Cretaceous Strike-Slip Dislocations in the Dalat Zone (Southeastern Vietnam)

Studies of ductile dislocations of the Jurassic terrigenous deposits and faulting kinematics of the Cretaceous injective structures in the Dalat zone show that the volcano-plutonic belt of southeastern Vietnam was formed under NE-striking sinistral dislocations affected by NNW transpression. This deformation took place in similar conditions as for the Cretaceous structure of the eastern margin of Asia. Due to the Indo-Eurasian collision, since the Cenozoic the Indo-Sinian Block has been displaced toward the southeast by a distance of more than 500 km from its primary position, causing transformation of the previously formed Cretaceous structures. Thus, the Cenozoic tectonogenesis in southeastern Vietnam should be studied with regard to the previous Cretaceous tectonic deformation.     

Effect of decompression and suction on macroscopic and microscopic behavior of a clay rock

The goal in this research was to analyze the effects of decompression and suction on the formation of cracks in a clay rock from the Andra (French National Radioactive Waste Management Agency) site at Bure (Meuse–Haute-Marne, France). The article investigates the relationship between the changes in the hydromechanical properties and the changes in microstructure and porosity. Concerning the effect of decompression, at the macroscopic scale, the study highlighted an important effect on the elastic modulus and permeability, but little effect at the microscopic scale except an evolution of mineralogy related to the oxidation of pyrite often present in layers where cracks develop. Concerning the effect of suction, at the macroscopic level, the results showed that, on drying path, the change in the properties of the material was very small, whereas, on wetting path, a large decrease in tensile strength and gas permeability was observed. At the microscopic level, observations with SEM and ESEM, and measurements with MIP, highlighted the evolution of microstructural organization as a function of suction, and the propagation and enlargement of cracks on wetting path, rather than on drying path.     

Agrochemical and polychlorobyphenyl (PCB) residues in the Mekong River delta, Vietnam

An environmental survey on pesticide residues and polychlorobyphenyl compounds (PCBs), encompassing more than 70 polar and non-polar compounds quantifiable by the techniques used, was performed in the Mekong River delta based on analyses of water, sediment and bivalve mollusc samples. Few polar compounds, such as diazinon and fenotrothion, were detected in water but a high number of non-polar chlorinated compounds, such as DDT, HCH, endosulfan and PCBs, were detected in sediments and biota. The highest concentrations measured were of DDT with an average 6.3 ng g(-1) dry weight (range 0.32-67 ng g(-1)) in sediments and 38.6 ng g(-1) (range 5.5-123 ng g(-1)) in molluscs' soft tissues. Amongst chlorinated compounds, DDT concentrations were followed in decreasing order by those of PCB, endosulfan, hexachlorocyclohexane and chlordane. Residues of organochlorine compounds originate from local usage of agrochemicals although with a likely contribution also of atmospheric deposition of residues (not measured) originated elsewhere. Concentrations of PCB and pesticide residues in the aquatic environment of the Mekong River delta are lower than values reported for other regions of Vietnam and Asia. Nevertheless, current concerns about the effects of chlorinated compounds on public health advise improved control of chemical residue discharges in order to abate environmental contamination.     

Distribution and Characteristics of Hazardous Geological Features in the Marine Coastal and Offshore Areas of Zhejiang Province,East China Sea

Newly acquired high-resolution shallow seismic profiles(7069 km in length) in the coastal and offshore areas of Zhejiang Province, East China Sea, China, have revealed eight marine hazardous geological features: shallow gas, sand ridges, erosion ditches, scarps, irregular bedrock features, underwater shoals, buried paleo-channels, and submarine deltas. Based on the seismic profiles, we have constructed a marine geological map of these hazardous features. Shallow gas accumulations are common and occur mainly in two separate nearshore regions that cover 4613 and 3382 km~2 respectively. There are also scattered shallow gas accumulations in the offshore area, typically accompanied by paleo-channels that occur mainly in the middle of the study area. Sand ridges, erosion ditches, scarps, and irregular bedrock features are found mainly in the northeast of the study area in association with each other. In the southeastern part of the study area, the sand ridges have a linear form and trend NW–SE, representing the western part of the linear sand ridges in the East China Sea. The maximum slope gradient is 1?, which suggests that this area is prone to landslides. These hazardous marine geological features are important to marine and engineering activities in this region.     

