Anisotropy of magnetic susceptibility of heated rocks

Heating produces changes, which does not always correspond to simple enhancement of the magnetic fabric. Two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating. The first diagonalizes the tensor from the difference between each tensor term before and after heating. The second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment. When the same magnetic fabric is obtained from several thermal steps, it cannot be related to randomly oriented ferrimagnetic minerals. Instead, the newly formed fabric must be related to characteristics of the pre-existing rock. By comparing this ferrimagnetic minerals fabric with the initial whole rock fabric, we can distinguish cases where heating simply enhances pre-existing fabric from those where thermal treatment induces a different fabric. Relative to the pre-heating fabric, this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction, related to petrostructural elements other than those defining the initial fabric.     

初始组构各向异性对砂土力学特性及临界状态的影响

采用离散元方法,利用半径扩展法和重力沉积法分别生成具有初始各向同性和各向异性内结构的试样,并开展三轴不排水压缩和拉伸试验,研究不同制样方法产生的初始各向异性对砂土宏微观力学特性及其临界状态的影响。运用组构张量对砂土的各向异性进行量化,分析不同初始组构各向异性对组构张量演化的影响并确定了组构张量的临界值。试验结果表明：初始组构各向异性对试样的剪胀性有重要影响,由于受重力影响形成初始各向异性,其各向异性程度越大、组构方向与加载方向越一致,剪胀性越显著;初始组构各向异性对试样的临界状态没有影响,砂土的组构张量具有唯一的临界状态值。     

西昆仑山前晚新生代沉积岩磁组构及构造意义

西昆仑山前晚新生代沉积岩磁组构特征表明，沉积岩原生磁组构受后期构造活动改变。磁组构测试结果表明晚新生代沉积岩生较明显变形，岩石磁化率椭球体指示磁面理较发育，反映岩石受压扁型变形为主。磁化率椭球体最小轴方向为NW，指示该区最大主压应力为NW，与区域构造分析结果相一致。     

Pore fabrics of ceramic models investigated by magnetic anisotropy

Ceramics was used as model to investigate the pore fabric originated during ductile deformation. The magnetic anisotropy was used to measure the connected pore fabric after pore saturating by the ferromagnetic fluid. The ductile deformation of the ceramics was investigated through measurement of magnetic anisotropy of dry specimens. The character and symmetry of the pore fabric are compatible with those of the deformations forming the fabric of the ceramics.     

Fabric evolution during hydromechanical loading of a compacted silt

A study was undertaken on a compacted silt to determine fabric modifications induced by suction and/or stress variations. The link between fabric and hydromechanical behaviour was also investigated. A suction-controlled oedometer, using air overpressure, was developed for this purpose and mercury intrusion porosimetry was employed to determine sample fabric. The initial samples fabric was made of macro and micropores. It was shown that suction increase produced a strong decrease in the macroporosity associated with an increase in microporosity. However, some macropores were not significantly affected by the suction increase; this phenomenon might be related to the initial fabric of the samples. Second, it appears that loading under saturated conditions also produces strong fabric modification: the higher the applied stress, the lower the macroporosity. Soil fabric depends on the maximum stress experienced by the soil. Finally, some tests have shown the influence of suction, as well as the role of the degree of saturation, on the deformation process and the mechanical behaviour. The test results show that in the case of unsaturated mechanical loading, all macropores are not destroyed by the mechanical loading. Copyright © 2004 John Wiley & Sons, Ltd.     

Study of ductile shear zones (Galicia, Spain) using texture goniometry and magnetic fabric methods

A joint magnetic and mica fabric study has been carried out on rocks from recognised shear zones in Galicia (Spain). The two main aims of the study are to determine strain gradients in a foliated material without any significant strain markers and to obtain a quantitative relationship between the magnetic susceptibility anisotropy ellipsoid and the mica fabric axes. The intensity parameters of the fabric on profiles intersecting the studied areas indicate the dominant influence of two major shear zones on the fabric development. The regional fabric shows less significant variations across minor (less developed) shear zones.The empirical relationship between magnetic fabric and strain reported by Rathore (1979, 1980) and Rathore and Henry (1982) is found to hold in the magnetic and mica fabrics comparison. The relationship found in this study is:     

