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1.
The interaction modalities of transfer zones connecting rift segments may be influenced by several factors. Amongst these, the location and architecture of transfer zones in narrow rifts has been repeatedly associated with the presence of inherited basement anisotropies. Sand-box models were made to investigate how the orientation, geometry and kinematics of transfer zones depend upon pre-existing basement anisotropies. Analogue models reproduced offset rift segments linked by transfer zones bordered by arcuate normal faults. Strike-slip faults are present inside the transfer zone, provided that angle α (between the extension direction and the axis of the transfer zone) is less than 50°. Narrower transfer zones, striking oblique to the extension direction, occur for angle φ (between the direction perpendicular to the extension direction and the direction of the basement anisotropy) less than 90°; wider transfer zones, subparallel to the extensional direction, occur for φ > 90°. Increasing the overstep induces narrower transfer zones striking subparallel to the extension direction. Similar geometrical and kinematical patterns have been found at transfer zones in narrow rifts. The comparison between experimental and natural data shows how the geometry, kinematics and orientation of natural transfer zones depends upon the trend of inherited anisotropies: transfer zones along inherited basement structures set at lower angles to the extension direction, and display lower aspect ratios, than transfer zones where inherited anisotropies are absent. 相似文献
2.
Recent interpretations of upper continental mantle seismic anisotropy observations have often relied on fabric measurements and calculated anisotropies of upper mantle xenoliths. Seismic ray paths of P and S waves, which provide information on azimuthal compressional wave anisotropy and shear wave splitting, are tens to hundreds of kilometers, whereas, xenoliths are usually only a few centimeters in diameter. To place better constraints on field-based anisotropy observations and to evaluate anisotropy information provided by xenoliths, it is important to examine anisotropy in large ultramafic massifs which have originated in the upper mantle. One such massif is the Twin Sisters Range located in the western portion of the North Cascades of Washington State, USA. The Twin Sisters massif, a slab of unaltered dunite, is 16 km in length, 6 km in width and 3 km thick. Exposed along its south and west sides are mafic granulite facies rocks, which likely represent lower continental crustal fragments. The ultramafic rocks are porphyroclastic in texture, consisting of strained, flattened porphyroclasts of olivine and enstatite and strain-free olivine mosaics. Olivine fabrics are typical of those formed at high temperatures and low strain rates. Petrofabrics and calculated anisotropies of individual samples vary throughout the massif, however, overall anisotropy of the body is significant, with maximum P and S waves anisotropies of 5.4% and 3.9%, respectively. The maximum delay time for split shear waves traveling through a 100-km-thick slab is 0.8 s and two directions of shear wave singularity are observed. The directions of maximum shear wave splitting and shear wave singularities do not coincide with the directions of maximum and minimum compressional wave velocity. In general, individual hand samples show significantly higher anisotropy than the overall anisotropy of the massif. It is concluded that simple averages of xenolith anisotropies are unreliable for use in the interpretation of field anisotropy observations. 相似文献
3.
为了理解各式各样的具有广义自相似性特征的尺度不变性系统, 提出了1个称之为GSI (generalized scale invariance) 的理论体系.它阐述了大小尺度可以相互关联而不需要引入任何1个特有(具体) 尺度的最普通情形.在二维线性GSI理论的基础上, 形成了2个重要的各向异性尺度不变性量化模型: SIG (scale invariant generator) 模型和S-A (spectrum-area) 模型.SIG模型通过在频率域中估计GSI理论中代表旋转和层化程度的尺度不变性生成元的参数来量化各向异性尺度不变性.而S-A模型通过从二维频率域中能谱密度大于P元素集的面积与P之间关系的非参数模型对各向异性尺度不变性进行量化.如果研究的对象是1个混合模式(多个不同尺度的过程或作用叠加而形成的), S-A模型不仅可以对异性尺度不变性进行量化, 还可以对该混合模式进行分解.系统阐述了GSI理论、SIG模型和S-A模型, 并将SIG模型和S-A模型结合提出了既能对混合模式进行分解又能对分解后模式的各向异性尺度不变性进行量化的模型. 相似文献
4.
