南极普里兹湾NP93－2柱样磁组构特征及其古气候意义

对采自东南极普里兹湾海区沉积物ＮＰ９３－２柱样系统的磁组构分析，结果表明：１．普里兹湾海洋沉积物具有较大的磁化率各向异性，较好地指示了沉积过程中水动力过程的变化；２．由磁化率各向异性主轴方向确定的古水流方面主要为近ＥＷ向及ＮＳ向，分别对应着南极绕极环流的方向及由融冰水的大量生成而导致的南极底层水形成的方向，这两种不同方向的水流在该海区不同时段的表现各不相同；３．依据表征磁组构的参数及古水流方向的变化，可将１２７００ａＢ．Ｐ．以来的南极地区的古气候分成５个古气候变化带．     

临夏盆地新近纪郭泥沟剖面磁组构特征及其环境意义

临夏盆地在东亚新生代地层、古气候、古生物研究方面占有举足轻重的地位.本文对盆地东部的郭泥沟剖面进行了详细的岩石磁学和磁组构研究,以揭示从早中新世到早上新世临夏盆地的沉积演化过程.郭泥沟剖面沉积物中的磁性矿物有磁铁矿、磁赤铁矿、赤铁矿和针铁矿,但剩磁载体以磁铁矿和赤铁矿为主.从上庄组和东乡组的褐红色粉砂质粘土到柳树组和何王家组的褐黄色粘土,赤铁矿含量呈现降低的趋势,与沉积物颜色变化一致.郭泥沟剖面沉积物磁组构类型为正常沉积磁组构.结合岩石磁学结果和磁组构参数特征可揭示临夏盆地早中新世-早上新世沉积的演化过程：早中新世上庄组为稳定湖相沉积,古水流方向为NNW,与南北向的大夏河方向一致;中中新世气候发生较明显的干湿波动,形成了东乡组的褐红色湖相粉砂质粘土夹粉砂、砂和青灰色泥灰质粘土条带,古水流方向主要为NNW,沉积过程主要受大夏河控制;中中新世晚期,受青藏高原构造运动影响,沉积相由湖相细粒沉积物转变为虎家梁组河流相砂砾层;同时,盆地的水动力条件也发生改变,晚中新世柳树组湖相沉积过程同时受南北向大夏河和东西向洮河控制,两个方向近垂直的河流共同作用导致柳树组内沉积各向异性度较低,面理和线理均不发育,磁化率最大轴偏角分布比较分散,磁组构确定的古流向为东西向和南北向;早上新世期间,由于受青藏高原隆升影响,沉积了何王家组下部的河流相砂砾层;受构造抬升影响,大夏河重新主导何王家组上部洪泛平原相沉积过程,水动力条件较为单一稳定,古流向主要为N向,与大夏河流向一致.     

风成沉积物磁组构与中国黄土区第四纪风向变化

通过对黄土高原几个剖面黄土样品磁化率各向异性的初步研究，发现了风成沉积物中的磁组构特征，且它的形成受沉积作用控制并与黄土高原形成时的古风场有关，即风成沉积物磁化率椭球体主轴方向及各轴比值与磁性颗粒分布排列方式亦即与空气动力条件相关．阐明了风成沉积物磁组构形成机制及其与古风向的关系，提出一种能够定量研究黄土高原形成时古风场的方法，为研究黄土高原形成演化和第四纪以来气候变化提供了基础数据．     

岩石磁性组构的研究述评

岩石样品磁化率各向异性,即磁性组构的研究,已逐渐引起人们注意。沉积岩的磁性组构与其沉积和压实过程有关;火山岩则取决于其熔岩流动情况;而变质岩则与塑性形变及重结晶等过程有关。可见,磁组构可以成为构造分析的工具。而这一手段将为经济建设做出贡献。对板岩形变与磁组构的关系,有人曾提出定量关系。笔者现正试图将断层剖面上各种岩石的磁组构,作为应变指标来说明古应力应变,进而为断层运动历史提供可靠证据。黄土的研究涉及古气候、磁性地层及第四纪地磁场的行为。除传统的古地磁研究,划分各个极性时,或探讨磁化率变化曲线与古气候的关系外,其磁性组构的研究还提供新的信息,将成为第四纪研究的一种手段。而岩石磁性组构这项新手段必将推动地球科学的发展。     

