黄土高原风成沉积物磁化率各向异性研究

中国黄土高原粉尘堆积以往注重从物源区携带粉尘的古风向,对粉尘沉积→固结期的古风向缺乏研究。本文评述了近年来应用黄土高原黄土、红粘土磁化率各向异性研究古季风风向变迁所取得的一些重要进展并指出磁化率各向异性最大轴和最小轴的方位和倾伏均与沉积→固结期风向变化密切相关。黄土磁组构结果与利用各种统计方法计算出的古风向在多个剖面与现代夏季风方向相似,由此推论黄土是冬季风带来的沙尘在经历夏季风携带的降雨洗礼后才固结而成。而在黄土之下的红粘土地层中的初步研究发现磁组构显示明显的西风带沉积特征,指出这一现象将在今后研究中继续深入探讨。     

220 ka以来萨拉乌苏河流域地层磁化率与气候变化

 萨拉乌苏河流域滴哨沟湾剖面磁化率变化结果表明:近220 ka来我国北方气候变化极不稳定,存在着不同尺度的频繁变化,特别是寒冷气候阶段变化尤为频繁,其中倒数第二次冰期存在5个气候旋回,末次冰期存在10个气候旋回。这些气候变化与深海氧同位素、极地冰芯反映的全球变化具有良好的对应关系,反映了该区气候变化与全球变化的一致性。控制本区气候变化的主要因素是全球冰量变化及太阳辐射影响的东亚季风变化。     

柴达木盆地大红沟剖面新生代地层岩石磁学 特征与环境演变

通过对柴达木盆地大红沟剖面新生代沉积物岩石磁学的研究,探讨青藏高原东北部的构造活动与气候变化。典型样品的热退磁曲线表明,磁铁矿含量随地层年代的变新而增大。沉积物的磁化率由老至新划分为5个阶段,随时代变新而不断增大,与磁铁矿含量的增大相吻合。柴达木盆地沉积物磁化率值的变化与气候变化关系密切:①约43Ma,磁化率增大是全球气候变冷变干导致沉积物中磁铁矿含量增加所致;②约33Ma,磁化率增大是全球气候加速变冷的结果;③22Ma左右,青藏高原整体隆升,控制中国大陆环境的气候系统由行星风系转变为季风风系,亚洲内陆开始干旱化,致使更多的磁铁矿得以保存,磁化率值再度增大;④14Ma青藏高原东北缘发生区域构造抬升,西北干旱区进一步扩大,东亚季风明显加强,同时全球冰量扩张,导致柴达木盆地沉积物的磁化率整体增大,而且波动幅度和频率增大。     

Post-emplacement tilting of lava flows inferred from magnetic fabric study: the example of Oligocene lavas in the Jeanne d’Arc Peninsula (Kerguelen Islands)

Statistical analysis of the magnetic fabric of samples from several successive lava flows emplaced under similar conditions can allow determination of the mean flow direction when magnetic fabric data from individual flows do not lead to reliable results. A difference between the obtained flow direction and the present dip direction indicates that the flows were tilted after emplacement. For 2 successive series of flows on the Jeanne d’Arc Peninsula presently NNW dipping, this method shows lava emplacement along a SSW–NNE direction. This indicates a gentle tilting acquired during a period of weak deformation in the whole archipelago. Additionally, the magnetic fabric data allow the reconstruction of the different conditions of emplacement of these two series of lava flows and of formation of the local thick conglomerate interbedded between these series.     

广东河台金矿田糜棱岩磁组构对剪切带变形与成矿的约束

河台金矿田是广东省内规模最大的金矿,由于金矿床都产于糜棱岩带中,因而被认为是典型的与剪切带有关的金矿。然而,对于Au矿化与剪切带变形条件之间的关系目前仍不十分清楚。高村和云西是河台金矿田两个代表性的金矿床,分别受ML11和ML9两条剪切带的控制。本文在对两条剪切带中的含矿和无矿糜棱岩的变形特征进行野外调查和室内岩（矿）相学观察的基础上,分别开展岩石磁学和磁化率各向异性（AMS）特征研究。糜棱岩x- T曲线显示了磁黄铁矿的居里温度,结合磁滞回线、等温剩磁（IRM）以及矿相学特征,表明河台金矿田糜棱岩几乎不含磁铁矿,载磁矿物主要为黑云母和磁黄铁矿,但是磁黄铁矿的贡献要远大于黑云母。两条糜棱岩带中不含矿糜棱岩的平均磁化率Km＜500×10-6 (SI)、磁化率各向异性度Pj、椭球体形状参数T、磁面理F和磁线理L没有明显差别。并且形成两条剪切带的主应力方向也近于一致,最大主应力方向σ1（K3）为近水平NNW- SSE向,最小主应力σ3（K1）为NEE- SWW向,倾伏角较缓。运动学涡度Wk值也近似,在0. 12~0. 65之间,小于0. 71。上述特征暗示两条含金剪切带都是在印支期NNW- SSE向近水平的强烈挤压作用下形成的,其中发育的糜棱岩为S型和SL型构造岩,具有强烈压扁的特征,并且在剪切变形过程中纯剪切作用所占的比重要明显大于简单剪切。然而,两条剪切带中含矿糜棱岩与无矿糜棱岩的AMS特征有明显的区别。含矿糜棱岩的K1近于直立,且Km＞500×10-6 (SI)和Pj值都明显大于无矿糜棱岩。因此,糜棱岩的变形强度与含矿性具有一定的正相关关系,而在燕山期叠加的脆韧性变形使得最大磁化率主轴（即最小主应力轴）变陡,更有利于热液的运移和最终成矿。     

