Magnetic and Gravity Investigations of Lisan Peninsula-Dead Sea(Jordan)

The Lisan Peninsula is located within the Dead Sea basin which represents the plate boundary between African and Arabian plates. This basin constitutes a good example of a pull-apart basin because of its large dimensions, its structural simplicity and its active subsidence . The gravity data reveal that the Dead Sea basin can be divided into segments, each of them about 30 km long in N-S direction , where the Lisan Peninsula represents the deepest one (9 km thick Pleistocene sediments ), overlying about 6 km thick Mesozoic sediments . In addition , 20 km of extension was predicted along the Dead Sea basin, which indicates that the Dead Sea basin should be about 3.3 Ma in age . Furthermore, the Precambrian basement under the Lisan area is characterized by high susceptibility contrast that is related to continuous tectonic activity in the region.     

Study on terrestiral noise in data of geomagnetic total field in North China

The terrestrial noise in the differencial data of geomagnetic total field recorded at 21h in North China was fitted by using the geomagnetic vector data from Beijing Observatory. The terrestrial noise level and its spatial feature in this region was discussed. A method, which may be useful for justifying the quality of fitting, was provided in this paper. The analysis indicates that the terrestrial noise is one of the important noise in tectonomagnetic study. It is shown that the susceptibility effect is quite small and the orientation effect is the main component in the terrestrial noise in North China. The rough spatial feature of the magnetic anomaly in crustal substance was infered from the spatial feature of terrestrial noise. The measurement precision and the source noise level of data at 21h in this region were obtained in the analysis of residual noise. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 441–447, 1993.     

苏北盆地XH 1＃钻孔沉积物磁化率与粒度组分相关性变化特征及其意义研究

对苏北盆地XH 1#钻孔沉积物磁化率与粒度组分的相关性进行了研究，研究结果表明，此钻孔沉积物磁化率与粒度组分的相关性在不同深度段内有着不同的表现:在350～247 m深度段内，磁化率大小与中值粒径大小表现为负相关、与4～12Φ各粒级组分百分含量表现为正相关；在234～0 m深度段内，磁化率大小与中值粒径大小表现为正相关，与1～3Φ粒级组分百分含量表现为密切的正相关；247～234 m深度段是上述两种相关性变化的一个过渡阶段。环境磁学实验结果也揭示出在上下两个深度段内磁性矿物组成上也存在差异。这种不同深度段内磁化率与粒度组分相关性上的大的变化，以及磁性矿物组成上的差异共同揭示出，在深度247～234 m这段沉积物的沉积过程中，苏北盆地内可能发生了一次重大的事件，正是这次重大事件的发生导致了碎屑沉积物来源的改变，出现了上下两部分磁化率大小与粒度组分相关性上的差异。     

Magnetic properties of soils from sites with different geological and environmental settings

Measurements of magnetic susceptibility of soils, reflecting magnetic enhancement of topsoils due to atmospherically deposited magnetic particles of industrial origin, are used recently in studies dealing with outlining polluted areas, as well as with approximate determination of soil contamination with heavy metals. One of the natural limitations of this method is magnetic enhancement of soils caused by weathering magnetically rich parent rock material. In this study we compare magnetic properties of soils from regions with different geological and environmental settings. Four areas in the Czech Republic and Austria were investigated, representing both magnetically rich and poor geology, as well as point-like and diffuse pollution sources. Topsoil and subsoil samples were investigated and the effect of geology and pollution was examined. Magnetic data including mass and volume magnetic susceptibility, frequency-dependent susceptibility, and main magnetic characteristics such as coercivity (Hc and Hcr) and magnetization (Ms and Mrs) parameters are compared with heavy metal contents. The aim of the paper is to assess the applicability of soil magnetometry under different geological-environmental conditions in terms of magnetic discrimination of dominant lithogenic/anthropogenic contributions to soil magnetic signature. Our results suggest that lithology represents the primary effect on soil magnetic properties. However, in case of significant atmospheric deposition of anthropogenic particles, this contribution can be clearly recognized, independent of the type of pollution source (point-like or diffuse), and discriminated from the lithogenic one. Different soil types apparently play no role. Possible effects of climate were not investigated in this study.     

