Analysis of gravity fields in the northwestern Himalayas and Kohistan region using deep seismic sounding data

A Bouguer gravity anomaly map of the NW Himalayas and parts of the Kohistan/Hindukush region has been prepared using all available gravity data. Analysis of the gravity field has been carried out along a profile extending from Gujranwala (located near the edge of the Indian shield) to the Haramosh massif in a NNE–SSW direction. The gravity profile is located close to the DSS profile shot under the USSR–India scientific collaborative programme. Velocity information available along different parts of the profile has been used to infer values of crustal and upper mantle density.
The observed gravity field (Bouguer) has been interpreted in terms of Moho depth and density contrast between the crust and the mantle. The Moho depth is interpreted as increasing from nearly 35 km near the edge of the Indian shield to 75 km (below sea-level) underneath the Haramosh massif. A similar model is applicable to a profile passing to the west of Nanga Parbat massif, from Gujranwala to Ghizar, through the Kohistan region. However, along this profile high-density lower-crustal rocks appear to have been emplaced in the upper part along the main mantle thrust. The nature of isostatic compensation prevailing underneath the Himalayas has been discussed, as has the theory of lithospheric flexure proposed by Karner & Watts and Lyon-Caen & Molnar. It is felt that although these ideas explain the broad features of the Moho configuration as observed in the NW Himalayas, there are significant departures. The role of tectonic forces in shaping the Moho and causing changes in the density of the crust cannot be denied.     

Regional vertical tectonics in the Eastern Mediterranean

Summary. New gravity observations from a systematic survey of the Eastern Mediterranean Sea and from a reconnaissance land survey in Central and Western Turkey have been compiled with existing data. Lack of sufficient geological and geophysical information precludes an analysis of the local anomalies or crustal structure; however, implications of the topography and gravity field at long wavelengths have been examined. Negative free-air anomalies characterize almost the entire Eastern Mediterranean basin and positive anomalies predominate in Turkey and the Aegean Sea. The change in sign coincides with the northern boundary of the African plate, and the wavelength and amplitude of the gravity variation are of the order of 1000 km and 100 mgal respectively. The lithosphere is probably unable to support such anomalies because the implied shear stresses are too large. The source of the anomalies is concluded to be in the asthenosphere where the low finite strength of material suggests that some sort of flow must exist to maintain the stresses. A good correlation is observed between the gravity and topography at wavelengths greater than 300 km; and the relationship is the same as that observed in the North Atlantic and the Central Pacific, as well as that computed for simple models of mantle convection. The gravity and topography of the Eastern Mediterranean can be explained in terms of flow in the upper mantle. This is the first region of subsidence for which this interpretation has been made.     

Numerical computation of the three-dimensional normal ellipsoidal field with a 10−2 m and 10−2 mgal accuracy

Summary. A new computation of the relevant geometrical and mechanical quantities pertaining to the three-dimensional normal ellipsoidal field (Bocchio) has been worked out with reference to a spatial grid ranging from 0 to 9000 m in dynamic height and from 0 to 90° in latitude, with a mesh size of 500 m and 1° respectively giving a better accuracy. An estimate of the latter at the upper end of the vertical interval gives 10⁻² m for the radii of curvature of the equipotential surfaces and 10⁻²mgal for normal gravity. Twenty-six functions of interest have been numerically computed including the metric tensor, Christoffel's symbols and the tensor of gravity gradients giving about 45 000 tabulated data.     

Structure and geological history of the Congo Basin: an integrated interpretation of gravity,magnetic and reflection seismic data

