地震波速度约束的大横向黏度变化的地幔对流研究

本文将横向黏度变化提高到3个量级,获得了地震波速度结构约束下的大横向黏度变化的地幔浅部的极型场和环型场对流图像.与小横向黏度变化下的结果相比,本文的结果具有显著的改善.对极型场对流图像,主要体现在本文结果能更清楚地解释太平洋板块、大洋洲和南美洲以及东太平洋洋中脊处的现今运动状态;对环型场对流图像,能更合理地解释北太平洋...     

The modern geoid and ancient plate boundaries

The non-hydrostatic geoid is dominated by three large anomalies: an area of high gravity potential in the equatorial Pacific; another stretching from Greenland through Africa to the southwest Indian Ocean; and a semi-continuous low region passing from Hudson's Bay through Siberia to India and on to Antarctica. None of these three high-amplitude (greater than 60 m) and long-wavelength anomalies corresponds to present-day plate boundaries. However, if the modern geoid is plotted over the positions of continents and plate boundaries at 125 Ma B.P. (reconstructed relative to hotspots) a strong correlation emerges. The modern geoidal low corresponds in position to the areas of subduction surrounding the Pacific 125 Ma ago. The geoidal high now centered on Africa is entirely contained within ancient Pangaea, and the equatorial Pacific high overlies the location of the spreading centers preserved in the magnetic anomalies of the central Pacific. The most plausible cause of the large geoidal undulations is lower mantle convection only weakly coupled to plate motions. The correspondence between modern geoid and ancient plate boundaries implies either that the coupling was much more intimate in the past, or that there is a lag of at least 100 Ma in response of the lower mantle to upper mantle conditions.     

Structure of the back-arc spreading basins and the fluid regime in the mantle

Seismic, geothermal, petrological and other data collected during the joint Soviet-Chinese-Japanese Project “Geotraverse: Pacific-China plain” are highly contradictory concerning their information on back-arc basins. The routine interpretation of the geothermal data leads, e.g. to the conclusion that the temperatures at depth are much higher than can be derived from other data. The discrepancies can be resolved by the back-arc spreading basins origin because of secondary mantle bulk or fluid convection. The inversion of the sign of the seismic velocity anomalies in the Pacific region at a depth of about 300 km can also be explained if active deep fluid regime is proposed. A new geotherm below the Mariana back-arc basin is proposed, and the velocity of the ascending mantle flow is estimated for this region.     

Chemical compositions and petrogenetic relationships in Apollo 15 mare basalts

Complex patterns of spreading centers were formed in Mesozoic time during the breakup of Gondwanaland and the Pacific tectonic plate. The approximate locus of each breakup can be identified by paleomagnetism, paleogeography, and plate tectonics. Each coincides with the present location of similarly complex patterns of intense positive gravity anomalies produced by rising and divergent mantle convection. Apparently the convection caused the breakups and the location and intensity of the convective pattern have not changed since Mesozoic time.     

Isotope and trace element geochemistry of MORB from the Nansen-Gakkel ridge at 86° north

The geochemistry of mid-oceanic ridge basalts from 86°N (Arctic Ocean) provides, for the first time, an insight into the composition of the mantle around the North Pole. Our data show the source region of the Arctic basalts to possess traces of an enrichment similar to the DUPAL signature. This is remarkable since up to now the DUPAL signature has been believed to be present only in Indian but not in Atlantic or Pacific MORB. These results also argue against a model of whole-mantle convection, in which upwelling of enriched material at the equator is balanced by downwelling of depleted material at the poles.     

板块绝对运动及地幔热对流

本文以板块绝对运动AM1-2模型为边界条件探讨了不同的瑞利数下地幔热对流模型.结果表明,瑞利数小于10000（529.41）时,地幔对流呈现以板块驱动图式,运动的极型场和环型场由板块运动激发,两种场占有差不多相同的功率.当瑞利数增加到接近或略超过最低临界值时（约1.5倍）,对流呈现出复杂状态:1.板块运动速率小于下伏地幔对流速率;2.区域性的双层对流环出现;3.对流谱成分发生变化;4.环型场仅在地幔很浅的区域中起作用,而在地幔深部对流图式中影响很小.     

