共查询到20条相似文献，搜索用时 15 毫秒
1.
采用远参考道和Robust技术,处理了华北地区14个地磁台站资料,得到了相干度超过0.8的地磁测深响应函数.并将其转换为大地电磁测深的响应函数,获取了105~107 s周期范围内的视电阻率和相位.应用ρ+理论对数据进行了一致性检验和反演,结果表明417km,850km深度附近可能存在电性间断面.同时采用基于一维最光滑模型的Occam反演方法得到了300~1000km范围的地幔电性结构,并与前人在其他地区的研究结果进行了对比.发现华北地区地幔过渡带的电导率在大兴安岭—太行山重力梯度带东西两侧表现不同,重力梯度带附近及西侧台站下方过渡带深度的电导率和北美的Tucson地区相当,而华北地区东部的电导率在地幔过渡带范围高出西侧约2~5倍,这很可能和太平洋板块的俯冲有关. 相似文献
2.
L.P. Vinnik R.A. Avetisjan N.G. Mikhailova 《Physics of the Earth and Planetary Interiors》1983,33(3):149-163
The method of detection of P-to-SV converted waves from distant earthquakes (Vinnik, 1977) was applied to sets of long-period records from a few seismograph stations in Europe and the west of North America. The results obtained suggest that the converted phases related to the major boundaries in the mantle can be reliably detected and the depths of conversion evaluated with an accuracy of a few kilometres. The depth of the olivine-spinel transition is close to 400 km and no difference between the estimates for the north of Europe and the west of North America is found. The depth of the boundary separating the upper and lower mantle is close to 640 km, which is 30 km less than in the recent Earth-reference models. Fine S velocity stratification of this transition changes laterally from a high-gradient layer 50 km thick, terminated at the bottom by a sharp discontinuity, to a gradient layer 100 km or more thick without the discontinuity. A striking anomaly of the mantle transition zone is found in the Rio Grande rift area where a well pronounced boundary is found at 510 km depth. 相似文献
3.
本文通过合成SH波理论地震图的方法,利用SS-S走时和SS波波形资料,研究了我国上地幔剪切波速度结构。初步结果表明,我国大陆上地幔可以分成两个独立不同的速度结构模型:一是青藏高原;另一是中国大陆东部。两部分均存在剪切波低速层,但埋藏深度不同,高原部分是100km,东部地区是60km,两部分的差异大约在350km以下趋于消失。在405km和660km深处存在剪切波的速度间断面。400km以下青藏高原和中国大陆东部地区剪切波的速度结构与北美洲、北大西洋西部、欧洲、阿尔卑斯带地区的结构一致,说明在这几个地区上地幔剪切波速度结构的横向变化在400km以下很小。 相似文献
4.
全球地磁感应测深能获得地幔转换带及下地幔上部的导电结构.但目前稀疏的地磁台站分布及部分台站的观测数据稳定性较差,影响了三维反演对地下电性结构的分辨力和反演可靠性.为此,区别于传统的L2-范数反演方法,本文提出并实现了基于L1-范数的地磁测深响应三维反演技术.在反演中,利用L1-范数度量数据预测误差,降低"飞点"数据的影响,将相关系数较小的C-响应估计也纳入反演数据中.三维正演模拟采用球坐标系下的交错网格有限差分法,反演采用有限内存拟牛顿法.文中利用指数概率密度分布函数构造非高斯噪声的合成数据,并采用棋盘模型对反演方法的可靠性进行了验证.之后,我们将本文提出的三维反演方法用于全球129个地磁观测台站的C-响应数据反演,结果表明在地幔转换带深部,中国东北地区为高导电异常,南欧和北非则均为高阻;夏威夷在900km以下为高导;菲律宾海及以东地区在转换带表现为明显的高阻,这些结果与前人研究结果一致.由于采用了更多的台站数据,我们的反演结果还发现一些新的异常:南美洲南端,转换带表现为明显的高导;澳大利亚东南部,地幔转换带深部,也存在一个明显的高导异常,这些异常分布和地震层析成像的低速区一致.因此,L1-范数三维反演能够充分利用全球C-响应数据信息,提高地磁测深对地球深部电性结构的分辨能力,更好的研究全球地幔电性结构. 相似文献
5.
在华北地区大地电磁测深工作的基础上,本文对北京——柏各庄剖面上的大地电磁观测资料,使用连续介质反演方法进行反演解释,获得了电性结构的最新结果。对构制的可接受地电模型的评价表明,本剖面内地壳中部普遍存在高导电带,上地幔内高导电带的深度变化较大,电阻率为1m的等值线深度,在沧县隆起为70多km,其西侧和东侧分别在60km和50km左右。上地幔高导电带的电阻率值由西而东有减小的趋势。这些结果与表层构造的对应关系说明,上地幔软流圈及地热场的横向不均一性,对地壳内部发生的构造运动有重要意义。 相似文献
6.
