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1.
The center-to-center method of strain analysis can be used to estimate flow lineation in high-silica ash-flow tuffs. It can be used as an alternative or supplement to other techniques for flow lineation identification, such as the examination of flow textures in thin sections and the measurement of anisotropy of magnetic susceptibility. The center-to-center method is a modification of a technique described by Fry (1979) and by Ramsay and Huber (1983) for determination of finite strain based on the spacing of particles within a deformed rock. In the present study, application of the method to an anticlustered array of phenocrysts in the flattening plane of an ash-flow tuff produces an ellipse with center to edge distances representative of the minimum distance between centers of phenocrysts in all directions within the flattening plane. The long axis of the ellipse corresponds to the maximum axis of finite strain; this direction is suggested to correspond to the flow lineation. The orientation of the stretching lineation was chosen both by eye and by least-squares analysis from center-to-center plots. The calculated orientation of the long ellipse axis can be varied by choice of a maximum distance between digitized objects which are included in the calculation. Comparison is made between late-stage flow lineations identified using the center-to-center method, and the AMS (anisotropy of magnetic susceptibility) method on samples of the high-silica Oligocene Bloodgood Canyon Tuff from the Mogollon-Datil volcanic field of southwestern New Mexico. Flow lineations based on center-to-center analyses of flattening plane-parallel rock slabs and thin sections agree well with AMS-derived flow lineations on most samples from which high-quality AMS lineations were obtained. Center-to-center analuses from flattening plane-perpendicular, lineation-parallel planes of ash-flow tuff produce ellipses inclined from 15° to 85° to the flattening plane, despite compaction of the ash, which should cause angles of inclination to be very low. The inclined ellipses may result from heterogeneities in grain size and distribution of phenocrysts in vertical sections of tuff, or from fragmentation of phenocrysts which occurred during the final stages of emplacement and compression. Center-to-center analyses on rock slabs rather than thin sections helps to avoid the effects of either textural heterogeneity and fragmentation of phenocrysts. With flow lineation identified by center-to-center analysis, petrographic examination of thin sections cut perpendicular to the flattening plane and parallel to the flow lineation allow for the identification of flow direction. 相似文献
2.
广东省两类不同成因类型花岗岩磁化率各向异性研究表明：Ⅰ型花岗岩的磁化率的数值较大，一致性较好；各向异性度比较小，磁化率椭球为纯压扁型．Ｓ型花岗岩的磁化率数值普遍较小，一致性较差；各向异性度比较大，磁化率椭球以兼具压扁型和拉长型为特征；岩体内部两种磁化率椭球（压扁型和拉长型）的３个主轴的方向具有较好的一致性．结合花岗岩的野外地质特征，作者认为，Ｓ型花岗岩在形成过程中受到了板块内部强烈的挤压和剪切作用，不同类型的源岩在强烈的挤压作用下局部地带发生破裂，发育了大规模的剪切推覆作用，在剪切热的作用下源岩物质被改造形成Ｓ型花岗岩；Ⅰ型花岗岩则是原始岩浆侵入到地壳上部形成，其形成的构造背景与当时板块的Ｂ型俯冲有关。 相似文献
3.
Summary Evidence for flow structure within hexagonal columns of basalt has been sought, using measurements of the anisotropy of magnetic susceptibility. The observed magnetic grain alignment indicates that the flow was essentially uniform and horizontal across the hexagons which were sampled. The flow probably occurred during emplacement of the lava; there is no evidence of convection within the hexagons, either from the degree of grain alignment or its orientation. 相似文献
4.