The long wavelength component of mantle convection

Theoretical models of mantle convection predict that the major temperature fluctuations within the mantle are confined to narrow horizontal boundary layers and vertical plumes. These fluctuations result in heterogeneities in seismic body wave velocities which could, in principle, be detected by seismic tomographic techniques. However, recent tomographic images of the spatial variations of temperature in the mantle are considerably “out of focus” in that only the longest wavelength components can be resolved. To assess this partial recovery of the total tomographic image, theoretical temperature fields have been generated with a numerical model of high Rayleigh number mantle convection and then Fourier analysed in two dimensions. Upon re-synthesizing the model temperature fields, the Fourier series expansions were truncated at various levels of resolution. The truncated expansions, containing only the long wavelength components of the model temperature fields, are compared to both the complete field and the tomographic images of the mantle. At the current level of resolution it seems unlikely that seismic tomography could distinguish between layered and whole-mantle convection. Estimates, based on current tomographic data, of long wavelength fluctuations of temperature and surface topography are predicted, in the case of whole-mantle convection, to represent approximately 10% of the total temperature variation, and approximately 50% of the total topographic relief. Thus topography at the core-mantle boundary may be more accurately inferred from seismic tomography than may the characteristic lateral temperature fluctuation which drives the convective circulation.     

The geochemistry of oceanic basalts in the vicinity of transform faults: Observations and implications

Consideration of geochemical data from basalts formed near major Atlantic and Pacific transform faults reveals two significant sets of observations. First, compared to basalts formed far from the transform, basalts near the ridge/transform intersection have, for the same MgO contents, higher abundances of TiO₂ and other incompatible elements, higher La/Sm and La/Yb ratios, and often higher FeO. These enrichments are distinct from and occur in addition to the more variable and fractionated compositions which have been frequently noted [10–13]. Modeling of this “transform fault effect” using data from the Tamayo/EPR intersection suggests the chemical systematics are caused by decreasing extents of melting as the transform is approached.Second, there are chemical discontinuities in the major element, trace element and isotopic chemistry of basalts across many transforms. These “transform discontinuities” occur in normal ocean crust as well as around hot spots.Consideration of the melting zone in the mantle suggests that the transform fault effect is a natural consequence of the ridge/transform plate boundary. The melting zone beneath a ridge segment must terminate across the transform, leading to lower extents of melting at the transform edge. The surface manifestation of the change in the melting zone may be affected by the age of the transform offset, the spreading rate, the transform spacing and the interaction of mantle flow with the local thermal structure; it may be obscured by episodic magma chamber processes and mantle heterogeneity.The significance of transform discontinuities depends on whether they persist with age. If they do not, then temporally variable crust-forming processes may produce changes along a flow line similar to those at zero age across a transform. If, on the other hand, a discontinuity persists with age, then the transform may be related to a fundamental discontinuity in the underlying mantle. Long-lived transform discontinuities would have profound implications for the nature of plate motions, mantle convection and mantle heterogeneity.     

Statistical analysis of isotopic ratios in MORB: the mantle blob cluster model and the convective regime of the mantle

A statistical examination of isotopic distributions for MORB from various ocean ridges leads to the “blob cluster model”, in which the oceanic crust accreting at ridges results from the mixing of two components within the ascending mantle. These are (1) upper mantle material and (2) discrete rising blobs of more radiogenic material. The blobs are fractionated to a variable degree and are distributed in the upper mantle circulation in a manner that is related to the spreading rate.(1) Themean values of the isotopic distributions allow us to calculate the probabilities of the two types of material within the mantle. The results show that theproportion of asthenospheric material in the mixtureincreases with the spreading rate, in agreement with the hypothesis of blob dilution within the upper mantle convection.Mass fluxes can be estimated for the rising blobs from these probabilities, which depend on the respective concentrations in the sources of the two types of material. If the blobs originate in the lower mantle, this flux estimation would suggest that a significant part of the lower mantle has been injected into the upper mantle during earth history.(2) Thestandard deviations of the distributions depend on the “efficiency” of the mixing process:the more imbricated are the asthenospheric and blob materials in the mixture,the smaller is theisotopic spread. This efficiency parameter is shown to increase with the spreading rate, as already suggested by previous comparisons between the East Pacific Rise and the Mid-Atlantic Ridge. Moreover, this feature may also be correlated with other data such as ridge bathymetric variations.     

