Simple features of mantle-wide convection and the interpretation of lower-mantle tomograms     

Norman H. Sleep 《Comptes Rendus Geoscience》2003,335(1):9-22

Simple fluid dynamic constraints aid in the interpretation of lower mantle tomograms. The geothermal gradient away from slabs and plumes is subadiabatic between the upper (lithosphere) and lower (D″) boundary layers by ～400 K. Slabs widen proportionally with the square root of the viscosity as they sink through the lower mantle. The time scale for the persistence of slab graveyards at the base of the mantle is comparable to the time for plate motions to reorganize ～120–200 Myr. At most a few starting plume heads currently exist in the lower mantle. Tomographic inversions may include more sophisticated numerical-modeling versions of these constraints.  相似文献   

青藏高原东缘地壳运动与深部过程的研究   总被引：26，自引：9，他引：26       下载免费PDF全文

熊熊  滕吉文 《地球物理学报》2002,45(4):507-515

由于青藏高原东部地区记录了高原约50 Ma演化历史中物质东流的构造史,因此受到地学界的广泛重视. 现代大地测量与地质研究结果给出了该区现代地壳运动的图像,为地球动力学数值模拟提供了重要的边界约束条件. 利用重力异常计算的高原及邻区地幔对流应力场与地表地壳运动格局的明显差异表征了高原东部地壳与地幔物质的运动解耦. 基于随深度变化地壳蠕变率的动力学模拟结果显示,高原东部地壳增厚与高原内部存在很大差异,高原东部地壳增厚主要表现为下地壳的增厚,并且地幔形变过程与地表变化也不一致,同样显示出地壳、地幔运动的解耦. 研究表明,下地壳低强度分布可能是导致这种解耦的重要原因,而了解高原东部地壳及上地幔物理力学性质对我们认识高原物质东流至关重要.  相似文献   

Ozone and PAN Formation Inside and Outside of the Berlin Plume – Process Analysis and Numerical Process Simulation     

U. Corsmeier  N. Kalthoff  B. Vogel  M.-U. Hammer  F. Fiedler  Ch. Kottmeier  A. Volz-Thomas  S. Konrad  K. Glaser  B. Neininger  M. Lehning  W. Jaeschke  M. Memmesheimer  B. Rappenglück  G. Jakobi 《Journal of Atmospheric Chemistry》2002,42(1):289-321

During the BERLIOZ field phase on 20 July 1998 a 40 km wide ozone-plume 30 to 70 km north of Berlin in the lee of the city was detected. The ozone mixing ratio inside the plume was app. 15 ppb higher than outside, mainly caused by high ozone precursor emissions in Berlin, resulting in a net chemical ozone production of 6.5 ppb h^–1, which overcompensates ozone advection of –3.6 ppb h^–1 andturbulent diffusion of –1.1 ppb h^–1. That means, although moreozone leaves the control volume far in the lee of Berlin than enters it at the leeside cityborder and although turbulent diffusion causes a loss of ozone in the leeside control volume the chemical production inside the volume leads to a net ozone increase. Using a semi-Lagrangian mass budget method to estimate the net ozone production, 5.0 ppb h^–1 are calculated for theplume. This means a fraction of about 20% of ozone in the plume is producedby local emissions, therefore called `home made' by the Berlin emissions. For the same area KAMM/DRAIS simulations using an observation based initialisation, results in a net production rate between 4.0 and 6.5 ppbh^–1, while the threefold nested EURAD model gives 6.0 ppbh^–1. The process analysis indicates in many cases goodagreement (10% or better) between measurements and simulations not only in the ozone concentrations but also with respect to the physical and chemical processes governing the total change. Remaining differences are caused by different resolution in time and space of the models and measurements as well as by errors in the emission calculation.The upwind-downwind differences in PAN concentrations are partly similar to those of ozone, because in the BERLIOZ case they are governed mainly by photochemical production. While in the stable boundary layer at night and windward of Berlin 0.1 to 0.3 ppb are detected, in the centre of the plume at noon concentrations between 0.75 ppb and 1.0 ppb are measured. The O₃/PAN ratio is about 80 to 120 and thus due to the relatively lowPAN concentrations significantly higher than found in previous studies. The low PAN formation on 20 July, was mainly restricted by the moderate nonmethane hydrocarbon levels, whereas high PAN concentrations of 3.0 ppb on 21 July, are caused by local production in the boundary layer and by large scale advection aloft.  相似文献   

