Fluid Geochemistry as Indicator of Tectonically-Related,Deep Water Circulations in the Sardinian Rift-Campidano Graben (Italy): New Insights from Environmental Isotopes

The EC funded Geochemical Seismic Zonation program (EEC GSZ Project 1996–1998) chose Sardinia as a low-seismicity site, in which the relationships between fluid geochemistry and seismo-tectonics had to be investigated and results compared with outcomes from other selected high-seismicity sites. A first article, examining the role of fault segmentation and seismic quiescence on the geochemical composition of groundwaters and gases, has already been presented (Angelone et al. 2005). This article deals with environmental isotopes which, together with selected hydrochemical data, give hints on tectonically-related fluid circulations. Four water-dominated hydrothermal systems were considered, all located along regional fault systems and discharging groundwaters belonging to the Na–HCO₃ and Na–Cl facies. In the considered systems, groundwater circulation takes place, principally, in the Palaeozoic Crystalline Basement (PCB), with the exception of the Logudoro system, where hydrological circuits develop in the Mesozoic Carbonate Platform (MCP). The high CO₂ contents, the non-attainment of fluid-rock equilibrium and the large lithological variability prevent the construction of a unique hydrogeological–geochemical conceptual model. In this case, stable isotopes provide a useful tool to describe the origin of fluids and their subterranean movements. Stable isotopes of water, integrated with hydrochemical data, indicate that fluids are derived from three main end members. The dominant component is a relatively recent local meteoric water; the second one is marine water; and the third one is a fossil freshwater, depleted in heavy isotopes with respect to modern rains. The latter end member entered the aquifer system in the past, when climatic conditions were greatly different from today. At least two circulation systems can be recognised, namely a shallow cold system and a deep hydrothermal system, as well as two distinct hydrological processes: (1) gravity-controlled descent of cold water towards greater depths and (2) convection linked to a thermal gradient, causing deep fluids to rise up from the hydrothermal reservoir towards the surface. The highly variable δ¹³C_TDIC values suggest the presence of two distinct CO₂ sources, namely, a biogenic one and a thermogenic one. The relation between the isotopic compositions of CO₂ and He indicates an increased mantle signature in uprising CO₂-rich fluids.     

山西地区应力场变化与地震的关系

收集了1970～2006年37年的小震初动符号建立了震源机制解数据库,研究了山西断陷带小震综合机制解时空演化特征.结果表明:山西断陷带小震综合机制解以走滑正断层为主,断陷带两端以拉张作用为主,中部地区以剪切作用为主,忻定盆地、太原盆地的区域应力场与华北区域应力场差异较大.利用沿山西断裂带布设的GPS监测网1996～2007年12期复测资料,分析了山西断裂带水平运动与地震活动的关系.结果表明:山西断裂带现主要受NWW-SEE向压应力场、NNE-SSW向张应力场的控制.1998～1999年有一次较为明显的应力扰动,空间上表现为北强南弱,接着发生了1999年11月1日大同—阳高5.6级地震.     

二维小波变换在消除相干噪声中的应用

相干噪声常使地震记录中的反射同相轴模糊不清,使得处理解释难以进行。而常规的各种ｆ－ｋ滤波技术不能对其有效地消除,且每种技术都有其局限性,容易产生人为假象,使信号发生很大畸变。我们的技术是利用二维小波变换,把二维地震信号变换到有关频率、时间、波数、空间的一个四维函数。根据信号和相干噪声在频率、时间、波数、空间等方面的局部差异来达到滤波的目的。经过对实际炮集数据的处理表明该方法比ｆ－ｋ滤波技术消除噪声的效果要好,信号的畸变程度更低,并且没有任何限制条件。     

Localization of temperature anomalies in the Upper Rhine Graben: insights from geophysics and neotectonic activity