Geophysical observations pertaining to solid-state convection in the terrestrial planets

We present a broad-based review of the observational evidence that pertains to or otherwise implies solid-state convection to be occurring (or have occurred) in the interiors of the terrestrial planets.For the Earth, the motion of the plates is prima facie evidence of large-scale mantle convection. Provided we understand upper-mantle thermal conductivity correctly, heat flow beneath the old ocean basins may be too high to be transported conductively from the upper mantle through the base of the lithosphere and therefore convection on a second smaller scale might be operative. The horizontal scale of plate dimensions implies, due to typical cell aspect ratios observed in convection, that the motion extends to the core-mantle boundary. Improved global data coverage and viscoelastic modeling of isostatic rebound due to Pleistocene deglaciation imply a uniform mantle viscosity, and thus indicate that whole-mantle convection could exist. Additionally, there is some seismic evidence of lithospheric penetration to depths deeper than 700 km. We discuss some salient features and assumption boundedness of arguments for convection confined to the upper mantle and for convection which acts throughout the mantle since the vertical length scale has a profound effect upon the relevance of geophysical observations. The horizontal form of mantle convection may be fully three-dimensional with complex planform and, therefore, searching for correlative gravity patterns in the ocean basins may not be useful without additional geophysical constraints. Many long-wavelength gravity anomalies may arise from beneath the lithosphere and must be supported dynamically, although thermal convection is not a unique explanation. Topography is an additional geophysical constraint, but for wavelengths greater than a few hundred kilometers, a general lack of correlation exists between oceanic residual gravity and topography, except at specific locations such as Hawaii. Theoretical calculations predict a complex relationship between these two observational types. Oceanic gravity data alone shows no regular planform and there is no correlation with any small-scale convective pattern predicted by laboratory experiments.All of the observational evidence argues against Martian plate tectonics occurring now or over much of the history of this planet, but lack of plate tectonics is not an argument against interior convection. The Tharsis uplift on Mars may have resulted from convective processes in the mantle, and the present-day gravity anomaly associated with Tharsis must be supported by the finite strength of the lithosphere or by mantle convection. Stresses imparted by the present topographic load would be greater than a kilobar, in excess of long-term finite strength. Observed fracture patterns are probably a direct result of this load, and the key question concerns the level of resultant strain relief. The global topographic and geomorphic dichotomy between the northern and southern hemisphere required a solid-state flow process to create the accompanying center-of-figure to center-of-mass offset.Lunar heat flow values, in analogy with oceanic heat flow on the Earth, strongly imply a convective mechanism of heat transport in the interior which, based on seismic Q values, is limited to the lower mantle. The presence of moonquakes in this region does not preclude solid-state convective processes. Lunar conductivity profiles provide no information on convection because of the difficulty in conductivity modeling, uniqueness of models, and the uncertainty in the conductivity-temperature relationship. The excess oblateness of the lunar figure over the hydrostatic value does not require convective support; in fact, such a mechanism is unlikely.The presence of a dipole magnetic field on Mercury does not provide a constraint on mantle convection unless its existence can be inextricably linked to a molten core. The non-hydrostatic shape of the equatorial figure, required for the observed $\text{3}\text{2}$ resonance between Mercury's rotational and orbital periods, is most likely related to surface processes, as opposed to convection. The $\text{3n}\text{2}$ resonance implies escape from a 2n resonance and, therefore, is related to the question of a molten core. Further dynamical data is needed to constrain interior models.Interpretation of limited radar imagery for the surface of Venus is enigmatic in terms of plate tectonics and therefore interior convection. Linear tensional and possibly compressional features are observed, but there are also crustal regions which appear to show large impact structures and are thus geologically old and may not have been recycled.     