下三叠统鲕粒灰岩中微生物矿化组构特征及意义 ——以川东北地区飞仙关组为例

PTB全球最大生物灭绝事件后,早三叠世被认为是微生物发育繁殖的黄金时期,浅水环境是微生物活动及微生物岩形成的最有利环境。借助高分辨率扫描电镜和能谱分析,在川东北地区开江—梁平海槽西侧下三叠统飞仙关组浅水碳酸盐岩台地边缘的鲕粒灰岩中发现了三种微生物矿化组构:空腔状微生物矿化组构、显微瘤状微生物矿化组构和纤状微生物矿化组构;通过沉积学、岩石学及地球化学的综合分析,认为显微瘤状微生物矿化组构是微生物新陈代谢时形成的纳米球状云质颗粒,纤状微生物矿化组构是微生物的胞外聚合物,而空腔状微生物矿化组构则属于微生物个体。这些微生物矿化组构的发现,为早三叠世微生物大爆发提供了一定的直观证据;推测大量的微生物活动是早三叠世碳酸盐快速沉积现象的直接诱因,并导致于二叠纪末发育的克拉通内裂陷——开江—梁平海槽在早三叠世被快速填平。     

岩土材料的各向异性强度准则

岩土材料的强度往往表现出很强的各向异性,而已有的各向同性强度准则不能够描述这一特性。提出一个岩土材料的各向异性强度准则。为了描述材料的各向异性,引入了一个由应力张量和组构张量的联合不变量表达的各向异性参数。该参数可以描述加载方向和材料组构方向的夹角。强度准则是基于材料在子午面和偏平面上的破坏特性而建立的,这为描述广义的材料强度各向异性提供了方便。与原各向同性强度准则相比,各向异性强度准则只引入了两个新的模型参数,而且所有的模型参数都可以通过常规的室内试验结果确定。该准则的预测结果与砂土、黏土、天然黏土和岩石的试验结果比较表明,它能够很好地描述岩土材料强度的各向异性     

Constitutive modelling of granular materials using a contact normal-based fabric tensor

This paper presents a fabric tensor-based bounding surface model accounting for anisotropic behaviour (e.g. the dependency of peak strength on loading direction and non-coaxial deformation) of granular materials. This model is developed based on a well-calibrated isotropic bounding surface model. The yield surface is modified by incorporating the back stress which is proportional to a contact normal-based fabric tensor for characterising fabric anisotropy. The evolution law of the fabric tensor, which is dependent on both rates of the stress ratio and the plastic strain, rules that the material fabric tends to align with the loading direction and evolves towards a unique critical state fabric tensor under monotonic shearing. The incorporation of the evolution law leads to a rotational hardening of the yield surface. The anisotropic critical state is assumed to be independent of the initial values of void ratio and fabric tensor. The critical state fabric tensor has the same intermediate stress ratio (i.e. b value) and principal directions as the critical state stress tensor. A non-associated flow rule in the deviatoric plane is adopted, which is able to predict the non-coaxial flow naturally. The stress–strain relation and fabric evolution of model predictions show a satisfactory agreement with DEM simulation results under monotonic shearing with different loading directions. The model is also validated by comparing with laboratory test results of Leighton Buzzard sand and Toyoura sand under various loading paths. The comparison results demonstrate encouraging applicability of the model for predicting the anisotropic behaviour of granular materials.

     

碳酸盐岩层序地层格架内组构选择性孔隙分布特征——以塔里木盆地塔中地区上奥陶统良里塔格组台地边缘相为例*

组构选择性孔隙是塔中地区良里塔格组台地边缘相油气储集层的重要孔隙类型,目前该类型的孔隙在层序地层格架内的分布特征仍不清晰。作者在前人建立的层序地层格架的基础上,通过大量的岩心、薄片、测井及地震资料的分析,在塔中地区良里塔格组台地边缘相识别出7类主要组构选择性孔隙: 粒内溶孔、粒间溶孔、铸模孔、粒间孔、窗格孔、生物体腔孔及格架孔。高频层序中,组构选择性孔隙在90%的礁(丘)滩复合体中上部的大气淡水渗流带—潜流带中成层集中发育,厚度10~40 m,且各类型孔隙纵向上分布特征不同。受研究区湿润的古气候、相对海平面的频繁波动、台地边缘礁滩相较大的沉积速率及较好的初始渗透性等因素控制,组构选择性孔隙的发育具有层位性、旋回性及区块差异性等分布规律。即组构选择性孔隙层主要发育在高位体系域内沉积的良三段—良一段中;纵向上发育5期,主要分布在进积型准层序组6、8及9中,其次是加积型准层序组5、7、10;自东向西,各井区孔隙层数量、厚度及物性均逐步降低,主要孔隙类型由粒内溶孔、铸模孔、粒间溶孔及体腔孔逐渐变为以窗格孔、粒内溶孔及粒间溶孔。     

