华北克拉通基性火山岩高场强元素对Ar-Pt界限及新生代地幔源区特征的示踪   总被引：5，自引：0，他引：5       下载免费PDF全文

刘勇胜  高山  骆庭川 《地球科学》1998,23(5):91

华北克拉通不同时代基性火山岩存在向富集方向演化的趋势,并出现明显的Ａｒ－Ｐｔ界限和新生代异常．元古宙基性火山岩比太古宙富集大部分高场强元素（ＨＦＳＥ）和轻稀土元素（ＬＲＥＥ）,ＲＥＥ分异程度显著增加,而且相容性接近的不相容元素（如Ｎｂ,Ｔａ）的相对不相容性发生了改变．新生代玄武岩ＨＦＳＥ及Ｎｉ高度富集,ＲＥＥ强烈分异,δ（Ｅｕ）值最高．Ａｒ－Ｐｔ界限可能与地幔熔融时物理条件（如氧逸度）的改变和地幔富集作用有关．新生代玄武岩独特的地球化学特征可能主要反映了地幔源区的特殊性———“似ＯＩＢ型”富集地幔,其形成既可能有携带远洋沉积物的俯冲古老玄武岩洋壳部分熔融残余的贡献,也可能有直接循环（如通过拆沉作用）的陆壳物质的贡献,同时还可能涉及了地幔柱活动的影响．  相似文献   

Laboratory modelling of shear behaviour of soft joints under constant normal stiffness conditions   总被引：1，自引：0，他引：1  

B. Indraratna  A. Haque  N. Aziz 《Geotechnical and Geological Engineering》1998,16(1):17-44

Shear behaviour of regular sawtooth rock joints produced from casting plaster are investigated under constant normal stiffness (CNS) conditions. Test results obtained in this investigation are also compared with the constant normal load (CNL) tests. It is observed that the peak shear stress obtained under CNL conditions always underestimates the peak shear stress corresponding to the CNS condition. Plots of shear stress against normal stress show that a nonlinear (curved) strength envelope is acceptable for soft rock joints subjected to a CNS condition, in comparison with the linear or bilinear envelopes often proposed for a CNL condition. Models proposed by Patton (1966) and Barton (1973) have also been considered for the predictions of peak shear stress of soft joints under CNS conditions. Although Patton's model is appropriate for low asperity angles, it overestimates the shear strength in the low to medium normal stress range at higher asperity angles. In contrast, while Barton's model is realistic for the CNL condition, it seems to be inappropriate for modelling the shear behaviour of soft joints under CNS conditions. The effect of infill material on the shear behaviour of the model joints is also investigated, and it is found that a small thickness of bentonite infill reduces the peak stress significantly. The peak shear stress almost approached that of the shear strength of infill when the infill thickness to asperity height ratio (t/a) reached 1.40. This paper also introduces an original, empirical shear strength envelope to account for the change in normal stress and surface degradation during CNS shearing. © Rapid Science Ltd. 1998  相似文献   

Quasi-isotropic approximation of ray theory for anisotropic media     

Matthias Zillmer  Boris Kashtan  & Dirk Gajewski 《Geophysical Journal International》1998,132(3):643-653

Wave propagation in weakly anisotropic inhomogeneous media is studied by the quasi-isotropic approximation of ray theory. The approach is based on the ray-tracing and dynamic ray-tracing differential equations for an isotropic background medium. In addition, it requires the integration of a system of two complex coupled differential equations along the isotropic ray.
The interference of the qS waves is described by traveltime and polarization corrections of interacting isotropic S waves. For qP waves the approach leads to a correction of the traveltime of the P wave in the isotropic background medium.
Seismograms and particle-motion diagrams obtained from numerical computations are presented for models with different strengths of anisotropy.
The equivalence of the quasi-isotropic approximation and the quasi-shear-wave coupling theory is demonstrated. The quasi-isotropic approximation allows for a consideration of the limit from weak anisotropy to isotropy, especially in the case of qS waves, where the usual ray theory for anisotropic media fails.  相似文献   

Sensitivity of frequency-dependent traveltimes to laterally heterogeneous, anisotropic Earth structure     

Li Zhao  & Thomas H. Jordan 《Geophysical Journal International》1998,133(3):683-704