The stratigraphic, paleogeographic and tectonic evolution of the intracratonic Congo Basin in Central Africa has been revised on the basis of an integrated interpretation of gravity, magnetic and reflection seismic data, together with a literature review of papers sometimes old and difficult to access, map compilation and partial reexamination of outcrop and core samples stored in the Royal Museum for Central Africa (RMCA). The Congo Basin has a long and complex evolution starting in the Neoproterozoic and governed by the interplay of tectonic and climatic factors, in a variety of depositional environments.This multidisciplinary study involving 2D gravity and magnetic modeling as additional constraints for the interpretation of seismic profiles appears to be a powerful tool to investigate sedimentary basins where seismic data alone may be difficult to interpret. The tectonic deformations detected in the Congo Basin after the 1970–1984 hydrocarbon exploration campaign in the Democratic Republic of Congo (DRC) have been attributed to crustal contraction and basement uplift at the center of the basin, following a transpressional inversion of earlier graben structures. Two‐dimensional gravity and magnetic models run along key seismic lines suggest the presence of evaporite sequences in some of the deeper units of the stratigraphic succession, in the lateral continuity with those observed in the Mbandaka and Gilson exploration wells. The poorly defined seismic facies that led to the previous basement uplift interpretation of the crystalline basement is shown to correspond to salt‐rich formations that have been tectonically de‐stabilized. These features may be related to vertical salt‐tectonics connected to the near/far‐field effects of the late Pan‐African and the Permo‐Triassic compressive tectonic events that affected this African part of Gondwana.     

Validation and fusion of different databases in preparation of high-resolution geoid determination

An up to date determination of a high-resolution geoid requires the use of best available databases concerning digital terrain model (DTM), bathymetry, global geopotential model and gravity field. The occasion to revisit methods to validate and merge different data sets has been created by a new project for the determination of a new European Geoid.
Since the computation of the latest European geoid and quasi-geoid model (EGG97), significant new or improved data sets have become available, such as new global geopotential models from CHAMP and GRACE missions, new national and global DTMs and new or upgraded gravity data sets.
In the context of the new European Gravity and Geoid Project (EGGP), within the IAG Commission 2, some data validation tests have been performed in the Italian zone.
In the area 19°× 17° wide, covering Italy, three kinds of tests have been performed: comparison among different DTMs in order to choose the best one to be used; comparisons in terms of geoid computation in some coastal areas, to evaluate bathymetry effects, and the validation of the EIGEN-CG01C and EIGEN-CG03C new global models up to degree and order 360.
These preliminary tests lead to the choice of SRTM DTM (integrated in no-data holes), with an added bathymetry derived by the Italian 1:25 000 official cartography near the coasts and the NOAA bathymetry in high seas. The validation of the new global models and the comparison with EGM96 model show that, in terms of geoid computation, the EGM96 yields better results. Moreover, the validation of new available land gravity data and the cross-validation of two sets of gravity data on sea have been completed.     

Modelling of ground deformation and gravity fields using finite element method: an application to Etna volcano

Elastic finite element models are applied to investigate the effects of topography and medium heterogeneities on the surface deformation and the gravity field produced by volcanic pressure sources. Changes in the gravity field cannot be interpreted only in terms of gain of mass disregarding the ground deformation of the rocks surrounding the source. Contributions to gravity changes depend also on surface and subsurface mass redistribution driven by dilation of the volcanic source. Both ground deformation and gravity changes were firstly evaluated by solving a coupled axisymmetric problem to estimate the effects of topography and medium heterogeneities. Numerical results show significant discrepancies in the ground deformation and gravity field compared to those predicted by analytical solutions, which disregard topography, elastic heterogeneities and density subsurface structures. With this in mind, we reviewed the expected gravity changes accompanying the 1993–1997 inflation phase on Mt Etna by setting up a fully 3-D finite element model in which we used the real topography, to include the geometry, and seismic tomography, to infer the crustal heterogeneities. The inflation phase was clearly detected by different geodetic techniques (EDM, GPS, SAR and levelling data) that showed a uniform expansion of the overall volcano edifice. When the gravity data are integrated with ground deformation data and a coupled FEM modelling was solved, a mass intrusion could have occurred at depth to justify both ground deformation and gravity observations.     

