Nb–Ta fractionation induced by fluid‐rock interaction in subduction‐zones: constraints from UHP eclogite‐ and vein‐hosted rutile from the Dabie orogen,Central‐Eastern China

Niobium and Ta concentrations in ultrahigh‐pressure (UHP) eclogites and rutile from these eclogites and associated high pressure (HP) veins were used to study the behaviour of Nb–Ta during dehydration and fluid‐rock interaction. Samples were collected through a ～2 km profile at the Bixiling complex in the Dabie orogenic belt, Central‐Eastern China. All but one eclogite away from veins (EAVs) display nearly constant Nb/Ta ratios ranging from 16.1 to 19.2, with an average of 16.9 ± 0.8 (2 SE), similar to that of their gabbroic protolith from the Yangtze Block. Nb/Ta ratios of rutile from the EAVs range from 12.7 to 25.3 among different individual grains, with the average values close to those of the corresponding bulk rocks. These observations show that Nb and Ta were not significantly fractionated by prograde metamorphism up to eclogite facies when no significant fluid‐rock interaction occurs. In contrast, Nb/Ta ratios of rutile from eclogites close to veins (ECVs) are highly variable from 17.8 to 49.8, which are systematically higher (by up to 17) than those of rutile from the veins. These observations demonstrate that Nb and Ta were mobilized and fractionated during localized fluid flow and intensive fluid‐rock interaction. This is strongly supported by Nb/Ta zoning patterns in single rutile grains revealed by in situ LA‐ICP‐MS analysis. Ratios of Nb/Ta in the ECV‐hosted rutile decrease gradually from cores towards rims, whereas those in the EAV‐hosted rutile are nearly invariable. Furthermore, the vein rutile shows Nb/Ta zoning patterns that are complementary to those in rutile from their immediate hosts (ECVs), suggesting an internal origin for the vein‐forming fluids. The Nb/Ta ratios of such fluids evolved from low values at the early stage of subduction to higher values at later supercritical conditions with increased temperature and pressure. Quantitative modelling was conducted to constrain the compositional evolution of metamorphic fluids during dehydration and fluid‐rock interaction focusing on Nb–Ta distribution. The modelling results based on our proposed multistage fluid phase evolution path can essentially reproduce the natural observations reported in the present study.     

对岛状融区房建冻害的几点认识——以沱沱河兵站楼房为例

座落在岛状融区中的青藏高原沱沱河兵站楼房,由于跨年度施工,楼房各部位季节冻深和融化速度不一,引起不均匀下沉,致使墙体产生严重裂缝。岛状融区是工程地质环境敏感区。在太阳辐射强烈的青藏高原,现行建筑规范中房屋采暖对冻深的影响系数偏小,建议对北外墙中段和角端应比规范规定系数分别增大0.2和0.3。     

华北地区水位下降是否会减缓气温上升——浅部地温影响的数值模拟分析

华北地区由于长期持续的地下水过量开采,导致了大面积地下水位大幅下降,引发地面塌陷、地下水质污染等一系列地质环境问题,这些现象早已为人们所熟知和关注.然而地下水位下降还会造成百米量级浅部地温及其梯度的变化,因此即使来自地球深部的大地热流密度没有变化,年度平均的从表浅部位通过地表实际传导进入大气的热流密度会减小,这是中外文献中尚未见讨论过的问题.我们通过数值模拟发现假定大地热流密度不变的条件下,华北数万平方公里地下水位下降会造成百米尺度内的地温降低,从而传入大气的热流密度降低40%以上,且会持续数百年以上的时间.这种长时间大范围的传导入大气的热流密度变化对环境会造成什么影响是一个十分值得关注的问题.这一预测在一定程度上得到了气象站地温观测数据的支持,但由于目前气象观测站只有3.2 m深度范围内的地温资料,累计不超过5、60年,中间还有10余年的间断,而且表浅深度地温受地表多种因素的影响也较大,这些资料难以对我们关心的地下水位下降引起流入大气的热流密度变化这一问题提供直接确凿的数据来进行分析,因此今后有必要开展对地下数十乃至数百米地温进行持续精确的监测工作.     