Upper mantle low anisotropy channels below the Pacific Plate

A new 3D anisotropic model has been obtained at a global scale by using a massive dataset of seismic surface waves. Though seismic heterogeneities are usually interpreted in terms of heterogeneous temperature field, a large part of lateral variations are also induced by seismic anisotropy of upper mantle minerals. New insight into convection processes can be gained by taking seismic anisotropy into account in the inversion procedure. The model is best resolved in the Pacific Plate, the largest and the most active tectonic plate. Superimposed on the large-scale radial (ξ parameter) and azimuthal anisotropy (of V_SV velocity) within and below the lithosphere, correlated with present or past Pacific Plate motions, are smaller-scale (<1000 km) lateral variations of anisotropy not predicted by plate tectonics. Channels of low anisotropy down to a depth of 200 km (hereafter referred to as LAC) are observed and are the best resolved anomalies: one east-west channel between Easter Island and the Tonga-Kermadec subduction zones (observed on both radial and azimuthal anisotropies) and a second one (only observed on azimuthal anisotropy) extending from the south-west Pacific up to south-east Hawaii, and passing through the Polynesia hotspot group for plate older than about 40 Ma. These features provide strong constraints on the decoupling between the plate and asthenosphere. They are presumably related to cracking within the Pacific Plate and/or to secondary convection below the rigid lithosphere, predicted by numerical and analog experiments. The existence and location of these LACs might be related to the current active volcanoes and hotspots (possibly plumes) in the Central Pacific. LACs, which are dividing the Pacific Plate into smaller units, might indicate a future reorganization of plates with ridge migrations in the Pacific Ocean.     

Distribution of Mantle Up-welling Determined from Plate Motions: A Case for Large-scale Bénard Convection

—?The number and geometric distribution of putative mantle up-welling centers and the associated convection cell boundaries are determined from the lithospheric plate motions as given by the 14 Euler poles of the observational NUVEL model. For an assumed distribution of up-welling centers (called “cell-cores”) the corresponding cell boundaries are constructed by a Voronoi division of the spherical surface; the resulting polygons are called “Bénard cells.” By assuming the flow-kinematics within a cell, the viscous coupling between the flow and the plates is estimated, and the Euler poles for the plates are computed under the assumption of zero-net-torque. The positions of the cell-cores are optimized for the HS2-NUVEL1 Euler poles by a method of successive approximation (“subplex”); convergence to one of many local minima occurred typically after ～20,000 iterations. Cell-cores associated with the fourteen HS2-NUVEL1 Euler poles converge to a relatively small number of locations (8 to 10, depending on interpretation), irrespective of the number of convection cells submitted for optimized distribution (from 6 to 50). These locations are correlated with low seismic propagation velocities in tomography, uniformly occur within hotspot provinces, and may specifically be associated with the Hawaiian, Iceland, Reunion/Kerguelen (Indian Ocean), Easter Island, Melanesia/Society Islands (South Pacific), Azores/Cape Verde/Canary Islands, Tristan da Cunha (South Atlantic), Balleny Islands, and possibly Yellowstone hotspots. It is shown that arbitrary Euler poles cannot occur in association with mantle Bénard convection, irrespective of the number and the distribution of convection cells. Nevertheless, eight of the observational Euler poles – including the five that are accurately determined in HS2-NUVEL1 (Australia, Cocos, Juan de Fuca, Pacific, and Philippine) – are “Bénard-valid” (i.e., can be explained by our Bénard model). Five of the remaining six observational poles must be relocated within their error-ellipses to become Bénard-valid; the Eurasia pole alone appears to be in error by ～115°, and may actually lie near 40°N, 154°E. The collective results strongly suggest Bénard-like mantle convection cells, and that basal shear tractions are the primary factor in determining the directions of the plate motions as given by the Euler poles. The magnitudes of the computed Euler vectors show, however, that basal shear cannot be the exclusive driving force of plate tectonics, and suggest force contributions (of comparable magnitude for perhaps half of the plates) from the lithosphere itself, specifically subducting slab-pull and (continental) collision drag, which are provisionally evaluated. The relationship of the putative mantle Bénard polygons to dynamic chaos and turbulent flow is discussed.     