Brooks B. Ellwood 《Earth and Planetary Science Letters》1978,41(3):254-264
Low-field anisotropy of magnetic susceptibility (AMS) has been determined for a total of 248 basaltic specimens taken from cross sections between the cooling interfaces of 6 subaerial lavas, 6 deep-sea lavas, and 6 intrusives (5 dikes and 1 sill). Statistically significant AMS clusters are exhibited by all the dikes examined and, based upon these clusters, derivation of emplacement direction becomes possible. Two lavas are observed to have statistically significant AMS clusters which can be used for flow direction determinations. The methods of emplacement and flow direction analysis are discussed as well as the statistics used. It is concluded that most of the dikes examined have low angle emplacement directions. A classification scheme for AMS data distributions is presented.The AMS analysis shows that intrusives and deep-sea lavas can be distinguished from subaerial lavas approximately 80% of the time by the random AMS ellipsoid orientations exhibited in subaerial lavas. Contrasts in the fluid properties, degassing, wall effects with subsequent distortion of the fluid, and grain interaction during the extrusion of subaerial lavas can be expected to distort magnetic grain alignment. Further effects such as convection and secondary processes contribute to yield the random distributions observed for most of these bodies. 相似文献
5.
We hypothesize that anisotropy in soil properties arises from pore-scale heterogeneity caused by the alignment of aspherical soil particles. We developed a method to predict the permeability tensor from particle shape and packing structure. Digital geometry maps were created for the pore space in regular cubic and random packs of particles with various aspect ratios using a numerical packing algorithm. The lattice-Boltzmann method was used to simulate saturated flow through these packs, and the effect of particle shape and degree of alignment on the permeability tensor was characterized. Results show that the degree of anisotropy in permeability depends not only upon particle shape and alignment, but also on the three-dimensional structure of the pack. In random packs, more oblate particles and higher degrees of particle alignment lead to reduced permeability perpendicular to the direction of particle alignment compared to the direction parallel to particle alignment. 相似文献
6.
根据含多畴磁铁矿岩石的热剩磁(TRM)及其磁化率各向异性(AMS)张量间的特征值的比值关系,推导出一种因AMS 而引起的TRM 方向偏离外磁场方向的具体校正公式,并将其成功地应用到中国月山闪长岩体及法国Flamanville 花岗岩体的古地磁研究中. 相似文献
7.
通过对风洞及室内模拟定向风条件下的风成沉积物磁化率各向异性与风向之间关系的研究,发现风成沉积物磁化率各向异性的长轴方向的偏角不仅与风向有关,而且与粉尘沉积量有关。风成沉积物磁化率各向异性的长轴方向的偏角在α95（α95≤20°）的区间范围内与粉尘沉积量大的风向一致．在干旱、半干旱区,现代风成沉积物（沙丘、黄土的表层土）的磁化率各向异性的长轴方向的偏角在此范围区间内与常年盛行风向一致,尤其与沉积物沉降时的盛行风向一致． 相似文献
8.
冰雷达技术已成为目前探测南极冰盖内部结构的主要技术手段.近年发展起来的多极化雷达技术可根据不同方向雷达反射功率的变化推断冰盖内部冰晶组构特征和变化规律,进而推断冰盖内部应力应变历史,这对于理解冰流机制和动力过程以及解释冰盖过去、现在和未来的变化规律具有非常重要的作用.本文从Maxwell方程出发,推导出适于介电常数各向异性的三维电磁波时域有限差分方程,进而建立模型模拟各向异性介质的响应输出及其时空分布特征.模拟结果表明： (1) 电磁波在各向异性介质中传播时,波前面为椭圆形,长轴位于介电常数小的主轴方向; (2) 横向各向异性介质反射波振幅在水平面内具有180°的变化周期.通过对现场常用三种天线装置类型模拟对比分析发现,不同天线类型各向异性层底界面反射波存在“时差”现象,并且时差大小和正负与上下层介电常数差异以及同层各向异性差异有关.在模拟计算的基础上,作者讨论了由于介电常数各向异性导致的“时间延迟”和水平面内振幅“周期性变化”的原因.模拟结果和结论对于南极冰盖冰雷达数据处理和解释工作具有指导意义. 相似文献
9.