不同温压下岩石弹性波速度、衰减及各向异性与组构的关系 总被引：16，自引：0，他引：16
结合岩石组构分析 ,阐述了岩石弹性波传播速度和衰减以及它们的各向异性与岩石组构之间的关系。在不同温压条件下对具有很强晶格优选方位的岩石样品的研究表明 ,随着围压的增加 ,波速和Q值均增大 ,但是在相互正交的 3个方向上 (垂直或平行于层理面及线理方向 )增大的速度并不相同 ,这与微裂隙的逐渐闭合密切相关。观测到的波速和Q值的各向异性具有不同的形成机理 ,波速各向异性主要与定向分布的微裂隙和主要矿物的晶格优选方位等构造因素有关 ;高围压下Q值各向异性与速度各向异性正好相反 ,可能是由于定向排列的矿物晶体沿不同方向其边界之间接触程度不同造成的。对岩石组构的研究不仅可以揭示岩体的变形机制、变形的动力学过程及其有关的热力学信息 ,还可以对宏观岩石的各种物理性质 ,尤其是力学特性 ,从微观机理上加以解释。文中特别强调了岩石组构分析对研究岩石物理性质的各向异性具有十分重要的意义。 相似文献
5.
The Jewel seam of the Cadomin area is complexly folded and cut by several thrust faults. Vitrinite reflectance anisotropy was determined from twenty oriented coal blocks, collected from various structural positions. Five of the blocks have uniaxial anisotropies, five have biaxial positive anisotropies, the remaining ten are biaxial negative. The biaxial coals display maximum reflectance axes parallel to nearby fold axes, which suggests a relationship between vitrinite anisotropy and deformation. The biaxial vitrinite reflectance ellipsoids result from superposition of tectonic strains on a primary, sedimentary burial related, uniaxial anisotropy. A tectonic stress field was probably present during the later stages of sedimentary burial and subsequent deformation.Coalification resulted largely from pre-deformational, sedimentary burial, but the final coalification stage was syn-deformational, as illustrated by the intersection of isorank surfaces and the Jewel seam and the biaxial vitrinite reflectance anisotropy. 相似文献
6.
Investigation of crack formation during loading of brittle rock 总被引：6，自引：1，他引：6
Conclusions The response of elastic/brittle targets to loading by conical-, wedge-and hemispherical-indenter shapes representing jackhammer bit tips was analyzed. Theoretical predictions of crack extent and direction are based on values of the maximum principal strains and the associated extension strain criterion in the target assuming a semi-infinite, homogeneous and isotropic solid.Corresponding static experiments using an MTS machine and dynamic percussive tests involving an encased circular target disk of Sierra granite have been conducted. The damage pattern consisting of a crater, a crushed zone, a region of multiple minor cracks as well a some longer fissures has been delineated. Comparison with theoretical predictions indicate that cracks are produced by maximum tensile strain, with their directions predicted well by the present theory. The upper bound predictions of the crack extent are also in reasonable accord. Deviations are due to the presence of inhomogeneities and anisotropies in the tests specimens. 相似文献
7.
塔里木盆地奥陶系生物礁储集层受到广泛关注。笔者优选塔里木盆地良里塔格地区奥陶系鹰山组顶部生物礁体(NYG-1)和一间房组底部生物礁体(NYG-2)为研究对象,通过详细的野外观察、实测和薄片鉴定,对生物礁的生物类型、岩性特征和沉积相单元进行分析。结果表明:塔里木盆地良里塔格地区鹰山组和一间房组的造礁生物主要由瓶筐石组成,与生物礁相关的岩性有障积礁灰岩、粘结礁灰岩、生屑粒泥灰岩、生屑泥粒灰岩、内碎屑泥粒灰岩、生屑颗粒灰岩、内碎屑颗粒灰岩和球粒颗粒灰岩。NYG-1礁体发育礁基、礁核、礁顶、礁翼和礁盖5个沉积相单元;NYG-2发育礁基、礁核与礁盖3个沉积相单元。从生物类型来看,鹰山组的礁为托盘—障积礁,为近似圆形的台内点礁,展布无明显的方向性;一间房组的礁为托盘障积—粘结礁,属台缘礁,常呈现出礁体迁移的特征。鹰山组和一间房组成礁模式的共同点是都经历了礁基的形成、礁体的发育和礁体的衰亡3个阶段,而差异主要表现在生物礁体的灭亡阶段: 对鹰山组生物礁而言,海平面的快速下降和高能水动力条件导致了瓶筐石的灭亡和内碎屑灰岩的产生;对一间房组生物礁而言,海平面的上升和浮游藻类生物大量发育造成造礁生物甁筐石的灭亡。以上研究对于建立符合塔里木盆地地区条件的礁体沉积模式和探索露头与井下生物礁对比研究有重要意义。 相似文献
8.
