Acquisition of long wavelength magnetic anomalies pre-dates continental drift

Using the low altitude MAGSAT satellite data, NASA recently published a new global magnetic map of anomalies. Using this new accurate map and paleomagnetic reconstructions of Gondwanaland, we point out that long wavelength anomalies were continuous in continents that are presently separated. Thus, continental drift would not significantly affect the geometry of these deep-source anomalies. Accordingly, the magnetization of the rocks responsible for these anomalies should mainly be induced.     

Late Mesozoic palaeomagnetism from Spitsbergen; implications for continental drift in the Arctic

Thermal and alternating-current demagnetization combined with ore microscopy and measurements of the temperature dependence of saturation magnetization have been carried out on some Mesozoic, probably Cretaceous, basaltic lavas from two areas (Seidfjell and Sørlifjell) at Spitsbergen. The experimental studies suggest that the Seidfjell locality has undergone extensive oxidations, which resulted in remagnetization. The estimated palaeomagnetic pole for this area is 77°N 107°E, which suggests a remagnetization, probably some time in the Late Tertiary. On the other hand the experimental data from the Sørlifjell locality suggest that the magnetization is primarily of deuteric origin. The mean palaeomagnetic pole position for this latter formation is at 75°N 235°E, which is significantly different from previously published European Mesozoic data. However, closing the Neo-Arctic basin by rotating Spitsbergen towards the Lomonosov Ridge, makes the suggested Cretaceous pole coincide with poles of similar age from North-America. This suggests that the estimated pole from Sørlifjell is a good approximation for a Late Mesozoic palaeomagnetic pole for Europe and it also confirms that the process of continental separation in the Arctic has taken place in Tertiary time.     

Abyssal hills,magnetic anomalies and the east pacific rise

Long ridges and troughs are characteristic of the East Pacific rise off North and Central America. The trends of the topography are parallel to the trends of magnetic anomalies but are not specifically related. Locally the topography, like the magnetic anomalies, shows a gross symmetry and the sea floor spreading hypothesis could account for that fact. Simultaneous rifting and accumulation of lava flows around opening rifts appear capable of generating this topography.     

The palaeomagnetism of muddy sediment cores from the Inner Sound,Northwest Scotland,and the glacial and post-glacial history of sedimentation in the area

Josephinite is a terrestrial iron-nickel alloy with an intergrown magnesium silicate, and arsenide and sulphide phases, and andradite garnet; several specimens have been found to contain elemental silicon and CaO · 2“FeO”. Josephinite is not awaruite, an iron-nickel mineral formed by serpentinization of ultramafic rocks. Because of its geologic setting and unique mineralogy we propose that josephinite might have originated in the region of the coremantle boundary, was transported via a deep-mantle “plume” and diatreme mechanism into lithosphere mantle that has been emplaced in the Klamaths by ophiolite obduction. Regardless of such a hypothesis, we report here the discovery of terrestrial silicon occurring with josephinite, which seems to preclude a lithosphere environment of origin for josephinite.     

Paleomagnetism and magnetic anomalies in Iceland

The paper reviews several aspects of paleomagnetic research on Icelandic rocks in past decades, with emphasis on studies of remanence directions in composite lava sections of 1–15 Ma age. We first describe briefly the physical basis of this research and experimental techniques, and list the major studies carried out so far. This is followed by a discussion of the internal consistency of directional results, effects of alteration on magnetic properties, rates of buildup of the lava pile, and the use of paleomagnetic reversals and excursions in stratigraphic mapping. Another section of the paper discusses some contributions which paleomagnetic research in Iceland has made to knowledge of properties of the geomagnetic field.A final section is devoted to regional magnetic surveys over Iceland and the surrounding shelf, giving examples of how their results have revealed various features of geological structures in that area. Knowledge of the magnetization of basement rocks aids in the interpretation of the magnetic anomalies, but more detailed and comprehensive studies of these structures are needed.     

Note on the palaeomagnetism of the Sekondi series,Ghana

From a collection of 39 oriented hand-samples at 16 sites, total N.R.M. directions at 12 sites from the Elmina Sandstone (Devonian or possibly Carboniferous) of the Ghana coast fall in a group. Their in-situ mean (D = 334°, $\text{I = +11}\text{1}\text{2}\text{°}$) is significantly divergent from the local geomagnetic field, and does not correlate with expected Palaeozoic remanence directions. A bedding-tilt test suggests that the magnetisation is secondary, and comparison with other African data suggests a Mesozoic (possibly Cretaceous) age. The remanence is only partially stable against thermal demagnetisation. The observations are consistent with a remanence originating at the time of faulting, tilting and uplift which marked the beginning of rifting of South America from Africa.     

