卫星重力场、地幔对流应力场与板块运动关系的探讨

本文采用GEM10C的前50阶系数,计算了全球自由空气重力异常和地幔对流产生的岩石层底部应力场.通过对全球重力异常、应力场的分析,着重讨论了板块运动与地幔对流间的关系.对各板块在地幔对流作用下受力的定性分析和定量计算表明,板块的运动主要由低阶应力场决定,板块底部多个地幔对流体（而不是一个以洋中脊为对流上涌中心的单个地幔对流）的共同作用,推动了现代板块的运动.     

罗斯海海域大地水准面异常的成因研究

本文利用数值法求解瞬时地幔对流问题以模拟大地水准面异常.利用两个较新的S波速度异常层析模型SEMUCB_WM1和TX2019slab,将其转换为密度异常作为控制方程的浮力驱动项;采取的黏度结构模型中,上下地幔的黏度比为1∶50.为了研究地幔不同结构对罗斯海海域大地水准面异常的影响,分别提取上、下地幔的密度异常正/负值,作为对流控制方程的输入项,计算相应的模拟大地水准面异常.将模拟大地水准面异常与观测值进行对比,发现罗斯海海域的大地水准面异常主要来自下地幔及上地幔的负密度(波速)异常,下地幔正密度异常对该区域大地水准面负异常也有一定的贡献.本文认为,地幔密度负异常在罗斯海海域大地水准面异常的形成中占据主导作用,地幔对流的动力学效应对该区域大地水准面异常的形成影响较弱.     

地幔对流的数值模拟及其与表面观测结果的关系

研究地幔介质的热与动力学过程(地幔对流)是了解发生在地表面的若干观测现象,诸如长波大地水准面异常、板块运动、地形特征和大地热流等形成原因的关键.相应的,一个现实合理的地幔对流模型必须与观测结果相容,并能对其作出合理的解释,这也是地幔对流研究与一般流体中热对流研究的区别     

Dupal异常研究的现状与趋势

Dupal异常是迄今为止在地球表面出现并反映地幔地球化学特征的最大同位素异常域.首先是在南半球现代大洋玄武岩中发现了以南纬30°为中心近环球分布的Dupal异常域,近年又在北半球的中国东部、日本海等非大洋玄武岩中发现了Dupal异常.现在认为Dupal异常源区既可赋存于岩石层地幔,亦可源自深部软流层地幔或更深,但对Dupal异常源形成机制仍无定论,对北半球的Dupal异常其源区是否来自南半球地幔还有不同认识.Dupal异常源可能是多成因的,不能排除星外物质冲击的可能性.南半球大洋地幔大规模同位素异常的存在对模拟地幔对流提出了制约,也为探索地幔地球化学特征提供了新途径.     

弹性岩石层,区域重力异常和上地幔小尺度对流

在小尺度地幔对流模型的基础上,建立了顾及岩石层与地幔耦合的地幔对流模型,考虑到岩石层对上地幔小尺度对流的弹性响应,将上地幔视为一均匀的等粘滞系数的牛顿粘滞流体,其对注能量来源于下地幔,用热流体动力学基本方程得到地上地幔对流在壳幔边界处的垂向应力作为弹性板弯曲方程的垂直加载,来耦合弹性岩石层和可流动地幔,推导了两者耦合下区域重力异常和上地幔小尺度对流的相关方程,用以反演上地幔小尺度对流场模式和岩石层底部拖力场格局,对比了有,无弹性岩石层影响的模型之间的差异,并对模型应用范围,特别要板块边界水平均造力可能产生的影响,作了进一步探讨。     

地幔动力学研究进展--地幔对流

分类而且系统地回顾了地幔对流研究的进展情况.发现虽然静态地幔对流模型(没有考虑地球自转或差异旋转效应)在拟合某些地表观测如重力异常、大地水准面异常以及地下应力场时取得了较满意的结果,但是却对于某些全球尺度的地质特征如0°、180°半球的非对称性显得难以适应,因此建议性地提出了今后地幔对流的可能发展方向.     