Discovery of anisotropy for upper-mantle P-wave velocities has resulted recently in many laboratory determinations of velocities for natural mantle material, either by direct measurement on rock samples or through calculations based on single-crystal elastic constants, fabrics and modal compositions Comparison of field and laboratory data provides gross constraints for convection processes and flow mechanisms. As natural peridotites yeidl a wide range of maximum seismic anisotropies, their depth of origin and deformation history are inferred from pyroxene geobarometry and geothermometry, and from stress estimates derived from neoblast grain sizes. Large maximum anisotropies (ca. 1.0 km/s) are typical of samples with fabrics strengthened by low-temperature/highstress deformation related to emplacement. Other highly anisotropic mantle material includes peridotites beneath continental rifts and thinned continental crust. Maximum anisotropies for ordinary suboceanic and infracontinental peridotites are generally in the range of the field data (0.3–0.6 km/s) and support a model with [010]ol nearly vertical (in accord with observations on ophiolites) and [100]ol subparallel to the spreading direction. On the basis of fabric data, seismic anisotropy should decrease with depth. 相似文献
9.
A sensitive instrument for measurement of the angular dependence of dielectric constant in a rotating rock specimen is described and the precautions necessary to obtain meaningful results are summarized. An important problem is the choice of length/diameter ratio for cylindrical specimens, so that complete three-dimensional information can be obtained from a single specimen; this ratio averages 0.86, but is found to be a function of dielectric constant and of electrode geometry. The measurements reported here have been concerned with weakly anisotropic rocks which lack visible fabrics, especially sandstones and basalts. A sandstone was used in a study of the statistical “noise” or scatter in anisotropy axes of a set of 52 similar specimens. The anisotropy was found to be much greater than expected from the finite numbers of grains in a sample. It is concluded that, in spite of careful drying of specimens, the anisotropy is dominated by thin grain coatings which have very high dielectric constants or may even be conducting. A basalt has been found in which the dielectric and magnetic anisotropies have different axes, apparently reflecting different grain alignment events in the history of the rock. However, this is the only example that has been found in which all of the information obtainable from dielectric anisotropy could not have been obtained more easily from magnetic anisotropy. 相似文献
10.
We present the mathematical deduction and properties of the mean traveltime curves for homogeneous elliptical anisotropic
media. These curves generalize their isotropic counterparts which have been introduced in the past as a simple data quality
analysis technique at the pre-inversion stage for 2D transmission experiments, allowing the inference of prior velocity models
to gain stability at the tomographic inversion. Also, the anisotropy parameters (maximum velocity, anisotropic direction and
ratio) are shown to affect the shape of these curves. The degree of asymmetry of the anisotropic mean traveltime curves (displacement
of the mean time and standard deviation minima from the middle of the gathering line) is related to the direction of anisotropy
which can then be visually estimated. Least squares’ fitting of the anisotropic theoretical models to their experimental counterparts
is an effective method to estimate at the pre-inversion stage a macroscopic elliptical anisotropic velocity model, valid at
the scale of the experiment, and able to match the experimental mean traveltime distribution.
Sensitivity analysis has shown that the mean curve is less prone to errors than the standard deviation curve. Parameter identification
from the standard deviation curve becomes unstable for noise levels higher than 5%; data errors produce smearing of the value
of the estimated anisotropy ratio and wrong directions of anisotropy biased towards zero degrees. Also, identification from
the mean traveltime curve becomes stable when the maximum velocity is well constrained. Finally, this methodology is illustrated
with the application to the Grimsel data set. Performing MTC analysis is always recommended since it does not need high numerical
requirements, and as shown in the sensibility analysis section, errors in data can be misinterpreted as geological anisotropies.
J.L. Fernández Martínez is a visiting professor at UC Berkeley, Department of Civil and Environmental Eng., CA 94720-1710. 相似文献
11.
