Silicon Isotope Geochemistry of Micro—Fine Disseminated Gold Deposits in SW Guizhou and NW Guangxi,China

Quartz was studied with respect to its silicon isotopic composition and cathodoluminescence in micro-fine disseminated gold deposits in SW Guizhou and NW Guangxi.The results showed that quartz in wall rocks.ores and that in association with hydrothermal silicification are distinctive in silicon isotopes and cathodoluminescence characters.Quartz in association with primary silicification is non-luminescent while that in wall rocks and associated with secondary silicification exhibits striking luminescence.Based on the dynamic fractionation of silicon isotopes,it is suggested that the mineralization was accompanied by rapid transport of aprimary siliceous fluid along the major deep fault system into subordinate faults before ore components deposited in favorable strata via penetration and metasomatism.Therefore,a deep origin is implicit for gold deposits of this type.     

Annual and seasonal variations in the low-latitude topside ionosphere

Annual and seasonal variations in the low-latitude topside ionosphere are investigated using observations made by the Hinotori satellite and the Sheffield University Plasmasphere Ionosphere Model (SUPIM). The observed electron densities at 600 km altitude show a strong annual anomaly at all longitudes. The average electron densities of conjugate latitudes within the latitude range ±25° are higher at the December solstice than at the June solstice by about 100% during daytime and 30% during night-time. Model calculations show that the annual variations in the neutral gas densities play important roles. The model values obtained from calculations with inputs for the neutral densities obtained from MSIS86 reproduce the general behaviour of the observed annual anomaly. However, the differences in the modelled electron densities at the two solstices are only about 30% of that seen in the observed values. The model calculations suggest that while the differences between the solstice values of neutral wind, resulting from the coupling of the neutral gas and plasma, may also make a significant contribution to the daytime annual anomaly, the E × B drift velocity may slightly weaken the annual anomaly during daytime and strengthen the anomaly during the post-sunset period. It is suggested that energy sources, other than those arising from the 6% difference in the solar EUV fluxes at the two solstices due to the change in the Sun-Earth distance, may contribute to the annual anomaly. Observations show strong seasonal variations at the solstices, with the electron density at 600 km altitude being higher in the summer hemisphere than in the winter hemisphere, contrary to the behaviour in NmF2. Model calculations confirm that the seasonal behaviour results from effects caused by transequatorial component of the neutral wind in the direction summer hemisphere to winter hemisphere.     

IMP 8 magnetosheath field comparisons with models

This study presents cross-sectional vector maps of the magnetic field derived from IMP 8 magnetometer in the magnetosheath at 30 R_e behind the Earth. In addition the vector patterns of the magnetosheath field for northward, southward, and east-west interplanetary magnetic field (IMF) directions are qualitatively compared with those obtained from the Spreiter-Stahara gas dynamic (GD) and Fedder-Lyon magnetohydrodynamic models (MHD). The main purpose is to display the cross-sectional differences in relation to the dayside merging with different IMF directions, allowing the reader to make direct visual comparisons of the vector patterns. It is seen that for east-west IMF directions, the data-based and MHD-based patterns differ noticeably in a similar way from the GD model, presumably reflecting the influence of dayside magnetic merging of the Earths magnetic field with the y-component of the interplanetary magnetic field. All three northward IMF cross sections show comparable field draping patterns as expected for a closed magnetosphere. For southward IMF case, on the other hand, differences between the three cross-sectional patterns are greater as seen in the field vector sizes and directions, especially closer to the magnetopause where more disturbed magneto-spheric conditions are known to be exist. The data comparisons with the MHD and GD models show that the differences result from the magnetic field-flow coupling and that the effects of dayside reconnection are present in IMP 8 magnetic field measurements.     

Preliminary results of measuring the crustal deformation in Qinghai-Xizang area using GPS technique

GPS repetition measurement tbta in Qinghai-Xizatlg (Tibetan) area in 1992 and 1994 have been used to determine the change rates of seven bascline vectors of Lhasa-Wenquan, etc. It is the first time to obtain the direct observation results of the large-scale crustal horizontal motions in this area. Thesc preliminary results also for the first time provide the direct observation evidence for some important geophysical and geological viewpoints, such as the northward gradual reduce of the effect of the northward push-pressing to Eurasian continent by Indian Plate in the Qinghai-Xizang area, having a southward strike slip movement of the Chuan-Dian diamond block, etc.     

