非滞后剩磁各向异性

由于非滞后剩磁各向异性和等温剩磁各向异性的参数直接与岩石中的携磁矿物颗粒有关,它比传统的磁化率各向异性更明确地指示岩石组构,在构造地质研究中有较大应用前景．本文介绍了非滞后剩磁及其各向异性的测定方法,并以一个实例阐述了它在构造地质中的应用．     

基于剩磁各向异性方法对华北下三叠统红层磁倾角浅化效应的研究

本文报道利用岩石剩磁组构对华北下三叠统红层进行磁倾角浅化效应的进一步识别与校正研究结果.首先,采用45°等温剩磁各向异性方法,即通过沿与样品原始水平面(即层面)呈45°夹角方向施加磁化场获得等温剩磁,并进行逐步热退磁,获得平行于层面和垂直于层面的等温剩磁分量随外加磁场和热退磁温度的变化趋势,计算获得浅化因子f=0.70.其次,应用高场等温剩磁各向异性方法,结合峰值为100 mT的交变退磁和120℃热退磁处理,分离获得碎屑赤铁矿对剩磁各向异性的贡献;由直接测量获得的单颗粒碎屑赤铁矿的各向异性度(a=1.35),计算获得f=0.59.该结果与前人对刘家沟组红层进行E/I法磁倾角浅化校正的结果(f=0.60)具有很好的一致性;表明华北下三叠统刘家沟组红层磁倾角浅化效应显著,其浅化因子为f=0.59;高场等温剩磁各向异性方法是红层磁倾角浅化校正的最有效方法.同时,如果有足够的独立样品数,且特征剩磁来自于单一剖面或有证据表明多条采样剖面之间未发生显著的相对运动,E/I法对红层磁倾角浅化因子的估计也是可信的.     

磁组构研究现状

磁组构技术作为一种快速、经济和无损性方法测量岩石组构方法,已被广泛用于地质和古今环境研究。随着测试仪器和磁学理论的不断完善,除磁化率各向异性（ＡＭＳ）测量外,近几年非磁滞剩磁各向异性（ＡＡＲ）和等温剩磁的各向异性（ＡＩＲ）的研究较多。同时随着扫描电镜等非磁方法研究和岩石磁性参数测量相结合,人们认识到磁性矿物组成对磁各向异性的重要控制作用。岩石应变磁组构测量在过去几年得到较大发展。岩浆岩的研究主要信     

华北地块早古生代地层单元的岩石磁学特征研究

通过对采自华北地块西部鄂尔多斯盆地边缘早古生代地层单元中的４４块定向岩芯样品的岩石磁学实验（饱和等温剩磁及剩磁矫顽力的测量，三轴磁化的饱和等温剩磁的系统热退磁和低温实验）研究，揭示出华北地块早古生代地层单元的各类岩石中，主要载磁矿物的构成具有磁铁矿或赤铁矿与中等居里温度的磁黄铁矿、磁赤铁矿等共存的特征；个别地层单元的岩石中以极低居里点的针铁矿为主要载磁矿物．     

岩石磁组构对剩磁稳定性的制约探讨:以印支地块中生代碎屑岩和拉萨林周盆地设兴组红层为例

岩石磁组构因能提供磁性矿物晶体形状、排列方式等赋存信息而被广泛应用于判别岩石剩磁是否受到了后期构造应力的显著影响;但常规岩石磁化率各向异性(AMS)是否能够准确限定岩石剩磁的稳定性,目前尚无深入探讨.本文以印支地块Nakhon Thai盆地中生代Nam Phong、Phu Kradung和Phra Wihan组三套碎屑岩样品及拉萨地块林周盆地设兴组红层样品为例,通过岩石磁化率组构和剩磁组构的对比分析发现,尽管Nam Phong组绝大多数样品和Phu Kradung组全部样品的AMS组构显示其具有铅笔状至强劈理过渡型构造变形组构特征,但高场等温剩磁各向异性(hf-AIR)显示其高矫顽力赤铁矿所携带的特征剩磁组构仍具有典型沉积组构特征,表明其以赤铁矿为主的载磁矿物未遭受后期构造应力的显著影响,仍然能够准确记录岩石形成时期的古地球磁场方向.另一方面发现有且仅有剩磁组构才是判别碎屑沉积岩特征剩磁是否遭受了后期构造应力影响的充分必要条件.也就是说,如果岩石剩磁组构(如以赤铁矿为主要载磁矿物的岩石hf-AIR组构)指示其原始沉积组构已被构造组构显著叠加或取代,则必然说明该岩石剩磁方向已受到构造应力...     

