地球磁极倒转与白垩纪超静磁带的非线性分析

地球磁场多次发生南北(正负)磁极位置的变换和白垩纪超静磁带(CNS)的异常现象,这已为大家所公认.但造成这种异常现象的原因,则是迄今未能很好解答的一个难题. 应用非线性理论对地球磁极倒转和白垩纪超静磁带进行了分析, 认为超静磁带事件意味着地球核幔相互作用和外核流体运动可能处于能量最低的状态,地球磁场系统通过不断地与外界交换物质和能量,维持一种空间或时间的有序结构.在121～83Ma期间,无外星撞击地球引起地磁极性倒转,可能是白垩纪超静磁带出现的原因之一.地球磁场极性的随机倒转具有混沌运动的自逆转特性,混沌理论给地磁极性倒转提出了一个简明的动力机制解释.     

地球基本磁场的形成与变化的探讨

本文在铁磁体假说的基础上,探讨地球基本磁场的形成与变化的原因.地球的偶极磁场是由于地球的回转和内核中特殊的磁化环境,使内核中心形成的饱和磁化的永磁球体（即磁核）产生的,磁核的大小和温度负相关.地球的非偶极磁场,由外核内几个可确定的磁偶极子产生,这些磁偶极子,是外核中液态金属的流动,切割磁核的磁场而产生的涡流形成的.     

白垩纪前的太平洋板块构造演化和地磁极性倒转时间表的扩展及对侏罗纪“宁静带”起源的推论

在西太平洋磁湾附近,由于海底扩张而形成的线性磁异常构造已经绘制成图。而新的航磁资料,可将M21到M28期间的磁湾更准确地勾画出来,并能分辨出侏罗纪“宁静带”中小幅度磁性构造线,这些磁性构造线是在M29以前的磁性倒转事件形成的。经过改正的侏罗纪地磁极性倒转时间表纳入了19个早于M29的倒转事件——序号是M30—M38(审者     

核幔界面反极性磁斑区和地磁场倒转

用国际参考地磁场模型ＩＧＲＦ１９００－２０００，在忽略地幔电导率的假设下，求出从地球表面直至核幔界面（ＣＭＢ）的深部地磁场分布．核幔界面磁场分布的重要特点之一是存在几块“反极性磁斑区”，即在南半球－Ｚ（向上）极性区的总体背景上有两块东西排列的＋Ｚ反极性磁斑区──南非斑区（ＳＡＦ）和南美斑区（ＳＡＭ），而在北半球＋Ｚ（向下）极性区的总体背景上也有两块－Ｚ反极性磁斑区──北极斑区（ＮＰＬ）和北太平洋斑区（ＮＰＡ）．在１９００～２０００年的１００ａ当中，南非斑区以０．２～０．３°／ａ的速度快速向西漂移，斑区面积增大了５倍，通过斑区的磁通量急剧增长了３０倍．与此相比，其他斑区的变化要小得多．从ＣＭＢ向上延伸，反极性斑区在地幔中形成烟筒状的“反极性磁柱”，其中南非反极性柱的高度随时间快速增加，从１９００年的２００ｋｍ增加到２０００年的９００ｋｍ．按照目前的增长速度估计，６００～７００ａ后，南非反极性柱将出露地表，那时，在南非将形成一个地磁场反极性区，这可能标志着一次新的地磁极移或地磁场倒转的开始．由此可以推论，地磁场倒转可能不是全球同时开始和同步发展的，倒转现象更象是首先在一个（或几个）区域出现，然后向周围扩展，继而     

古地磁学在地磁场起源研究中的应用

地磁场起源及其倒转是地球科学的难题之一。究其原因一方面是由于无法直接观测地球内部发生的物理过程,另一方面是由于缺乏理论与实验相结合的综合研究。本文以磁流体力学为基础,将古地磁学与αω发电机理论结合在一起进行分析和研究。得出了如下新观点:(1)洛仑兹力在地核发电过程起负反馈作用;(2)较差旋转控制着地磁场西向漂移,(3)α作用使地磁极偏离地球自转轴     

地球磁场极性倒转的限制

地磁极性倒转是地球磁场的重要特征之一.研究极性转换过程中地球磁场的时空分布规律是认识地磁倒转机制的重要途径.国际上利用湖相和海相沉积物以及火山岩体对转换场形态学作了大量的研究工作,但由于湖相沉积物的剩余磁性受生物扰动和化学变化影响大,所得结果可靠性较低;深海沉积物则     

