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1.
Introduction The northwest of China includes Tarim, Junggar and Qaidam basins, and Kunlun, Tianshan, Altun and Qilian mountains, as well as the north part of the Tibetan Plateau. For a long time, the study of lithosphere structures and dynamics in this area has been a popular topic in geoscience, and has yielded many results. For example, TANG (1994) and LI et al (1998) suppose that the tectonic structure of Tarim Basin is various with geological periods, which changes many times betwee… 相似文献
2.
假设地震层析成像提供的地震波速异常对应于上地幔物质的密度异常分布，而该密度异常直接源于上地幔热对流相应的温度扰动. 在给定边界条件下，利用三维傅里叶变换，在波数域内求解控制流体行为的运动方程和连续性方程，得到上地幔小尺度对流流场. 利用密度异常驱动上地幔小尺度对流的数学 物理模型，采用胥颐、刘福田等提供的地震层析成像数据计算得到了我国西北及周边地区上地幔对流模式. 结果表明，对流流场的顶部在岩石圈较薄的盆地区域呈现上升发散流动特征，如塔里木盆地、柴达木盆地、哈萨克斯坦块体及准噶尔盆地；岩石圈较厚的山脉则对应了会聚下降的流动特征，如天山山脉、昆仑山山脉和祁连山山脉. 同时，塔里木盆地处于拉张状态，驱动其上地幔物质南下向青藏高原北部西昆仑运动，以及北上向天山下部流动，这可能是天山隆升的原因之一. 相似文献
3.
Numerical experiments have been carried out on two-dimensional thermal convection, in a Boussinesq fluid with infinite Prandtl number, at high Rayleigh numbers. With stress free boundary conditions and fixed heat flux on upper and lower boundaries, convection cells develop with aspect ratios (width/depth) λ? 5, if heat is supplied either entirely from within or entirely from below the fluid layer. The preferred aspect ratio is affected by the lateral boundary conditions. If the temperature, rather than the heat flux, is fixed on the upper boundary the cells haveλ ≈ 1. At Rayleigh numbers of 2.4 × 105 and greater, small sinking sheets are superimposed on the large aspect ratio cells, though they do not disrupt the circulation. Similar two-scale flows have been proposed for convection in the earth's mantle. The existence of two scales of flow in two-dimensional numerical experiments when the viscosity is constant will allow a variety of geophysically important effects to be investigated. 相似文献
4.
The paper presents results obtained in experiments on a horizontal layer heated from below in its central part and cooled from above; the layer models the oceanic asthenosphere. Flow velocity and temperature profiles are measured and the flow structure under boundary layer conditions is determined (at Rayleigh numbers Ra > 5 × 105). The flow in the core of a plane horizontal layer heated laterally and cooled from above develops under conditions of a constant temperature gradient averaged over the layer thickness. The flow core is modeled by a horizontal layer with a moving upper boundary and with adiabatic bounding surfaces under conditions of a constant horizontal gradient of temperature. Exact solutions of free convection equations are found for this model in the Boussinesq approximation. Model results are compared with experimental data. Temperature and flow velocity ranges are determined for the boundary layer regime. Based on the experimental flow velocity profiles, an expression is found for the flow velocity profile in a horizontal layer with a mobile upper boundary heated laterally and cooled from above. Free convection velocity profiles are obtained for the asthenosphere beneath a mid-ocean ridge (MOR) with a mobile lithosphere. An expression is obtained for the tangential stress at the top of the asthenosphere beneath an MOR and the total friction force produced by the asthenospheric flow at the asthenosphere-lithosphere boundary is determined. 相似文献
5.