Tidal gravity in Britain: tidal loading and the spatial distribution of the marine tide

Summary. Tidal gravity measurements have been made at six sites in Britain with two nulled LaCoste and Romberg Earth tide gravitymeters. The M ₂ observations from these and two further sites are compared with calculations of the tidal loading from the seas around the British Isles and the major oceans. Models of the M ₂ marine tides are convolved with Green's functions for appropriate radially stratified Earth models. The differences between the M ₂ observations and the theoretical calculations are less than 0.6 μ gals and it is shown that these differences contain further information concerning the errors in the marine tide models. The M ₂ marine tides on the north-west European continental shelf are reasonably well known and this allows a useful test of the feasibility of using tidal gravity measurements for the inverse ocean tide problem in areas where the ocean tides are less well known. The differential gravity loading signal between pairs of gravity stations is shown to be important for considerations of the uniqueness and accuracy of the inverse problem. M ₂ tidal gravity loading maps for the British Isles and Europe have been produced which are of use in making corrections to various geodetic measurements.     

Geodetic singularities in the gravity field of a non-homogeneous planet

Summary. An inverse geodetic singularity problem is considered for a non-homogeneous spherical planet. The singularity condition is expressed in terms of the density distribution and of the geometrical parameters of the configuration; the condition for the density distribution is deduced which gives rise to singularities of parabolic type in the external gravity field of the planet. The structure of the gravity field in the neighbourhood of the singularities is investigated in detail together with the behaviour of the gradients of the disturbances in the geodetic coordinates.     

Towards understanding the lithospheric structure of the southern Chilean subduction zone (36°S–42°S) and its role in the gravity field

The Southern Andes differ significantly from the Central Andes with respect to topography and crustal structures and are, from a geophysical point of view, less well known. In order to provide insight into the along-strike segmentation of the Andean mountain belt, an integrated 3-D density model was developed for the area between latitudes 36°S and 42°S. The model is based on geophysical and geological data acquired in the region over the past years and was constructed using forward density modelling. In general, the gravity field of the South American margin is characterized by a relatively continuous positive anomaly along the coastline and the forearc region, and by negative anomalies along the trench and the volcanic arc. However, in the forearc region of the central part of the study area, located just to the south of the epicentre of the largest ever recorded earthquake (Valdivia, 1960), the trench-parallel positive anomaly is disrupted. The forearc gravity anomaly differences thus allow the study area to be divided into three segments, the northern Arauco-Lonquimay, the middle Valdivia-Liquiñe, and the southern Bahía-Mansa-Osorno segment, which are also evident in geology. In the proposed model, the observed negative gravity anomaly in the middle segment is reproduced by an approximately 5 km greater depth to the top of the slab beneath the forearc region. The depth to the slab is, however, dependent upon the density of the upper plate structures. Therefore, both the upper and lower plates and their interaction have a significant impact on the subduction-zone gravity field.     

中国中部地区城市影响范围划分方法的比较

随着城市化、区域一体化的不断推进,各种类型的城市与区域规划陆续推出。其中,识别城市的腹地范围、促进城市与腹地的协同发展是其关键内容。因此,城市影响范围的划分具有重要的现实意义。已有的研究成果多以单一年份的静态研究及单一方法的应用研究为主,缺乏时间序列上的动态演变分析,更无多种方法综合集成分析。正是基于以上考虑,在详细梳理与比较不同理论的差异性基础上,采用引力模型和改进场模型两种方法对中国中部地区的城市影响范围进行测度和比较研究,细致比较这两种基于不同视角的模型方法在识别城市影响范围的优势与劣势,进而提出综合集成研究的基本框架,分析中国中部地区城市影响范围及其动态变化特征。希望能为合理选择和运用城市影响范围的分析方法,有效制定区域等级体系规划、城市空间组织方案提供科学参考。     

社区人居环境吸引力研究——以大连市为例

城市人居环境研究中一个重要的理论问题就是人居环境吸引力。本文提出人居环境吸引力和引力场的概念,构建了社区人居环境满意度指标体系,运用引力势能模型计算出184个社区的人居环境引力势能,借助MapInfo软件中的Vertical Mapper工具得到大连市人居环境引力势能等值线图和空间分布图,研究表明:(1)运用引力场理论对人居环境评价研究,是一种有效的评价方法;(2)大连市社区人居环境供求关系在空间上形成了"两心一带"分布格局;(3)大连市社区人居环境引力势能空间分异明显,大致呈东北西南向"带状"分布,在中心广场、西安路锦辉商场等值线由同心圆状向外逐渐递减;(4)社区人居环境供求关系与引力势能成正比,与社区与引力势能中心的距离呈反比,城市空间发展结构、海岸带是大连人居环境引力势能空间分布的驱动机制。     