The anisotropy of magnetic susceptibility (AMS) and mean particle size in the silt fraction have been measured in surface sediments and down selected cores from the Vema Channel. Results show that a function (Fs) which represents magnetic grain long axis alignment is highly correlated with variation in mean size of the silt fraction. An increase in alignment is caused by an increase in bottom water velocity which corresponds to an increase in mean particle size. Best alignments and coarsest particle sizes are found in the axis of the Vema Channel. In contrast, poor particle alignment and fine silt mean characteristics the shallower region east of the channel axis, consistent with lower current velocities.Core CH 115-61 was collected near the Level of Least Motion at the transition between northward-flowing Antarctic Bottom Water (AABW) and southward-flowing North Atlantic Deep Water. A positive relationship exists between silt size and AMS in the core. Fluctuations in particle alignment and mean silt size indicate three or four periods of increased velocity during the past 150,000 years.Geographic orientation of the maximum AMS axes in cores CH 115-60 and -61 was accomplished using the declination of remanent magnetism and an axial dipole as the reference azimuth. The data show that the larger magnetic grains in these sediments are aligned normal to the bathymetric trend of the channel which suggests that these grains are aligned by traction transport. The result indicates that AABW in the Vema Channel has flowed along an azimuth of approximately N30°E for the past 150,000 years. 相似文献
10.
岩浆岩的磁组构与岩浆流动、侵位时应力和冷凝后遭受后期构造改造作用有关.本文分析了北京房山岩体东山口-凤凰亭剖面岩石磁化率各向异性(AMS)变化.热磁曲线和磁滞回线分析显示,岩石的主要载磁矿物为多畴磁铁矿颗粒.AMS磁化率椭球体呈压扁状,磁面理发育且产状陡倾,磁化率各向异性度P值平均值高达1.189.自边缘相(东山口)到中心相(凤凰亭),岩石磁化率椭球的扁率E值和形状因子T值逐渐减小,说明磁组构主要反映岩浆流动和岩体侵位时近NNE-SSW方向的挤压作用,在侵位冷凝后并没有遭受到明显的构造改造. 相似文献
11.
The observation of a magnetic susceptibility ellipsoid whose maximum axis corresponds to the minimum axis of petrofabric (pole of bedding or schistosity) is referred to as an inverse magnetic fabric. The investigation of the magnetic properties of some ferroan carbonate monocrystals and paramagnetic limestones demonstrates thatc-axis preferred orientation of paramagnetic carbonates results in a maximum susceptibility parallel to the flattening direction.Inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones. A mineralogical model based on the property of single-domain grains to have a zero susceptibility parallel to their long axis is proposed. However, more complex cases are also encountered. 相似文献
12.
Mourad Essalhi Stanislas Sizaret Luc Barbanson Yan Chen Yannick Branquet Dominique Panis Pierre Camps Pierre Rochette Angels Canals 《Earth and Planetary Science Letters》2009,277(3-4):501-513
The present study aims to apply the AMS method (Anisotropy of Magnetic Susceptibility) at a regional scale to track the fluid circulation direction that has produced an iron metasomatism within pre-existing dolomite host rock. The Urgonian formations hosting the Zn–Pb mineralizations in La Florida (Cantabria, northern Spain) have been taken as target for this purpose. Sampling was carried out, in addition to ferroan dolomite host rock enclosing the Zn–Pb mineralizations, in dolomite host rock and limestone to make the comparison possible between magnetic signals from mineralized rocks, where fluid circulation occurred, and their surrounding formations. AMS study was coupled with petrofabric analysis carried out by texture goniometry, Scanning Electron Microscopy (SEM) observations and also Shape Preferred Orientation (SPO) statistics. SEM observations of ferroan dolomite host rock illustrate both bright and dark grey ribbons corresponding respectively to Fe enriched and pure dolomites. SPO statistics applied on four images from ferroan dolomite host rock give a well-defined orientation of ribbons related to the intermediate axis of magnetic susceptibility K2. For AMS data, two magnetic fabrics are observed. The first one is observed in ferroan dolomite host rock and characterized by a prolate ellipsoid of magnetic susceptibility with a vertical magnetic lineation. The magnetic susceptibility carrier is Fe-rich dolomite. These features are probably acquired during metasomatic fluid circulations. In Fe-rich dolomite host rock, ?c? axes are vertical. As a rule, (0001) planes (i.e. planes perpendicular to ?c? axes) are isotropic with respect to crystallographic properties. So, the magnetic anisotropy measured in this plane should reflect crystallographic modification due to fluid circulation. This is confirmed by the texture observed using the SEM. Consequently, AMS results show a dominant NE–SW elongation interpreted as the global circulation direction and a NW–SE secondary elongation that we have considered as sinuosities of the fluid trajectory. The second type of magnetic fabric is essentially observed in the limestone and characterized by an oblate form of the ellipsoid of magnetic susceptibility, a horizontal magnetic foliation and mixed magnetic susceptibility carriers. It is interpreted as a sedimentary fabric. 相似文献
13.