长江中游地区洪泛沉积物与正常河流沉积物磁组构特征对比研究

在地层剖面中,洪泛沉积物与正常的河道沉积物的沉积学特征非常相似,仅靠沉积学特征很难将两者区分. 为了识别古洪灾事件,因而从环境磁学角度建立两种沉积物的识别标志是非常必要和有意义的. 本文通过对1998年长江黣洲湾溃口扇及其附近的长江现代河漫滩沉积物进行磁组构参数测量和对比分析,并结合长江下游河漫滩沉积物的磁组构参数特征,揭示出长江中游正常河流沉积物与河流溃口沉积物的磁组构参数特征具有明显的不同,并且它们与沉积时的沉积环境有着密切的关系. 因此,对长江中游地区沉积物磁组构参数的测试及磁化率各向异性量值椭球体形态的研究,是目前分辨洪泛沉积物和正常的河道沉积物的一个较有效的方法.     

现代风成沉积物磁化率各向异性与风向关系的研究

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．     

库车坳陷克拉苏河新生代沉积岩磁组构特征与古流向分析

塔里木盆地库车坳陷克拉苏河沿岸发育良好的新生界地层.我们对克拉苏河沿岸新生界剖面分A、B两段进行系统采样,获得定向岩芯样品1700余块. 岩石磁学研究表明,新生代沉积岩中磁性矿物以赤铁矿为主,含有少量磁铁矿;磁组构测试结果显示,两段剖面磁面理均较磁线理发育,最小磁化率主轴近于直立,显示原生沉积组构. A剖面磁化率主轴к₁的偏角指示古流向. 库姆格列木群沉积时的古水流方向为NEE-SWW向,至苏维依组沉积时,流向变为NNE-SSW向. 河流古流向在苏维依组形成时发生急剧变化,说明南天山此时可能发生了明显的隆升,且以北西部隆升为主;B剖面为吉迪克组、康村组和库车组粗粒碎屑岩,磁组构显示к₁-к₂构成的磁面理与层理面小角度相交,к₃呈叠瓦状分布,可以用来指示古水流方向.从吉迪克组至库车组下段,古流向均为NNW-SSE,但逐渐偏东,暗示天山在此期间处于缓慢的隆升期或东西部处于同步隆升,且西部隆升速度快于东部.     

现代湖泊沉积物中磁性矿物的研究及其环境意义

通过固城湖沉积物中高分辨率的岩石磁学取样及其实验研究，取得了环境磁学参数及各种磁参数的比值随深度的变化曲线，结合其他气候指标和同位素年龄结果，表明湖泊沉积中磁参数及其比值的变化能与末次晚冰期以来的古气候变化很好地相对应，尤其是在４．００ｍ和１２．０３ｍ的气候转折点上，沉积物中环境磁参数及其比值同样出现明显的变化，有力地说明沉积物中磁参数及其比值能很好地记录湖泊沉积物沉积时的古气候变化，并且可用来认识古环境突变的界线．     

岩石磁组构在构造混杂岩带和韧性剪切带研究中的应用——以西天山地区为例

本文以西天山地区为例,讨论了岩石磁组构在构造混杂岩带和韧性剪切带中的作用和功能．指出岩石磁组构的某些参数能反映混杂岩带的变形类型,磁化率椭球体三个轴的方位在混杂岩基质中分布有规律,而在岩块中没有规律．在韧性剪切带中,磁组构的各向异性度Ｐ值绝大部分大于１．１,并有变化．磁面理的产化变化可用来求出韧性剪切带的位移量和变形路径．磁组构的Ｌ—Ｆ参数图可确定韧性剪切带的类型,最小主磁化率的方位可用来确定韧性剪切带的剪切指向．     

北京房山岩体的磁组构特征及其 对岩体侵位的约束

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.     

