SIGN REVERSALS IN THE TRANSIENT METHOD OF ELECTRICAL PROSPECTING (ONE-LOOP VERSION)*

The transient electromagnetic fields observed by the MPPO-1 equipment in practice over geological structures cannot always be adequately explained by Maxwell's equation when the imaginary part of the conductivity of the rocks is neglected. If, for any reason, the conductivity of the rocks is a complex function of frequency such that the imaginary part is appreciable the model calculations must include this. This explains why some observed transient electromagnetic decay curves change sign with time.     

用板块构造学说对中国部分地区构造发展的初步分析

为便于在大陆上研究板块构造,作者提出了几项原则,作为参考:1.板块接触带时常表现为一条大断层或断裂带,延伸至少数百公里,经常超过一千公里;2.在两个相邻的板块上,沉积岩相和古生物群有显著的划分;3.有混杂堆积的出现;4.有蛇绿岩带的出现;5.有蓝片岩的出现;6.侵入岩与喷出岩具有规律性的分布;7.地震震中的分布;8.两个相邻板块所指极向的不同移动轨迹. 根据上述原则,结合中国地质情况进行分析,作者认为我国有几条山脉和地区可能是板块构造接触带.这些是:1.秦岭东西构造带,2.台湾省东岸,3.西藏的雅鲁藏布江,4.金沙江上游,5.龙门山及“康滇地轴”,6.祁连山北部边缘. 在工作中也遇到了以下几个有待进一步研究的问题:1.大陆板块对大陆板块互相碰撞的说法,似应作一定的修改.因为几乎所有大陆上的俯冲带都是沿着地槽的一边或两边发生的,而不是两个大陆板块直接相互接触.它和大陆边缘的海洋板块对大陆板块的移动,极为相似。2.超基性岩是否能侵入到大陆地壳,还是只能生成于地幔?3.如果说超基性岩只出现于地槽,则古老地块中有超基性岩出现时,是否可以说,这是以前地槽沉积的变质岩,而不是古老的岩浆岩体,例如秦岭的大华群和大别山的淮阳地盾等。     

Petrogenesis of Mesozoic, Peraluminous Granites in the Lamoille Canyon Area, Ruby Mountains, Nevada, USA

Two groups of closely associated, peraluminous, two-mica graniticgneiss were identified in the area. The older, sparsely distributedunit is equigranular (EG) with initial     

NUTRIENT LOAD ESTIMATION METHODS FOR RIVERS

Pollutant load estimation is essential for watershed management and water pollution control. For most watersheds, only sparse water quality measurements (e.g. monthly data) are available. The influence of input data on the accuracy of non-point source pollution load estimation is studied using the water quality and stream flow data from a small watershed in Hong Kong. Comparison and analysis of the results using 8 different methods show that the accuracy of stream-flow runoff is the single most important factor for the calculation of pollutant load. Based on the results, the stream flow correction coefficient is advanced to provide a more reliable load estimation. The improved method of pollutant load estimation can be easily applied in practice since the stream-flow runoff can be measured by hydrological station or estimated with various hydrological methods.     

Tectonometamorphic evolution of the Chuncheon amphibolite, central Gyeonggi massif, South Korea

Abstract The Chuncheon amphibolite, part of the Gubongsan Group which overlies the Yongduri gneiss complex, is interlayered with calc-silicate rock, marble, quartzite, biotite schist and quartzofeldspathic gneiss in the central Gyeonggi massif, South Korea. Metamorphic pressures and temperatures estimated from the amphibolite are 5.5–10.6 kbar and 615–714°C. These P—T conditions are close to those defined by the reaction curve between kyanite and sillimanite, and suggest medium-pressure-type metamorphism of the Chuncheon amphibolite. For two metapelites intercalated with the amphibolite, temperatures are estimated to be 607–699° C, consistent with those obtained from the amphibolite. On the other hand, pressures estimated from these metapelites are significantly different, 4–6 kbar and 9–13 kbar, when rim and core compositions of garnet are, respectively, used. These P—T estimates obtained from the amphibolite and metapelite suggest a nearly isothermal decompression of 3–7 kbar during denudation. Rapid decompression is likely on the basis of the results of mineral chemistry, phase equilibria and geothermobarometer. Moreover, in conjunction with the occurrence of kyanite in the adjacent Gyeonggi gneiss complex, P—T estimates of the Chuncheon amphibolite and metapelite suggest a clockwise P—T—t path. This evolutionary path may be related to the amalgamation of continents during the late Proterozoic event which corresponds to the Jinningian orogeny in the Qinling belt of China.     