Azimuthally averaged power spectra are widely used in the Curie point depth (CPD) estimation with the implicit assumption that the magnetization distribution is random and uncorrelated. However, the marine magnetic anomalies are caused by bands of normal and reverse magnetization and show obvious trends. To investigate the effects of the anisotropy of marine magnetic anomalies on the CPD estimates, we develop 3D fractal striped magnetization models to produce lineated marine magnetic anomalies for the first time. We analyze the spectra anisotropy of the lineated magnetic anomalies of the synthetic fractal striped magnetization models and investigate its effects on the CPD estimates. The synthetic models and actual data show that the spectra of the lineated marine magnetic anomalies are directionally anisotropic. The amplitude response is strong and the slope of the logarithmic spectrum is large in a direction perpendicular to the stripes of magnetic anomalies, whereas the amplitude response is weak and the slope of the logarithmic spectrum is small in a direction parallel to the stripes of magnetic anomalies. The depth estimates in the perpendicular direction are close to the actual values, whereas the depths estimates in the parallel direction are significantly lower than the actual values. The actual marine magnetic anomalies of the South China Sea exhibit an anisotropic power spectrum that is consistent with the spectral anisotropy of magnetic anomalies of the synthetic fractal striped magnetization models. 相似文献
14.
Summary The magnetic anisotropy of foliated rocks of several types has been measured by the torque-meter method, and shows that the alignment of long axes of magnetic grains in rocks normally follows the pattern of foliation evident in field observations. In a sharp fold in a lit-par-lit formation the magnetic anisotropy indicated an otherwise undetected lineation independent of the bedding and superimposed upon the foliation determined by the layering. In two adamellites, each with two alignment patterns separated by an angle of 30° the magnetic data are shown to be consistent with two foliations but not with one foliation plus a lineation. Magnetic anisotropy data can be ambiguous for rocks in which two or more grain alignment processes have operated, but combined with other observations magnetic measurements can provide a valuable new tool in the study of rock fabrics. 相似文献
15.
Magma flow revealed by magnetic fabric in the Okavango giant dyke swarm, Karoo igneous province, northern Botswana 总被引：1，自引：0，他引：1
C. Aubourg G. Tshoso B. Le Gall H. Bertrand J.-J. Tiercelin A.B. Kampunzu J. Dyment M. Modisi 《Journal of Volcanology and Geothermal Research》2008,170(3-4):247-261
To determine the magma flow direction of the giant, 179 Ma Okavango dyke swarm of northern Botswana, we measured the anisotropy of magnetic susceptibility (AMS) of 23 dykes. Dykes are located in two sections (Shashe and Thune Rivers), which are about 300 km and 400 km from the presumed magma source respectively; the Nuanetsi triple point. We collected samples from the margins of the dykes in order to use the imbrication of magnetic foliation to determine magma flow direction. About half of the magnetic fabric in the dykes is inverse, i.e. with the magnetic foliation perpendicular to the dyke plane. Lateral flow to the west and vertical flow is in evidence in the Shashe section. However, the overall analysis of normal and inverse magnetic fabric data supports that lateral flow to the west was dominant in the Shashe section. Across the Thune section, a poorly defined imbricated magnetic foliation also suggests lateral flow to the west. 相似文献
16.
