Mesozoic and early Cenozoic tectonic convergence-to-rifting transition prior to opening of the South China Sea

We have investigated Mesozoic geological problems around the South China Sea (SCS) based on gravimetric, magnetic, seismic, and lithofacies data. Three-dimensional analytical signal amplitudes (ASA) of magnetic anomalies clearly define the inland tectonic boundaries and the residual Mesozoic basins offshore. The ASA suggest that the degree of magmatism and/or the average magnetic susceptibility of igneous rocks increase southeastwards and that late-stage A-type igneous rocks present along the coast of southeast China possess the highest effective susceptibility. The geophysical data define Mesozoic sedimentary and tectonic structures and reveal four major unconformities [Pz/T–J, T–J/J, J/K, and Mesozoic/Cenozoic (Pz, Palaeozic; T, Triassic; J, Jurassic; K, Cretaceous)], corresponding to regional tectonic events revealed by nine palaeogeographic time slices based on prior geological surveys and our new fieldwork. Showing both sedimentary and volcanic facies and regional faults, our palaeogeographic maps confirm an early Mesozoic northwestward-migrating orogeny that gradually obliterated the Tethyan regime, and a middle-to-late Mesozoic southeastward migration and younging in synchronized extension, faulting, and magmatism. Three major phases of marine deposition developed but were subsequently terminated by tectonic compression, uplift, erosion, faulting, rifting, and/or magmatism. The tectonic transition from the Tethyan to Pacific regimes was completed by the end of the Middle Triassic (ca. 220 Ma), reflecting widespread Mesozoic orogeny. The transition from an active to a passive continental margin occurred at the end of the Early Cretaceous (ca. 100 Ma); this was accompanied by significant changes in sedimentary environments, due likely to an eastward retreat of the palaeo-Pacific subduction zone and/or to the collision of the West Philippine block with Eurasia. The overall Mesozoic evolution of southeast China comprised almost an entire cycle of orogenic build-up, peneplanation, and later extension, all under the influence of the subducting palaeo-Pacific plate. Continental margin extension and rifting continued into the early Cenozoic, eventually triggering the Oligocene opening of the SCS.     

重磁三维自动反演软件系统的分析与设计

由于三维反演计算非常耗时,导致目前的重磁三维自动反演系统的工作方式效率低。为了改善工作效率和提高反演结果的质量,提出实时可视化自动反演方式,设计并开发了一个重磁三维实时可视化反演系统。该系统在三维反演计算的过程中,将中间反演结果同时进行成图可视化,使得工作人员能及时分析和评价中间反演结果并能修改参数,以此来介入和引导复杂的反演过程,快速有效地得到比较理想的反演结果。该系统具有实时性、交互性和引导性特点。对该系统进行了分析和设计,并给出了其中的关键技术与设计方案。     

Study of the crust and mantle using magnetic surveys by Magsat and other satellites

Among the first measurements made from near-Earth orbiting satellites were measurements of the magnetic field. The sources of that field lie both within the Earth, in its core and crust, and in the surrounding ionosphere and magnetosphere. This article summarizes some of the methodology and results for studies of the Earth’s mantle and crust. Mantle conductivity studies can be made either by studying signals impressed on the Earth from outside, e.g., the ionosphere or magnetosphere, or by studying signals originating in the core and transmitted through the mantle. Crustal field studies begin with a careful selection of the data and subsequent removal of core and external fields by some sort of filtering. Average maps from different local times sometimes differ, presumably due to the remaining presence of fields of external origin. Several techniques for further filtering are discussed. Where large-area aeromagnetic maps are available, crustal maps derived from satellite data can be compared with upward continued data. In general, the comparisons show agreement, with some differences, particularly in and near the auroral belts. The satellite data are further reduced by various methods of inverse and forward modelling, sometimes including reduction to the pole (RTP). These techniques are generally unstable at the equator. Common methods of stabilizing the inversions include principle components analysis and ridge regression. Because of the presence of the core field, the entire crustal contribution from the field is not known. Also, there is a basic nonuniqueness to the inverse solutions. Nevertheless, magnetizations that are interpretable can be derived.     