低纬度磁异常的转换与处理

我国的南海大部分海域位于磁赤道带附近,属于低磁纬度区域,以水平磁化为主,磁性体产生的△T异常特征与中国大陆广大地区的△T异常特征差别较大,且南北跨度大,达两千余公里,通常的化极技术在该区域很难取得良好的效果,虽然化赤方法在低纬度地区算法稳定,并通常能取得较好的结果,但是目前针对化赤异常的解释技术却相对较少;本文结合南海地区的航磁数据,对几种常用的化极技术进行了对比与分析,并对化赤技术进行了简要的介绍.     

Continental magnetic anomalies and the evolution of the Scotia arc

Linear belts 50–100 km in width of long-wavelength positive magnetic anomalies exceeding 500 nT are observed on continental blocks of the Scotia arc. The most developed is the West Coast Magnetic Anomaly which may be traced for more than 1300 km along the Antarctic Peninsula. Comparison of magnetic profile data after low-pass filtering and reduction to pole reveals a striking similarity between the individual anomaly belts. Correlation of the anomalies with positive gravity anomalies, seismic refraction data and geology indicates that the sources are linear batholiths intruded during Mesozoic/Cenozoic subduction. The anomaly belts are truncated at the block margins reflecting Cenozoic fragmentation of a cuspate convergent margin. An early Cenozoic reconstruction, based on the assumption that the batholiths once formed a continuous curvilinear feature, shows a good alignment of Mesozoic strato-tectonic terrains and is compatible with the known history of Scotia Sea opening.     

地壳磁异常的全球模型

从全球磁场减去地核主磁场和变化磁场及其感应磁场后,就得到岩石圈磁场,又称地壳磁场,或地磁异常.地壳磁场是地磁场的重要组成部分,对导航、通信、地球物理勘探等有重要意义.本文简要综述近年来在地壳磁场基本信息获取方面取得的重大进展,特别是Oersted、CHAMP等卫星磁侧的最新成果,介绍在此基础上建立的全球磁异常模型,展望地壳磁场研究的发展趋势和应用前景.     

Comments on the interpretation of linear magnetic anomalies

Summary In response to the recent interpretation of marine magnetic anomalies in terms of the crustal spreading hypothesis, a summary is made of other factors including structural, initial cooling, post cooling, and possible spontaneous processes which may cause at least some of the characters of linear magnetic anomalies. Arguments are developed by presentation ofQ values for 571 specimens from fourteen Icelandic lavas and six dikes, and by theQ value, stability factor, and oxidation variation between the cooling faces of one of these lavas. None of the processes described can alone readily account for the observed symmetry of magnetic anomalies with respect to mid-oceanic ridges, but are presented to encourage examination and development of mechanisms other than geomagnetic polarity changes superimposed on a crustal generating system, as the causes of at least some oceanic magnetic anomalies.     

Future of the interpretation theory of gravity and magnetic anomalies

The general methodology of the interpolation theory of potential fields (gravity and magnetic anomalies), and, primarily, the three fundamental ideas underlying this methodology—(I) approximation, (II) linearization, and (III) optimization-are the matter of the utmost importance. The rationale of these ideas and the ways of their practical implementation are discussed in this paper.     

Morphology,magnetic anomalies and basalt magnetization at the ends of the Galapagos high-amplitude zone

Detailed bathymetric and magnetic data, complemented by nine dredge stations, define the eastern and western limits of a belt of high-amplitude magnetic anomalies associated with the Galapagos hot spot. The hypothesis of “magnetic telechemistry” was tested and locally confirmed. High amplitudes correspond to high remanence, susceptibility, FeO^T, TiO₂, and presumably titanomagnetite concentration. The average remanence of surface samples in the high-amplitude zone is 0.027 emu/cm³ (range, 0.009–0.085 emu/cm³), about 4 times that of the normal-amplitude zone. Magnetic amplitudes are only 2–2.5 times higher, however. If the greater TiO₂/FeO^T ratio of high-amplitude zone basalts also characterizes the titanomagnetites, remanence in the high-amplitude zone may fall off more rapidly with depth in the crust as a result of reheating. Alternatively, small pillows of high remanence are more common than larger pillows at the top of the high-amplitude zone crust; FeTi basalt may also be concentrated in the upper part of the crust. Anomaly amplitudes are highest at the ends of the zone, particularly in the east. As asthenosphere crystal slushes presumably flow away from the Galapagos plume, progressive crystal fractionation may enrich residual magmas in FeO^T and TiO₂. The Galapagos FeTi zone terminates abruptly against transform fractures at both ends, perhaps because subaxial flow is dammed at the transforms. The FeTi-producing crystal slushes have advanced east and west at speeds up to 10 cm/yr since they first appeared at the spreading axis at least 6.6 m.y. B.P. Their progressive advance was connected with the progressive southward jumps of the spreading axis east of the Galapagos hot spot, and northward jumps to the west.     