The gravity field of a dipping plate in Greece

Summary. The surface gravity anomalies for a three-dimensional density model of a dipping lithospheric plate under the Aegean island arc and the Aegean Sea in Greece in the eastern Mediterranean Sea have been calculated. Such a dipping plate has been suggested in geophysical investigations. The calculated gravity maximum over the dipping plate is located in the area of the largest observed Bouguer anomalies inside the island arc. It is suggested that this should be taken into account in studies of crustal structure and gravity in the area.     

Yucatán karst features and the size of Chicxulub crater

The buried Chicxulub impact structure is marked by a dramatic ring of sinkholes (called cenotes if containing water), and adjacent less prominent partial rings, which have been shown to coincide with maxima in horizontal gravity gradients and a topographic depression. These observations, along with the discreteness and spacing of the features, suggest a formation mechanism involving faulting in the outer slump zone of the crater, which would thus have a diameter of approximately 180 km.
An opposing view, based primarily on the interpretation of gravity data, is that (he crater is much larger than the cenote ring implies. Given the association of the known cenote ring with faults, we here examine northern Yucatan for similar rings in gravity, surface features and elevation, which we might expect to be associated with outer concentric faults in the case of a larger, possibly multiring, structure.
No such outer rings have been found, although definite patterns are seen in the distribution of karst features outside the crater rim. We explain these patterns as resulting mainly from deformation related to the block fault zone that parallels tbe shelf edge of eastern Yucatan.     

基于引力势力范围的郑州城市多中心空间形态识别

识别和分析城市多中心结构的空间形态,对优化城市空间结构具有重要意义。对引力模型的多中心综合实力参数和距离参数测定方法进行优化,首先将能够反映居民对空间结构感知体验的空间句法可达性,与人口经济和社会活动空间分布结合构建评价体系,定量计算多中心综合实力;然后将能够表达居民出行轨迹空间特征和偏好规律的词向量相关性,作为距离参数引入引力模型;最后计算引力强度并划分势力范围,对多中心发展目标的现实演化状态进行分析和评价。通过对比实验发现：优化引力模型与传统引力模型分析的势力范围存在一定差异,经空间统计和实地调查验证,优化模型对多源数据的运用更充分,考虑的因素更全面,其空间形态分析结果较为准确;从识别与规划多中心势力范围差异看,二者在郑州市核心城区的势力范围较为相似,均为早期规划或自然演化形成的已建成多中心,而部分外围多中心的识别与规划位置不同,空间形态差异明显,为未建成多中心,还远未形成其应有的分散疏解作用。     

极地航空重力测量及其应用进展

在论述航空重力测量技术在目前重力场信息获取中不可替代性的基础上,回顾并简述了国际、国内航空重力测量技术的发展历史,介绍了航空重力测量的基本原理,总结了航空重力测量技术在极地航空地球物理勘探任务中的应用。指出航空重力测量技术是目前唯一兼顾高精度高分辨率并且能覆盖全南极区域的观测手段,在完善南极地区全球重力场模型、研究南极地区地质构造、质量变化等任务中起着非常重要的作用。     

内蒙古耕地重心移动及其驱动因子分析

简单回顾了内蒙古垦荒历史,并用内蒙古自治区近５０年的统计数据计算出１９４９年～１９９６年不同年份的全区耕地重心及人口重心,在此基础上得到了内蒙古耕地重心及人口重心均向东北方向移动的结论,分析了耕地重心移动的驱动因子,提出了土地持续利用的几点建议。     