地幔对流与岩石层板块的相互耦合及影响──（Ⅰ）球腔中的自由热对流

板块运动是地幔对流的主要证据之一．同时，作为地球动力系统中一个相对独立部分，板块自身的存在和运动对地幔内部物质的流动形态有巨大影响．地幔内部的流动由两部分组成：一是由内部非绝热温度差异造成的自由对流解；另一部分是由在地表运动的板块所激发．作为系列工作的第一部分，本文研究球腔中的自由热对流问题．得到了对地幔对流研究有实际意义的下边界为自由、上边界为刚性情况下的临界瑞利数值，不同的瑞利数时球腔内流场和温度场的分布形态等．     

Plate-kinematic Constraints from Mantle Bénard Convection

General kinematic implications for plate tectonics are determined for Rayleigh-Bénard convection of the mantle. The continuum of all possible configurations of Bénard polygons is probed by large random samples of global configurations (450,000 to 54,000,000), for each of which the Euler poles are determined on the basis of viscous coupling across the asthenosphere. Two computationally related methods lead first, to Euler pole restrictions for fourteen plates, and second, to restrictions on the Bénard cell configuration. Result No. 1: Euler poles occur in global preference-patterns, which are determined exclusively by the shape of the plate. The observational HS2-NUVEL1 model poles occur near regions preferred by Bénard convection (Eurasia excluded); the agreement is best for the most accurate observational poles. Result No. 2: Seven specific mantle Bénard cells are indicated by present-day plate motions. The upwelling centers correlate with hotspot domains; the major global subduction zones correlate with Bénard model downwelling. This result is independent of the Euler pole accuracy used in its determination, and is consistent with the distribution of low seismic p-wave propagation velocities determined by tomography, and with shear-wave splitting analysis within the asthenosphere. Conclusions: The results suggest that the bulk mantle is divided into less than ten Bénard convection cells globally (cf., Fohlmeister and Renka, 2002), each of which extends from the asthenosphere to the core-mantle boundary; turbulent flow, and other perturbations of the Bénard kinematics appear to be limited. These primally poloidal flow kinematics provide basal shear forces as a major component in driving plate tectonics, and are specifically configured for the directions of plate motions. The Bénard model is incomplete without a dynamic contribution from the lithosphere, which represents a separate convection layer of the distinct polar kinematics of rigid plates. The complete hybrid mechanism for driving plate tectonics includes lithospheric buoyancy dynamics, specifically from the subducting Pacific plate slabs to compensate for plate-slowing due to the back-flow sector of the Hawaiian convection cell, and collision-drag dynamics principally for smaller plates or continental margins.     

青藏高原-天山地区岩石层构造运动的地幔动力学机制

利用全球重力大地水准面异常、板块绝对运动及全球地震层析成像数据,计算了青藏高原-天山地区岩石层下部地幔大尺度对流格局以及此种尺度对流驱动下岩石层内应力场分布;同时,利用区域均衡重力异常数据反演青藏高原中、北部到天山地区上地幔小尺度对流模型．结果表明,大尺度的地幔物质运移过程可能驱动着中国大陆岩石层整体从西部以南北方向为主的运动转向东部地区以北东和南东方向的运动;而该区域上地幔小尺度上升流动支持了现代青藏高原和天山地区的抬升运动．提出和讨论了青藏高原隆升的“断离隆升-挤压隆升-对流隆升”三阶段模式,并探讨了大陆岩石层构造运动的地幔深部动力学背景．     

Hotspot distribution, gravity, mantle tomography: evidence for plumes

Thermal convection is the motor of Earth dynamics and therefore is the link between plate motions, hotspots, seismic velocity variations in the mantle, and anomalies of the gravity field. Small scale mantle anomalies, such as plumes, do, however, generally escape detection by tomographic methods. It is attempted to approach the problem of detection in a somewhat statistical manner. Correlations are sought between spherical harmonic expansions of the fields under study: the hotspot distribution, mantle velocity variations, gravity, heat flow. Using spherical harmonic representations of global fields implies integration and averaging over the whole globe. Thus, although relationships may remain masked in the space domain by a multitude of effects, tendencies may become visible in the spectra or in appropriate averages.The main results are the following: There is a significant long wavelength (n=2,3) negative correlation between the hotspot density and the P-wave velocity variation in the lower mantle. Positive hotspot density of degree 2 to 9 generally correlates with low seismic velocity in all depths of the upper mantle and with positive gravity. This fits well with plume-type convection. These results are also confirmed regionally for a number of individual mid-ocean ridges and hotspots. The hotspot density and the free air anomalies are distinctly positive above regions of low velocity extending to great depth. The effect is not distinct at ridges with shallow velocity anomalies. In a general way, we suggest that the antipodal upwellings (Pacific, Africa) are divided by downwelling currents around the shrinking Pacific. Plate boundaries can easily move away from their past connections with the deeper mantle. Small scale plume currents seem to be depicted in the hotspot expansion. © 1999 Elsevier Science Ltd. All rights reserved.     