地幔对流与深部物质运移研究的新进展

现代固体地球科学已经认识到,地幔对流不再是少数动力学家的假想,它是地幔热动力系统的主要构架.地幔对流和板块运动驱动机理关系的研究已经从简单的主动或被动驱动的讨论转向对统一热动力系统的探讨.包括地幔热柱在内的地幔对流的深入研究不仅成为研究地幔热动力系统演化的主线,也成为研究大陆形成和演化驱动机理的主线.与此同时,以地震层析成像为主体的地震、地球物理观测资料和以地幔岩石化学组份为主体的地球化学观测成为认识地幔对流的强有力的工具.然而,地球化学和地球物理观测之间存在明显的差异,一些依赖于地球化学数据构思的新的热动力学框架对地幔对流的研究构成了强烈的挑战.     

大尺度地幔动力学研究的现状和展望

这篇综述讨论大空间、大时间尺度的地幔动力学近几十年的发展和现状,着重讨论了相关的观测及其动力学意义.这些观测包括现在地球的板块运动的基本特性,中、长波重力异常及大地水准面异常,地震层析成像得到的地幔结构,以及过去10亿年超级大陆Pangea和Rodinia的形成、裂解和演化,及火山岩浆活动.关于地球动力学模型的讨论是围绕着这些相关的观测而进行的.涉及到的一些主要问题包括以下.第一,地幔动力学研究显示,地震层析成像得到的下地幔的二阶结构(比如核幔边界附近的LLSVP结构),和俯冲带的快速异常体,可以解释为过去1亿年左右的板块运动和地幔对流的结果;第二,地幔三维结构作为地幔对流的驱动力,是导致中、长波重力及大地水准面异常的直接原因;结合地幔动力学模拟,观测的大地水准面异常对地幔黏性结构提供了强有力的约束,很可靠的结果之一是下地幔的黏性比上地幔要高至少一个量级,并且最近的研究确定软流圈的存在;第三,过去10亿年大陆块体经历过的Rodinia和Pangea两期超级大陆的形成和破裂是地幔动力学在地表的反映.地幔结构在Pangea形成过程中是一阶结构(即一个半球是冷的下降流,而另一个半球是热的上涌流)主导的,而现在的二阶为主导的地幔结构是Pangea形成后,破裂前或破裂过程中才形成的;地幔动力学和其他研究支持地幔结构在一阶和二阶间转换的1-2-1模型;第四,板块构造在地球上的起源和动力机制依然是充满争议和不确定的课题,但是这些问题同时也是重要的地球动力学基本问题.     

卫星重力资料揭示的新疆天山地区构造动力学状态

利用最新的地球重力场模型,计算了新疆及邻近地区的自由空气重力异常、大地水准面扰动异常、地壳和上地幔平均密度异常以及地幔对流引起的岩石层底界面粘滞应力场分布．根据计算结果,对新疆天山地区的构造动力学特征进行了分析和推断,认为天山处于地幔对流形成的挤压沉降环境中,在南北不对称的挤压应力作用下快速隆升,挤压应力场中心在天山以南．这种应力场特征有利于塔里木板块向天山之下俯冲的观点．天山南北两侧的准噶尔盆地南缘和塔里木盆地北缘,是地壳内质量缺失区,是由于南北两侧地壳向天山下挤压而弯曲造成的．中国天山东段的深部密度分布特征与中段和西段的不同,可能是地幔对流分布的东西向差异造成的．      

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.     

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.     

欧亚地区均衡残差大地水准面和上地幔强度

首先计算了欧亚地区均衡残差大地水准面．基于地幔热对流的内负荷理论和最新全球层析成像结果，探讨了欧亚地区中波长均衡残差大地水准面的地球动力学意义．研究结果表明，中波长均衡残差大地水准面主要受上地幔粘滞度和岩石层强度的影响，进而得出欧亚地区一些古老地盾和构造稳定地区的上地幔与年轻山脉及构造活动地区的上地幔结构存在着差异．这个差异主要是占老地盾和构造稳定地区，如波罗的海地盾、中西伯利亚地台、东欧等区域，冷却的上地幔已穿透地幔较深，上地幔与岩石层之间耦合较好;而年轻山脉和构造活动区，如帕米尔、天山、贝加尔活动带、青藏高原、日本海周围地区，在上地幔可能存在着热物质即粘滞度很低的软流层，上地幔与岩石层耦合程度较差，甚至有可能解耦．从欧亚地区上地幔属性的差异，可以解释该地区的一些地球动力学问题．     

Tomographic structure of East Asia:Ⅱ.Stagnant slab above 660 km discontinuity and its geodynamic implications

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.     