The European Cenozoic Rift System hosts major temperature anomalies in Central Europe. In its central segment, the Upper Rhine Graben (URG), temperatures range from 75°C to nearly 150°C at a depth of 2000 m. Different hypotheses have been suggested to explain the localization of these anomalies. Our review and comprehensive interpretation of gravimetric and magnetic data, as well as neotectonic activity patterns, suggests that low-density, mostly magnetic and fractured granitic basement is systematically associated with major temperature anomalies. Further analyses provide insight into different heat transport processes contributing to the localization of these anomalies. Magnetic and gravity anomalies are known to represent lithological variations associated with the pre-Permian. We show their spatial relationship with positive temperature anomalies in the URG. Correlation between magnetics and temperature reveal a mean contribution of heat production to the temperature anomaly of about 10–15°C. A slightly higher mean value is obtained from correlation between gravity and temperature, which may be attributed to effects resulting from fracture porosity. The spatial relationship between temperature anomalies and neotectonic patterns indicates compressional shear and uplift regime for the major anomalies of the central segment of the URG. This is in agreement with different numerical models indicating free convection on fracture zones linked to faults. Our findings show that about 15–25% of the temperature anomaly can be attributed to variation in heat production. Hydrothermal circulation convection along faults, activated by the tectonic context, may explain the remaining 75–85% of the temperature anomalies.     

Graben width controlling syn-rift sedimentation: the Palaeogene southern Upper Rhine Graben as an example

Eocene to Early Oligocene syn-rift deposits of the southern Upper Rhine Graben (URG) accumulated in restricted environments. Sedimentation was controlled by local clastic supply from the graben flanks, as well as by strong intra-basinal variations in accommodation space due to differential tectonic subsidence, that in turn led to pronounced lateral variations in depositional environment. Three large-scale cycles of intensified evaporite sedimentation were interrupted by temporary changes towards brackish or freshwater conditions. They form three major base level cycles that can be traced throughout the basin, each of them representing a stratigraphic sub-unit. A relatively constant amount of horizontal extension (ΔL) in the range of 4–5 km has been estimated for the URG from numerous cross-sections. The width of the rift (L _f ), however, varies between 35 and more than 60 km, resulting in a variable crustal stretching factor between the bounding masterfaults. Apart from block tilting, tectonic subsidence was, therefore, largely controlled by changes in the initial rift width (L ₀). The along-strike variations of the graben width are responsible for the development of a deep, trough-like evaporite basin (Potash Basin) in the narrowest part of the southern URG, adjacent to shallow areas in the wider parts of the rift such as the Colmar Swell in the north and the Rhine Bresse Transfer Zone that delimits the URG to the south. Under a constant amount of extension, the along-strike variation in rift width is the principal factor controlling depo-centre development in extensional basins.     

辽东太子河流域石炭—二叠纪岩相古地理及铝土矿成矿地质条件

晚石炭—早二叠世的辽东太子河盆地是一个周边为平移断层的断陷盆地。以东西向垂直运动为主体的构造活动控制了盆地的发展和演化。根据构造特征、沉积相和古地磁与华北腹地石炭一二叠系的差异,太子河地区的石炭一二叠系应是华北型石炭一二叠系的一个分支,作者称“辽东亚型”。古地理演化表现为从晚石炭世早期的扇三角3洲→峡湾海岸→碳酸盐台地的快速海进,直至早二叠世末的三角洲→冲积平原→湖泊体系的缓慢海退,堆积了近陆特征明显的、近尔西展布的各沉积相带。随时代渐新,堆积物中碎屑组分成熟度趋于增高,大地构造背景从部分卷入大陆边缘活动带过渡为大陆板块克拉通内部,均反映本区的断陷活动由次稳定向稳定型转化。铝土矿成矿物质主要来源于北部古陆结晶岩石,系红土(钙红土)—沉积成因。G层铝土矿形成于泻湖边缘环境;B臣则位于近海湖泊的滨—浅湖地带,同受控于北纬41°20′的同沉积断裂,与北东向断裂复合凹地为成矿最佳场所。     