Edge-driven convection 总被引：23，自引：0，他引：23
We consider a series of simple calculations with a step-function change in thickness of the lithosphere and imposed, far-field boundary conditions to illustrate the influence of the lithosphere on mantle flow. We consider the effect of aspect ratio and far-field boundary conditions on the small-scale flow driven by a discontinuity in the thickness of the lithosphere. In an isothermal mantle, with no other outside influences, the basic small-scale flow aligns with the lithosphere such that there is a downwelling at the lithospheric discontinuity (edge-driven flow); however, the pattern of the small-scale flow is strongly dependent on the large-scale thermal structure of a much broader area of the upper mantle. Long-wavelength temperature anomalies in the upper mantle can overwhelm edge-driven flow on a short timescale; however, convective motions work to homogenize these anomalies on the order of 100 million years while cratonic roots can remain stable for longer time periods. A systematic study of the effect of the boundary conditions and aspect ratio of the domain shows that small-scale, and large-scale flows are driven by the lithosphere. Edge-driven flow produces velocities on the order of 20 mm/yr. This is comparable to calculations by others and we can expect an increase in this rate as the mantle viscosity is decreased. 相似文献
6.
A. P. Trubitsyn 《Izvestiya Physics of the Solid Earth》2013,49(5):668-674
The formation of the thermal cross section of the lithosphere and mantle upon the interaction between the mantle convection and the immobile continent surrounded by the oceanic lithosphere is studied by numerical modeling. The convective temperature and velocity fields and then the averaged geotherms for subcontinental and suboceanic regions up to the boundary with the core are calculated from the solution of convection equations with a jump in viscosity in the continental zone. Using the experimental data on the solidus temperature in the rocks of the upper mantle, the average thickness of the continental and oceanic lithosphere is estimated at 190 and 30 km, respectively. The effect of a hot spot formed in the subcontinental upper mantle at a depth of 250–500 km, which has not been previously noted, is revealed. Although the temperature in this zone is typically assumed to be close to adiabatic, the calculations show that it is actually higher than adiabatic by up to 200°C. The physical mechanism responsible for this effect is associated with the accumulation of convective heat beneath the thermally insulating layer of the continental lithosphere. The revealed anomalies can be important in studying the phase and mineral transformations at the base of the lithosphere and in the regional geodynamical reconstructions. 相似文献
7.
本文通过地震层析成像研究获得了华北克拉通及其东邻地区（30°N-50°N,95°E -145°E）1°×1°的P波速度扰动图像.结果显示,在西太平洋俯冲带地区,上地幔中西倾的板片状高速异常体与其上方的低速异常区构成俯冲带与上覆地幔楔的典型速度结构式样.俯冲板片高速体在约300~400 km深度范围内被低速物质充填,暗示俯冲板片可能发生了断离.在华北克拉通地区的上地幔中发现三个东倾排列的高速异常带.在此基础上,本文构建了华北克拉通及其东邻西太平洋活动大陆边缘地区的上地幔速度结构模式图,并据此探讨克拉通岩石圈减薄与西太平洋活动大陆边缘的深部动力学联系.本文认为,太平洋板片的俯冲（断离）,触发热地幔物质上涌并在上覆地幔楔中形成对流,使克拉通岩石圈受到改造（底侵与弱化）.随着俯冲板片后撤,地幔楔中的对流场以及对岩石圈改造的影响范围均随之东移,最终导致华北克拉通岩石圈自下而上、从西向东分三个阶段依次拆沉减薄.这一模式能很好地解释现今克拉通岩石圈自西向东呈台阶状减薄的深部现象. 相似文献
8.
Tomographic structure of East Asia:Ⅱ.Stagnant slab above 660 km discontinuity and its geodynamic implications 
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下载免费PDF全文 P-wave arrival times of both regional and teleseismic earthquakes were inverted to obtain mantle structures of East Asia.No fast(slab) velocity anomalies was not find beneath the 660-km discontinuity through tomography besides a stagnant slab within the transition zone.Slow P-wave velocity anomalies are present at depths of 100-250 km below the active volcanic arc and East Asia.The western end of the flat stagnant slab is about 1 500 km west to active trench and may also be correlated with prominent surface topographic break in eastern China.We suggested that active mantle convection might be operating within this horizontally expanded "mantle wedge" above both the active subducting slabs and the stagnant flat slabs beneath much of the North China plain.Both the widespread Cenozoic volcanism and associated extensional basins in East Asia could be the manifestation of this vigorous upper mantle convection.Cold or thermal anomalies associated with the stagnant slabs above the 660-km discontinuity have not only caused a broad depression of the boundary due to its negative Clapeyron slope but also effectively shielded the asthenosphere and continental lithosphere above from any possible influence of mantle plumes in the lower mantle. 相似文献
9.