Constraints on the dynamics of mantle plumes from uplift of the Hawaiian Islands

The ∼0.2 mm/yr uplift of Hawaiian islands Lanai and Molokai and Hawaiian swell topography pose important constraints on the structure and dynamics of mantle plumes. We have formulated 3-D models of mantle convection to investigate the effects of plume-plate interactions on surface vertical motions and swell topography. In our models, the controlling parameters are plume radius, excess plume temperature, and upper mantle viscosity. We have found that swell height and swell width constraints limit the radius of the Hawaiian plume to be smaller than 70 km. The additional constraint from the uplift at Lanai requires excess plume temperature to be greater than 400 K. If excess plume temperature is 400 K, models with plume radius between 50 and 70 km and upper mantle viscosity between 10²⁰ and 3×10²⁰ Pa s satisfy all the constraints. Our results indicate that mantle plume in the upper mantle may be significantly hotter than previously suggested. This has important implications for mantle convection and mantle melting. In addition to constraining plume dynamics, our models also provide a mechanism to produce the observed uplift at Lanai and Molokai that has never been satisfactorily explained before.     

Convection model of post-glacial uplift

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.     

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.     

Factors responsible for the high position of the Siberian platform

A new model accounting for the origin of anomalously high elevations of the Siberian platform (SP) topography is presented. It is shown that the traditional interpretation of these topographic anomalies is at variance with the available evidence for the geological history of the SP development. The ideas elaborated in the paper are based on the concept of the formation of a mantle plume that has led to the supply of large volumes of molten material into the upper crust and surface basalt eruptions. A new approach is proposed for the construction of a density model of the Siberian upper mantle. A density model of the crust based on the available seismic and petrological data is constructed at the first stage. The calculated anomalous gravity field produced by this model is then subtracted from the observed field. The resulting residual mantle anomalies are used, together with seismological data, for the construction of an upper mantle density model. The formation of the present SP topography is shown to have been controlled by the thickening of the crust due to underplating caused by the development of a giant mantle plume at 251 Ma.     

Two-dimensional mantle flow beneath a rigid accreting lithosphere

This study considers two-dimensional mantle flow beneath a rigid lithosphere. The lithosphere which forms the upper boundary of a convecting region moves with a prescribed uniform horizontal velocity, and thickens with distance from the accreting plate boundary as it cools. Beneath the lithosphere, the mantle deforms viscously by diffusion creep and is heated radiogenically from within. Solutions for thermal convection beneath the lithosphere are obtained by finite-difference methods. Two important conclusions have resulted from this study: (1) convective patterns of large aspect ratio are stable beneath a rigid moving lithosphere; (2) even for a lithosphere velocity as small as 3 cm/yr. and a Rayleigh number as large as 10⁶, mantle circulation with large aspect ratio is driven dominantly by the motion of the lithosphere rather than by temperature gradients within the flow. Gravity, topography and heat flow are determined and implications for convection in the upper mantle are discussed.     

Small-scale upper mantle convection and orogeny of Tianshan Mountains in China

In this study, from the travel time data recorded in the Tianshan passive seismic array experiment, we present the P-wave velocity structure of the upper mantle down to 660 km along the Kuqa-Kuitun pro-file in terms of seismic tomography technique. Based on the P-wave velocity model, we derive the corresponding 2D upper mantle density model. The 2D small-scale convection of the upper mantle underneath the Tianshan Mountains in China driven by the density anomalies is simulated using the hybrid finite element method combining with the marker-in-cell technique. The main features of the upper mantle convection and the reciprocation between the convection and mountain building are in-vestigated. The results manifest that (1) in the upper mantle underneath the Junggar basin and North Tianshan exists a counterclockwise convection, which scale is ~ 500 km; (2) underneath the Tarim ba-sin and South Tianshan exists a clockwise northward convection, which is relatively weak; (3) the convective velocity at the top of the upper mantle underneath the Tianshan Mountains in China should not be less than 20 mm/a, while considering the dependent of convective velocity on the viscosity; (4) the northward extrusion of the Tarim block plays a key role in the Cenozoic Tianshan mountain building and the present-day tectonic deformation of the Tianshan range is related closely to the upper mantle convection; and (5) the northward subduction of the Tarim block does not influence obviously the up-per mantle convection.