Particle shape effect on macro‐ and micro‐behaviors of monodisperse ellipsoids

The microscopic and macroscopic behaviors of assemblages of monodisperse ellipsoids with different particle shapes were studied using the discrete element method. Four samples were created with 1170 identical prolate ellipsoids. The samples were compressed isotropically to 100 kPa. Then triaxial compression tests were carried out to very large strains until the ultimate state was reached. This paper presents typical macroscopic result including stress–strain relationship and volumetric behavior. In addition, the fabric of the samples was examined at the initial state, at the peak shear strength state, and at the ultimate state. We studied the evolution of three vector‐typed micromechanical arguments with strain including the particle orientation, branch vector, and normal contact force. The normal contact force (micromechanical argument) was found to have a direct relationship with the principal stress ratio (macroscopic parameter). The angles between these vectors were also investigated. The maximum angle between vectors is related to particle shape. The results indicate that the distributions and the maximum values of these angles do not change with loading. Copyright © 2008 John Wiley & Sons, Ltd.     

Undrained Monotonic Response and Instability of Medium-Dense Sandy Soil

A series of undrained monotonic triaxial compression tests were performed on natural, medium-dense (relative density (RD) = 50%) Chlef sand containing 0.5% of non-plastic fines, under different confining pressures of 50 kPa, 100 kPa, and 200 kPa. This article focuses on distinctive states of the monotonic undrained response of sands, namely the critical state, the phase transformation state, the quasi-steady state, and the state of undrained instability (onset of flow liquefaction). Specimens were prepared using dry funnel pluviation and wet deposition to investigate the effect of the initial sand fabric on these states. The present data suggest that the initial fabric of the sand appears to have a significant effect on the undrained behavior in the early stages of shearing, with its influence vanishing at large strains. Wet deposition specimens demonstrate considerably larger undrained instability state strength than their dry funnel pluviation counterparts, and a unique critical state locus is reached by both dry funnel pluviation and wet deposition.     

陕西凤县铅洞山铅锌矿床的矿石组构成因意义的分析

本矿床的矿石组构类型可概括为四类:同生沉积组构、成岩组构、后生叠加组构和后期变形改造组构。详细研究表明,以同生沉积和成岩作用形成的组构为主,如浸染状、层纹状、条带状、角砾状构造和莓球状、半自形细粒状结构等,其中部分受到后期构造变形改造、但后生成矿组构极为少见。矿床的地球化学特征研究提供了成矿物质来源于深部地层的信息。因此,从矿石组构特征及成矿物质来源两个方面分析,认为本矿床为海底喷流沉积—成岩弱改造型层控矿床.     

Sampling‐surface orientation and clast macrofabric in periglacial colluvium

To isolate the influence of sampling‐surface orientation on the macrofabric of periglacial colluvial deposits, clast orientation measurements were obtained from seven paired horizontal and vertical exposures in turf‐banked solifluction lobes on Niwot Ridge, Colorado Front Range. Most samples form moderately strong, upslope‐plunging clusters aligned with the local slope orientation. Fabrics obtained from vertical faces were stronger than those from horizontal exposures in six of the pairs. Near‐horizontal sampling surfaces yield less biased results than vertical exposures, owing to operator perceptions, procedural difficulties, and the relative thinness of the layer affected by colluvial processes. Sampling procedures must be standardized before comparative studies of colluvium can yield reliable results. Copyright © 2001 John Wiley & Sons, Ltd.     

Magnetic fabric and its significance in the Florianópolis dyke swarm, southern Brazil

Magnetic fabric was determined by applying the anisotropy from the low-field magnetic susceptibility (AMS) technique in 62 mafic dykes from the Mesozoic Florianópolis (Santa Catarina Island) dyke swarm, southern Brazil. These dykes cut the crystalline basement rocks, which are mainly Proterozoic. They are vertical or subvertical in dip and trend mainly NE, although NW-trending dykes are also found. Dykes are tholeiitic in composition and are geochemically similar to those from the Ponta Grossa swarm. Thicknesses vary from 0.3 to 60 m. Polished sections show that titanomagnetites carry the AMS in these dykes. Hysteresis parameters show that the magnetic minerals fall in the PSD range. Two types of magnetic fabric are recognized. Type I is characterized by K ₁- K ₂ parallel to the dyke wall, representing magma flow within the dykes; type II, with K ₁- K ₃ parallel to the dyke wall, was found in four dykes. Type I is found in 94 per cent of the dykes, and approximately 20 per cent of these have K ₁ inclinations of less than 30°, suggesting horizontal or subhorizontal flow. About 80 per cent have K ₁ inclinations of greater than 30°, due to inclined to vertical flow. The comparison of AMS studies from both the Florianópolis and the Ponta Grossa dykes suggests a source position closer to Santa Catarina Island than the Ponta Grossa arch.     

