Oxfordian magnetostratigraphy of Britain and its correlation to Tethyan regions and Pacific marine magnetic anomalies

A suite of 11 sections through the Oxfordian (Upper Jurassic) strata in the Dorset and Yorkshire regions of England and the Isle of Skye in Scotland yielded magnetic polarity patterns directly calibrated to the ammonite biostratigraphy of the Boreal and the Subboreal faunal provinces. The sections include the leading candidate for the global stratotype (GSSP) for the Callovian–Oxfordian stage boundary. The mean Oxfordian paleomagnetic pole derived from the Dorset and Yorkshire sections is 71.3°N, 172.6°E (δp = 4.2°, δm = 6.1°). The integrated magneto-biostratigraphic scale is consistent with results from the Sub-Mediterranean faunal province and extends the polarity pattern to the base of the Oxfordian. After adjusting for the estimated durations of ammonite subzones from cycle stratigraphy, the magnetostratigraphy confirms models for marine magnetic anomalies M30 through to M37, including some of the short-duration features recorded by deep-tow magnetic surveys in the western Pacific. The Callovian–Oxfordian boundary (base of Quenstedtoceras mariae Zone) occurs in a normal-polarity zone that is correlated to the youngest part of polarity chron M37n of this extension to the M-sequence.     

The nature of magnetic anomalies in subduction zones

The analysis of the magnetic survey data suggests the presence of a frontal zone of intense magnetic anomalies in a number of the Pacific island-arc systems. These zones with amplitudes of 100–300 nT are observed within the Kuril–Kamchatka and Aleutian island arc systems, in Southern and Central America, and Alaska. As demonstrated by the solution of the inverse problem and petromagnetic investigation of the rocks, these zones are presumably related to the serpentinite bodies which form as a result of the hydration of the upper mantle peridotites by the oceanic water penetrating through a system of cracks and fractures into the subducting slab at its bend. The rock magnetic studies show that magnetite is the main carrier of magnetization in these serpentinite bodies. Hydration of the subducting slab also causes hydration of the mantle rocks of the overriding plate with the formation of the magnetized serpentinite wedge. The decompaction of ultrabasic rocks under hydration is marked by a decrease in the gravity field and velocities of elastic waves. As the subducting plate loses water, it becomes embrittled and becomes the localization region for the epicenters of the strongest earthquakes. Magnetic survey can be used for revealing the potential sources of catastrophic earthquakes.     

Interpretation of magnetic anomalies of dikes using correlation factors

The magnetic anomaly due to a buried dike consists of the sum of two easily separated elementary functions. These functions, which have simple symmetry, are called even and odd functions. The correlation factors (r _0,1 for the even andr _0,2 for the odd function) between least-squares residual anomalies from even and odd functions are computed. Correlation values are used to determine the depth to the top and the half-width of the dike. The method also includes the determination of the index parameter and the amplitude coefficient. The validity of the method is tested against a theoretical and a field example where the parameters of the latter were determined by other investigators in comparing the results.     

泉州四至井水氡异常及中国台湾东南部(包括附近海域)地震的对应关系

对泉州四至井2006年1月至2008年12月水氡观测数据进行分析与处理,依据2倍中误差、短期阶升突跳、短期和中长期趋势异常等多种判别法,对其水氡异常进行判别。结果表明:泉州四至井水氡氡值基本保持春夏高、秋冬低的弓形年变化动态图像,出现的水氡异常对中国台湾东南部及附近海域有较好的对应关系,并对2006年7月28日花莲6.0级、2006年12月26日恒春7.2级、2007年9月7日台东6.3级地震进行案例分析,有关研究结果对寻求泉州四至井水氡异常对中国台湾地区某些敏感对应点的映震能力具有一定的现实意义。     

Magnetic smooth zones and slope anomalies: a mechanism to explain both

Marginal magnetic smooth zones occur only along continental margins where these are oblique (not normal) to the regional fracture-zone trend.This relationship suggests a simple geometric mechanism which can generate marginal smooth zones and abyssal smooth zones during times of typical reversal history.The mechanism provides an explanation of “slope anomalies” and other smooth-zone characteristics, and casts doubt on the validity of using smooth zones as evidence for long fixed polarity intervals.     