Composition and assemblage characteristics of magnetic minerals in layer S<Subscript>5-1</Subscript> of Xifeng and Duanjiapo sections

Relatively strongly magnetic fine components (< 30μm, XS-4J and DS-4J) which are most environmentally sensitive were separated from layer S_5-1 in the Xifeng and Duanjiapo loess sections and analyzed by MPV-3 for their morphometric characteristics and reflectance, SEM-ESD for their element contents and XRD for their mineral phases, respectively. The results showed that minerals in both samples are dominated by detrial Fe-Ti oxides of aeolian origin. In sample XS-4J the reflectance and iron contents of magnetic minerals are usually high. In addition to magnetite (Fe₃O₄), maghemite (γFe₂O₃) and hematite (Fe₂O₃), some Fe-high oxide (72.25 wt%–86.67 wt%), ilmenite (FeTiO₃), and magnetite-ulvöspinel [Fe(FeCr)O₄, Fe (FeNi)O₄] were also detected. In sample DS-4J obvious negative linear correlations were found between Ti and Fe, and the contents of Mn, Si, Al and Ca are usually high and the minerals are dominated by magnetite (maghemite), goethite (FeOOH) and limonite (containing Si and OH). In addition, the signs of corrosion of magnetic minerals and newly crystallized magnetite (maghemite) were recognized. Differences in the composition and assemblage characteristics of magnetite minerals between XS and DS reflect significant differences in source rocks and preserving conditions.     

Analysis of geomagnetic field along seismic profile P4 of the International Project POLONAISE'97

This paper presents relative secular variations of the total intensity of the geomagnetic field against a background of results of magnetic anomaly interpretation along seismic profile P4. Profile P4 crosses a Variscan folding zone in the Paleozoic Platform (PLZ), the Trans-European Suture Zone (TESZ), and the Polish part of the East European Craton (EEC). Secular geomagnetic field variations measured in 1966–2000 along a line adjacent to seismic profile P4 were analysed. The study of secular variations, reduced to the base recordings at the Belsk Magnetic Observatory, showed that the growth of geomagnetic field at the East European Craton was slower than in the Trans-European Suture Zone and the Paleozoic Platform.A 2D crustal magnetic model was interpreted as a result of magnetic modelling, in which seismic, geological and geothermal data were also used. The modelling showed that there were significant differences in the magnetic model for geotectonic units, which had been earlier determined based on deep seismic survey data. It should be noted that a fundamental change of trend of the relative secular variations was observed at the slope of the Precambrian Platform. After analysing the geomagnetic field observed along profile P4, the hypothesis that the contact between Phanerozoic and Precambrian Europe lies in Poland's territory can be proven.     

Centrifuge modelling of the influence of crustal fabrics on the development of transfer zones: insights into the mechanics of continental rifting architecture

Centrifuge analogue experiments are used to model the reactivation of pre-existing crustal fabrics during extension. The models reproduced a weakness zone in the lower crust whose geometry was varied in order to investigate its role in controlling the architecture of rift segments and related transfer zones. The typical rift system geometry was characterised by two offset rift segments connected by a major transfer zone in which boundary faults were oblique to the extension vector and displayed a significant transcurrent component of movement. The transfer zone was also characterised by cross-basin faults with both trend and strike-slip component of movement opposite to that displayed by the master faults. Typically, different structural patterns were obtained by changing the offset angle φ between the rift segments, supporting that the structural pattern at transfer zones is strongly influenced by the orientation of pre-existing discontinuities with respect to the stretching vector. In the models, the aspect ratio (ratio of length vs. width) of the transfer zone shows a positive correlation with the offset angle (i.e., the more the inherited fabric is parallel to the extension direction, the longer and narrower the transfer zones). In case of staircase offset of the rift segments (φ=90°), the structural pattern was characterised by two isolated rift depressions linked by a narrow transfer zone in which border faults with alternating polarity overlapped. Prominent rise of the ductile lower crust was also observed at the transfer zone. Many of these geometrical features display striking similarities with natural rift systems. The results of the current experiments provide useful insights into the mechanics of continental rift architecture, supporting that rift propagation, width and along-axis segmentation may be strongly controlled by the reactivation of pre-existing pervasive crustal fabrics.     