Quartz microfabric of the Laxfordian Canisp Shear Zone,NW Scotland

The Canisp Shear Zone transects layered Lewisian gneisses near Lochinver, NW Scotland. It is a vertical ductile shear zone with a dextral shear sense, formed during Laxfordian amphibolite facies metamorphism, transposing the layering to new foliation and linear structures. Minerals in the layered gneisses show little or no shape fabric, while a strong shape fabric defines the foliation. For quartz, this shape fabric is accompanied by development of a preferred crystal orientation with fabric patterns reflecting the geometry of the shear deformation. The quartz fabric shows a pole-free area around the lineation with the c-axes concentrated in an asymmetric cross-girdle or a point maximum perpendicular to the shear plane, and a monoclinic symmetry consistent with the shear sense.     

Emplacement kinematics of nepheline syenites from the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt,west of Khariar,NW Orissa: Evidence from meso- and microstructures

Nepheline syenite plutons emplaced within the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt west of Khariar in northwestern Orissa are marked by a well-developed magmatic fabric including magmatic foliation, mineral lineations, folds and S-C fabrics. The minerals in the plutons, namely microcline, orthoclase, albite, nepheline, hornblende, biotite and aegirine show, by and large, well-developed crystal faces and lack undulose extinction and dynamic recrystallization, suggesting a magmatic origin. The magmatic fabric of the plutons is concordant with a solid-state strain fabric of the surrounding mylonites that developed due to noncoaxial strain along the Terrane Boundary Shear Zone during thrusting of the Eastern Ghats Mobile Belt over the Bastar Craton. However, a small fraction of the minerals, more commonly from the periphery of the plutons, is overprinted by a solid state strain fabric similar to that of the host rock. This fabric is manifested by discrete shear fractures, along which the feldspars are deformed into ribbons, have undergone dynamic recrystallization and show undulose extinction and myrmekitic growth. The shear fractures and the magmatic foliations are mutually parallel to the C-fabric of the host mylonites. Coexistence of concordant solid state strain fabric and magmatic fabric has been interpreted as a transitional feature from magmatic state to subsolidus deformation of the plutons, while the nepheline syenite magma was solidifying from a crystal-melt mush state under a noncoaxial strain. This suggests the emplacement of the plutons synkinematic to thrusting along the Terrane Boundary Shear Zone. The isotopic data by earlier workers suggest emplacement of nepheline syenite at 1500 +3/−4Ma, lending support for thrusting of the mobile belt over the craton around that time.     

Superposition of plane strain on an initial sedimentary fabric: an example from Laksefjord, North Norway

Shape fabric determinations in deformed grits, microconglomerates and conglomerates in the Landersfjord region of the Laksefjord Nappe, Finnmark, North Norway, have demonstrated the existence of prolate fabric ellipsoids. The correlation between shape fabric symmetry and the orientation of the principal fabric axes suggests that the fabrics are the result of a tectonic plane strain superimposed roughly coaxially on an initially oblate sedimentary fabric. A plot of the fabric ellipsoids on a three axis diagram allows the effect of the initial fabric to be removed, and the amount of strain to be determined. A contour map of the strain shows it to increase to the north and west. Models are proposed for the origin of the strain.     