Resolvability of the interval between inversions using marine magnetic anomalies based on the Rao-Cramer inequality

It has been indicated that the Rao-Cramer inequality can be used to estimate the resolvability of intervals with opposite geomagnetic field polarity based on marine anomalies and for measurement planning. Specifically, it has been elucidated that the width of a detected interval of one polarity is determined more exactly than its center.     

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.     

Interpretation of gravity and magnetic anomalies caused due to spherical bodies

Summary The method of continuation has been used to obtain the master curves for gravity and magnetic anomalies caused by spherical bodies. The procedure to calculate the depth of burial and radius of spherical bodies has been outlined.     

基于重磁异常相关分析的场源位置及属性识别方法

针对传统的重磁对应分析在场源区域外趋于高相关度的缺点,本文提出了一种基于相关系数和垂向导数的重磁场源位置及属性识别方法,给出了一个新的重磁场源平面位置及属性判别参数（GMCP）,该参数能够有效地缩小重磁场源的识别范围.判别参数GMCP值非零分布范围反映了场源的规模.GMCP值的正负反映了场源属性,当GMCP大于零时为正相关,代表了该区域存在高磁高密度或低磁低密度的重磁同源地质体;当GMCP小于零时为负相关,代表了该区域存在高磁低密度或低磁高密度的重磁同源地质体;GMCP趋于零,则表示无重磁同源地质体.两组不同干扰水平的同源复杂组合模型实验、两组不同源组合模型试验以及南海中央海盆实际重磁数据处理结果均验证了该重磁场源位置及属性识别方法的正确性和有效性.

     

Geoid anomalies across fracture zones and the thickness of the lithosphere

Recent advances in the measurement and interpretation of geoid height anomalies provide a new way to estimate the thickness of the oceanic lithosphere as a function of crustal age. GEOS-III satellite altimetry measurements show abrupt changes in sea level across fracture zones which separate areas of lithosphere with different ages. These changes have the correct location, amplitude, and wavelength to be caused by the combined gravitational attraction of the relief across the fracture zone and the isostatic support of this relief. Eight profiles of geoid height and bathymetry across the Mendocino fracture zone are inverted to determine the depth of the isostatic compensation, assuming that the compensation occurs in a single layer. These depths are then interpreted with a thermal boundary layer model of lithospheric growth. To explain satisfactorily the geoid measurements, the thermal diffusivity of the upper mantle must be 3.3 × 10^?3 cm² s^?1 and the thickness of the lithosphere, defined as the depth at which the geotherm reaches 95% of its maximum value, must be9.1km m.y.^?1/2 × t^1/2, where t is lithospheric age.     

The effects of anisotropy of marine magnetic anomalies on the Curie point depth estimates from spectral analysis

Azimuthally averaged power spectra are widely used in the Curie point depth (CPD) estimation with the implicit assumption that the magnetization distribution is random and uncorrelated. However, the marine magnetic anomalies are caused by bands of normal and reverse magnetization and show obvious trends. To investigate the effects of the anisotropy of marine magnetic anomalies on the CPD estimates, we develop 3D fractal striped magnetization models to produce lineated marine magnetic anomalies for the first time. We analyze the spectra anisotropy of the lineated magnetic anomalies of the synthetic fractal striped magnetization models and investigate its effects on the CPD estimates. The synthetic models and actual data show that the spectra of the lineated marine magnetic anomalies are directionally anisotropic. The amplitude response is strong and the slope of the logarithmic spectrum is large in a direction perpendicular to the stripes of magnetic anomalies, whereas the amplitude response is weak and the slope of the logarithmic spectrum is small in a direction parallel to the stripes of magnetic anomalies. The depth estimates in the perpendicular direction are close to the actual values, whereas the depths estimates in the parallel direction are significantly lower than the actual values. The actual marine magnetic anomalies of the South China Sea exhibit an anisotropic power spectrum that is consistent with the spectral anisotropy of magnetic anomalies of the synthetic fractal striped magnetization models.     

磁法探测地下核爆炸初探

从核核查角度探讨了地下核爆炸产生磁异常的机理,并对磁法探测地下核爆炸可行性进行了分析研究,结合我们的一些试验研究认,为通过分析处理地下核爆炸的磁异常可以达到核核查目的.     