塔里木盆地中生代古地磁研究及构造意义

采用主成分分析方法、线性谱法和LINFIND方法，分离了塔里木盆地中生代岩石的多磁成分，并采用岩石磁学和褶皱检验等方法研究了剩磁稳定性．对于塔里木盆地三叠系、侏罗系的岩石其携磁矿物以磁铁矿为主，具有两组次生磁性成分和一组原生剩磁成分；原生剩磁成分的解阻温度为550℃，而两组次生成分的解阻温度分别为175－250℃和300一375℃，可能为生物剩磁和次生氧化形成的剩磁．白垩纪岩石的携磁矿物以亦铁矿为主，同样具有两组次生剩磁和一组原生剩磁；两组次生剩磁分别为现代地磁场的粘滞剩磁和岩石变形过程中形成的构造剩磁．塔里木盆地中生代早期极移不明显，处于一个相对平静时期；侏罗纪至白垩纪盆地则主要表现有一定规模的南移，伴有顺时针旋转运动。盆地白垩纪的古纬度与现在纬度相比，仍存在17°－20°左右的纬度等，这一纬度差是通过白垩纪以后塔里木板块的北向漂移和板块北部造山带的压缩及边界的大型走滑作用来缩小；另外，压实作用也可能是由垩纪磁倾角变低的一个原因。     

巴黎盆地下侏罗统的岩石磁学研究及其意义

通过下佛罗统磁性地层学研究,获得了一些能记录岩石形成时的地磁场的样品,根据常规的古地磁方法,等温剩磁和热退磁实验,说明这些岩石的主要载磁矿物为磁铁矿.磁滞回线参数的测量表明多数载磁矿物磁铁矿为假单磁畴,少量样品则可能为假单磁畴与多磁畴的混合物.通过与北美古生代重磁化岩石的磁滞回线参数比值对比,说明重磁化与非重磁化岩五磁滞回线参数比值的分布趋势有明显差别,这一差别可能用于判定岩石的重磁化和非重磁化性质.     

巴黎盆地下侏罗统的岩石磁学研究及其意义

通过下佛罗统磁性地层学研究,获得了一些能记录岩石形成时的地磁场的样品,根据常规的古地磁方法,等温剩磁和热退磁实验,说明这些岩石的主要载磁矿物为磁铁矿.磁滞回线参数的测量表明多数载磁矿物磁铁矿为假单磁畴,少量样品则可能为假单磁畴与多磁畴的混合物.通过与北美古生代重磁化岩石的磁滞回线参数比值对比,说明重磁化与非重磁化岩五磁滞回线参数比值的分布趋势有明显差别,这一差别可能用于判定岩石的重磁化和非重磁化性质.     

四川盆地早白垩世红层磁倾角偏低研究

对四川盆地东北部巴中地区和西部雅安地区早白垩世红层分别采集9块手标本,进行了详细的古地磁学研究.系统退磁获得两地的特征剩磁,其中巴中地区平均方向为,倾斜校正之前:D_g/I_g=25.3°/19.0°,k=18.6,α_(95)=8.8°;倾斜校正之后:D_s/I_s=25.8°/18.9°,k=24.3,α_(95)=7.6°.雅安地区平均方向为,倾斜校正之前:D_g/I_g=24.5°/45.0°,k=15.7,α_(95)=9.0°;倾斜校正之后:D_s/I_s=356.7°/35.6°,k=28.5,α_(95)=6.6°.磁化率各向异性实验结果显示两地均未受到显著的构造应力影响.等温剩磁各向异性(AIR)实验结果,巴中地区IRMz/IRMLx平均值为0.8194,表明18%的压实率;雅安地区IRMz/IRMx平均值为0.8909,表明11%的压实率,计算得到巴中和雅安地区校正后的磁倾角分别为22.7°和38.8°.根据等温剩磁各向异性(AIR)实验和Tauxe and Kent(2004)提出的EI校正法得到的结果表明,四川盆地早白垩世陆相碎屑岩层中存在沉积压实作用造成的磁倾角偏低现象,而且川东北巴中地区的偏低程度强于川西雅安地区.     