The objective of this study is to understand and predict the alteration of porous rock by crystallization of salts. Samples
of different rocks have been tested according to the EN 12370 standard test. Two parameters are proposed to evaluate the alteration
of a rock during these tests. The alteration index AI represents the cycle in which the first damages occur. The alteration
velocity AV represents the alteration rate at the end of the experiment, when the decay has become regular. These parameters
can be estimated with the help of a microstructural study of the rocks. The most relevant intrinsic parameters of the stones
for this estimation are capillary coefficient, evaporation coefficient, tensile strength and P-waves velocity. An evaluation
of the alteration pattern is also proposed depending on the eventual heterogeneities and anisotropies of the rocks. The influence
of the dimension and shape of the samples is also discussed. 相似文献
12.
韧性剪切带中片麻岩和超高压榴辉岩变形特征及其与地震波速各向异性的关系：来自中国大陆科学钻探（CCSD）680～1200米岩心的证据 总被引：12，自引：3，他引：12
中国大陆科学钻探(CCSD)680-1200米区段发育了多个韧性剪切带,带中主要岩石类型包括片麻岩和超高压榴辉岩。片麻岩中的变形石英、面理化榴辉岩中的拉长石榴石和绿辉石的应变轴比都表现为X>Y>Z,Flinn系数分别为0.11-0.27、0.22-0.23和0.23-0.24。随着糜棱岩化作用的增强,变形石英的C轴组构由Z轴极密逐渐向Y轴极密和叶理面上的大圆环带转变。在常温常压下测试了样品的波速,计算出片麻岩Vp和Vs的各向异性分别为30.17％-60.97％和11.52％-35.79％,榴辉岩Vp和Vs的各向异性分别为0.17％-11.19％和2.41％-6.70％。影响各向异性的主要因素有岩石的结构构造、矿物的晶格优选方位(LPO)、形态优选方位(SPO)和定向微裂隙。随着糜棱岩化作用的增强,岩石的P波各向异性逐 渐升高。变形岩石中的黑云母、石英、绿辉石的LPO和SPO是地震波各向异性的主要控制因素。饱水后的片麻岩样品的P波各向异性明显低于干燥片麻岩样品。在东海钻井中的强反射带主要是由于不同岩层之间的波阻抗差异而造成的,榴辉岩/强退变榴辉岩和黑云斜长片麻岩之间的接触界面会产生较强的地震深反射。此外,与LPO相关的地震波各向异性会增强地震波的反射,所以韧性剪切带中的糜棱岩化片麻岩可能是地震反射的良好载体。韧性剪切带中岩石弹性波速度的强各向 相似文献
13.
Carlos J. Archanjo Patrick Launeau Maria Helena B. M. Hollanda José Wilson P. Macedo Dunyi Liu 《International Journal of Earth Sciences》2009,98(8):1793-1807
Anisotropy of magnetic susceptibility (AMS) applied to an alkaline granite from Meruoca (NE Brazil) recorded weak anisotropies,
typically below 4%, and a considerable dispersion of the AMS axes. Red-clouded feldspars and clots of metasomatic minerals
enclosed in magmatic crystals indicate that hydrothermal fluids altered the granite. U–Pb isotopic data show high-common Pb
on zircons but allowed the calculation of a mean SHRIMP age of 523 ± 9 Ma attributed to the magmatic crystallization. Growth
of fine oxides by late fluid–rock interactions was responsible for the scattering of AMS. Rock magnetic data indicate they
consist mainly of an oxidized magnetite and (titano)hematite. Shape preferred orientation of mafic aggregates measured in
granite quarries shows that the pluton preserves a gently dipping magmatic foliation. AMS in some quarries with a well-defined
magmatic fabric, however, remains highly dispersed. When AMS mimics the mafic shape fabric, only magnetic foliations share
a common orientation. Locally, AMS grounded in coarse Ti-poor magnetite associated with titanite develops a consistent subhorizontal
oblate fabric that agrees with tectonic models suggesting that the cupola of the pluton has been exposed by erosion. 相似文献
14.