内核地震波速各向异性的成因

地球内核是轴对称各向异性的,其对称轴与地球的极轴之间有１１°左右的夹角,本 文根据地球内核相对于外部地球有差异转动这一观测结果,利用晶体生长理论,对内核地震波 速度各向异性的成因进行了探讨．当从熔融状态结晶时,晶体的生长速度与晶体和熔融态之 间相对运动的线速度成正比涸此当固态内核在液态外核中生长时,沿赤道方向的生长速度比 两极方向快．在万有引力场的作用下内核始终保持近似球形,生长速度较快的赤道附近的物 质会向两极区域流动,形成轴对称的流变场。这一轴对称的流变场伴随着轴对称的应力场,使 得构成地球内核的ｈｃｐ型铁晶体的ｃ轴沿着内核自转轴的方向排列,导致观测到的地球内核地 震波速度各向异性。作为推论,内核相对于外部地球可能同时存在着进动和章动。     

地球内核对地磁场涨落和倒转的影响

考虑到内地核较小(其半径为外核的1/3),许多地球发电机模型将内核完全忽略(Hollerbach et al.,1992),要不然就将其处理为不导电的绝缘体(Zhangand Busse,1990;Glatzmaier and Roberts,待出版)。在我们以前的稳态模型(Hollerbach and Jones,1993)中,曾考虑了有限导电内地核的某些效应,尤其是引起内外核之间的电磁耦合效应。本文中,我们包括了一种规定的浮力,从地球物理学的观点讲,这样更加现实,而且所得到的解与时间有关,而不再是与时间无关。在有限导电的内核中的磁场不再是瞬间地调整到外核中的场值,而是有一个它自己固有的几千年的扩散时间尺度。从而外核中较大一些、快一些的地磁涨落有效地被内核平均掉了,产生了相对稳定的外部偶极磁场。我们推测地磁场倒转的发生只能是一次特大的磁场涨落的结果,其幅度足够大,持续时间足够长,使得磁场在整个内核中也发生倒转。     

扬子地块奥陶系碳酸盐岩重磁化机制探讨

碳酸盐岩是记录古地磁场信息的重要载体,然而,广泛存在的重磁化现象制约了碳酸盐岩在古地磁研究中的应用,其重磁化机制亟待解决.本文对采自贵州羊蹬地区的319块奥陶系碳酸盐岩定向样品作了详细的古地磁学和岩石磁学研究,其结果表明,94%样品(A类)记录了单一剩磁分量A,其解阻温度低于450℃;在地理坐标系下的平均方向为Dg/Ig=3.1°/48.1°(α₉₅=2.9°),对应的古地磁极(87.0°N,2.8°E,A₉₅=3.0°)与扬子地块古近纪-第四纪的古地磁极重合.6%样品(B类)记录了两个磁化分量,其高温分量(450℃~585℃)与A分量显著不同,但明显远离扬子块体早古生代古地磁极;低温分量(< 450℃)与A分量类似.说明羊蹬剖面奥陶系碳酸盐岩记录了两期重磁化.A分量和B低温分量的主要载磁矿物为磁黄铁矿(胶黄铁矿),B高温分量的主要载磁矿物为磁铁矿.这些磁性矿物都是成岩后的次生矿物.其中,解阻温度高于450℃的磁铁矿可能受晚燕山期造山运动影响生成;磁黄铁矿(胶黄铁矿)等矿物可能与印度板块与欧亚大陆碰撞引起的喜马拉雅造山运动所产生的流体作用有关,以后一期重磁化为主.新生代早期青藏高原隆升产生的流体在流经东南缘的碳酸盐岩等沉积岩层时,与原岩发生相互作用,使磁黄铁矿、胶黄铁矿、磁铁矿等磁性矿物生长并获得化学剩磁,造成了广泛重磁化.     

塔里木地块库车坳陷早白垩世古地磁结果及磁倾角偏低的成因探讨

运用主成分分离及线性区段等方法 ,使早白垩世样品明显分离出二组磁组分 .叠加剩磁为喜山期重磁化 ,特征剩磁明显偏离现代地磁场方向 ,经倾斜校正后 ,有很好的一致性并通过了倒转检验 ,给出塔里木地块库车坳陷早白垩世巴西盖组古地磁新数据 .综合已有的古地磁结果 ,获得了塔里木地块早白垩世平均剩磁方向及平均古地磁极 ,阐明了塔里木地块早白垩世磁倾角明显偏低这一现象 .分析导致磁倾角偏低的诸多因素 ,认为压实作用可能是导致磁倾角偏低的重要因素之一 .     