On the reoccurrence of site specific response

The period and amplitude variations of local peaks in the Fourier amplitude spectra of free-field strong ground motion recorded at five stations in San Fernando Valley of metropolitan Los Angeles, California, are described, searching for peaks that reoccur during different earthquakes. The data suggest that some local peaks reoccur (about 50% of the time), during shaking by small local earthquakes (peak ground velocities, v_max<10–20 cm/s). During large strong motion amplitudes (v_max>20 cm/s), these peaks are shifted towards longer periods (by nonlinear response of soils) or disappear. The data also suggest that densification and settlement of soil, minutes and hours following the strong shaking may contribute towards fluctuations in the effective stiffness of the shallow surface layers.     

中卫活动断裂带地震破裂危险区演化特征模拟

通过建立的岩石破裂危险度KR及断层滑动危险系数KF2个判别指标,对中卫一同心活动断裂带进行粘弹性有限元数值模拟,得到该断裂带在现代构造应力场作用下大震后的应力演变及地震破裂危险区长度、范围、峰值、衰减规律等的演化特征。结果表明,该断裂带大震后应力场的调整主要产生于震后300～400年间,震后地震破裂危险区转移到断裂带的中西段,并逐渐向东迁移,范围越来越集中,最终转移到断裂带的弧形顶点部位(红谷梁附近)。     

结构双层多重调谐质量阻尼器(DMTMD)控制策略的鲁棒性评价

基于定义的二类优化目标函数,评价双层多重调谐质量阻尼器(DMTMD)控制策略对漂移频率系数(DFR)摄动的鲁棒性。数值研究表明,使用第二类优化准则设计的DMTMD、双重调谐质量阻尼器(DTMD)和多重调谐质量阻尼器(MTMD)比使用第一类优化准则设计的DMTMD、DTMD和MTMD具有更高的对DFR摄动的鲁棒性。而且,使用第二类优化准则设计的总数为4的DMTMD、DTMD和总数为11的MTMD具有近似相同的对DFR摄动的鲁棒性。     

A three‐dimensional transmitting boundary formulated in Cartesian co‐ordinate system for the dynamics of non‐axisymmetric foundations

A three‐dimensional transmitting boundary is formulated in the Cartesian co‐ordinate system. It is developed for the dynamic soil–structure interaction problems of arbitrary shape foundations in laterally heterogeneous strata overlying rigid bedrock. Dynamics of a rectangular rigid surface foundation on a homogeneous stratum is analysed by a hybrid approach in which the finite region including foundation is modelled by the conventional finite element method and the surrounding infinite region by the newly developed transmitting boundary. To demonstrate its strength, the present method is applied to a rectangular foundation in a horizontally heterogeneous ground consisting of two distinct regions divided by and welded along a vertical plane. Copyright © 2000 John Wiley & Sons, Ltd.     

Fe,Mn, P and S Speciation in Sediments from the Capivara Hydroelectric Dam Lake (Brazil) as an Indicator of Anthropogenic Influences

The conservation of large water resources is essential for the preservation of human life. The quantification and, more importantly, the speciation of chemical substances that indicate the presence of anthropogenic contamination in water resources are of great importance. This paper presents the results of analysis for the determination of organic, inorganic and total phosphorus, pseudo‐sulfur, and iron and manganese, in five fractions, in water sediments collected from the Capivara Hydroelectric Power Plant, Brazil. A study on the seasonal variation of these parameters was conducted, with data having been collected in the winter and in the summer, at two sites along the dam, 5 km apart, close to the city of Primeiro de Maio. Phosphorous was found in sediments and adjacent soil in the organic form (OP), and was used as an indicator of anthropogenic influence on the reservoir banks. Speciation of potentially toxic Mn showed that it is present in the exchangeable fraction of the 0–5 cm depth layer (sediment/water interface), making its transfer to the water column possible. Results from this study showed that domestic and industrial effluent treatment measures are needed for the preservation of the quality of aquatic environments.     

Seismic self‐centering steel beam‐to‐column moment connections using bolted friction devices

This paper first presents the force–deformation relationship of a post‐tensioned (PT) steel beam‐to‐column connection constructed with bolted web friction devices (FDs). This paper then describes the test program conducted in the National Center for Research on Earthquake Engineering, Taiwan, on four bolted FDs and four full‐scale PT beam‐to‐column moment connection subassemblies using the FDs. Tests confirm that (1) the hysteretic behavior of four bolted FDs is very stable, (2) the friction coefficient between the steel plate and the brass shim is about 0.34, (3) the proposed force–deformation relationships reasonably predict the experimental responses of the PT connections under cyclically increasing deformations up to a beam peak rotation of 0.05 rad, and (4) the decompression moments do not degrade as beam cyclic deformations increase. Copyright © 2007 John Wiley & Sons, Ltd.