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1.
太阳系内类地行星具有相似的岩石层包围金属核的圈层结构,在行星幔的热演化历史起源方面具有同时性和同源性,并且都在早期变形重力位能加热的基础上随放射性热能衰减而冷却。但是,由于半径、密度、粘度以及表层构造属性等物理条件的差异,其热演化历史各具特色。依据基本的热对流和热传导方程,我们计算分析了类地行星热物理条件差异对行星幔热演化历史的影响。计算表明,类地行星热演化的早期,行星幔热对流是主要的散热方式。半径较大的行星表面热流密度大,平均散热量也大。半径较小的行星内部温差小,粘滞系数高,对流能力低,提早进入传导散热状态,且传导散热的岩石层也比大行星厚。不同边界层热物理条件下,类地行星幔热演化历史会分别出现逐渐冷却的平稳式、包含热柱上涌的波动式、行星幔幕次翻转的周期式等特点不同的热演化过程。火星内部曾经存在的地幔热柱构造与火星地幔热动力学演化过程密切相关。我们从火星地幔热动力学演化模型出发,定量计算与地幔热柱构造演化相关的地幔热动力学演化特征,通过三维球壳数值模拟,研究了火星地幔热演化历史上可能存在的热柱活动造成的火星热演化历史的非单调变化,火星地幔对流环结构随时间的演变方式,以及与边界相关的地幔热柱对火星地形的影响。 相似文献
2.
基于海洋地质地球物理观测建立的板块构造理论意味着板块和浅部地幔共同演化,然而地幔底部尤其是大型横波低速异常区(LLSVP)与板块(尤其微板块)运动和演化之间是否存在关联仍有争议。一些研究认为LLSVP长期保持稳定,而另一些模型则认为它与各级板块存在相互作用。为此,本文通过总结前人成果,并基于近期发表的板块重建和地幔对流模型进行进一步分析,探讨微板块运动和LLSVP的演化关系。模拟结果表明,微板块与大板块类似,俯冲后通常会下沉至核幔边界。微幔块会推动地幔底部热的物质聚集并形成大的热化学结构。该热化学结构与层析成像揭示的LLSVP基本吻合。下地幔径向流速场和温度场的二阶结构与地表速度场散度的二阶结构随时间的移动轨迹相似,表明深浅部圈层的耦合演化,但是下地幔结构演化一般会滞后于浅表。在微幔块推挤之下,地幔柱优先沿着地幔底部热化学结构的边缘形成,且有时会被推至热化学结构的内部。地幔柱上升至浅部后,能够导致岩石圈弱化甚至裂解或板块边界跃迁,形成微板块。因此,地幔底部LLSVP不是稳定或静止的,而是与微板块动态协同演化,并通过地幔柱与浅表板块边界发生遥相关,从而控制微板块生成场所。 相似文献
3.
全球地幔三维结构模型及动力学研究新进展 总被引：10，自引：0，他引：10
介绍了地幔三维地震模型及地球动力学最新进展，特别是１９９５年７月ＩＵＧＧ第２１届大会展示的新成果。地幔三维速度分布主要由全球数字地震台网资料求得。１００ｋｍ深度速度分布主要与板块构造有关，３５０ｋｍ深度显示了大陆与海洋的差异，１９００ｋｍ深度表现环太平洋的高速异常带。 相似文献
4.
Based on the high-resolution body wave tomographic image and relevant geophysical data, we calculated the form and the vertical
and tangential velocities of mantle flow. We obtained the pattern of mantle convection for East Asia and the West Pacific.