甘肃西山坪遗址岩石磁学性质及其研究意义探讨

对甘肃西山坪遗址剖面样品进行磁化率、频率磁化率、热磁分析、非磁滞剩磁和等温剩磁等一系列岩石磁学实验测试,分析环境磁学参数特征变化,试图探讨西山坪遗址研究意义,为今后深入研究该沉积序列蕴含的古环境记录提供了岩石磁学基础.结果表明：西山坪遗址剖面沉积物中磁性矿物主要以低矫顽力的磁铁矿和磁赤铁矿为主,高矫顽力磁性矿物赤铁矿含量极其有限,越向剖面表层,强磁性矿物含量逐渐增加.磁性矿物颗粒表现出黄土的典型特征,以单畴、准单畴为主,含有一定量的超顺磁颗粒,颗粒度越向剖面表层越细.从物源上看,遗址沉积物主要来自近源的黄土风尘堆积,主要是风成堆积,并混合大量人类活动信息,给今后综合研究岩石磁学与人类活动特征的关系有一定借鉴意义.     

Magnetostratigraphy of the lower member of the hadar formation (Ethiopia): Evidence for a short normal event in the mammoth subchron

Summary Rock magnetism and magnetostratigraphy of the lower part of the Hadar Formation (Afar, Ethiopia) is presented after analysis of multiple new collection of samples from over 84 horizons. The Hadar Formation is composed of lacustrine, lake margin, fluvial and flood plain sediments and known for important Pliocene vertebrate faunas including Australopithecus afarensis. Hysteresis measurements, thermomagnetic analysis, growth and decay of isothermal remanent magnetisation are used to unravel the complex magnetic mineralogy of the different representative lithologies. Ferrimagnetic minerals of magnetite or titanomagnetite in composition, in the stable pseudo-single domain (PSD) size range are found to be the main carriers of the remanence. In most sites the characteristic remanence was isolated using stepwise thermal demagnetisation. The overall mean direction for about 72 horizons (434 samples) is D=358·6°, I=7° (k=17·9, α₉₅=4°) implying some 14° of inclination shallowing, related to sediment compaction due to the very rapid sedimentation history of the site. Five successive polarity zones (N1-R1-N2-R2-N3) are identified and correlation with the lower Gauss chron of the astronomically calibrated geomagnetic polarity time scale (GPTS) is proposed using the existing⁴⁰Ar/³⁹Ar ages. This implies the existence of a short normal polarity event (N2), identified on six different sites, within the reversed Mammoth subchron, called the Kada-Hadar event. The age calculated for the Kada-Hadar event, using linear interpolation of the dated horizons, assuming a constant rate of sedimentation is 3.246 Ma and its duration is about 8 kyr.     

Paleosecular variations 12–20 Kyr as recorded by sediments from Lake Moreno (Southern Argentina)

We present results of paleomagnetic and sedimentological studies carried out on three cores Lmor1, Lmo98-1, Lmor98-2 from bottom sediments of Lake Moreno (south-western Argentina), and integrate them with data from our previous studies. Measurements of directions (declination D and inclination I) and mass specific intensity of natural remanent magnetization (NRM intensity), magnetic susceptibility (specific, χ and volumetric, κ), isothermal remanent magnetization (IRM), saturation of isothermal remanent magnetization (SIRM), and back field remanent coercivity (B_0CR) were performed. The stability of the NRM was investigated using alternating-field demagnetization. The results show that these sediments meet the criteria required to construct a reliable paleomagnetic record. The cores were correlated very well based on magnetic parameters, such as χ and NRM intensity, as well as with lithological features. Tephra layers were identified from the lithological profiles and magnetic susceptibility logs. We obtained the D and I logs of the characteristic remanent magnetization for the cores as a function of shortened depth. The data from the three cores were combined to form a composite record using the Fisher method. A comparison between stacked inclination and declination records of Lake Moreno and those obtained in previous works on Lake Escondido and Lake El Trébol shows good agreement. This agreement made it possible to transform the stacked curves into time series spanning the interval 12–20 kyr. The results obtained improved our knowledge of SV and the behaviour of the geomagnetic field and also allowed us to determine the range of past inclination variations from −70° to −45° for the southern hemisphere, where data are scarce.     