A. G. Green 《Pure and Applied Geophysics》1978,116(6):1262-1273
AP-wave velocity model for the upper mantle beneath eastern and southern Africa is proposed. The top 250 km of the model is characterized by relatively low velocities similar to those deduced for the upper mantle beneath the western United States of America. At greater depths, the velocities gradually change to normal mantle values. 相似文献
7.
A two dimensional velocity model of the upper mantle has been compiled from a long-range seismic profile crossing the West Siberian young plate and the old Siberian platform. It revealed considerable horizontal and vertical heterogeneity of the mantle. A sharp seismic boundary at a depth of 400 km outlines the high-velocity gradient transition zone, its base lying at a depth of 650 km. Several layers with different velocities, velocity gradients and wave attenuation are distinguished in the upper mantle. They likewise differ in their inner structure. For instance, the uppermost 50–70 km of the mantle are divided into blocks with velocities from 7.9–8.1 to 8.4–8.6 km s?1.Comparison of the travel-time curves for the Siberian long-range profile with those compiled from seismological data for Europe distinguished large-scale upper mantle inhomogeneities of the Eurasian continent and allowed for the correlation of tectonic features and geophysical fields. The velocity heterogeneity of the uppermost 50–100 km of the mantle correlates with the platform age and heat flow, i.e., the young plates of Western Europe and Western Siberia have slightly lower velocities and higher heat flows than the ancient East European and Siberian platforms. At greater depths (150–250 km) the upper mantle velocities increase from the ocean to the inner parts of the continent. The structure of the transition zone differs significantly beneath Western Europe and the other parts of Eurasia. The sharp boundary at a depth of 400 km, traced throughout the whole continent as the boundary reflecting intensive waves, transforms beneath Western Europe into a gradient zone. This transition zone feature correlates with positions of the North Atlantic-west Europe geoid and heat-flow anomalies. 相似文献
8.
PRELIMINARY STUDY ON THE CONDUCTIVITY STRUCTURE OF THE CRUST AND UPPER MANTLE: THIRD REPORT ON THE GEOSCIENCE TRANSECT FROM XIANGSHUI, JIANGSU, TO MANDAL, NEI MONGOL 
下载免费PDF全文
下载免费PDF全文 The features of deep conductivity structure along Xiangshui-Mandal Geoscience Transect are described in this paper basing on newly obtained magnetotelluric data. Large resistivity contrasts can be found on both borders of North China platform. The depth of the crustal high conductivity layer changes abruptly from 21 km to 34 km beneath Nei Mongol fold system on the northwestern end of the transect. While it is absent beneath Subei-Jiaonan terrane on the southeastern end of the transect 相似文献
9.
Robert P. Massé 《Pure and Applied Geophysics》1987,125(2-3):205-239
From an analysis of many seismic profiles across the stable continental regions of North America and northern Europe, the crustal and upper mantle velocity structure is determined. Analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques. TheP wave velocity structure beneath the Canadian Shield is virtually identical to that beneath the Baltic Shield to a depth of at least 800 km. Two major layers with a total thickness of about 42 km characterize the crust of these shield regions. Features of the upper mantle of these region include velocity discontinuities at depths of about 74 km, 330 km, 430 km and 700 km. A 13 km thickP wave low velocity channel beginning at a depth of about 94 km is also present.A number of problems associated with record section interpretation are identified and a generalized approach to seismic profile analysis using many record sections is described. TheS wave velocity structure beneath the Canadian Shield is derived from constrained surface wave data. The thickness of the lithosphere beneath the Canadian and Baltic Shields is determined to be 95–100 km. The continental plate thickness may be the same as the lithospheric thickness, although available data do not exclude the possibility of the continental plate being thicker than the lithosphere. 相似文献
10.
11.
The geomagnetic field components are periodically measured at repeat stations. The main objective of the repeat stations is to provide data for tracing the secular variation of the geomagnetic field components. Secular variation at the repeat station is generally different from that at geomagnetic observatory used in the data reduction. The effect of the secular variation differences on geomagnetic data reduction was estimated for the regions of Europe, North America (below 60°N) and Australia, respectively, during the period of 2000-2010. These estimations were obtained by using the monthly mean values of north, east and vertical components of geomagnetic field, recorded at geomagnetic observatories. The effects were calculated by using observatories pairs, with distances from 350 km (in Europe) to 3100 km (in North America and Australia). The maximal effects were found to be the smallest in east component in Europe and North America, and vertical component in Australia; the effects increase with time from a central reduction epoch and they are not constant during mentioned eleven years; they were less than 1 nT only in Europe, for distances between the observatories up to 1000 km in all three components and for periods spanning ±1 month from a central epoch. It was found that their year to year variability is mostly due to the non-eliminated external field residuals in the observatories monthly means; their effect is up to 3 nT for ±3 months from a central epoch. Further, the real effects were compared to those modeled by IGRF-12 model. The maximal differences between the real and the modeled values are below 4.5 nT in all three components, for ±3 months from a central epoch. 相似文献
12.