Using magnetic susceptibility to find Precambrian impact ejecta: A proposal

Layers with high magnetic susceptibility contain heavy minerals that are the most likely to preserve signatures of impact in metamorphosed Precambrian rocks. The following impact indicators are resistant to alteration and metamorphism and are likely to be found in heavy mineral layers: impact diamond, SiC, shocked zircon, high-pressure TiO₂ phases, reidite, and high-pressure chromite. Coesite and stishovite could also be present but are not likely to be preserved in rocks metamorphosed above the greenschist facies. Looking for layers with high magnetic susceptibility will also identify layers that might contain concentrations of zircon for dating.     

Rock-magnetic properties of Neogene sediments, northern flank of Tianshan Mountains

Analyses of rock-magnetic properties of Neogene sediments of the Taxihe section, northern Tianshan Mountains, show that the section can be classified into three categories including lacustrine facies, fluvial facies and alluvial facies, which correspond to the lower, middle and upper of the Taxihe section respectively. The magnetic minerals of the lacustrine facies may be affected by the process of weath- ering, lithogenesis and biolithogenesis besides the source of the sediments. The natural remanence intensities are between 10-3 A/m and 10-2 A/m. The minerals are dominated by magnetite and the high coercive magnetic mineral may be goethite. The magnetic grains are the mixture of PSD SD or SD SP. The natural remanence intensities of the strata of fluvial facies are between 10-2 A/m and 10-1 A/m, about ten times that of the lacustrine facies. The magnetic minerals are mainly magnetite and hematite, and the magnetic grains are mainly PSD. The characteristic remanence (ChRM) carriers are magnetites. In the alluvial facies, the natural remanence intensities are mostly less than 1×10-2 A/m. The magnetic minerals of the series are dominated by magnetite and hematite, almost the same as the fluvial facies. But the difference is that most of the stepwise demagnetization can reveal two components and the ChRM carriers are hematites. The magnetic grains are PSD in terms of the hysteresis parameters.     

磁法在煤火探测中的应用

对采自宁夏汝箕沟煤田上覆盖层的原岩样品在温度作用下的磁性变化进行了系统研究.通过分析煤火区不同燃烧阶段围岩的温度变化情况,对原岩样品进行了模拟“燃烧”试验,试验结果表明,岩石磁性在不同温度作用条件下存在明显差异,从而从岩石磁性的角度讨论了磁法勘探圈定煤田火烧区的可行性.最后,采用人机交互法对地面实测磁异常剖面进行反演,有效地圈定了煤火区着火点的位置及范围.     

Mesozoic extension in the Basque–Cantabrian basin (N Spain): Contributions from AMS and brittle mesostructures

In this work we analyse and check the results of anisotropy of magnetic susceptibility (AMS) by means of a comparison with palaeostress orientations obtained from the analysis of brittle mesostructures in the Cabuérniga Cretaceous basin, located in the western end of the Basque–Cantabrian basin, North Spain. The AMS data refer to 23 sites including Triassic red beds, Jurassic and Lower Cretaceous limestones, sandstones and shales. These deposits are weakly deformed, and represent the syn-rift sequence linked to basins formed during the Mesozoic and later inverted during the Pyrenean compression. The observed magnetic fabrics are typical of early stages of deformation, and show oblate, triaxial and prolate magnetic ellipsoids. The magnetic fabric seems to be related to a tectonic overprint of an original, compaction, sedimentary fabric. Most sites display a NE–SW magnetic lineation that is interpreted to represent the stretching direction of the Early Cretaceous extensional stage of the basin, without recording of the Tertiary compressional events, except for sites with compression-related cleavage.Brittle mesostructures include normal faults, calcite and quartz tension gashes and joints, related to the extensional stage. The results obtained from joints and tension gashes show a dominant N–S to NE–SW, and secondary NW–SE, extension direction. Paleostresses obtained from fault analysis (Right Dihedra and stress inversion methods) indicate NW–SE to E–W, and N–S extension direction. The results obtained from brittle mesostructures show a complex pattern resulting from the superposition of several tectonic processes during the Mesozoic, linked to the tectonic activity related to the opening of the Bay of Biscay during the Early Cretaceous. This work shows the potential in using AMS analysis in inverted basins to unravel its previous extensional history when the magnetic fabric is not expected to be modified by subsequent deformational events. Brittle mesostructure analysis seems to be more sensitive to far-field stress conditions and record longer time spans, whereas AMS records deformation on the near distance, during shorter intervals of time.