Vicksburg loess is characterized by preferred orientation of constituent grains, which on the average dip 4° toward the west (N 80–85°W). This investigation was undertaken to study, quantitatively, the relationship between natural fabric anisotropies of Vicksburg loess and the orientation of applied stress distribution Results of the study indicate the fabric anisotropies in Vicksburg loess are reflected by definite variation in triaxial shear strength of dry and moist specimens.
In two series of triaxial tests, ultimate strength of the loess is maximum where σ1 is perpendicular to grain orientation, and it is reduced where the principal stresses are 45° to the fabric plane. In this respect, Vicksburg loess may serve as a structural model for granular earth materials in illustrating the influence of fabric on ultimate strength. Moreover, the Mohr-Coulomb fracture line consists of two line segments, with an increase in slope at higher confining pressure. This characteristic suggests that poorly-cemented sands, or sandstones, and silts, or siltstones, may undergo two failures: one at small strains where cement bonds are disrupted and the other at larger strains where internal shearing resistance of granular components is exceeded. 相似文献
15.
Fatima Martin-Hernandez Sara Guerrero-Suárez 《International Journal of Earth Sciences》2012,101(3):637-647
Hematite is one of the most important carriers of remanent magnetization in natural samples. Its strong magnetocrystalline
anisotropy makes it difficult to determine one single value for the magnetic anisotropy constant and other magnetic properties.
In particular, the anisotropy of hematite within its basal plane is controversial because an assumed triaxial anisotropy compatible
with the crystallographic structure has not always been detected. This study presents a comparative analysis of rock magnetic
properties, compositional analysis and determination of the magnetic anisotropy constant. Different models with anisotropy
constant within the basal plane ranging from 0 to 13 (J m−3) are considered in the evaluation of the factors that control the presence or absence of a triaxial anisotropy. A linear
relationship between saturation magnetization (M
s
) and coercitivity (B
c
) in measurements at increasing temperatures is observed in samples where anisotropy is either uniaxial or biaxial while those
with significant triaxial anisotropies have a power–law relationship between M
s
and B
c
. 相似文献
16.
The evolution of the microstructure of an assembly of cohesionless granular materials with associated pores, which carry the overall applied stresses through frictional contacts is a complex phenomenon. The macroscopic flow of such materials take place by the virtue of the relative rolling and sliding of the grains on the micro‐scale. A new discrete element method for biaxial compression simulations of random assemblies of oval particles with mixed sizes is introduced. During the course of deformation, the new positions of the grains are determined by employing the static equilibrium equations. A key aspect of the method is that, it is formulated for ellipse cross‐sectional particles, hence desirable inherent anisotropies are possible. A robust algorithm for the determination of the contact points between neighbouring grains is given. Employing the present methodology, many aspects of the behaviour of two‐dimensional assemblies of oval cross‐sectional rods have been successfully addressed. The effects of initial void ratio, interparticle friction angle, aspect ratio, and bedding angle on the rolling and sliding contacts are examined. The distribution of normals to the rolling and sliding contacts have different patterns and are concentrated along directions, which are approximately perpendicular to one another. On the other hand, the distribution of all contact normals (combined rolling and sliding) are close to that of rolling contacts, which confirm that rolling is the dominant mechanism. This phenomenon becomes more pronounced for higher intergranular friction angle. Characteristics of the rolling and sliding contacts are also discussed in the context of the force angle, which is the inclination of contact force with respect to the contact normal. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
17.
Results from a travel-time analysis of three-component shear-wave (S-wave) data recorded in southwest Ireland during a controlled source seismic experiment have been used to investigate the magnitude of crustal anisotropy. The data used were recorded from 20 in-line shots on three-component short-period stations deployed at approximately 1-km spacing along two parallel profiles. Analysis of the travel-time differences between vertically and horizontally polarised S-waves recorded on vertical, radial and transverse seismometer components was undertaken using seismic phases travelling near the Earth's surface (Sg) and reflected from the Moho (SmS). Travel-time differences between the components for both phases scatter largely within the range ± 0.2 s, which is about the uncertainty in the measurements, with no observed coherent variation with shot-receiver offset. Synthetic S-wave seismograms were also computed from 1-D S-wave velocity models with varying degrees of anisotropy in the upper and in the lower crusts. Travel-time differences of Sg and SmS phases picked from these synthetic seismograms confirm that for anisotropies with probable symmetries of magnitude 1–2% in either the upper or lower crust should result in an observable variation of the travel-time differences between the transverse and radial, and transverse and vertical components with source–receiver offset. The study shows that crustal anisotropy does not contribute significantly to the marked anisotropy recently deduced from SKS and SKKS measurements in Ireland, which is therefore confirmed to reside at sub-crustal and deeper mantle levels. 相似文献
18.