Some important results and understandings are gained from the images of the vertical velocity of mantle flow for East Asia
and the West Pacific. There is an upwelling plume beneath East Asia and West Pacific, which is the earth’s deep origin for
the huge rift valley there. We have especially outlined the tectonic features of the South China Sea, which is of the “工”
type in the upper mantle shield type in the middle and divergent in the lower; the Siberian clod downwelling dives from the
surface to near Core and mantle bounary (CMB), which is convergent in the upper mantle and divergent in the lower mantle;
the Tethyan subduction region, centered in the Qinghai-Tibet plateau, is visible from 300 to 2 000 km, which is also convergent
in the upper mantle and divergent in the lower mantle. The three regions of mantle convection beneath East Asia and the West
Pacific are in accordance with the West Pacific, Ancient Asia and the Tethyan structure regions. The mantle upwelling originates
from the core-mantle boundary and mostly occurs in the middle mantle and the lower part of the upper mantle. The velocities
of the vertical mantle flow are about 1–4 cm per year and the tangential velocities are 1–10 cm per year. The mantle flow
has an effect on controlling the movement of plates and the distributions of ocean ridges, subduction zones and collision
zones. The mantle upwelling regions are clearly related with the locations of hotspots on the earth’s surface.
Translated from Geology in China, 2006, 33(4): 896–905 [译自: 中国地质] 相似文献
5.
Yaoling Niu 《地学前缘(英文版)》2018,9(5):1265-1278
In studying the petrogenesis of intra-plate ocean island basalts(OIB) associated with hotspots or mantle plumes, we hypothesized that the two large-low-shear-wave-velocity provinces(LLSVPs) at the base of the mantle beneath the Pacific(Jason) and Africa(Tuzo) are piles of subducted ocean crust(SOC)accumulated over Earth's history. This hypothesis was formulated using petrology, geochemistry and mineral physics in the context of plate tectonics and mantle circulation. Because the current debate on the origin of the LLSVPs is limited to the geophysical community and modelling discipline and because it is apparent that such debate cannot be resolved without considering relevant petrological and geochemical information, it is my motivation here to objectively discuss such information in a readily accessible manner with new perspectives in light of most recent discoveries. The hypothesis has the following elements:(1) subduction of the ocean crust of basaltic composition to the lower mantle is irreversible because(2) SOC is denser than the ambience of peridotitic composition under lower mantle conditions in both solid state and liquid form;(3) this understanding differs from the widespread view that OIB come from ancient SOC that returns from the lower mantle by mantle plumes, but is fully consistent with the understanding that OIB is not derived from SOC because SOC is chemically and isotopically too depleted to meet the requirement for any known OIB suite on Earth;(4) SOC is thus the best candidate for the LLSVPs, which are, in turn, the permanent graveyard of SOC;(5) the LLSVPs act as thermal insulators, making core-heating induced mantle diapirs or plumes initiated at their edges, which explains why the large igneous provinces(LIPs) are associated with the edges of the LLSVPs;(6) the antipodal positioning of Jason and Tuzo represents the optimal momentum of inertia, which explains why the LLSVPs are stable in the spinning Earth. 相似文献
6.
Mantle convection modeling of the supercontinent cycle： Introversion,extroversion, or a combination？
The periodic assembly and dispersal of continental fragments,referred to as the supercontinent cycle,bear close relation to the evolution of mantle convection and plate tectonics.Supercontinent formation involves complex processes of"introversion"(closure of interior oceans),"extroversion"(closure of exterior oceans),or a combination of these processes in uniting dispersed continental fragments.Recent developments in numerical modeling and advancements in computation techniques enable us to simulate Earth’s mantle convection with drifting continents under realistic convection vigor and rheology in Earth-like geometry(i.e.,3D spherical-shell).We report a numerical simulation of 3D mantle convection,incorporating drifting deformable continents,to evaluate supercontinent processes in a realistic mantle convection regime.Our results show that supercontinents are assembled by a combination of introversion and extroversion processes.Small-scale thermal heterogeneity dominates deep mantle convection during the supercontinent cycle,although large-scale upwelling plumes intermittently originate under the drifting continents and/or the supercontinent. 相似文献
7.