西太平洋海山玄武岩的岩石学特征及与上覆富钴结壳的关系

通过对西太平洋海山玄武岩和玄武质火山角砾岩的岩石学特征及其气孔充填物成分变化的研究,讨论了海山玄武岩与上覆富钴结壳的关系。富钴结壳的基岩火山岩主要为碱性玄武岩和玄武质火山角砾岩。碱性玄武岩中气孔充填物为S型和C型,S型和C型气孔充填物不共存在同一个气孔中。S型气孔充填物的物质成分富Si、Fe、K、Mg、Al,Co含量一般为0.17%~0.46%,其成分与碱性玄武岩的组成类似,其物质来源可能与碱性玄武岩有关。C型气孔充填物为O、Ca、C及少量Si、Al、Mg、Fe、K等元素,C型气孔充填物可能与海水中物质的沉积有关。由Si、Al、Fe、Mg、K等元素变化曲线反映出的S型气孔充填物沉积过程与富钴结壳早期的沉积过程具有相似的趋势,说明富钴结壳早期沉积物的物质来源可能与碱性玄武岩有关,富钴结壳物质来源可能是碱性玄武岩和海水共同作用的结果。     

豫西地区秦岭造山带武当群火山岩和沉积岩锆石U-Pb年龄

在南秦岭地体中广泛分布着武当群变质沉积-火山岩组合的基底岩石,是东秦岭造山带重要的岩石单元,但对其形成时代的认识存在分歧.笔者通过对豫西地区武当地体北部武当群上部杨坪组变质沉积岩和下部双台组变质中酸性火山岩锆石u-Pb年龄的测定,认为杨坪组变质沉积岩沉积物源主要由晚元古代(860~640 Ma)和早元古代(2260~1800 Ma)、少量晚太古代和中元古代地壳物质构成,与扬子陆块具有亲缘关系.沉积时代应晚于640 Ma.变质中酸性火山岩形成时代为730~780 Ma之间,峰值(755±6)Ma,与双台组新元古代变质基性火山岩为同期岩浆作用的产物,成因上推测与Rodinia超大陆裂解作用有关.     

越南红河流域沉积物的镜下碳屑分析实验室处理方法对比研究

本文设计4个不同的实验方法,以越南红河流域原始森林区、河漫滩区和水下三角洲区3个不同沉积环境下的9个样品为实验对象,对镜下碳屑不同的实验室处理方法进行对比研究,实验发现样品用六偏磷酸钠分散法(方法A)和盐酸分散法(方法B)处理后,镜下碳屑形态不易分辨,杂质多,而用氢氟酸处理法(方法C)和附加醋酸酐混合液处理法(方法D),镜下碳屑的形态清晰,杂质少,碳屑数量容易统计,但是方法D中利用醋酸酐与浓硫酸混合液去纤维素与染色过程会使纤维素碳化,结果显示碳屑浓度明显比用前3种方法处理的浓度高。以上实验结果表明方法C是最优的。另外,本研究在进行碳屑统计时,利用NIS­Elements Br 3.0软件的自动统计计数功能,不仅可以减小人为计数产生的误差,还可以实现碳屑的多参数统计,有利于古环境、古气候、古植被变化的多信息解译。     

Using stable isotopes to estimate young water fractions in a heavily regulated,tropical lowland river basin

The young water fraction of streamflow (Fyw), an important hydrological variable, has been calculated for the first time, for a monsoon-fed coastal catchment in northern Vietnam. Oxygen stable isotopes (δ¹⁸O) from six river sites in the Day River Basin (DRB) were analysed monthly, between January 2015 and December 2018. River δ¹⁸O signatures showed sine wave variability, reflecting the amount effect and tropical (dry-rainy) seasonality of the region. The δ¹⁸O composition of precipitation ranged from −12.67 to +1.68‰, with a mean value of −5.14‰, and in-streamflow signatures ranged from −11.63 to −1.37‰ with a mean of −5.02‰. Fractions of young water (Fyw) were calculated from the unweighted and flow-weighted δ¹⁸O composition of samples. Unweighted Fyw ranged between 29 ± 8% and 82 ± 21% with a mean value of 51 ± 19%, and was not significantly different from flow-weighted Fyw (range between 33 ± 25% and 92 ± 73%, mean 52 ± 36%). Both unweighted and flow-weighted Fyw were highest in the middle of stream and lowest in downstream sites, capturing the impacts of landuse changes, hydrology and human activities in the catchment. Our calculations imply that more than a half of rainwater reaches the DRB river mainstream within the first 3 months. The Fyw is much higher than the global average (of one-third) and insensitive to discharge due to the combination of a humid catchment with high rainfall, low storage capacity, flat landscape and an intensive drainage system in the DRB. Also the low discharge sensitivity of Fyw in the DRB implies that the regional hydrology is severely altered by humans.     

Constraining the cosmological time variation of the fine - structure constant

The variation of the fine-structure constant α = e ² / ħc can be probed by comparing the wavelength of atomic transitions from the redshift of quasars in the Universe and laboratory over cosmological time scales t ~ 10¹⁰ yr. After a careful selection of pairs of lines, the Thong method with a derived analytical expression for the error analysis was applied to compute the α variation. We report a new constraint on the variation of the fine-structure constant based on the analysis of the C_IV, N_V, Mg_II, Al_III, and Si_IV doublet absorption lines. The weighted mean value of the variation in α derived from our analysis over the redshift range 0.4939 ≤ z ≤ 3.7 is = ( 0.09 ± 0.07)×10⁻⁵. This result is three orders of magnitude better than the results obtained by earlier analysis of the same data on the constraint on Δα/α .