M. A. G. M. Manthilake N. Miyajima F. Heidelbach V. Soustelle D. J. Frost 《Contributions to Mineralogy and Petrology》2013,165(3):495-505
The effect of alumina and water solubility on the development of fabric in orthopyroxene in response to simple shear deformation has been investigated at a pressure of 1.5 GPa and a temperature of 1,100 °C using the D-DIA apparatus. The microstructure observations at these conditions indicate that dislocation glide is the dominant deformation mechanism. In MgSiO3 enstatite and hydrous aluminous enstatite, partial dislocations bounding the stacking faults in [001] glide parallel to the (100) (or) the (100) [001] slip system. Electron backscattered diffraction analysis of anhydrous aluminous enstatite, however, indicates operation of the (010) [001] slip system, and microstructure analysis indicates dislocation movement involving [001] on both (100) and {210} planes. The strong covalent bonding induced by the occupation of M1 and T2 sites by Al could have restricted the glide on (100), activating slip on {210}. The resulting seismic anisotropies (~2 %) in orthopyroxene are weaker compared to olivine (~9.5 %), and reduced anisotropy can be expected if orthopyroxene coexists with olivine. Weak anisotropy observed in stable cratonic regions can be explained by the relatively high abundance of orthopyroxene in these rocks. 相似文献
19.
Klaus Ullemeyer Siegfried Siegesmund Patrick N.J. Rasolofosaon Jan H. Behrmann 《Tectonophysics》2006,414(1-4):97
A representative suite of deformed, metamorphic rocks from the TRANSALP reflection seismic traverse in the Eastern Alps was studied in the laboratory with respect to elastic properties and whole-rock texture. Compressional wave (P-wave) velocities and their anisotropies were measured at various experimental conditions (dry, wet, confining pressure), and compared to the texture-related component of anisotropy. Here ‘texture’ refers to crystallographic preferred orientations (CPOs), which were determined by neutron texture goniometry. In gneisses and schists P-wave anisotropies are mainly controlled by the microcrack fabric. In marbles and amphibolites CPO contributes very significantly to anisotropy. At 200 MPa confining pressure the degree of anisotropy is between 5% and 15%, depending on rock composition and/or CPO intensity. Special emphasis was also put on discussing possible effects of fluids on seismic velocity and anisotropy. Distributions of water-filled microcracks and pores are distinctly anisotropic, with maximum contribution to bulk rock velocity mostly parallel to the foliation pole. Decreasing P-wave velocity and increasing anisotropy of immersed samples may be explained by crack-induced changes of the elastic moduli of bulk rock. The main conclusion regarding interpretation of TRANSALP data is that strong reflections in the deep Alpine crust are probably due to marble–gneiss and metabasite–gneiss contacts, although P-wave anisotropy and boundaries between zones of ‘dry’ or ‘wet’ series may contribute to reflectivity to some extent. 相似文献
20.
Chris Roth 《Mathematical Geology》2000,32(3):277-300
Some years ago, earth scientists came to realize that knowing more about the geology of an orebody or an oil reservoir makes it easier to make the appropriate decisions concerning mine planning or reservoir exploitation. Geostatistical techniques for simulating lithofacies—that is, the geometry of the geology—were developed as a result of this. These methods should be able to produce geological images that respect not only the anisotropies of the different lithofacies but also their spatial layout relative to one another. While indicator variograms ensure that anisotropies are respected, another tool needs to be incorporated in the simulation technique to reflect the relative spatial layout of the different lithofacies. We propose to use the concept of edge effects that define the position of one lithofacies relative to another. Simple tests using direct and cross indicator variograms confirm the presence or absence of edge effects. We investigate if and how edge effect information can be incorporated in the different indicator simulation techniques—sequential indicator simulations, simulated annealing, the truncated Gaussian method and plurigaussian simulations. Results show that the choice of simulation method must be guided by the edge effect characteristics of the experimental lithologic data. 相似文献