High-frequency polarity swings during the Gauss-Matuyama reversal from Baoji loess sediment

Paleomagnetic records of the Gauss-Matuyama reversal were obtained from two loess sections at Baoji on the Chinese Loess Plateau. Stepwise thermal demagnetization shows two obvious magnetization components. A low-temperature component isolated between 100 and 200–250°C is close to the present geomagnetic field direction, and a high-temperature component isolated above 200–250°C reveals clearly normal, reversed, and transitional polarities. Magnetostratigraphic results of both sections indicated that the Gauss-Matuyama reversal consists of a high-frequency polarity fluctuation zone, but the characteristic remanent magnetization directions during the reversal are clearly inconsistent. Rock magnetic experiments demonstrated that for all the specimens with normal, reversed, and transitional polarities magnetite and hematite are the main magnetic carriers. Anisotropy of magnetic susceptibility indicates that the studied loess sediments have a primary sedimentary fabric. Based on virtual geomagnetic pole latitudes, the Gauss-Matuyama reversal records in the two sections are accompanied by 14 short-lived geomagnetic episodes (15 rapid polarity swings) and 12 short-lived geomagnetic episodes (13 rapid polarity swings), respectively. Our new records, together with previous ones from lacustrine, marine, and aeolian deposits, suggest that high-frequency polarity swings coexist with the Gauss-Matuyama reversal, and that the Gauss-Matuyama reversal may have taken more than 11 kyr to complete. However, we need more detailed analyses of sections across polarity swings during reversals as well as more high-resolution reversal records to understand geomagnetic behavior and inconsistent characteristic remanent magnetization directions during polarity reversals.     

Geomagnetic reversal frequency since the Late Cretaceous

Models of geomagnetic reversals as a stochastic or gamma renewal process have generally been tested for the Heirtzler et al. [1] magnetic polarity time scale which has subsequently been superseded. Examination of newer time scales shows that the mean reversal frequency is dominated in the Cenozoic and Late Cretaceous by a linearly increasing trend on which a rhythmic fluctuation is superposed. Subdivision into two periods of stationary behavior is no longer warranted. The distribution of polarity intervals is visibly not Poissonian but lacks short intervals. The LaBrecque et al. [2] polarity time scale shows the positions of 57 small-wavelength marine magnetic anomalies which may represent short polarity chrons. After adding these short events the distribution of all polarity intervals in the age range 0–40 Myr is stationary and does not differ significantly from a Poisson distribution. A strong asymmetry develops in which normal polarity chrons are Poisson distributed but reversed polarity chrons are gamma distributed with indexk = 2. This asymmetry is of opposite sense to previous suggestions and results from the unequal distribution of the short polarity chrons which are predominantly of positive polarity and concentrated in the Late Cenozoic. If short-wavelength anomalies arise from polarity chrons, the geomagnetic field may be more stable in one polarity than the other. Alternative explanations of the origin of short-wavelength marine magnetic anomalies cast doubt on the inclusion of them as polarity chrons, however. The observed behavior of reversal frequency suggests that core processes governing geomagnetic reversals possess a long-term memory.     

On the identification of magnetostratigraphic polarity zones

Polarity zones of sedimentary sections reflect a pattern of alternating polarity of the geomagnetic field recorded by the remanent magnetization of rocks. Unfortunately, this pattern can have been modified by the variable sedimentation rate, which complicates the identification of polarity zones against the reference geomagnetic polarity time scale. To avoid this obstacle, the present paper suggests a transform applied to both the sequence of levels of polarity reversal horizons and the sequence of ages of polarity reversals before computing their cross-correlation. This transform usually reduces the impact of the variable sedimentation rate so that a sequence of more than eight polarity reversal horizons may be identified without biostratigraphic constraints. Numerical experiments involving random processes to simulate both the duration of polarity reversals and the sedimentation rate proved, however, that not all the parts of a hypothetical stratigraphic section spanning the past 165 Ma would be equally suitable for dating by magnetic polarity stratigraphy. A program performing both the compilation of polarity zones from the directions of the primary magnetization sampled along a section and subsequent identification of these polarity zones is made available online.     