山西吉县沃曲桃园下三叠统刘家沟组红层的古地磁研究

本文对鄂尔多斯盆地东南缘一个背斜剖面的早三叠世红层样品进行了古地磁研究.对逐步热退磁矢量序列进行主成分分析以及各磁组分的解阻温度谱分析,估计了携磁矿物.分离出4种磁成分:镜铁矿携带的沉积或沉积后剩磁;赤铁矿携带的化学剩磁;等温剩磁和粘滞剩磁.特征剩磁（最高解阻温度T_u≥670℃）通过倒转检验、（递增）褶皱检验,平均方向为偏角D=-25°,倾角I=41°,相应的极位置为65°N、356°E.     

Paleomagnetic and sedimentological studies at Lake Tahoe,California-Nevada

Three closely spaced 6-m piston cores were taken in the central part of Lake Tahoe. Cores were split into two complete replicates for paleomagnetic study and the remaining sections were used for stratigraphic and mineralogical analysis.Stratigraphic correlation of the cores is based on two distinctive horizons (volcanic ash and diatomite) and upon three different sedimentological regimes dominated by (1) poorly bedded silts and muds, (2) well bedded graded units, and (3) finely laminated silts. These correlations served as the standards for the evaluation of the paleomagnetic data. Extrapolation of¹⁴C dates obtained in the upper sections of the Lake Tahoe sediments suggests that the lower sections of the cores may reach ages of 25,000–30,000 years B.P.X-ray, optical, Curie point, and hysteresis measurements show that magnetite is the only important magnetic mineral in the sediments and occurs in the size range of 10 μm. Hematite is essentially absent. Based on large changes in the declination and inclination of the natural remanent magnetism (NRM) within single graded layers the paleomagnetic signature is a post-depositional remanent magnetism (PDRM). This PDRM is believed to be caused by magnetic orientation during compaction.Paleomagnetic measurements show three regimes that are correlated with the stratigraphic regimes. NRM declination and inclination data show good correlation between the three cores and agree well with the correlations based on sediment character. NRM intensity variations are due largely to the variations in magnetite content and its occurrence as either single detrital grains or as inclusions within the larger silicates. Thus the variation in paleo intensity was not determined.Comparisons of Lake Tahoe data with that from Mono Lake show fair correlations of declination and inclination. The occurrence of a short-wavelength, high-amplitude event in the lower section of the Lake Tahoe cores may provide confirmation of the Mono Lake geomagnetic excursion.     

Paleomagnetism of carboniferous formations of the Russian platform: Reinvestigation of old collections

The collections of Carboniferous rocks from sections of the Russian Platform (Gzhelian, Moscovian, Bashkirian, and Visean stages) are studied. The new mean paleomagnetic poles are obtained from the Gzhelian, Moscovian, and Visean layers of the Carboniferous of the Russian Platform. In the redbed Gzhelian and Moscovian rocks, the natural remanent magnetization (NRM) components with the inclination shallowing are revealed, which is due to the presence of the large hematite particles or particle aggregates associated with the interaction between the magnetic and clay particles. Based on the obtained determinations and the results contained in the World paleomagnetic database, the trajectory of the apparent polar wander path (APWP) for the East European Platform is constructed in the interval from the Devonian to Early Permian. The Carboniferous kinematics of the East European Platform is estimated.     

Inclination shallowing and preferred transitional VGP paths

Transitional VGP paths recorded in sediments cluster into two antipodal preferred longitude bands that tend to lie 90° away from their site longitudes, the latter also being clustered. VGP paths obtained from lava flow sequences, though much fewer, appear not to show these biases, suggesting a rock-magnetic influence on VGP paths recorded in sediments. Inclination shallowing of detrital magnetic remanence, enhanced under low transitional field strengths, is the most likely candidate.

We illustrate the effects of inclination shallowing by applying a simple shallowing model (tan I_R = f tan I_A, where I_A is the inclination of the magnetic remanence and I_A is the inclination of the ambient field) with field variation to hypothetical data sets. Shallowing-induced clustering increases as f decreases and becomes extreme as f approaches 0.1.