Bryony A.R. Youngs Gregory A. Houseman 《Physics of the Earth and Planetary Interiors》2007,160(1):60-74
In this study, we examine the development of topography on a thin dense layer at the base of the lower mantle. The effect of the convecting mantle above is represented as a traction acting on the upper surface of the layer. Topography on the layer boundaries is predicted by a balance of dynamic flow stress and external traction. The nature of boundary topography depends on the magnitude of the driving tractions and the density variation within the layer. If we assume that the layer density is greatest beneath areas of mantle downwelling and decreases to a minimum beneath areas of mantle upwelling (the layer is thermally coupled to the convection in the overlying mantle) then its upper boundary develops a cusp-like peak beneath the upwelling mantle. The height of this peak is potentially much greater than the layer thickness. If, however, the layers are effectively coupled by viscous shear then internal density gradients of the opposite sign may be established. In this case, we observe solutions where the layer is completely swept away beneath areas of mantle downwelling leaving steep-sided ‘islands’ of dense material. This mechanism therefore provides a possible explanation for steep-sided anomalously slow regions at the base of the mantle observed by seismic methods (e.g. beneath south Africa) or for discrete ultralow velocity zones detected at the core-mantle boundary beneath locations of surface hotspots. The magnitude of the upper boundary driving tractions compared to the density gradient within the layer is the key parameter that determines the nature of flow in, and consequently boundary topography of, the layer. The deflection of the core-mantle boundary is small compared with that of the top of the dense layer, but a change in sign of the ratio of these deflections is observed as the magnitude of the driving tractions changes relative to the magnitude of the internal density gradient. We compare seismic measurements of core-mantle boundary topography and D′′ topography with the predictions of this model in an attempt to constrain model parameters, but no clear correlation seems to exist between D′′ thickness and CMB topography. 相似文献
10.
Whether in the mantle or in magma chambers, convective flows are characterized by large variations of viscosity. We study the influence of the viscosity structure on the development of convective instabilities in a viscous fluid which is cooled from above. The upper and lower boundaries of the fluid are stress-free. A viscosity dependence with depth of the form ν0 + ν1 exp(?γ.z) is assumed. After the temperature of the top boundary is lowered, velocity and temperature perturbations are followed numerically until convective breakdown occurs. Viscosity contrasts of up to 107 and Rayleigh numbers of up to 108 are studied.For intermediate viscosity contrasts (around 103), convective breakdown is characterized by the almost simultaneous appearance of two modes of instability. One involves the whole fluid layer, has a large horizontal wavelength (several times the layer depth) and exhibits plate-like behaviour. The other mode has a much smaller wavelength and develops below a rigid lid. The “whole layer” mode dominates for small viscosity contrasts but is suppressed by viscous dissipation at large viscosity contrasts.For the “rigid lid” mode, we emphasize that it is the form of the viscosity variation which determines the instability. For steep viscosity profiles, convective flow does not penetrate deeply in the viscous region and only weak convection develops. We propose a simple method to define the rigid lid thickness. We are thus able to compute the true depth extent and the effective driving temperature difference of convective flow. Because viscosity contrasts in the convecting region do not exceed 100, simple scaling arguments are sufficient to describe the instability. The critical wavelength is proportional to the thickness of the thermal boundary layer below the rigid lid. Convection occurs when a Rayleigh number defined locally exceeds a critical value of 160–200. Finally, we show that a local Rayleigh number can be computed at any depth in the fluid and that convection develops below depth zr (the rigid lid thickness) such that this number is maximum.The simple similarity laws are applied to the upper mantle beneath oceans and yield estimates of 5 × 1015?5 × 1016 m2 s?1 for viscosity in the thermal boundary layer below the plate. 相似文献
11.