13.
这篇文章给出了内蒙、甘肃、宁夏某些地区的大地电磁测深结果。这些结果显示出,在地震带内地壳和上地幔电导率分布的横向变化是很明显的,而在比较稳定的鄂尔多斯地台内电导卑分布的横向变化较小,地壳和上地幔的电导率结构比较简单。基于这些结果发现,上部地壳(10—15公里)低阻层在大震震中区较为发育,这可能与大震有关。我国西北地区已有的大地电磁测深结果显示出,莫霍面並不是一个清楚的电性分界面,而某些地区的地壳中部低阻层似乎与康拉界面相应,这就使得后者成为一个明显的电性分界面。 相似文献
14.
Electromagnetic study of lithospheric structure in the marginal zone of East European Craton in NW Poland 总被引：1，自引：0，他引：1
Waldemar Jóźwiak 《Acta Geophysica》2013,61(5):1101-1129
The marginal zone of the East European Platform, an area of key importance for our understanding of the geotectonic history of Europe, has been a challenge for geophysicists for many years. The basic research method is seismic survey, but many important data on physical properties and structure of the lithosphere may also be provided by the electromagnetic methods. In this paper, results of deep basement study by electromagnetic methods performed in Poland since the mid-1960s are presented. Over this time, several hundred long-period soundings have been executed providing an assessment of the electric conductivity distribution in the crust and upper mantle. Numerous 1D, 2D, and pseudo-3D electric conductivity models were constructed, and a new interpretation method based on Horizontal Magnetic Tensor analysis has been applied recently. The results show that the contact zone is of lithospheric discontinuity character and there are distinct differences in geoelectric structures between the Precambrian Platform, transitional zone (TESZ), and the Paleozoic Platform. The wide-spread conducting complexes in the crust with integral conductivity values reaching 10 000 S at 20–30 km depths are most spectacular. They are most likely consequences of geological processes related to Caledonian and Variscan orogenesis. The upper mantle conductivity is also variable, the thickness of high-resistive lithospheric plates ranging from 120–140 km under the Paleozoic Platform to 220–240 km under the East European Platform. 相似文献
15.
上地幔温度是控制地幔流变性和动力学过程的关键参数之一.本文利用高分辨率S波地震层析成像速度结果,基于岩石温度与地震波速度的关系,研究了华北地区上地幔50~300 km深度范围内的温度分布和"热"岩石圈厚度.为了验证结果的可靠性,本文用计算的上地幔60 km深度处的温度作为底面约束,得到了相应的地表热流.计算地表热流与观测地表热流之间符合程度较好,相对误差大部分都在地表热流观测误差范围之内.通过对上地幔的温度分布进行分析,我们研究发现:(1)在上地幔浅部,温度与地表构造之间有很好的对应关系.在小于170 km的深度上,温度呈现东高西低的分布态势.温度较高的区域集中在东部的河淮盆地、渤海湾盆地、华北平原和中部陆块的交界处、西部鄂尔多斯高原北缘的银川―河套地堑以及阴山地区,同时,这些地区的岩石圈厚度也相应较薄,大约为80~100 km;(2)西部的鄂尔多斯高原是整个华北地区岩石圈地幔温度最低的地区,比东部地区低200~400 ℃,岩石圈厚度相应最厚,平均岩石圈厚度达到140~150 km,最厚处超过160 km.(3)在170 km以下的软流圈地幔部分,温度分布发生反转,西部温度高于东部,表明东、西部陆块在地质历史时期经历了不同的深部地幔动力学过程. 相似文献
16.
We document strong seismic scattering from around the top of the mantle Transition Zone in all available high resolution explosion seismic profiles from Siberia and North America. This seismic reflectivity from around the 410 km discontinuity indicates the presence of pronounced heterogeneity in the depth interval between 320 and 450 km in the Earth’s mantle. We model the seismic observations by heterogeneity in the form of random seismic scatterers with typical scale lengths of kilometre size (10-40 km by 2-10 km) in a 100-140 km thick depth interval. The observed heterogeneity may be explained by changes in the depths to the α-β-γ spinel transformations caused by an unexpectedly high iron content at the top of the mantle Transition Zone. The phase transformation of pyroxenes into the garnet mineral majorite probably also contributes to the reflectivity, mainly below a depth of 400 km, whereas we find it unlikely that the presence of water or partial melt is the main cause of the observed strong seismic reflectivity. Subducted oceanic slabs that equilibrated at the top of the Transition Zone may also contribute to the observed reflectivity. If this is the main cause of the reflectivity, a substantial amount of young oceanic lithosphere has been subducted under Siberia and North America during their geologic evolution. Subducted slabs may have initiated metamorphic reactions in the original mantle rocks. 相似文献
17.