玉树地震的地磁预测研究

利用玉树地震前玉树周围500 km范围内的地磁观测数据,采用地磁垂直分量加卸载响应比、地磁垂直分量日变化幅度逐日比、地磁垂直分量日变化空间相关、低点位移等方法讨论了震源区地磁场变化与地震的相关性,并发现它们之间有较好的对应关系。     

地磁房建设中的磁性检测

德都地震台从2006年4月25日开始进行台站的数字化改造,改造的主要内容是进行地磁绝对观测室的建设和地磁相对记录室的建设。众所周知,地磁观测用房建设难度大,而难度大主要来自地磁观测场地和地磁观测设施的磁性是否合格。本文就是对德都地震台建设地磁房期间有关磁性检测和检测结果的介绍。利用G856磁力仪和CTM-DI磁通门磁力仪,进行野外和台站的材料磁性检测,方便、快捷、准确,是保证地磁观测用房建筑材料磁性合格的好方法。     

MT探测居里面深度的可行性研究

居里面是地球内部铁磁性物质向顺磁性物质转换的界面,在温度略低于居里点时物质磁化率会快速升高,这被称为Hopkinsin峰.对于地球内部而言Hopkinsin峰是只有几百米至几公里厚的薄层,由于其与居里温度的关系,因此其底界面的深度可以作为居里面深度的估计.传统的居里面深度探测方法包括谱分析方法、等层模型方法和温度－深度剖面法.这些方法是人们研究地球内部热结构和居里面深度的重要手段,但是谱分析和等层模型的结果均有一些固有的缺点,如横向分辨率太低等;而地热方程的结果则受地表因素影响十分严重,并且地球内部的热源分布也不是十分清楚,这导致了其结果是不可靠的.本文提出用MT方法探测居里面深度,通过对几种简单一维模型进行的正反演数值试验,论证了该方法的可行性.结果表明,用MT方法研究居里面性质,不但可以得到居里面深度,还可以得到居里面顶部Hopkinsin峰所对应介质的电学和磁学性质,但必须同时反演岩层的电阻率和磁导率,才能获得较为可靠的居里面深度估计.     

国际地磁参考场在我国地磁基础研究中的应用

国际地磁参考场资料在我国得到广泛应用。利用国际地磁参考场资料，我国学者研究了高斯分析、地球磁场模型及其源场可能位置、重磁关系、核幔耦合、地磁场能量、地球非偶极子磁场西向漂移等。在绘制中国地磁等值图中也利用了某些国际地磁参考场资料。     

The geomagnetic field at the Paleozoic/Mesozoic and Mesozoic/Cenozoic boundaries and lower mantle plumes

The data on the amplitude of variations in the direction and paleointensity of the geomagnetic field and the frequency of reversals throughout the last 50 Myr near the Paleozoic/Mesozoic and Mesozoic/Cenozoic boundaries, characterized by peaks of magmatic activity of Siberian and Deccan traps, and data on the amplitude of variations in the geomagnetic field direction relative to contemporary world magnetic anomalies are generalized. The boundaries of geological eras are not fixed in recorded paleointensity, polarity, reversal frequency, and variations in the geomagnetic field direction. Against the background of the “normal” field, nearly the same tendency of an increase in the amplitude of field direction variations is observed toward epicenters of contemporary lower mantle plumes; Greenland, Deccan, and Siberian superplumes; and world magnetic anomalies. This suggests a common origin of lower mantle plumes of various formation times, world magnetic anomalies, and the rise in the amplitude of geomagnetic field variations; i.e., all these phenomena are due to a local excitation in the upper part of the liquid core. Large plumes arise in intervals of the most significant changes in the paleointensity (drops or rises), while no correlation exists between the plume generation and the reversal frequency: times of plume formation correlate with the very diverse patterns of the frequency of reversals, from their total absence to maximum frequencies, implying that world magnetic anomalies, variations in the magnetic field direction and paleointensity, and plumes, on the one hand, and field reversals, on the other, have different sources. The time interval between magmatic activity of a plume at the Earth’s surface and its origination at the core-mantle boundary (the time of the plume rise toward the surface) amounts to 20–50 Myr in all cases considered. Different rise times are apparently associated with different paths of the plume rise, “delays” in the plume upward movement, and so on. The spread in “delay” times of each plume can be attributed to uncertainties in age determinations of paleomagnetic study objects and/or the natural remanent magnetization, but it is more probable that this is a result of the formation of a series of plumes (superplumes) in approximately the same region at the core-mantle boundary in the aforementioned time interval. Such an interpretation is supported by the existence of compact clusters of higher field direction amplitudes between 300 and 200 Ma that are possible regions of formation of world magnetic anomalies and plumes.