根据横跨中国境内天山的库车—奎屯宽频带流动地震台阵和区域地震台网记录的近震和远震P波走时数据，利用地震层析成像方法重建了沿该地震台阵剖面下方400 km深度范围内地壳上地幔的P波速度结构.结果表明：沿新疆库车—奎屯剖面，天山地壳具有明显的横向分块结构，且南、北天山地壳显示了较为强烈的横向变形特征，表明塔里木地块对天山地壳具有强烈的侧向挤压作用；在塔里木和准噶尔地块上地幔顶部有厚度约60～90 km的高速异常体，塔里木—南天山下方的高速异常体产生了较为明显的弯曲变形，而准噶尔—北天山下方的高速异常体向南一直俯冲到中天山南侧边界下方300 km的深度，两者形成了不对称对冲构造；在塔里木和准噶尔地块下方150～400 km深度存在上地幔低速体，其中塔里木地块一侧的上地幔低速物质上涌到南天山地块的下方；在塔里木—南天山200～300 km深度范围的上地幔存在高速异常体，它可能是地幔热物质向上迁移过程融断的塔里木岩石圈的拆离体. 上述结果表明，塔里木地块的俯冲可能涉及整个岩石圈深度，但其前缘仅限于南天山的北缘；青藏高原隆升的远程效应可能不但驱动塔里木岩石圈向北俯冲，同时还造成天山造山带南侧上地幔物质的涌入；天山造山带上地幔广泛存在的低速异常有助于其上地幔的变形，而上地幔物质的强烈非均匀性应有助于推动天山造山带上地幔小尺度地幔对流的形成；根据研究区地壳上地幔速度结构特征推断，新近纪以来天山快速隆升的主要力源来自青藏高原快速隆升的远程效应，相对软弱的上地幔为加速天山造山带的变形和隆升创造了必要条件. 相似文献
12.
近年来新的研究成果反应出鄂尔多斯地块岩石圈并不是一个具有深根的完整的刚性块体,尤其在鄂尔多斯北部以及河套地堑发现有大范围的下地壳-上地幔低速低阻物质,如果这一情况属实,那么人们对鄂尔多斯地块的认识将发生大的变化.为此,我们在华北克拉通西部布设了一条穿过鄂尔多斯地块、河套地堑和阴山造山带的南北向大地电磁剖面,试图通过深部电性结构的探测提供更多信息.该剖面全长约850 km,共布设54个宽频测点和17个长周期测点.二维和三维反演结果均表明:鄂尔多斯地块内部以38°N为界,南部和北部电性结构存在明显差异.鄂尔多斯地块南部地壳至上地幔150 km深度范围内整体表现为高阻,具有刚性克拉通的特征;鄂尔多斯地块北部到河套地堑之间下地壳出现低阻层,特别是鄂尔多斯北端与河套地堑接壤地段,深部存在一个规模较大的下地壳-上地幔低阻异常体,该异常体从河套地堑开始,横向上向南延伸到鄂尔多斯地块内部约200 km,纵向上从下地壳向下延伸到上地幔(约100 km深度).根据该异常体的空间特征,参考该区地震波低速异常体的分布,我们认为鄂尔多斯北部及河套地堑中下地壳到上地幔存在热物质,其原因与深部的构造活动有关(软流圈热物质上涌、侧向流动等),这一情况可能反映出鄂尔多斯地块北部岩石圈深部正处于被改造(或者破坏)阶段,这对进一步认识青藏高原东北缘与华北克拉通之间的深部关系具有一定的启示作用. 相似文献
13.