鄂尔多斯盆地马家沟组准层状岩溶角砾岩特征及其成因*

鄂尔多斯盆地奥陶系马家沟组广泛发育岩溶角砾岩,结构特征复杂。近期研究发现,这些角砾岩的组构和分布特征难以能单纯地用经典的风化壳岩溶模式解释,各类角砾岩的成因机理值得进一步探究。文中基于大量岩心及薄片观察,对苏里格气田东区奥陶系马家沟组上组合不同层位角砾岩进行归纳和判识,明确其空间展布和序列结构特征,分析形成机理。研究发现: (1)根据成因与发育环境,苏格里气田东区奥陶系马家沟组上组合角砾岩可划分为5类,分别是近原地解离角砾岩(B1)、近地表堆积角砾岩(B2)、洞穴堆积角砾岩(B3)、洞顶(壁)碎裂角砾岩(B4)和坍塌角砾岩(B5),它们具有纵向多层叠置、横向可对比的“准层状”特征; (2)不同类型角砾岩的纵向配置和演化与周期性的向上变浅沉积序列和暴露溶蚀有关,共归纳为3种类型,即B1主发育型、B2-B3-B4组合发育型、B5主发育型; (3)准层状角砾岩形成于早成岩期,受控于古地貌起伏背景下的高频海平面变化,由暴露溶蚀透镜体横向迁移连片和纵向多旋回叠置而成。研究结果不仅有助于了解鄂尔多斯盆地马家沟组多样化的岩溶角砾岩特征与成因,也为白云岩早成岩期岩溶研究提供了新的素材。     

内蒙古黄旗海全新世湖泊沉积物粒度分析及其沉积学意义

对内蒙古黄旗海湖心、湖滨2个现代沉积剖面在前期磁组构研究的基础上,进行了沉积物粒度分析。结果显示,2个剖面从下到上都表现出粗颗粒含量逐渐增加的趋势,反映水动力逐渐增强。对比之前的磁组构结果发现,靠近湖心的HQW剖面,粒度与磁组构具有较好的对比性。HQW剖面的磁各向异性度等磁组构参数从下到上表现出逐渐增大的趋势,显示矿物定向和水流流速逐渐增强。另外,在20cm、40cm、65cm深度附近,粒度与磁组构参数都发生突然变化,表明存在水动力增大的异常沉积事件。HQE剖面的磁组构未能有效指示异常沉积事件,但其粒度分析中的细砂百分比含量和中值粒径在这些位置附近明显减小。若这是由同一异常沉积事件造成的,则表明异常沉积事件的沉积过程比较复杂,在研究异常沉积事件时不同的沉积部位需使用不同的研究方法。     

Advective heat transfer and fabric development in a shallow crustal intrusive granite — the case of Proterozoic Vellaturu granite,south India

Syntectonic plutons emplaced in shallow crust often contain intermediate-to low-temperature deformation microstructures but lack a high-temperature, subsolidus deformation fabric, although the relict magmatic fabric is preserved. The Proterozoic Vellaturu granite emplaced at the eastern margin of the northern Nallamalai fold belt, south India during the late phase of regional deformation has a common occurrence of intermediate-to low-temperature deformation fabric, superimposed over magmatic fabric with an internally complex pattern. But high-T subsolidus deformation microstructure and fabric are absent in this pluton. The main crystal plastic deformation and fluid enhanced reaction softening was concentrated along the margin of the granite body. Resulting granite mylonites show Y-maximum c-axis fabric in completely recrystallized quartz ribbonds, dynamic recrystallization of perthites, and myrmekite indicative of fabric development under intermediate temperature (∼ 500–400°C). The weakly-deformed interior shows myrmekite, feldspar microfracturing and limited bulging recrystallization of quartz. The abundance of prism subgrain boundaries is indicative of continuing deformation through low-temperature (∼ 300°C). The relative rates of cooling influenced by advective heat transfer and deformation of the pluton seem to control the overall subsolidus fabric development. The rapid advective heat transfer from the interior in the early stages of subsolidus cooling was followed by slow cooling through intermediate temperature window as a well-developed phyllosilicate rich mylonitic skin around the granite body slowed down conductive heat loss. Low-T crystal plastic deformation of quartz was effected at a late stage of cooling and deformation of the shallow crustal granite body emplaced within the greenschist facies Nallamlai rocks.     