日本海西部及中国东部地区深源地震在华北、东北有感的研究     

高山泰  于岫嵋  杨清福  郑雅琴 《东北地震研究》2002,18(1):13-23

本世纪以来 ,在日本海西部和中国东部地区深震带发生三次有感深源强震。震后调查发现深震在震中附近无感 ,而在远离震中区、偏离板块俯冲方向的东北南部和大华北地区强烈有感 ,且有感范围广、烈度低、衰减慢、振动中心不明确、没有破坏性等。研究表明有感范围严格受郯庐构造带和地幔上拱区控制 ,微观震中与有感范围的几何中心 (即宏观震中 )相距 4 30～ 1 0 2 0km ,两者连线偏离板块插入方向约 4 0°,认为这是由太平洋板块俯冲增强时 ,深震带上的张应力除以 2 5 5°走向向SWW方向平推外 ,同时又以大于 4 5°的仰角上撅造成的。从深震活动到东部地区上地幔隆起 ,再到郯庐带活动 ,都是由太平洋板块插入、俯冲、上撅、顶托同一动力学过程形成。  相似文献   

Permo–Triassic intraplate magmatism and rifting in Eurasia: implications for mantle plumes and mantle dynamics     

A. M. Nikishin  P. A. Ziegler  D. Abbott  M. -F. Brunet  S. Cloetingh 《Tectonophysics》2002,351(1-2)

At the transition from the Permian to the Triassic, Eurasia was the site of voluminous flood-basalt extrusion and rifting. Major flood-basalt provinces occur in the Tunguska, Taymyr, Kuznetsk, Verkhoyansk–Vilyuy and Pechora areas, as well as in the South Chinese Emeishen area. Contemporaneous rift systems developed in the West Siberian, South Kara Sea and Pyasina–Khatanga areas, on the Scythian platform and in the West European and Arctic–North Atlantic domain. At the Permo–Triassic transition, major extensional stresses affected apparently Eurasia, and possibly also Pangea, as evidenced by the development of new rift systems. Contemporaneous flood-basalt activity, inducing a global environmental crisis, is interpreted as related to the impingement of major mantle plumes on the base of the Eurasian lithosphere. Moreover, the Permo–Triassic transition coincided with a period of regional uplift and erosion and a low-stand in sea level. Permo–Triassic rifting and mantle plume activity occurred together with a major reorganization of plate boundaries and plate kinematics that marked the transition from the assembly of Pangea to its break-up. This plate reorganization was possibly associated with a reorganization of the global mantle convection system. On the base of the geological record, we recognize short-lived and long-lived plumes with a duration of magmatic activity of some 10–20 million years and 100–150 million years, respectively. The Permo–Triassic Siberian and Emeishan flood-basalt provinces are good examples of “short-lived” plumes, which contrast with such “long lived” plumes as those of Iceland and Hawaii. The global record indicates that mantle plume activity occurred episodically. Purely empirical considerations indicate that times of major mantle plume activity are associated with periods of global mantle convection reorganization during which thermally driven mantle convection is not fully able to facilitate the necessary heat transfer from the core of the Earth to its surface. In this respect, we distinguish between two geodynamically different scenarios for major plume activity. The major Permo–Triassic plume event followed the assembly Pangea and the detachment of deep-seated subduction slabs from the lithosphere. The Early–Middle Cretaceous major plume event, as well as the terminal–Cretaceous–Paleocene plume event, followed a sharp acceleration of global sea-floor spreading rates and the insertion of new subduction zone slabs deep into the mantle. We conclude that global plate kinematics, driven by mantle convection, have a bearing on the development of major mantle plumes and, to a degree, also on the pattern of related flood-basalt magmatism.  相似文献   

Petrology of ultramafic nodules from São Tomé Island, Cameroon Volcanic Line (oceanic sector)     