羌塘坳陷石油地质走廊剖面重磁异常处理模拟及地质解释

利用“重磁视深度滤波”方法对羌塘石油地质走廊南、北两条剖面上的实测重磁数据进行处理解释,提取两条剖面位场异常中含有的地质-地球物理断面的初始特征,然后使用最新的“重磁模拟解释系统（GM-SYS）”,以初始特征为模拟初值,以地震、地电和地质资料为约束,进行重磁剖面模拟反演,获得了羌塘盆地南、北两条剖面精细的地质结构解释断面图和栅状图。结合剖面域内的其它资料对走廊域内几个重要地质问题进行了初步的分析和解释,对羌塘盆地石油地质走廊域内的地层、基底、断裂及火山岩分布有了进一步的认识,为整个羌塘盆地区域地质解释及油气远景评价提供了新的解释依据。     

A mathematical model of concentrate solids content for the wet drum magnetic separator

Low concentrate density from wet drum magnetic separators in dense medium circuits can cause operating difficulties due to inability to obtain the required circulating medium density and, indirectly, high medium solids losses. The literature is almost silent on the processes controlling concentrate density. However, the common name for the region through which concentrate is discharged—the squeeze pan gap—implies that some extrusion process is thought to be at work. There is no model of magnetics recovery in a wet drum magnetic separator, which includes as inputs all significant machine and operating variables.A series of trials, in both factorial experiments and in single variable experiments, was done using a purpose built rig which featured a small industrial scale (700 mm lip length, 900 mm diameter) wet drum magnetic separator. A substantial data set of 191 trials was generated in this work. The results of the factorial experiments were used to identify the variables having a significant effect on magnetics recovery.It is proposed, based both on the experimental observations of the present work and on observations reported in the literature, that the process controlling magnetic separator concentrate density is one of drainage. Such a process should be able to be defined by an initial moisture, a drainage rate and a drainage time, the latter being defined by the volumetric flowrate and the volume within the drainage zone. The magnetics can be characterised by an experimentally derived ultimate drainage moisture. A model based on these concepts and containing adjustable parameters was developed. This model was then fitted to a randomly chosen 80% of the data, and validated by application to the remaining 20%. The model is shown to be a good fit to data over concentrate solids content values from 40% solids to 80% solids and for both magnetite and ferrosilicon feeds.     

探测未爆炸弹的地球物理技术综述

采用综合地球物理勘查技术探测未爆炸弹(UXO)具有快速、定位准确和成本低廉的优势.综述了国内外广泛应用的地面高精度磁测法、电磁法以及地质雷达和微重力法在探测UXO中的特点和效果,介绍了国外航空磁测法及航空电磁法探测UXO的进展,并根据某些探测实例阐述了在具体实施过程中应注意的技术问题;提出了适合中国UXO探测的一些措施和建议.     

重磁解释中健全的极大似然法及三维反演

本文介绍了JOAO.B.C.SILVA提出的重磁解释中健全的(Robust)极大似然反演方法的基本原理,讨论了重力二度体非线性几何参数的反演情况,并将该方法扩展到三维重、磁非线性几何参数反演中。重点讨论了场值中有较大随机干扰、有不同类型的地质噪音源影响下的反演结果。理论反演结果表明,该方法有比最小二乘反演方法更接近实际的解,从而提高了解的可靠性,对于重磁局部异常或孤立异常有较好的反演效果。