Electrical conductivity of the earth's crust and upper mantle 总被引：1，自引：0，他引：1
Gerhard Schwarz 《Surveys in Geophysics》1990,11(2-3):133-161
This review summarizes recent results of electrical resistivity studies of the earth's crust and upper mantle. Where available, the data are discussed in the context of further regional geophysical information. Electrical resistivity is very sensitive to a wide range of petrological and physical parameters, e.g., to carbon, fluids, volatiles and enhanced temperatures, making electrical resistivity methods a powerful tool in crust and upper mantle investigations. Yet, the general increase in resistivity data of the crust and mantle has not ended the battle of explanations for anomalous crustal conductivities. 相似文献
18.
The paper presents a review and analysis of new seismic data related to the structure of the mantle beneath the East European platform. Analysis of observations of long-range profiles revealed pronounced differences in the structure of the lower lithosphere beneath the Russian plate and the North Caspian coastal depression. The highest P-velocities found at depths around 100 km are in the range 8.4–8.5 km s?1. Deep structure of the Baltic shield is different from the structures of both these regions. No evidence of azimuthal anisotropy in the upper mantle was found. A distribution of P-velocity in the upper mantle and in the transition zone consistent with accurate travel-time data was determined. The model involves several zones of small and large positive velocity gradients in the upper mantle, rapid increases of velocity near 400 and 640 km depths and an almost constant positive velocity gradient between the 400 and 640 km discontinuities. The depth of the 640 km discontinuity was determined from observations of waves converted from P to SV in the mantle. 相似文献
19.
《中国科学:地球科学(英文版)》2021,(9)
The Circum-Pacific subduction zone is a famous gold metallogenic domain in the world, with two important gold metallogenic provinces, the North China Craton and Nevada, which are related to the destruction of the North China Craton and the Wyoming Craton, respectively. Their ore-forming fluids were possibly derived from the stagnant slab in the mantle transition zone. The oceanic lithospheric mantle usually contains serpentine layers up to thousands of meters thick. During plate subduction, serpentine is dehydrated at depths of 200 km and transformed into high-pressure hydrous minerals, known as Phases A to E, which carries water to the depth of 300 km. The overlying big mantle wedge is hydrated during the breakdown of these hydrous facies in the mantle transition zone. The dehydration of the subducted slab in the big mantle wedge releases sulfur-rich fluid, which extracts gold and other chalcophile elements in the surrounding rocks, forming gold-rich fluid. Because the cratonic geotherm is lower than the water-saturated solidus line of lherzolite, the fluid cannot trigger partial melting. Instead, it induces metasomatism and forms pargasite and other water-bearing minerals when it migrates upward to depths of less than 100 km in the cratonic lithospheric mantle, resulting in a water-and gold-rich weak layer. During the destruction of craton, the weak layer is destabilized, releasing gold-bearing fluids that accelerate the destruction. The ore-forming fluids migrate along the shallow weak zone and are accumulated at shallow depths, and subsequently escape along deep faults during major tectonic events, leading to explosive gold mineralization. The ore-forming fluids are rich in ferrous iron, which releases hydrogen at low pressure through iron hydrolysis. Therefore, decratonic gold deposits are often reduced deposits. 相似文献
20.
M.J. Drury 《Physics of the Earth and Planetary Interiors》1978,17(2):P16-P20
Modelled conductivity variations with depth in the upper mantle obtained from geomagnetic induction and magnetotelluric results are compared to predicted conductivity variations obtained from laboratory measurements on dry material assumed to occur in the mantle. Higher than predicted conductivities at the base of the oceanic lithosphere suggest the presence of highly conductive partial melt. Using a known relationship between observed conductivity and the conductivity and volume fraction of the fluid, estimates of the melt volume fraction have been made assuming the melt to be in continuously connected network. In the sub-oceanic asthenosphere these values range from approximately 0.45 to 9%, whereas in sub-continental asthenosphere the partial melt volume fraction appears to be too low to increase the bulk conductivity. The partial melt content of the sub-continental asthenosphere may reach a few percent if the melt exists in isolated pockets. The apparent difference in melt content in sub-oceanic and sub-continental asthenosphere is discussed in terms of the different velocities of Pacific-type plates (which carry no continental block) and Atlantic-type plates (which carry a continental block). 相似文献