钻井岩心古水流方向确定方法和原理

为了实现钻井岩心信息最大化的应用.首先利用古地磁技术对钻井岩心进行原始方位恢复,然后通过测定岩心上具有古水流指向性的沉积构造的参数,或者通过测定岩心磁性组构判定古流向,最后经过岩心校正得出研究区的古水流方向.文中以钻井岩心上的斜层理为例,推导出其在岩心坐标系中的倾向和倾角的表达式,经过公式校正得到真倾向和真倾角的表达式,确定古流向.最终将井位、岩心恢复及古水流计算结果综合反映,获得每个井位的古水流方向,进而通过连片处理分析,可获得某区块的局部及整体的古水流趋势方向,预测砂体的展布方位.本文为钻井岩心古水流方向的确定提供了研究方法,实现了岩心信息最大化利用,可以指导油田的进一步勘探开发.     

Variation of magnetic properties in sedimentary rocks hosting the Foum Zguid dyke (southern Morocco): Combined effects of re-crystallization and Fe-metasomatism

The effects of dyke intrusion on the magnetic properties of host sedimentary rocks are still poorly understood. Therefore, we have evaluated bulk magnetic parameters of standard palaeomagnetic samples collected along several sections across the sediments hosting the Foum Zguid dyke in southern Morocco. The study has been completed with the evaluation of the magnetic fabric after laboratory application of sequential heating experiments.The present study shows that: (1) close to Foum Zguid dykes, the variations of the bulk magnetic parameters and of the magnetic fabric is strongly related with re-crystallization and Fe-metasomatism intensity. (2) The thermal experiments on AMS of samples collected farther from the dyke and, thus, less affected by heating during dyke emplacement, indicate that 300–400 °C is the minimum experimental temperature necessary to trigger appreciable transformations of the pre-existing magnetic fabrics. For temperatures higher than ca. 580 °C, the magnetic fabric transformations are fully realized, with complete transposition of the initial fabric to a fabric similar to that of samples collected close to the dyke. Therefore, measured variations of the magnetic fabric can be used to evaluate re-crystallization temperatures experienced by the host sedimentary rock during dyke emplacement. The distinct magnetic behaviour observed along the cross-sections strongly suggests that samples collected farther from the dyke margins did not experience thermal episodes with temperatures higher than 300 °C after dyke emplacement. (3) AMS data shows a gradual variation of the magnetic fabric with distance from the dyke margin, from sub-horizontal K₃ away from the dyke to vertical K₃ close to the dyke. Experimental heating shows that heat alone can be responsible for this strong variation. Therefore, such orientation changes should not be unequivocally interpreted as the result of a stress field (resulting from the emplacement of the dyke, for instance). (4) Magnetic studies prove to be a very sensitive tool to assess rock magnetic transformations, thermally and chemically induced by dyke intrusion in hosting sediments.     

Modelling the relationship between magnetic fabric and strain in polymineralic rocks

We present a model that is applicable to the relatively frequent case of rocks in which the magnetic fabric is dominated by uniaxial paramagnetic minerals, and in which the deformation (pure shear) corresponds to the March-Fernandez model. Borradaile et al.'s procedure for the isolation of the magnetic fabric of monocrystals of anisotropic components allows us to obtain the magnetic properties equivalent to those of all the component minerals if they should be artificially aligned. In the case of real component minerals with similar shape parameters, χ (dependent on dimension ratio), these properties will correspond to those of a theoretical equivalent mineral. Therefore, finite strain and orientation tensors may be determined from magnetic fabric measurements of such polymineralic rocks.     