Depositional processes and stratigraphic architecture within a coarse-grained rift-margin turbidite system: The Wollaston Forland Group,east Greenland

The Wollaston Forland Basin, NE Greenland, is a half-graben with a Middle Jurassic to Lower Cretaceous basin-fill. In this outcrop study we investigate the facies, architectural elements, depositional environments and sediment delivery systems of the deep marine syn-rift succession. Coarse-grained sand and gravel, as well as large boulders, were emplaced by rock-falls, debris flows and turbulent flows sourced from the immediate footwall. The bulk of these sediments were point-sourced and accumulated in a system of coalescing fans that formed a clastic wedge along the boundary fault system. In addition, this clastic wedge was supplied by a sand-rich turbidite system that is interpreted to have entered the basin axially, possibly via a prominent relay ramp within the main fault system. The proximal part of the clastic wedge consists of a steeply dipping, conformable succession of thick-bedded deposits from gravity flows that transformed down-slope from laminar to turbulent flow behaviour. Pervasive scour-and-fill features are observed at the base of the depositional slope of the clastic wedge, c. 5 km into the basin. These scour-fills are interpreted to have formed from high-density turbulent flows that were forced to decelerate and likely became subject to a hydraulic jump, forming plunge pools at the base of slope. The distal part of the wedge represents a basin plain environment and is characterised by a series of crude fining upward successions that are interpreted to reflect changes in the rate of accommodation generation and sediment supply, following from periodic increases in fault activity. This study demonstrates how rift basin physiography directly influences the behaviour of gravity flows. Conceptual models for the stratigraphic response to periodic fault activity, and the transformation and deposition of coarse-grained gravity flows in a deep water basin with strong contrasts in slope gradients, are presented and discussed.     

A k - *epsiv Turbulence Closure Model For The Atmospheric Boundary Layer Including Urban Canopy

A numerical model for the computation of the wind field,air temperature and humidity in the atmospheric boundary layer (ABL) including the urbancanopy was developed for urban climate simulation. The governing equations of the modelare derived by applying ensemble and spatial averages to the Navier–Stokes equation, continuityequation and equations for heat and water vapour transfer in the air. With the spatial averagingprocedure, effects of buildings and other urban structures in the urban canopy can be accounted for byintroducing an effective volume function, defined as the ratio between the volume of air in acomputational mesh over the total volume of the mesh. The improved k - model accounts for the anisotropyof the turbulence field under density stratification. In the improved k - model, the transportof momentum and heat in the vertical direction under density stratification is evaluated based onthe assumption of a near-equilibrium shear flow where transport effects on the stresses andheat fluxes are negligible. The heating processes at surfaces of buildings and ground are alsomodelled. The comparison of the computational results obtained with the present modeland existing observational data and numerical models shows that the present model is capableof predicting the structure of turbulence in the urban canopy layer under density stratification.Numerical experiments with the new model show that the flow behaviour of the air in the urbancanopy layer is strongly affected by the existence of buildings and density stratification.     

伊通地堑断层系统与构造样式

与走滑相关的盆地构造一般比较复杂,主要表现为断层系统较复杂和构造样式类型丰富。通过对三维地震资料进行系统的构造解析,结合区域地质和钻井资料的分析表明,伊通地堑的断层系统在平面上主要由近东西向的张性正断层和北东走向的张剪性断层组成,反映盆地具右旋走滑的应力体制;断层在演化序列上分为4个类型,与盆地的形成和演化之间存在良好的对应关系。地堑构造样式以"基底卷入型"为主,从盆地的动力学机制的角度,它属于"走滑—拉分型",进一步可将它概括为5种类型的构造样式,而且每一个断陷都以一种构造样式占主体,其展布具有明显的规律性。构造样式的差异是造成不同断陷具有独特油气分布规律的主导因素。