The occurrence of Greigite in sediments from Loch Lomond

Greigite has been identified in the sediments of Loch Lomond using X-ray diffraction. Greigite is the dominant magnetic mineral in the oldest sediments recovered, while magnetite predominates in the younger sediments which had previously been used for palaeomagnetic secular variation studies. A layer of sediment in between these two magnetic mineral regimes has very low magnetic concentrations, probably as a consequence of magnetite dissolution associated with sulphide rich pore-waters produced at the time of a marine incursion. The greigite largely oxidises once the sediment is exposed to air, but if freeze dried the greigite becomes surprisingly stable. Following freeze drying the greigite can be heated to 280°C in air before it alters and loses its strong ferrimagnetic properties.     

The evaluation of eco-environmental susceptibility to human activity in Yulin region

The eco-environmental susceptibility is to demonstrate the reaction of a ecosystem under the same influences of the human activity. It comprise many influencing factors which have a complicated relation to the eco-environmental susceptibility. In this paper, the Analysis of Hierarchy Process (AHP) is used to determine the weight of each factor to the eco-environmental susceptibility through analyzing the relationship between human activity and environmental deterioration. And the weighted summation method is used to calculate the eco-environmental susceptibility of different divisions in Yulin region. The result shows that the loess hilly and gully area is more subjective to deteriorating eco-environment than the desert area because of the strong human activity and severe soil erosion. The project is supported by the National Natural Science Foundation of China     

Refined technique for susceptibility resolution into ferromagnetic and paramagnetic components based on susceptibility temperature-variation measurement

The measurement of the temperature variation of magnetic susceptibility can be used for the separation of ferromagnetic and paramagnetic susceptibility components. The method suggested by Hrouda (1994) assumes a hyperbolic dependence of paramagnetic, susceptibility and constant ferromagnetic susceptibility in the temperature interval used for the separation. Our new method works with a paramagnetic hyperbola again, but assumes that the ferromagnetic susceptibility temperature variation is represented by a linear relationship in the resolution interval, as indicated by the investigation of monomineralic ferromagnetic fractions.     

Transverse zones controlling the structural evolution of the Zipaquira Anticline (Eastern Cordillera,Colombia): Regional implications

We report paleomagnetic, magnetic fabric and structural results from 21 sites collected in Cretaceous marine mudstones and Paleogene continental sandstones from the limbs, hinge and transverse zones of the Zipaquira Anticline (ZA). The ZA is an asymmetrical fold with one limb completely overturned by processes like gravity and salt tectonics, and marked by several axis curvatures. The ZA is controlled by at least two (2) transverse zones known as the Neusa and Zipaquira Transverse Zones (NTZ and ZTZ, respectively). Magnetic mineralogy methods were applied at different sites and the main carriers of the magnetic properties are paramagnetic components with some sites being controlled by hematite and magnetite. Magnetic fabric analysis shows rigid-body rotation for the back-limb in the ZA, while the forelimb is subjected to internal deformation. Structural and paleomagnetic data shows the influence of the NTZ and ZTZ in the evolution of the different structures like the ZA and the Zipaquira, Carupa, Rio Guandoque, Las Margaritas and Neusa faults, controlling several factors as vergence, extension, fold axis curvature and stratigraphic detatchment. Clockwise rotations unraveled a block segmentation following a discontinuos model caused by transverse zones and one site reported a counter clockwise rotation associated with a left-lateral strike slip component for transverse faults (e.g. the Neusa Fault). We propose that diverse transverse zones have been active since Paleogene times, playing an important role in the tectonic evolution of the Cundinamarca sub-basin and controlling the structural evolution of folds and faults with block segmentation and rotations.     

Correlation of strain with anisotropy of magnetic susceptibility (AMS)

Existing correlations between strain and anisotropy of low-field magnetic susceptibility (AMS) have been re-assessed using a single parameter to express both anisotropies. TheP parameter (Hrouda, 1982) shows potential as a powerful single expression of the intensity of strain and of AMS. Previous correlations are improved by use of this parameter. Cautious optimism is justified for correlations between strain and susceptibility in a certain strain window between a lower limit (excluding the incomplete overprint of predeformation anisotropy) and an upper limit (excluding the effects of saturation anisotropy). For successful correlations the influence of stress-controlled recrystallisation should be minimal and the mineralogical sources of susceptibility must predate deformation.