Equilibrium Compositions of Coexisting Garnet and Orthopyroxene: Experimental Determinations in the System FeO-MgO-Al2O3-SiO2, and Applications

We have determined the Fe-Mg fractionation between coexistinggarnet and orthopyroxene at 20–45 kb, 975–1400?C,and the effect of iron on alumina solubility in orthopyroxeneat 25 kb, 1200?C, and 20 kb, 975?C in the FMAS system. The equilibriumcompositions were constrained by experiments with crystallinestarting mixtures of garnet and orthopyroxene of known initialcompositions in graphite capsules. All iron was assumed to beFe²⁺. A mixture of PbO with about 55 mol per cent PbF₂ provedvery effective as a flux. The experimental results do not suggest any significant dependenceof K_D on Fe/Mg ratio at T 1000?C. The lnK_D vs. l/T data havebeen treated in terms of both linear and non-linear thermodynamicfunctional forms, and combined with the garnet mixing modelof Ganguly & Saxena (1984) to develop geothermometric expressionsrelating temperature to K_D and Ca and Mn concentrations in garnet. The effect of Fe is similar to that of Ca and Cr³⁺ in reducingthe alumina solubility in orthopyroxene in equilibrium withgarnet relative to that in the MAS system. Thus, the directapplication of the alumina solubility data in the MAS systemto natural assemblages could lead to significant overestimationof pressure, probably by about 5 kb for the relatively commongarnetlherzolites with about 25 mol per cent Ca+Fe²⁺ in garnetand about 1 wt. per cent Al₂O₃ in orthopyroxene.     

Discrimination between coastal subenvironments using textural characteristics

The objective of this study was to discriminate between modern beach subenvironments based on textural characteristics obtained using the graphical (percentile) method, the moment method, and the log-hyperbolic distribution (LHD). A total of 126 surface sedimentation units were sampled at the nodes of a 21 x 6 rectangular grid (1000 m²) on a carbonate sand beach, Oahu, Hawaii. Sampling was conducted at low energy conditions from the lower foreshore to the backshore. Non-parametric discriminant analysis was used as an objective tool in defining distinct subenvironments. Confidence bands around the canonical variates derived from the graphic mean, sorting, skewness and kurtosis indicated four separate subenvironments (lower foreshore, mid-foreshore, upper foreshore and backshore). Three distinct subenvironments were identified using the mean, sorting (standard deviation) and skewness measures derived by the method of moments. A similar subenvironment distinction was obtained using five statistics of the LHD (gamma, γ; nu ν; delta, δ; tau, τ; and xi, ξ). No significant difference was noted in textural characteristics between the upper foreshore and backshore zones, and these zones were grouped into one subenvironment. These results indicate that different process scenarios would be needed to explain different subenvironment partitioning based simply on the approach adopted. Discriminant analysis indicated that fewer subenvironment samples were misclassified and separation distances between subenvironments in bivariate canonical plots were greater for the standard moment measures compared with the statistics derived from fitting the computationally intensive LHD model. Examination of the mass frequency grain size distributions indicated that the LHD was generally the most appropriate model. These observations were confirmed by the hyperbolic shape triangle which indicated that the LHD rather than the more commonly used log-normal distribution was generally optimal in describing sediments. These results support the use of the LHD statistical measures in subenvironment discrimination over the graphic-inclusive measures.     

The influence of salt withdrawal subsidence on palaeosol maturity and cyclic fluvial deposition in the Upper Triassic Chinle Formation: Castle Valley, Utah

Integrated fluvial sequence stratigraphic and palaeosol analysis can be used to better reconstruct depositional systems, but these approaches have not been combined to examine halokinetic minibasins. This study characterizes the temporal and spatial patterns of lithofacies and palaeosols in a sequence stratigraphic framework to reconstruct a model of minibasin evolution and identify halokinetic influences on fluvial deposition. This research documents fluvial cycles and stratigraphic hierarchy, palaeosol maturity and apparent sediment accumulation rates in the Chinle Formation within the Big Bend minibasin. This study also uses palaeosols to help identify fluvial aggradational cycle (FAC) sets. The Chinle is divided into two hectometre‐scale (10² m) fluvial sequences, six decametre‐scale (10¹ m) FAC sets, and variable numbers of metre‐scale FACs depending on proximity to the minibasin. Ten pedotypes representing 225 palaeosol profiles are recognized. The pedotypes include palaeosols similar to modern Entisols, Inceptisols, Aridisols, Vertisols and Alfisols. A maturity index (1–5) is assigned to each pedotype to assess its variability in palaeosol development. Estimated palaeosol development time is used to approximate apparent sediment accumulation rates. Increased subsidence resulted in a greater number and thicker FACs, thicker FAC sets and fluvial sequence sections, and lithofacies associations reflecting more rapid sedimentation along the minibasin axis. Palaeocurrent indicators converge towards the minibasin axis and indicate that it formed and drifted through time. Relative palaeosol maturity is inversely related to stratal thickness, and decreases towards the minibasin where episodic burial by fluvial sediment was more frequent. Metre‐scale FACs are most abundant towards the minibasin axis, and locally have Entisols and Inceptisols developed upon their upper boundaries reflecting increased sediment accumulation rates. Areas outside the minibasin are characterized by fewer FACs that are associated with more mature palaeosols. Palaeosol‐derived apparent sediment accumulation rates are as much as two orders of magnitude greater within the minibasin than in marginal areas. The combined stratigraphic, palaeocurrent and palaeosol evidence is used to develop a model for the evolution of the Big Bend minibasin that illustrates the halokinetic affect on fluvial and landscape processes.