Estimating open boundary conditions from coastal tidal observations by adjoint approach

A numerical study for estimating the tidal open boundary conditions of a shelf current modrl from tb coastal tidal observations is presented. The method is based on the optimal control/adjoint method. A lrast square fitting of the model state to simulated data is used. Two ideal domains and coastlines are considered. Using the IAP shallow. water model and its adjoint model, some identical twin experiments are carried out to test efficiency and lirnilsd of the method. The results show that the adjoint method can efficiently estimate the open boundary conditions well for gulf/bay like domains. The adjoint method seems to have great potential to improve the accuracy of tide and shelf current modeling in coastal regions. Project supported hy the National Natural Science Fuundation of China (Grant No. 49376256)     

Processes of the deposition and vicissitude of Mu Us Desert, China since 150 ka B.P.

“Milanggouwan stratigraphical section” named lately takes down 27 cycles of alternately evolutionary histories of desert depositions in the Mu Us area with the fluviolacustrine facies and palaeosols since 150 ka B. P. Studies show that the sedimentary form was induced by the growth and decline and confrontation each other between the winter monsoon and the summer monsoon of East Asia in the past 150 ka. Project supported by the National Natural Science Foundation of China (Grant No. 49473192).     

New seismic images of the crust in the central Trans-Hudson Orogen of Saskatchewan

A reprocessing program to enhance the correlation between the surface geology and the seismic data has been completed for seismic line 9 (eastern 100 km) and line 10 in the central region of the Trans-Hudson Orogen of Saskatchewan, Canada. The new seismic images through lateral continuity of reflectivity provide sufficient detail to resolve the discrepancy between the low-dipping, layer-parallel and dextral-reverse nature of the Sturgeon-Weir shear zone (line 9) observed in the field and its steeply dipping (apparent) normal displacement character interpreted on the basis of the initial processing. Furthermore, the new interpretation provides a strong confirmation of the role of Pelican Thrust as a major detachment zone — the main `sole thrust' — along which juvenile allochthons have been carried across the Archaean microcontinental block. The images are also refined enough to suggest: (a) a boundary within the Pelican Thrust between its internal and external suites; (b) a possible boundary separating a lower (older?) Archaean basement from its upper (younger?) counterpart; and (c) sub-Moho events (M2) which reveal possible involvement of the upper mantle in the collisional tectonic process in addition to the well defined Moho (M1) which probably represents the youngest of the post-collisional detachments.     

Last Glacial snowlines in the Tibetan Plateau: an argument against an extensive coalescing icesheet

Two opposing theories are circulating with regard to the extent of the Last Glacial ice cover in the Tibetan Plateau. One says that only less than 20% of plateau was covered with ice, and another insists that the plateau be completely covered with an extensive coalescing icesheet. The extent of the ice cover is thought to be significant in shaping global climatic systems, and a further discussion on this issue may help to understand the earth's surface feedback mechanisms to the global climates. This paper focuses on the Last Glacial snowline reconstruction and uses the reconstructed snowline to argue against the existence of an extensive coalescing icesheet. The reconstructed Last Glacial snowlines suggest that the snowlines dropped 500–700 m in the western and northern marginal mountains and about 1000 m in the southern and eastern marginal mountains of the Tibetan Plateau. However, the magnitude of the snowline dropping decreases dramatically towards the interior of the plateau, from 300–400 m in those mountains adjacent to the marginal mountains to about 100 m in the driest area in the interior. This means that the snowlines were too high and associated glaciers were too limited to extend to the vast intermountainous basins. To be blamed are weakened summer monsoons and lowered condensation elevations, both of which were probably responsible for not bringing in an adequate amount of precipitation into the interior for developing an extensive coalescing icesheet. The relatively high radiation in these relatively low latitudes could be a major negative force to prevent the snow and ice from forming a coalescing icesheet. In contrast, the enhanced plateau blockade to the monsoons may have helped to significantly lower the snowlines and expand the glaciers in the outer slopes of the southern and eastern marginal mountains. The westerlies may have greatly helped those glaciers in the western and northern marginal mountains. This revised version was published online in July 2006 with corrections to the Cover Date.     

穿过张家口—渤海地震带西缘的折射剖面所揭示的地壳上地幔构造与速度结构

近期完成的北北东向的繁峙～太仆寺旗宽角反射／折射剖面,由山西断隆进入内蒙地轴,它穿过了山西地震带的大同～阳高震区和北西西向张家口～渤海地震带的西北缘。其主要目的是探测研究张～渤地震带及其两侧壳幔构造与速度结构的差异及其与地震活动的相关性。结果表明：张～渤地震带壳内界面及莫霍面较其两侧有１．０～２．０ｋｍ的不同程度的上隆;在阳原～张家口之间地壳结构纵向与横向非均匀性明显,上地壳和下地壳上部有局部的速度逆转,下地壳下部的低速层由山西断隆一直延伸至内蒙地轴,上地幔顶部的速度梯度明显增大;在阳原、怀安和张北附近分别存在着延伸至莫霍面的地壳深断裂带;在内蒙地轴一系列壳内界面不明显。