Transient surface deflections associated with non-continuous subduction are studied through the use of a sequence of idealized numerical models of time dependent mantle convection. The major concerns of this study are the magnitude and duration of dynamically supported topographic fluctuations resulting from either the initiation or termination of a subduction zone at the Earth's surface. The former is modelled by prescribing an initial temperature structure which mimics the presence of a cold slab of lithosphere immersed in a warm uniform-temperature mantle, while the latter is induced by thermally detaching this slab from the upper surface of the convection cell. The model “slabs” are thus defined thermally, rather than mechanically, and their negative buoyancy induces convective flow in the neighbouring mantle. The full hydrodynamic equations governing natural convection are solved numerically in order to follow the evolution of the temperature and velocity fields with time. The resulting model flows are, therefore, dynamically self-consistent, and differ from previous kinematic models in that the flow velocities are determined at each instant from the evolving thermal field, rather than prescribed as boundary conditions. As the lithospheric slab sinks into the mantle, the induced flow produces normal stresses, which in turn result in a broad topographic depression. Subsequently, as the slab-surface distance increases, the topography rebounds at a rate which is strongly dependent upon the imposed temperature contrast between the slab and the surrounding mantle material. Assuming that these depressions are filled with sediments, if a subsequent episode of subduction were to begin before the initial depression was eliminated, new sediments would be superimposed upon the old. Repeated episodes of subduction at a continental margin may, therefore, be an important factor contributing to repeated cycles of platform sedimentation. 相似文献
8.
Seismic observations have shown structural variation near the base of the mantle transition zone(MTZ)where subducted cold slabs,as visualized with high seismic speed anomalies(HSSAs),flatten to form stagnant slabs or sink further into the lower mantle.The different slab behaviors were also accompanied by variation of the "660 km" discontinuity depths and low viscosity layers(LVLs) beneath the MTZ that are suggested by geoid inversion studies.We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics.A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ.We found that the viscosity reduction of subducted crustal material leads to a separation of crustal material from the slab main body and its transient stagnation in the MTZ.The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20-30 My from the start of the plate subduction.The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model.Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals,dehydration should occur at the phase transformation depth,~660 km.The variation of the discontinuity depths and highly localized low seismic speed anomaly(LSSA) zones observed from seismic P waveforms in a relatively high frequency band(~1 Hz) support the hypothesis of dehydration from hydrous slabs at the phase boundary.The LSSAs which correspond to dehydration induced fluids are likely to be very local,given very small hydrogen(H~+) diffusivity associated with subducted slabs.The image of such local LSSA zones embedded in HSSAs may not be necessarily captured in tomography studies.The high electrical conductivity in the MTZ beneath the northwestern Pacific subduction zone does not necessarily require a broad range of high water content homogeneously. 相似文献
9.
In the available numerical models, mantle plumes are represented by homogeneous ascending streams of thermal convection. Pulses are considered to be possible only in thermochemical plumes within the compositionally inhomogeneous mantle. We show that pulses can also occur under regular thermal convection in the homogeneous mantle. As the intensity grows, the flow in the tail of a thermal plume first begins pulsing and then the plume breaks up into a set of sequentially emerging thermals. For the present-day mantle, the pulsation periods for plumes in the lower mantle can range up to 10 Ma and about 1 Ma in the upper mantle. 相似文献
10.
地球内部物理和演化的几个核心论题：I.地球内部结构和物理性质 总被引：1，自引：0，他引：1
本文综述研究地球内部结构和物理特性的几种常见规方法和主要研究结果,并首重讨论地球物理状态方程,地震成象,综合反演,高温高压实验和有关对比研究方法,均匀各向同性球对球模型仍不失其参考意义,但最新研究结果表明,地球内部状态是非均匀和各向异性的,横向不均匀对称地球模型仍不其参考意义,但最新研究结果表明,地球内部状态是非均匀和各向异性的,横向不均匀性主要表现在上地幔部分,下地幔和液态外核似乎比较均匀,但核－幔边界过渡带(D″)可能代表一个内含非均匀化学边界的热边界层,其形态起伏和横向变化影响地球模壁的球对称性。3-D地震成象实质上反映地震波建与温度异常的关系．而温度变化又会引起密度异常,因而密度变化是控制地幔对流的关键参数之一。 相似文献
11.