  相似文献   

Seismic reflection and transmission coefficients of a self-similar interface   总被引：1，自引：0，他引：1  

Kees Wapenaar 《Geophysical Journal International》1998,135(2):585-594

The derivation of seismic reflection and transmission coefficients is generally based on the assumption that the medium parameters behave as step functions of depth, at least in a finite region around the interface. However, outliers observed in well logs generally behave quite differently from step functions. In this paper we represent an interface by a self-similar singularity, embedded between two homogeneous half-spaces, and we derive its frequency-dependent normal-incidence reflection and transmission coefficients. For ω  → 0 the expressions for the coefficients reduce to those for a discrete boundary between two homogeneous half-spaces; for ω → ∞ they become frequency-independent. These asymptotic expressions have a relatively simple form and depend on the singularity exponent α .
  The exact as well as the asymptotic expressions are used to evaluate the time-domain reflection and transmission responses of a self-similar interface. Finally, we use a numerical method to model the response of a smoothed version of a self-similar interface (note that the velocity of a smoothed singularity remains finite). It turns out that smoothing has hardly any effect on the response, provided that the smoothing does not affect the scales corresponding to the seismic frequency range.  相似文献   

某古岩溶铀矿床中铀,钼,铼的赋存状态     

吴俊奇  闵茂中 《矿物学报》1998,18(3):325-330

本文采用电子探针、透射电镜和化学物相法,查明某古岩溶铀矿床中铀主要呈显微和超显微状沥青铀矿、铀石,其次呈分散吸附状和铀酰（次生）矿物形态存在;钼主要呈蓝钼矿、钼钙矿、钼华、钼铅矿等氧化物,其次呈胶硫钼矿等形态存在;铼主要呈吸附状,其次可能在黄铁矿中呈二硫化物包裹体形态产出。最后讨论了该三个元素赋存状态与该铀矿床形成地球化学环境之间的关系。  相似文献   

三维结线动力无穷元及其特性研究     

陈健云  林皋 《岩土力学》1998,19(3):14-19

研究了文献[2]提出的结线动力无穷元的特性及其参数确定。由于这种单元将衰减函数定义在无穷向结线上,从而不仅可同时考虑多种波动形式,而且对于不同介质,不必通过改变节点坐标分布的形式实现介质间的过渡,而直接以结线上参数的改变来实现,因而可以更好地模拟无穷波动场,使用起来更方便．更容易推广。  相似文献   

三维复杂域非结构化六面体网格全自动生成方法   总被引：3，自引：0，他引：3  

陈尚法  曹新红 《岩土力学》1998,19(4):46-51

使用自适应有限单元法在三维复杂域内求解水工及岩土工程问题,讨论了应用行波法在多介质三维复杂块体内六面体单元网格的自动生成过程。其特点是对不规则区域具有较强的适应性,并考虑了节理单元的自动生成。  相似文献   

岩土工程数值模拟新方法   总被引：13，自引：1，他引：12  

唐辉明  滕伟福 《地质科技情报》1998,(Z2)

论述了岩土工程数值模拟的新进展和近年来新出现的数值模拟方法,重点介绍了无网络伽辽金法（ＥＦＧＭ）和数值流形法（ＭＭ）的原理及其在岩土工程中的应用,指出了今后研究的重点方向。  相似文献   

Remarkable response amplification of building frames due to resonance with the surface ground     

Izuru Takewaki 《Soil Dynamics and Earthquake Engineering》1998,17(4):211-218

Seismic response of a building structure is influenced greatly by soil-structure interaction. This fact has been demonstrated in the past earthquakes. It is shown that tuning of the natural period of a building structure with that of a surface ground causes remarkable response amplification of the building structure. Introduction of an overall system as a building-pile-soil system is inevitable to investigate such a tuning effect. It is demonstrated to be essential to define a design earthquake at a bedrock level in order to guarantee the structural safety of building structures under seismic disturbances. Comparison of the response due to input of double the upward-propagating wave (an outcropping motion) into the bedrock without any viscous boundary with that due to input of an within motion into the bedrock without any viscous boundary is also shown in order to investigate the effect of input motions on the response of the superstructure.  相似文献