塔里木盆地苏盖特布拉克剖面寒武系第二统第三阶微生物岩结构类型*

塔里木盆地阿克苏地区寒武系第二统第三阶肖尔布拉克组主要由微生物白云岩组成,代表性的剖面在阿克苏市西南90 km处的苏盖特布拉克。由于成岩作用改造严重,对这套微生物碳酸盐岩的特征和形成环境的认识还存在很多分歧,制约了勘探工作。通过详细野外勘察和室内研究,将肖尔布拉克组微生物岩分为4种结构类型: 凝块结构、纹层结构、砂屑结构和骨架结构。首次对凝块结构和纹层结构进行了亚类型的划分,其中凝块结构可划分为蠕虫状、网状和斑点状3种亚类型,纹层结构可划分为致密纹层、短薄纹层、颗粒纹层以及单纹层、纹层组、复合纹层。肖Ⅰ段—肖Ⅲ段发育凝块石白云岩、凝块—层纹石白云岩和层纹石白云岩,肖Ⅳ段发育凝块石微生物丘,肖V段下部发育网状结构凝块石白云岩,肖V段上部发育砂屑白云岩和肾形菌骨架岩。白云石化作用、溶蚀作用、重结晶作用是改变肖尔布拉克组微生物岩结构最重要的成岩作用,且该组下部比上部遭受了更强的成岩作用改造;微生物岩结构对成岩作用改造的抵抗能力为: 砂屑结构>纹层结构>凝块结构。根据微生物岩结构,推测肖Ⅰ段—肖Ⅲ段形成于潮坪环境,肖Ⅳ段和肖V段下部形成于深水潮下环境,肖V段上部形成于浅水潮下环境。以上成果为认识塔里木盆地肖尔布拉克组微生物岩的平面分布规律和今后开展被成岩作用强烈改造的白云岩型微生物岩的研究提供了一个重要参考。     

Pompeian red ware: Processing archaeological ceramic data

A program of ceramic analysis, by which archaeologically excavated pottery can be efficiently and economically grouped by fabric and geographic origin, has been devised. the fabric is segregated by xeroradiography, macroscopic observation, and heavy-mineral analysis before it is identified petrologically. the program has enabled determination of the effects of certain aspects of trade patterns and economics on life in the ancient city of Roman-Period Caesarea Maritima. This program was tested on an 84-sherd corpus of Pompeian Red Ware. Xeroradiography, heavy-mineral analysis, and petrology each segregated the sherds into two fabrics, one of which contained 83 sherds. This latter fabric had not been reported in earlier studies of Pompeian Red Ware which concentrated on samples from England and Europe. However, the similarities between this new fabric and another known fabric suggest that the Caecuban and Falernian regions west of Terracina and east of Capua should be the source of this ware. an astonishing result was that other known fabrics were not found at Caesarea Maritima. A potential chronological hypothesis was formulated to account for this observation.     

Magnetic fabrics and rock magnetism of Proterozoic dike swarm from the southern São Francisco Craton, Minas Gerais State, Brazil

Magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a Proterozoic dike swarm from the southern São Francisco Craton (SFC; Minas Gerais State, SE Brazil). Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of remanent magnetization (ARM). The latter was performed imposing both anhysteretic (total (AAR) and partial pAAR)) and isothermal remanence magnetizations (AIRM). Partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site. In most sites, AMS is dominantly carried by ferromagnetic minerals, however, in some sites, the paramagnetic contribution exceeds 70% of bulk susceptibility. Rock magnetism and thin section analysis allow classifying the dikes as non-hydrothermalized and hydrothermalized. Magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5×10⁻³ (SI). Ti-poor titanomagnetites up to pure magnetite pseudo-single-domain (PSD) grain sizes carry the majority of magnetic fabrics for non-hydrothermalized dikes whereas coarse to fine grained Ti-poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes.Three primary AMS fabrics are recognized which are coaxial with ARM fabric, except for two dikes, from both non-hydrothermalized and hydrothermalized dikes. Normal AMS fabric surprisingly is not dominant (31%). The parallelism between AMS, pAAR_0–30, pAAR_30–60 and pAAR_60–90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late-stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse-grained early crystallized Ti-poor titanomagnetites. This fabric is interpreted as magma flow in which the analysis of K_max inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes (K_max<30°). Intermediate AMS fabric is the most important (41%) in the investigated swarm. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. ARM determinations for these sites also remained intermediate except for two dikes. In one of them, AIRM fabric resulted in normal AMS fabric while for the other AAR fabric resulted in inverse AMS fabric. A combination of AMS and ARM fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event, probably related to Brasiliano orogenic processes after dike emplacement. InverseInverse AMS fabric is a minority (four dikes). ARM determinations also remained inverse suggesting a primary origin for inverse AMS fabric.