The spatial distribution of recent (under 2 Ma) volcanism has been studied in relation to mantle hotspots and the evolution of the present-day supercontinent which we named Northern Pangea. Recent volcanism is observed in Eurasia, North and South America, Africa, Greenland, the Arctic, and the Atlantic, Indian, and Pacific Oceans. Several types of volcanism are distinguished: mid-ocean ridge (MOR) volcanism; subduction volcanism of island arcs and active continental margins (IA + ACM); continental collision (CC) volcanism; intraplate (IP) volcanism related to mantle hotspots, continental rifts, and transcontinental belts. Continental volcanism is obviously related to the evolution of Northern Pangea, which comprises Eurasia, North and South America, India, Australia, and Africa. The supercontinent is large, with predominant continental crust. The geodynamic setting and recent volcanism of Northern Pangea are determined by two opposite processes. On one hand, subduction from the Pacific Ocean, India, the Arabian Peninsula, and Africa consolidates the supercontinent. On the other hand, the spreading of oceanic plates from the Atlantic splits Northern Pangea, changes its shape as compared with Wegener’s Pangea, and causes the Atlantic geodynamics to spread to the Arctic. The long-lasting steady subduction beneath Eurasia and North America favored intense IA + ACM volcanism. Also, it caused cold lithosphere to accumulate in the deep mantle in northern Northern Pangea and replace the hot deep mantle, which was pressed to the supercontinental margins. Later on, this mantle rose as plumes (IP mafic magma sources), which were the ascending currents of global mantle convection and minor convection systems at convergent plate boundaries. Wegener’s Pangea broke up because of the African superplume, which occupied consecutively the Central Atlantic, the South Atlantic, and the Indian Ocean and expanded toward the Arctic. Intraplate plume magmatism in Eurasia and North America was accompanied by surface collisional or subduction magmatism. In the Atlantic, Arctic, Indian, and Pacific Oceans, deep-level plume magmatism (high-alkali mafic rocks) was accompanied by surface spreading magmatism (tholeiitic basalts). 相似文献
12.
Walter Alvarez 《地学学报》2001,13(5):333-337
Recent evidence indicates that beneath the Caribbean a tongue of sublithosphere mantle is flowing from the Pacific to the Atlantic, dragging the overlying lithosphere eastward: (i) Shear-wave splitting results from beneath the Andean subduction zone and Venezuela suggest mantle flow eastward through the Caribbean. (ii) Volcanic chemistry in Central America indicates a slab source beneath Nicaragua, but a different source in Costa Rica, above the proposed Pacific outflow. (iii) An extinct volcanic arc accreted to the margins of the Caribbean swept eastward through the Caribbean gap between North & South America. The 1982 'continental undertow' model requires shallow-mantle flow through the Caribbean gap from the Pacific to the Atlantic, if continents have deep roots and if shallow-mantle flow beneath oceans is decoupled from convection at deeper levels. The new evidence from the Caribbean is thus compatible with the continental undertow model, and perhaps with other models involving decoupled shallow flow. 相似文献
13.
It has been thought that granitic crust,having been formed on the surface,must have survived through the Earth’s evolution because of its buoyancy.At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction,subduction erosion, and continental subduction.Granitic rocks,the major constituent of the continental crust,are lighter than the mantle at depths shallower than 270 km,but we show here,based on first principles calculations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone,and thus can be subducted in the depth range of 270-660 km.This suggests that there can be two reservoirs of granitic material in the Earth,one on the surface and the other at the base of the mantle transition zone(MTZ).The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust.Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle.This could explain the anomalous seismic-wave velocities observed around 660 km depth.The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation,chemical discontinuities between granitic material and the surrounding mantle,or a combination thereof. 相似文献
14.