Anomalous induction zones in the Czech Republic in relation to large scale european anomalies

Summary Induction response has been investigated at 57 field stations covering the Bohemian Massif (BM) and the Bruno-Vistulicum (BV), and at 93 temporary observing sites across the West Carpathians (WCP). Induction response data at 958 localities covering mainly the Central European area were also collected and analysed. Components of transfer functions (TF), moduli and arguments of corresponding induction vectors are analyzed for individual profiles. Pseudo-sections of induction parameters at profiles are used to visualize the period dependence of induction response and computer generated contour maps of TF components define zones of anomalous induction and their relation to large scale anomalies. Depths of anomalous field source are estimated by separating variations into internal and external parts. Correlations among induction response characteristics and the results of other geophysical investigations are discussed.     

Some magnetic fabric characteristics of sheared zones

In this paper the magnetic susceptibility anistropyy technique has been evaluated for the purpose of identifying shear zones in deformed regions and for detecting movements on shear planes. The magnitude parameters and orientations of the susceptibility ellipsoid show distinct characteristic behaviour on traversing deformed zones. The actual shear lines are characterized by the very high values of anistrropy and anisttropy gradients in their vicinities. The orientational data give clear indication of any relative rotations across questionalbe boundaries. Furthermore, a semi-analytical method is outlined for determining detailed pice-meal movements along shear lines as well as giving the regional movement direction. It is not possible to quantify the movements since too many parameters are unknown.     

A revised correlation of Mesozoic polarity chrons and calpionellid zones

Investigation of four sections of Tithonian to Valanginian pelagic limestone have led to refinement of the correlation of calpionellid zones to the magnetic polarity time scale. The correlations are self-consistent but differ slightly from those previously published. The discrepancy with the published correlation from the Bosso section [1] has been resolved by re-evaluation of the biostratigraphy of this sequence.The revised correlation places the base of theChitinoidella Zone in the lower part of polarity chron CM21n, the base of Zone A near the top of CM20n, the A/B boundary at the base of CM18, theB/C boundary in the upper part of CM17, theC/D boundary at the top of CM16 and theD/E boundary at the top of CM14.     

New advances of seismic tomography and its applications to subduction zones and earthquake fault zones: A review

Abstract There have been significant advances in the theory and applications of seismic tomography in the last decade. These include the refinements in the model parameterization, 3-D ray tracing, inversion algorithm, resolution and error analyses, joint use of local, regional and teleseismic data, and the addition of converted and reflected waves in the tomographic inversion. Applications of the new generation tomographic methods to subduction zones have resulted in unprecedentedly clear images of the subducting oceanic lithosphere and magma chambers in the mantle wedge beneath active arc volcanoes, indicating that geodynamic systems associated with the arc magmatism and back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. High-resolution tomographic imagings of earthquake fault zones in Japan and California show that rupture nucleation and earthquake generating processes are closely related to the heterogeneities of crustal materials and inelastic processes in the fault zones, such as the migration of fluids. Evidence also shows that arc magmatism and slab dehydration may also contribute to the generation of large crustal earthquakes in subduction regions.     

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。     

Age of the source of the Jarrafa gravity and magnetic anomalies offshore Libya and its geodynamic implications

The interpretation of the Jarrafa magnetic and gravity highs, NW Libyan offshore, suggests that it may be caused by a body of high-density and high magnetization. Analysis of their power spectra indicates two groups of sources at: (1) 2.7 km depth, probably related to the igneous rocks, some of which were penetrated in the JA-1 borehole, (2) 5 km depth, corresponding to the top of the causative body and (3) 10 km depth, probably referring to the local basement depth. The boundary analysis derived from applied horizontal gradient to both gravity and magnetic data reveals lineaments many of which can be related to geological structures (grabens, horsts and faults).The poor correlation between pseudogravity fields for induced magnetization and observed gravity fields strongly suggests that the causative structure has a remanent magnetization (D = −16°, I = 23°) of Early Cretaceous age, fitting with the opening of the Neo Tethys 3 Ocean.Three-dimensional interpretation techniques indicate that the magnetic source of the Jarrafa magnetic anomaly has a magnetization intensity of 0.46 A/m, which is required to simulate the amplitude of the observed magnetic anomaly. The magnetic model shows that it has a base level at 15 km.The history of the area combined with the analysis and interpretation of the gravity and magnetic data suggests that: (1) the source of the Jarrafa anomaly is a mafic igneous rock and it may have formed during an Early Cretaceous extensional phase and (2) the Jarrafa basin was left-laterally sheared along the WNW Hercynian North Graben Fault Zone, during its reactivation in the Early Cretaceous.