Bioturbation: minimal effects on the magnetic fabric of some natural and experimental sediments

Magnetic fabric measurements have been performed on three suites of sediment samples to contrast biologically disturbed and undisturbed sediments. The first of these analyses includes samples from laminated, relatively undisturbed and bioturbated, totally disturbed horizons found in an Athabasca Oil Sand core. The second includes samples from biologically undisturbed sediments from Mammoth Cave in Kentucky. The third includes evaluation of the evolution of a magnetite horizon which was established by the author in December of 1982 in a biologically active tidal flat on Sapelo Island, Georgia. Changes in this magnetite horizon were monitored by periodic subsampling.Results from these suites of samples indicate that the measured magnetic fabric in bioturbated sediments may exhibit primary sedimentary characteristics similar to undisturbed sediments. Such primary indicators include the anisotropy of magnetic susceptibility (AMS) parametersV used by Graham, always greater than 45°,Q used by Rees and Hamilton, always less than 0.69, and a subhorizontal magnetic foliation plane. I infer that the presence of apparently primary fabric distributions in these bioturbated sediments indicates that physical, not biological factors are responsible for the observed fabric. These factors include dewatering and simple, low magnitude compaction. The data indicate that it cannot be assumed, a priori, that poor fabric distributions are the result of bioturbation. On the contrary, for the Sapelo Island experiments, the magnetic fabric actually improved with bioturbation and the Athabasca bioturbated magnetic fabric is very similar to that from the Athabasca laminated sediment.Data from the Sapelo Island, Georgia, tidal flat experiment are interpreted to indicate that long-axis AMS alignments develop within the sediment. Such alignments are the result of water movement through sediment which exhibits increased porosity due to high rates of burrowing. It is also observed that RM precision improves during the experiment in spite of bioturbation.     

Magnetic fabric, welding texture and strain fabric in the Nuraxi Tuff, Sardinia, Italy

Anisotropy of magnetic susceptibility (AMS) has been used to interpret flow directions in ignimbrites, but no study has demonstrated that the AMS fabric corresponds to the flow fabric. In this paper, we show that the AMS and strain fabric coincide in a high-grade ignimbrite, the Nuraxi Tuff, a Miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well-defined magnetic fabric. Natural remanent magnetization (NRM) data indicate that the magnetization of the tuff is homogeneous and was acquired at high temperatures by Ti-magnetite crystals. Comparison between the magnetic fabric and the deformation features along a representative section shows that AMS and anisotropy of isothermal remanent magnetization (AIRM) fabric are coaxial with and reproduce the shape of the strain ellipsoid. Magnetic tests and scanning electron microscopy observations indicate that the fabric is due to trails of micrometer-size, pseudo-single domain, magnetically interacting magnetite crystals. Microlites formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff. The fabric was thus acquired after deposition, before late rheomorphic processes, and accurately mimics homogeneous deformation features of the shards during welding processes and mass flow.     

Subaqueous sand blow deposits induced by the 1995 Hyogo-ken Nanbu Earthquake, Japan

Abstract The 1995 Hyogo-ken Nanbu Earthquake (Mw 6.9) occurred in the region around Kobe City and Awaji Island in south-west Japan. Co-seismic liquefaction caused subsidence of the land and damage to sea wall caissons on the man-made Port Island at Kobe City. A zone 2–3 m wide behind the caissons of the northern wharf on the island subsided into the intertidal zone and a sandy deposit settled into this subsided zone. The depos-it consists of upward-fining sequences that are subdivided into three parts, in ascending order: graded coarse- to medium-grained sand, parallel-laminated fine- to very-fine-grained sand, and massive mud. Grain fabric analysis (employing the anisotropy of magnetic susceptibility method and microscopic measurement) of these sequences shows that there is a remarkable contrast in grain fabric between the lowest portion of the graded sand division and the laminated sand division. The former has a high q -value (magnetic lineation/foliation) and a unimodal orientation of elongate grains in the horizontal plane, but random orientation in the vertical plane. Conversely, the latter is characterized by a low q -value and a grain fabric in which the long axes of the grains have random orientations and are nearly parallel to the plane of deposition. This result shows that the main depositional processes changed from a combination of flow and allied processes to the force of gravity. As still water is essential for gravity to be the dominant factor in deposition, this deposit is regarded as subaqueous sand blow deposits. If this interpretation is correct, the grain fabric produced by gravity alone is a useful criterion for distinguishing between subaqueous sand blow deposits and other liquefaction-induced deposits.