利用地球重力场模型确定南极大地水准面

利用我国最新地球重力场模型 WDM94,给出了南极 (纬度范围为 - 60°～ - 90°)大地水准面高和平均空间重力异常。为了全面总结分析南极大地水准面特征 ,收集了国外最新地球重力场模型 OSU91 ( 360阶次 )和 JGMOSU( 360阶次 ) ,计算了相应的大地水准面高和平均重力异常。其结果分别与 WDM94的结果作了比较 ,WDM94与 OSU91和 JGMOSU的大地水准面高标准差分别为± 1 .90 m和± 2 .0 9m,平均空间重力异常标准差分别为± 8.97mgal和± 9.32 mgal     

冰芯中MSA迁移假说的延伸和完善

MSA在冰芯深层的“迁移”现象的存在 ,关系到能否应用大气中 MSA季节特征进行极地冰芯断代 ,并以冰芯分析结果确定大气中生物硫化物含量的方法基础。本文展示各类最有代表性的冰雪 MSA剖面 ,对提出冰芯深层 MSA“迁移假说”的理论依据进行探讨 ,并根据最新资料 ,说明冰川中 MSA的“迁移”和重新分布 ,在表层粒雪和渗浸 -冻结冰层中同样可能发生。对“迁移假说”的延伸补充在于 ,粒雪中的 MSA迁移是在积雪中空气与外界贯通的“开放”条件下进行的 ,渗入雪层中的融水将 MSA溶出后 ,在晶粒间宏观向下输送 ,在阳离子集中的层位发生反应 ,生成盐类冰点的改变使之发生“冻结”,重新分布。而渗浸 -冻结冰层中的迁移机制可能和深层冰川冰中的情况接近 ,即在气体与外界隔绝条件下 ,主要在“封闭”的晶间脉状纹理中以“微观”形式进行。南极半岛 MSA迁移过程需要很长时间 ,而表层渗浸 -冻结冰层中 MSA迁移过程很快出现 ,说明冰中脉状纹理的加快形成、较高冰川温度和冰层中的较大含水量等因素会对 MSA“迁移”进程起促进作用。最后 ,对 MSA“迁移”情况下 ,如何进行有关冰芯 MSA计算进行了讨论 ,对设计实验验证并完善迁移理论提出了设想。     

Tectonic evolution of the North Sea basin: crustal stretching and subsidence

Summary. The lithospheric stretching model for the formation of sedimentary basins was tested in the central North Sea by a combined study of crustal thinning and basement subsidence patterns. A profile of crustal structure was obtained by shooting a long-range seismic experiment across the Central Graben, the main axis of subsidence. A seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin. The data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure, and the final model showed substantial crustal thinning beneath the graben. Subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central North Sea since Jurassic times. Changes in water depth were quantified using foraminiferal assemblages where possible, and observed basement subsidence paths were corrected for sediment loading, compaction and changes in water depth through time. The seismic model is shown to be compatible with the observed gravity field, and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium. Both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the Central Graben during the mid-Jurassic to early Cretaceous extensional event. This extension appears to have occurred over crust already slightly thinned beneath the graben, and the seismic data suggest that total extension since the early Permian may have been more than 100km. The data presented here may all be explained using a simple model of uniform extension of the lithosphere.     

Gravity studies of the Rockall and Exmouth Plateaux using SEASAT altimetry

Abstract SEASAT altimetric measurements are used to determine the gravity anomalies across two passive continental margins: the western margin of the Rockall Plateau, UK, and the Exmouth Plateau off north-west Australia. The small gravity anomalies observed over the starved western margin of the Rockall Plateau require the existence of a major density contrast within the crust, as well as the Moho, and show that the elastic thickness is less than 5 km at the time of rifting. The gravity anomaly over the Exmouth Plateau is compared with the gravity anomaly calculated from the sediment loading of a thin elastic plate, taking account of the variation in crustal thickness. This comparison shows that the Exmouth Plateau also has a small effective elastic thickness of 5 km, even for loads emplaced between 60 and 120 Myr after rifting. Elastic thicknesses of about 5 km have also been reported for other sedimentary basins, and are to be expected if the rheological properties of the crust and mantle depend on the ratio of the present temperature to the melting temperature. Flexural effects are therefore likely to be of minor importance in sedimentary basins.