《地学前缘(英文版)》2020,11(5):1571-1579
Mantle plumes originating from the Core-Mantle Boundary(CMB) or the Mantle Transition Zone(MTZ) play an important role in material transfer through Earth's interior.The hotspot-related plumes originate through different mechanisms and have diverse processes of material transfer.Both the Morganian plumes and large low shear wave velocity provinces(LLSVPs) are derived from the D " layer in the CMB,whereas the Andersonian plumes originate from the upper mantle.All plumes have a plume head at the Moho,although the LLSVPs have an additional plume head at the MTZ.We compare the geochemical characteristics of various plumes in an attempt to evaluate the material exchange between the plumes and mantle layers.The D" layer,the LLSVPs and the Morganian plumes are consisted of subducted slab and post-perovskite from the lower mantle.Bridgmanite would crystallize during the upwelling process of the LLSVPs and the Morganian plumes in the lower mantle,and the residual is a basalt-trachyte suite.Unlike the Morganian plumes,the crystallization in the LLSVPs is insufficient that material accumulates beneath the MTZ to form a plume head.Typically,the secondary plumes above the plume head occur at the edge of the LLSVPs because it is easier for bridgmanite crystal separating from the plume head at the edge,and the residual material with low density upwells to form the secondary plumes.Meanwhile,Na and K are enriched during the long-term crystallization process,and then the basalt-phonolite suite appears in the LLSVPs.The geochemical characteristics of Andersonian plumes suggest that the basalt-rhyolite suite is the major component in the upper mantle.Meanwhile the basalt-rhyolite suite also appears in the LLSVPs and the Morganian plumes because of the assimilation and contamination in the plume head beneath the Mono. 相似文献
15.
Both seismology and geochemistry show that the Earth's mantle is chemically heterogeneous on a wide range of scales. Moreover, its rheology depends strongly on temperature, pressure and chemistry. To interpret the geological data, we need a physical understanding of the forms that convection might take in such a mantle. We have therefore carried out laboratory experiments to characterize the interaction of thermal convection with stratification in viscosity and in density. Depending on the buoyancy ratio B (ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly), two regimes were found: at high B, convection remains stratified and fixed, long-lived thermochemical plumes are generated at the interface, while at low B, hot domes oscillate vertically through the whole tank, while thin tubular plumes can rise from their upper surfaces. Convection acts to destroy the stratification through mechanical entrainment and instabilities. Therefore, both regimes are transient and a given experiment can start in the stratified regime, evolve towards the doming regime, and end in well-mixed classical one-layer convection. Applied to mantle convection, thermochemical convection can therefore explain a number of observations on Earth, such as hot spots, superswells or the survival of several geochemical reservoirs in the mantle. Scaling laws derived from laboratory experiments allow predictions of a number of characteristics of those features, such as their geometry, size, thermal structure, and temporal and chemical evolution. In particular, it is shown that (1) density heterogeneities are an efficient way to anchor plumes, and therefore to create relatively fixed hot spots, (2) pulses of activity with characteristic time-scale of 50–500 Myr can be produced by thermochemical convection in the mantle, (3) because of mixing, no ‘primitive’ reservoir can have survived untouched up to now, and (4) the mantle is evolving through time and its regime has probably changed through geological times. This evolution may reconcile the survival of geochemically distinct reservoirs with the small amplitude of present-day density heterogeneities inferred from seismology and mineral physics. 相似文献
16.
西北太平洋各边缘海及其相应俯冲系统受深部构造活动等地质条件的控制,热流变化较大。在收集整理该区域最新的热流数据基础上,重点探讨西北太平洋俯冲带热结构相关理论、边缘海大洋岩石圈热演化理论模型和局部高异常热流的影响因素,总结了西北太平洋边缘海热流所反映的地质意义。研究结果表明,在西北太平洋“沟-弧-海”体系中,从“沟”到“弧”再到“边缘海”,热流密度呈“低-高-较高”的变化趋势,弧后地区整体表现为“均一高热”特征;千岛海沟、日本海沟和琉球海沟热流密度值在30.0 mW/m2左右,而对应的岛弧值为其2~3倍。弧后热流大小受到汇聚型俯冲带热结构的影响,俯冲带脱水作用导致的弧后上地幔黏度变化,地震速度降低,岩石圈弹性厚度减薄,引起小尺度地幔对流,形成弧后“均一高热”的热状态。热流的时空分布与岩石圈年龄也有关,随着岩石圈年龄增大,地表热流密度值会随之降低,热流密度值大小和离散性与其形成时间大致呈负相关。鄂霍次克海形成时代(30~65 Ma)较早,其热流密度值(86.8 mW/m2)和离散性(标准差3.727)相对较低;冲绳海槽目前还处于扩张阶段,其热流密度值(139.0 mW/m2)和离散性(标准差7.001)较高。浅层的地下水循环、断裂活动,深层的地幔部分熔融岩浆活动、弧后小尺度地幔对流、俯冲带拐角流等对局部异常热流起到一定程度的控制作用。 相似文献
17.