Factors influencing the calculation of periodic secondary circulation in a tidal river: numerical modelling of the lower Sacramento River,USA 
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下载免费PDF全文 Secondary circulation is the component of three‐dimensional (3D) flow in river channels perpendicular to the primary flow direction. Secondary circulation calculated from acoustic Doppler current profiler (ADCP) transects is sensitive to the calculation method and is affected by the transect angle relative to the mean flow direction and variations in the flow direction along a transect. To quantify bounds on transect alignment relative to river flow for field data collection and examine tidal time‐scale variability in secondary circulation, the 3D hydrodynamic model UnTRIM was applied to simulate the hydrodynamics in the lower reach of the Sacramento River (CA, USA). Secondary circulation was calculated using the Rozovskii and the zero net discharge methods on repeated transects extracted from the model results in regions of both relatively uniform and complex flows. When the depth‐averaged flow direction along a transect varied by more than about 5 °, occurring when the transect was as little as 10 to 20 ° out of normal to the mean flow direction, the Rozovskii method produced more realistic secondary circulation than the zero net discharge method. Analysis indicated that ADCP transects should be within 20 ° of perpendicular to the mean flow direction when calculating secondary circulation. Secondary circulation strength around two tidally influenced bends generally increased with increasing flow and broke down near slack water. However, the strength of the secondary circulation was not only a function of the flow magnitude, but also depended on the direction of the water flow and the transect location relative to the river curvature, which varied with the tidal flow direction. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
17.
Anisotropy of the apparent resistivity variation rate near the epicentral region for strong earthquakes 总被引：1，自引：0，他引：1
IntroductionThe geo-electrical anisotropy before earthquake is of special significance for the prediction of earthquake location and for the research on crustal stress status near the epicentral region. The anisotropy has been studied from various aspects (CHEN, et al, 1983; QIAN, et al, 1985, 1996; LU, et al, 1990a; MAO, et al, 1995; RUAN, et al, 1999). But the anisotropic phenomena presented by geo-electrical data are not practically satisfactory and not clear as compared with the res… 相似文献
18.
Magnetic properties of two apparently unrelated lava suites (one tholeiitic and the other alkalic) coexisting in the central parts of the Peninsula of Baja California, Mexico, were measured in this study. Macroscopic indicators and measurements of anisotropy of magnetic susceptibility (AMS) were combined to infer the flow direction on those lavas. These measurements were used to investigate the likelihood of the existence of an abnormally long tholeiitic lava flow in this part of the Peninsula. The obtained results indicate that the tholeiitic flows in the region constitute regular length flows, and are likely to have been issued through vents located within the area of study. Additionally, comparison of the magnetic signature of tholeiitic and alkalic lavas reveals a systematic difference in some of the magnetic parameters (bulk susceptibility and Curie temperatures). In particular, it is proposed that the narrow range of Curie temperatures characteristic of each lava type can be used as a reliable proxy for the identification of each lava type in the region. All of these findings show that the alkalic and tholeiitic lavas of the region are more closely related to each other than previously suspected.Editorial responsibility: R. Cioni 相似文献
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20.
本文分析了2001年2月和3月期间Cluster Ⅱ穿越磁层顶前后的观测资料,检测到13个通量传输事件（FTEs）.用多颗卫星磁场测量资料的最小方差分析（MVAB）方法确定FTE的管轴方向（其中6个方向较可靠）.FTE管轴方向的分布和低纬处不同,在磁顶法线坐标系LMN中对M轴有较大偏离,比较靠近L轴.deHoffmann Teller(HT)分析指出,13个FTEs都存在一个很好的HT参考系,表明它们以一个准稳的MHD结构运动.对垂直于管轴方向的运动分析表明FTEs并不一定和背景等离子体一起对流,它们可快于或慢于背景流,但FTEs的运动和背景流基本沿相同方向,其间可有一不大的夹角.在HT坐标系中,10个FTEs的等离子体速度接近零, 其他3个FTEs的等离子体速度约为局地Alfven波速的14%,都不符合Walen关系.其中北半球事件的Walen曲线为正斜率,南半球事件为负斜率,这说明等离子体沿磁力线（北半球顺着磁场,南半球逆着磁场）流向磁层. 相似文献