Al₂O₃) and pseudomorphs after spinel and plagioclase in the MMG, suggesting a medium‐P granulite facies condition (0.8–1.3 GPa at > 850 °C). An unusually low‐variance M2 assemblage, Hbl + Czo + Ky ± St + Pg + Rt ± Ab ± Crn, occurs in the matrix of all rock types. The presence of relict plagioclase inclusions in M2 kyanite associated with clinozoisite indicates a hydration reaction to form the kyanite‐bearing M2 assemblage during cooling. The corundum‐bearing phase equilibria constrain a qualitative metamorphic P–T condition of 1.1–1.9 GPa at 550–800 °C for M2. The M2 minerals were locally replaced by M3 margarite, paragonite, plagioclase and/or chlorite. The breakdown of M2 kyanite to produce the M3 assemblage at < 0.5 GPa and 450–500 °C suggests a greenschist facies overprint during decompression. The P–T evolution of the FPM may represent subduction of an oceanic plateau with a granulite facies lower crust and subsequent exhumation in a Pacific‐type orogen.