18.
A passive seismic experiment across the Longmenshan (LMS) fault belt had been conducted between August 2006 and July 2007 for the understanding of geodynamic process between the Eastern Tibet and Sichuan basin. We herein collected 3677 first P arrival times with high precision from seismograms of 288 teleseismic events so as to reconstruct the upper mantle velocity structure. Our results show that the depth of the Lithosphere–asthenosphere boundary (LAB) changes from 70 km beneath Eastern Tibet to about 110 km beneath Longquanshan, Sichuan Basin, which is consistent with the receiver function imaging results. The very thin mantle part of the lithosphere beneath Eastern Tibet may suggest the lithosphere delamination due to strong interaction between the Tibetan eastward escaping flow and the rigid resisting Sichuan basin, which can be further supported by the existences of two high-velocity anomalies beneath LAB in our imaging result. We also find there are two related low-velocity anomalies beneath the LMS fault belt, which may indicate magmatic upwelling from lithosphere delamination and account for the origin of tremendous energy needed by the devastating Wenchuan earthquake. 相似文献
19.
Seismic images under 60 hotspots: Search for mantle plumes 总被引：10，自引：0，他引：10
The mantle plume hypothesis is now widely known to explain hotspot volcanoes, but direct evidence for actual plumes is weak, and seismic images are available for only a few hotspots. In this work, we present whole-mantle tomographic images under 60 major hotspots on Earth. The lateral resolution of the tomographic images is about 300 km under the continental hotspots and 400–600 under the oceanic hotspots. Twelve plume-like, continuous low-velocity (low-V) anomalies in both the upper and lower mantle are visible under Hawaii, Tahiti, Louisville, Iceland, Cape Verde, Reunion, Kerguelen, Amsterdam, Afar, Eifel, Hainan, and Cobb hotspots, suggesting that they may be 12 whole-mantle plumes originating from the core–mantle boundary (CMB). Clear upper-mantle low-V anomalies are visible under Easter, Azores, Vema, East Australia, and Erebus hotspots, which may be 5 upper-mantle plumes. A mid-mantle plume may exist under the San Felix hotspot. The active intra-plate volcanoes in Northeast Asia (e.g., Changbai, Wudalianchi, etc.) are related to the stagnant Pacific slab in the mantle transition zone. The Tengchong volcano in Southwest China is related to the subduction of the Burma microplate under the Eurasian plate. Although low-V anomalies are generally visible in some depth range in the mantle under other hotspots, their plume features are not clear, and their origins are still unknown. The 12 whole-mantle plumes show tilted images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow. In most cases, the seismic images under the hotspots are complex, particularly around the mantle transition zone. A thin low-V layer is visible right beneath the 660-km discontinuity under some hotspots, while under a few other hotspots, low-V anomalies spread laterally just above the 660-km discontinuity. These may reflect ponding of plume materials in the top part of the lower mantle or the bottom of the upper mantle. The variety of behaviors of the low-V anomalies under hotspots reflects strong lateral variations in temperature and viscosity of the mantle, which control the generation and ascending of mantle plumes as well as the flow pattern of mantle convection. 相似文献
20.
Two large cold masses in the deep mantle have been delineated by using long-wavelength seismic tomographic models in conjunction with mineralogical experimental data at high pressure. These cold anomalies are found under the western Pacific and the Americas with temperatures more than 1000 degrees below the ambient mantle temperature. These strong cold anomalies existing in the lower mantle today would suggest that there might have existed not too long ago a substantial temperature jump across a thermal boundary layer between the upper and lower mantle. Numerical simulations in an axisymmetric spherical-shell model incorporating the two major phase transitions have shown that very large pools of cold material with temperatures of around 1500 K can be flushed down to the core–mantle boundary during this tumultuous gravitational instability. A correlation is found between the current locations of these very cold masses and regions of past subduction since the Cretaceous. Correlation analysis shows that the slab mass-flux into the lower mantle does not behave in a steady-state fashion. These findings may support the idea of a strong gravitational instability with origins in the transition zone, as suggested by numerical models of mantle convection. 相似文献