Effects of plate and slab viscosities on the geoid

The effects of plate rheology (strong plate interiors and weak plate margins) and stiff subducted lithosphere (slabs) on the geoid and plate motions, considered jointly, are examined with three-dimensional spherical models of mantle flow. Buoyancy forces are based on the internal distribution of subducted lithosphere estimated from the last 160 Ma of subduction history. While the ratio of the lower mantle/upper mantle viscosity has a strong effect on the long-wavelength geoid, as has been shown before, we find that plate rheology is also significant and that its inclusion yields a better geoid model while simultaneously reproducing basic features of observed plate motion. Slab viscosity can strongly affect the geoid, depending on whether a slab is coupled to the surface. In particular, deep, high-viscosity slabs beneath the northern Pacific that are disconnected from the surface as a result of subduction history produce significant long-wavelength geoid highs that differ from the observation. This suggests that slabs in the lower mantle may be not as stiff as predicted from a simple thermally activated rheology, if the slab model is accurate.     

黏滞分层地幔中密度异常驱动对流模型的研究

在地震层析成像计算的地幔密度异常直接驱动地幔对流的新方法的基础上,发展了在上、下地幔不同黏性结构框架下,密度异常驱动地幔对流的物理模型.利用 Grands和S12 WM13等地震层析成像模型推得的地幔密度异常分布,设置板块绝对运动极型场为运动上边界,考虑深度660km地震波不连续面为界的上、下地幔之间存在黏滞性的差异,直接反演了不同黏滞系数的双层地幔结构下地幔对流的模式.研究中选取地幔平均密度为ρ=5500kg/m3, 上层地幔平均黏滞系数为μ=1021Pa·s,计算了上、下地幔黏滞系数之比为1∶1, 1∶10, 1∶100和1∶1000时地幔大圆剖面、以及区域剖面上的流场.结果表明,两种模型在球谐展开1～13阶的范围内其对流的基本格局相似.当下地幔黏滞性超过上地幔的100倍时,下地幔流场速度与上地幔的流场速度相比显著减小,但是对流仍然表现出单层对流环的基本格局.论文还用 240km深度球面上的对流格局讨论了对流和全球构造之间的关系.     

长白山天池火山地震活动与西北太平洋俯冲带内中深源地震关系的研究

根据长白山火山活动的地球动力学背景,综合分析西北太平洋俯冲带俯冲作用所引起的中深源地震与长白山火山地震活动及温泉流体化学气体释放变化趋势的关系。认为在北纬35°以北,西北太平洋板块俯冲作用引起的地幔对流的扰动作用下,长白山火山活动从1999年至目前大体经历了3个阶段:1999年6月至2002年5月为扰动起始阶段,2002年6月至2004年为扰动增强阶段,2005年至目前为扰动衰减阶段。在分析了3个阶段内长白山火山地震活动及温泉气体释放特征的基础上,讨论了长白山天池火山目前的活动状态。从日本海至中国东北深源地震区地震活动期和平静期存在的韵律性活动过程出发,对长白山天池火山未来喷发的可能性进行了分析     

Resolution tests of global geodynamic models by travel-time tomography

This study presents results of a 2-D tomographic inversion of synthetic data that examines the ability of seismic tomography to reveal structures created by mantle dynamic processes. Our seismic velocity anomaly model is based on the density heterogeneities obtained from models of thermal and thermo-chemical convection. Both layered and whole-mantle models are employed to produce the synthetic input anomalies. We investigate the resolving power of the inversion of P and pP arrival times, and assess the influence of parameterisation and regularisation (damping). We show that the effect of regularisation is substantial and that the optimum damping depends upon the wavelength of the input structures. The resolution of the inversion decreases considerably at depths greater than 1000 km, therefore the ability of the kinematic inversion to distinguish between whole-mantle and layered flows (coupled via thermal coupling) may be limited.