We have used the model to ‘de-shallow’ the available set of transitional VGP sediment records for various values of f. The probability that the observations arise from inclination shallowing of a uniform random distribution of paths increases as f decreases. When f drops to 0.13 there is a 50% chance of getting at least as much grouping as observed. To decide if inclination shallowing is a dominant factor in the clustering, we need to know whether such extreme shallowing is widespread in sedimentary records under transitional field conditions. Field and laboratory redeposition data are not yet adequate to resolve this question.     

Paleomagnetism of Pleistocene widespread tephra deposits and its implication for tectonic rotation in central Japan

Abstract   A single layer of widespread tephra deposits possibly can provide an instantaneous record of the past geomagnetic field and potentially can indicate even a small-scale tectonic rotation compared to a range of geomagnetic secular variations. We report paleomagnetic data of the Ebisutoge–Fukuda tephra, which is dated at approximately 1.8 Ma and is distributed in central Japan between the Osaka–Kyoto area and the Boso Peninsula. The Fukuda volcanic ash layer and its correlative ash deposits in the Osaka–Kyoto area, near Lake Biwa and in the Mie and Niigata areas yield identical site mean declinations of approximately −170° after tilt correction, whereas moderate inclination shallowing is observed in the upper unit at several localities. Anisotropy measurements both of low-field magnetic susceptibility and of anhysteretic remanent magnetization suggest that the inclination shallowing results from the biased alignment of magnetic grains, which were deposited in the fluvial environment. The source volcanic unit, Ebisutoge pyroclastic deposits in the Takayama area, yields a mean declination of approximately −155°, showing clockwise deflection from the magnetic directions of the correlative tephra deposits. These results suggest that no significant rotation occurred between the Osaka–Kyoto, Mie and Niigata areas, but that the Takayama area suffered a clockwise rotation in respect to the other areas during the Quaternary. This rotation might have been caused under an east–west stress field associated with the collision of the Okhotsk Plate with the Eurasia Plate.     

中国东部红土的磁性及其环境意义

通过中国东部红土剖面的环境磁学参数（磁化率、频率磁化率、非磁滞剩磁、饱和等温剩磁等）测量,获得了红土剖面磁性矿物浓度、粒度和类型等特性随深度的变化曲线以及红土经连二亚硫酸钠一柠檬酸钠一重碳酸钠溶液（ＤＣＢ）处理后的磁性参数变化．根据红土剖面环境磁学参数及其磁参数比值的变化可将红土分为３个层段,各层段的磁性矿物特征存在明显的差异．证实了红土剖面中的磁性载体主要是磁赤铁矿、赤铁矿和针铁矿,并分离出了球粒状磁颗粒．认为红土磁性矿物的数量、粒度、类型等的变异指示了其形成时的环境特征,其频率磁化率和ＤＣＢ处理的磁化率损失量指示了红土成壤化作用的强弱,可作为在红壤区研究过去全球变化的一种新途径．     

扬子克拉通太古代崆岭群角闪岩相与麻粒岩相岩石的岩石磁学研究

提供了扬子克拉通太古代基底崆岭群２０件代表性岩石样品的密度与磁性参数的测量结果．结果表明，变碎屑岩与ＴＴＧ（英云闪长－奥长花岗－花岗间长质）片麻岩（１３个样品）的磁化率与饱和剩磁普遍大于斜长角闪岩和辉长岩．前者的磁化率与饱和剩磁平均值分别为１２１３×１０^－６ＳＩ与１９．９４Ａ／ｍ，而后者的则为８０２×１０^－６ＳＩ与１０．７７Ａ／ｍ．变碎屑岩与ＴＴＧ片麻岩呈明显的亚铁磁性状态；斜长角闪岩和辉长岩则以顺磁性或顺磁性与亚铁磁性混合状态分布为主．热磁分析结果表明，变碎屑岩与ＴＴＧ片麻岩的剩磁载体以磁铁矿与磁赤铁矿为主；斜长角门岩和辉长岩则明显含有磁黄铁矿与次要的磁铁矿．变碎屑岩与ＴＴＧ片麻岩的磁性具有很强的非均一性．推断崆岭群中的变碎屑岩为麻粒岩相变质级，视深度相当于大陆下地壳；而斜长角闪岩的变质级明显低于变碎屑岩，可能为角门岩相，视深度相当于中地壳．变碎屑岩与ＴＴＧ片麻岩磁性的强非均一性可能与该区后期广泛发生的混合岩化作用密切相关。