S. V. Gavrilov 《Izvestiya Physics of the Solid Earth》2009,45(5):437-443
A new approach to analytical and numerical study of the process of the post-glacial uplifting of the Earth’s surface was proposed within the framework of a viscous model. Displacement of the Earth’s surface is considered as the motion of the density boundary due to chemico-density convection. It is shown that the incorporation of the non-Newtonian rheology at observed velocities of post-glacial uplifts requires an obligatory presence of faults in the lithosphere and gives rise to quasi-uniform motion of the mantle material, whose viscosity under the lithosphere is, on the average, sufficiently small and amounts to ～1019 Pa. The study of the stability of the constructed model of the post-glacial uplift considered as the chemico-density convection relative to the thermal convection shows that the velocity of thermal convection developing in the presence of a quasiuniform mantle flow related to the post-glacial recovery is ～1 m/yr. 相似文献
14.
A detailed comparison between fully dynamic and kinematic plate formulations has been made in models of mantle convection. Plate velocity is computed self-consistently from fully dynamic plate models with temperature- and stress-dependent viscosity and preexisting mobile faults. In fully dynamic models, the flow is driven solely by internal buoyancy, while in kinematic models the flow is driven by a combination of the prescribed surface velocity and internal buoyancy. Only a temperature-dependent viscosity, close to the effective viscosity determined from the fully dynamic models, is used in the kinematic models. The two types of models give very similar temperature structures and slab evolutionary histories when the effective viscosity and surface velocity are nearly identical. In kinematic plate models, the additional work introduced by the prescribed velocity boundary condition is apparently dissipated within the lithosphere and has little influence on the convection under the lithosphere. In models with periodic lateral boundary conditions, slabs sink into the lower mantle at an oblique angle and this contrasts with the vertical sinking which occurs with reflecting boundary conditions. Models show that we can simulate fully dynamic models with kinematic models under either periodic boundary conditions or reflecting boundary conditions. 相似文献
15.
Mean S-wave residuals from 46 earthquakes within and on the margins of the Tibetan Plateau exhibit systematic lateral variations that do not correlate well with elevation or with simple aspects of the geologic history. The earliest S waves come from earthquakes in western Tibet, the Karakorum, and the western Himalaya, and the latest come from earthquakes in north-central Tibet. Although S-waves from earthquakes in the Himalaya tend to be early, the east-west variation in residuals across Tibet is at least as large as the north-south difference between the Himalaya and northern Tibet. If the variations in residuals are a reflection of temperature variations in the upper mantle associated with convection, then upwelling beneath north-central Tibet seems to be flanked by downwelling in western, eastern, and probably southern Tibet. This convective flow might reflect the detachment and removal of thickened mantle lithosphere beneath Tibet. 相似文献
16.
用高分辨率地震体波速度成像以及相关的地球物理资料，计算地幔垂直流动形式及流动速度，得到全球地幔流垂直运动模式.从全球尺度来看，地幔流基本可划分为以下几个区域：欧亚大陆—澳大利亚、北美洲—南美洲为两个大规模下降流区域，西印度洋—非洲及大西洋、中南太平洋及东太平洋为两个大规模地幔上升流区域.地幔上升流起源于核幔边界，主要表现在地幔中部和上地幔下部.地幔垂直流动速度约每年1～4cm.地幔流动对地表板块运动、海洋中脊和中隆、俯冲带和碰撞带的分布起着控制作用.地幔上升流与地表现代热点有密切关系.从东亚尺度看，地幔流大体分为三个区域：东亚边缘裂谷系和西太平洋边缘海为上升流、西伯利亚地幔深度表现为物质下降流、青藏高原—缅甸—印度尼西亚特提斯俯冲带地幔下降流，这三个区域地幔流动与地表的西太平洋构造域、亚洲构造域和特提斯构造域相吻合.勾勒出南海地区构造特征：从上到下的大体结构是上部呈“工"字型、中间为圆柱型、底部呈盾形的地幔上升流. 相似文献
17.