Interpretation of gravity and magnetic anomalies near Naples,Italy, using computer techniques

Some aspects of the analytical procedure for an automatic fitting of gravity and magnetic anomalies are discussed. An example of application relative to the Campania volcanic district near Naples, Italy, is reported. Gravity anomalies in that area mainly reflect the depth to the carbonate basement rocks. A small gravity high over Vesuvius may indicate buried volcanic layers. Magnetic anomalies near L. Patria and Castel Volturno are interpreted to be caused by buried intrusive rocks of higher susceptibility. Computer models of subsurface rock units with appropriate densities and susceptibilities have been generated to match the observed anomalies.     

Late Cretaceous magnetic anomalies in the North Atlantic

An identification of anomalies 31–34 is presented for the North Atlantic. North of the Azores-Gibraltar Ridge this implies a revision of the identification of the magnetic anomalies older than anomaly 26. DSDP site 10 in the western North Atlantic appears to be located on the old end of anomaly 33. The relative spacings of anomalies 29–34 in the North and South Atlantic, North and South Pacific and Indian Oceans are compared and the estimated relative widths of the magnetic polarity intervals in the Late Cretaceous are revised.     

Exploration of magnetic anomalies of the Baikal rift zone

The paper presents the results of an experimental study of natural magnetic field anomalies in the Baikal rift zone.     

Studies of crustal magnetic anomalies of the British Isles

Andrew Jackson examines the subtle but significant question of the distinction between induced and remanent magnetism, using the British Isles as an example. This continuing project formed part of his Bullerwell Lecture given in 2006.     

A large volume magnetic field free space for thermal demagnetization and other experiments in palaeomagnetism

Summary Three orthogonal coil systems have been constructed within a cube of side 2.5 m to cancel the geomagnetic field to less than 1 gamma over a large volume. The north-south and vertical components are cancelled by systems of four square coils whilst a square Helmholtz pair cancels the east-west component. Cancellation to 1 part in 10⁵ (0.5 gamma in Canberra) is achieved within a sphere of diameter 35 cm, more than 20 times the field-free volume which could be obtained using conventional Helmholtz coils within the available space. Three fluxgate detectors monitor the residual field and their output, taken through a feedback circuit, allows compensation for field variations. The residual field can be maintained to less than 0.5 gamma over the working volume of about 22,000 cm³ during the critical part of a laboratory experiment.     

云南地区地壳磁异常与地质构造

根据美国地球物理数据中心的地磁场模型（NGDC-720）,研究云南地区地壳磁异常的空间变化,包括磁异常及其梯度的分布、磁异常随高度的衰减、不同波长成分对总体磁异常的贡献.比较磁异常与重力异常、大地热流、地震活动等地球物理信息的异同特征,探讨地壳磁异常与地质构造的关系.研究结果表明：丽江-小金河、红河断裂带以西的三江、滇西造山带的异常较弱,菱形地块的异常相对较强.研究区域的地壳磁异常主要是弱磁性基底背景下叠加的浅源磁性体产生.卫星磁异常显示滇中坳陷区具有清晰的偶极场特征.沿红河断裂带分布的正负磁异常带与断裂构造走向一致.围限菱形地块的丽江-小金河断裂、红河断裂、康定-奕良-水城断裂和弥勒-师宗-水城断裂带是磁异常的强弱过渡带.强烈地震发生的地点、大地热流值高的地区,地壳磁异常为负值或相对较弱.

     

重磁弱异常评价与提取技术

本文阐述了重磁弱异常概念,提出了当代采集方式下重磁弱异常可信度的判断标准,并提出针对重磁弱异常的处理新技术,该技术由消除干扰处理技术和弱异常提取技术组成,实际应用效果显著。随着地质勘探向精细方向发展,重磁弱异常将得到更加广泛的关注。