Composite magnetic fabric deciphered using heating treatment

In a number of AMS studies, the presence and deciphering of composite magnetic fabrics is of major importance for a correct interpretation of the data. On the basis of several examples from intrusive rocks (diorites and dolerites) we show that the use of laboratory heatings can help to extract at least one component of the composite magnetic fabrics usually present. The procedure includes comparison of the fabrics measured after stepwise laboratory heating with the fabrics determined by tensor difference and by linear regression analysis. In the diorite samples, the measured AMS results from the superimposition of different component fabrics and does not correspond exactly to any of these fabrics. In these dykes, isolated magnetic fabric during thermal treatment corresponds to that of the main magnetic mineral (Ti-poor titanomaghemite) and reveals an unknown structure. In volcanic flow or doleritic dykes, a “parasitic” fabric related to late or post-magmatic evolution superimposed to the flow fabric can produce important scattering of the AMS principal directions. Decomposition of magnetic fabric during thermal treatment allows isolation of the flow fabric.     

从郭扎错断裂构造特征探讨阿尔金断裂带西延问题

阿尔金断裂带在大陆动力学研究中具有十分重要的地位,但其西延过郭扎错后的走势存在争议。郭扎错断裂为北东东向的线性构造带,具多期活动性。笔者从宏观到微观详细论述了该断裂构造的几何学、运动学特征,并结合动力变质作用、沉积盆地、岩石地层分布及变形年代学等资料将其划分为韧性左行平移(J3-K1)、韧脆性正-平移(E1-N1)、韧脆性逆-平移(N2)和脆性左行滑落(Q)等4个阶段。综合分析地质调查资料、地球物理场资料及卫片影像特征,认为郭扎错断裂与阿尔金断裂带是在同一动力学系统中形成的具有相似运动学、动力学特征的相关线性构造,应属同一断裂系统。因此,阿尔金断裂带并非西止于拉竹龙,亦非由郭扎错北侧转向北西,而是经郭扎错继续向南西延伸,过龙木错、羌臣摩河后,由空喀山口进入克什米尔。     

东天山康古尔塔格金矿田控矿构造特征

利用构造面理统计、岩组分析和岩石磁化率各向异性参数测量,研究了东天山康古尔金矿田控矿构造的特征。结果表明,金矿田发育一组构造面理,具有典型的对称型岩组图;磁组构特征的变化明显反映出矿田韧性变形具北强南弱的特点;矿田韧性断裂具有压扁应变的特征,磁面理产状与韧性断裂产状相一致。因此,康古尔金矿田韧性断裂应是起源于共轴变形的韧性挤压带。     

新疆东天山却勒塔格地区韧变形带识别及其意义

却勒塔格韧变形带位于新疆东天山却勒塔格一带,构造上属哈萨克斯坦-准噶尔板块中的准南陆缘活动带.变形带由下石炭统干墩组组成,为一套糜棱岩化火山岩、火山碎屑沉积岩.带内向南陡倾的片理极为发育,片理面上广泛分布方向各异的透入性拉伸线理,形成构造动力变质片岩.动力变形变质岩磁化率各向异性度P≥1.10 以上,磁面理很发育,F 值最高可达 1.23,磁化率椭球扁率 E>1,磁椭球的形状因子 T>0.X光岩组分析显示:构造岩的绿泥石和绢云母发生明显的定向,组构对称.根据区域对比,却勒塔格韧性变形带是中天山秋格明塔什—黄山巨型韧性挤压带的一部分,属其北部边缘过渡带范围.与邻区小热泉子铜矿和土屋铜矿具有相似的地质构造条件和成矿环境.其作为区域巨型韧性挤压带的一部分,起源于东天山觉罗塔格晚石炭世碰撞期共轴挤压造山作用,属共轴应变韧性挤压带,应变类型属压扁应变.