吕苗苗 《CT理论与应用研究》2019,28(2):175-186
青藏高原因其复杂的结构和演化历史,一直都是研究大陆碰撞、构造运动及其动力学的热点区域。本文采用三重震相波形拟合技术,基于中国地震观测台网和大型流动台阵记录到的某地震P波垂向记录,获得了包括拉萨、南羌塘和松潘甘孜地块在内的青藏高原上地幔P波速度结构。结果表明:①拉萨和南羌塘地块下方地幔过渡带存在高速异常,推测是俯冲的印度板片滞留体,过渡带底部的板片残余温度较低,使得660-km相变滞后约3～8km。而松潘甘孜地块下方过渡带同样存在高速异常,可能是欧亚岩石圈发生拆沉进入地幔过渡带所致。这说明印度板块俯冲作用的影响已经到达地幔过渡带,其俯冲前缘位于班公怒江缝合带附近。②从拉萨、南羌塘到松潘甘孜地块,200km之上的地幔岩石圈高速盖层速度由南向北逐渐减小,松潘甘孜地块则出现盖层缺失。推测受小规模地幔对流或者热不稳定性的影响,在南羌塘和松潘甘孜地块,增厚的欧亚岩石圈发生拆沉作用,岩石圈被减薄和弱化,造成羌塘地块上地幔低速和松潘甘孜地块上地幔高速盖层缺失。拆沉的冷的欧亚岩石圈可能部分停留在410-km上方,使得410-km抬升约10km,部分沉入地幔过渡带,表现为松潘甘孜地块地幔过渡带中存在高速异常。低温造成660-km下沉约8km,导致地幔过渡带增厚。 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.
利用地震层析成像结果分析了中国西部地区的上地幔速度结构,发现青藏高原北部至东南边缘上地幔顶部速度普遍偏低;随着深度的增加,低速区主要分布在羌塘、松潘—甘孜和云南西部地区,而印度大陆、塔里木、柴达木、鄂尔多斯和四川盆地均显示出较高的速度.上述速度分布与青藏高原及周边地区的岩石层结构和深部动力性质密切相关:其中羌塘地区的低速异常反映了青藏北部的地幔上涌和局部熔融,起因于印度大陆岩石层的向北俯冲;松潘—甘孜地区的低速异常与青藏东部的深层物质流动及四川盆地刚性岩石层的阻挡有关;而滇西地区的低速异常可能受到印缅块体向东俯冲作用的影响.以上三个区域构成青藏高原和周边地区的主要地幔异常区.相比之下,印度大陆、塔里木、柴达木、鄂尔多斯和四川盆地的高速异常反映了大陆构造稳定地区的岩石层地幔特点.根据速度变化推测,地幔上涌和韧性变形并非贯穿整个青藏高原,而是主要集中在羌塘、松潘—甘孜和滇西地区,上述构造效应不仅导致岩石层厚度减薄且引发了火山和岩浆活动. 相似文献
20.
上地幔小尺度对流是控制区域地球动力学过程的主要机制之一,蒙古-贝加尔地区的一些区域动力学过程被认为与上地幔小尺度对流相关.本文目的在于利用重力资料研究蒙古-贝加尔地区的上地幔小尺度对流,并探讨其与构造动力学的关系.基于区域均衡重力异常与上地幔小尺度对流的相关方程,本文利用区域均衡重力异常资料反演了蒙古-贝加尔地区上地幔小尺度对流流场及作用于岩石层底部的应力场.结果显示,蒙古-贝加尔地区地幔流场及对流应力场呈现非常复杂的图像,流场及应力场分布与地表构造具有很好的相关性.西伯利亚地台和蒙古褶皱带下地幔流场和对流应力场均较弱,这与这些地区现今较弱的构造活动性是一致的.贝加尔裂谷区下存在地幔上升流,对流应力场呈拉张状态,但应力场的幅值较小(约8 MPa),表明地幔对流不是贝加尔裂谷开裂的主要控制因素.Hangay高原、阿尔泰和戈壁-阿尔泰下存在地幔上升流,对流应力场为拉张状态,这一方面可能构成Hangay高原隆升的深部动力机制,另一方面,也为Amurian板块西边界划分提供了动力背景. 相似文献