R. Caldeira  J. M. Munh 《Journal of African Earth Sciences》2002,34(3-4)

Ultramafic xenoliths were found in recent alkali basalts from São Tomé Island. These include spinel peridotites (lherzolites, harzburgites and dunites) and pyroxenites (orthopyroxenites and clinopyroxenites). Textures and mineral compositions indicate that pyroxenites originated from crystal/liquid separation processes operating on magmas similar to those giving rise to their present host rocks whereas spinel peridotite xenoliths had an accidental origin; Fo (>89) and Ni (>0.36 wt.%) contents in olivines, Mg# (91–95) of orthopyroxenes and low Ti in clinopyroxene (primary crystals: TiO₂<0.06 wt.%) and in spinel (TiO₂<0.1 wt.%) are within the range reported for abyssal peridotites, indicating São Tomé spinel peridotites represent refractory residues of melting. Nevertheless, the lack of correlation between mineral chemistry and modal composition suggests that spinel peridotite xenoliths are not simple residues and were affected by infiltration of fluid/melts within the mantle. The wide temperature range obtained for spinel peridotites (700 to >1150 °C) is compatible with a long period of pre-entrainment cooling supporting Fitton's [Tectonophysics 94 (1983) 473] hypothesis that proposes oceanic lithosphere uprising in the Cameroon Volcanic Line prior to the initiation of the current thermal regime, related to São Tomé magmatism. The association of upper mantle (peridotite) xenoliths with igneous cumulates (pyroxenites) suggests that the spinel peridotite suite originated in the uppermost mantle above the São Tomé magma storage zone(s), probably in a region of high strain rate, near the boundary between the mantle and the overlying oceanic crust.  相似文献   

Effect of a Viscosity Interface at 1000 km Depth on Mantle Circulation     

ČIžková  Hana  Čadek  Ondŕej 《Studia Geophysica et Geodaetica》1997,41(3):297-306

We present the preliminary results of axisymmetric numerical simulations of thermal convection in the mantle with a phase transition boundary at 660 km depth and a viscosity interface at 1000 km depth. The results, obtained for Ra = 2 × 10 ⁶ , are compared with the case when both the phase and the viscosity boundaries are located at the same depth of 660 km.  相似文献   

早前寒武纪洋壳的地质记录及其板块构造意义   总被引：5，自引：0，他引：5  

李江海 《地质科技情报》1997,16(1):10-16

元古宙蛇绿岩的不断发现表明威尔逊旋回至少在早元古代已经明显起作用，部分太古宙绿岩带由不同的构造单元拼合而成，并发育不同于显生宙蛇绿岩的大洋壳岩石组合，地幔柱在早太古代构造演化过程中起重要作用，是板块构造的重要补充机制，地球早期热产量较高，可能是造成板块规模较小，洋壳较厚，板块运动速度较快的原因，并以缓倾角俯冲为特征。  相似文献   

地幔热柱成矿作用研究进展   总被引：15，自引：0，他引：15  

孙爱群  牛树银 《地质科技情报》1997,16(1):65-71

地幔热柱构造是地球内部热传导的基本形式，只是地球的不同演化阶段其表现形式和强度有所不同。研究表明，燕山期强烈的地幔热柱活动沟通了深部成矿物质来源，金、银等元素随地幔热柱多级演化以气态→气—液混合相向上运移，在地幔热柱演化的三级单元——幔枝构造的有利部位聚集成矿。并据幔枝构造的成矿模式提出了华北地区燕山、秦岭、太行、胶辽等主要成矿集中区都是河淮地幔亚热柱控制下的幔枝成矿系列。成矿作用均伴随有强烈的构造—岩浆活动，成矿时代多集中在２６０～１２０Ｍａ。金、银多金属矿床具明显的相似性、同源性和深源性  相似文献   

Ergodicity and the ‘scale effect’     

Garrison Sposito 《Advances in water resources》1997,20(5-6)

Solute plume spreading in an aquifer exhibits a ‘scale effect’ if the second spatial concentration moment of a plume has a non-constant time-derivative. Stochastic approaches to modeling this scale effect often rely on the critical assumption that ensemble averages can be equated to spatial averages measured in a single field experiment. This ergodicity assumption should properly be evaluated in a strictly dynamical context, and this is done in the present paper. For the important case of trace plume convection by steady groundwater flow in an isotropic, heterogeneous aquifer, ergodicity does not obtain because of the existence of an invariant function on stream surfaces that is not uniform throughout the aquifer. The implications of this result for stochastic models of solute transport are discussed. © 1997 Elsevier Science Ltd. All rights reserved  相似文献