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.     

Long wavelength thermal convection between non-conducting boundaries

Non-linear Rayleigh-Bénard convection in a fluid layer is considered as a model of convection in the Earth's upper mantle. Previous studies have shown that when the temperature is held fixed at one of the boundaries of the layer, convection takes place in cells of width of the order of the layer depth or less. We investigate the effects of a different thermal boundary condition, in which the flux of heat is held fixed on both layer boundaries; then if this flux is just greater than that required for the onset of convection, motion takes place on horizontal scales much greater than the layer depth. An analytical treatment of the equations, based on an expansion in the depth-to-width ratio of the cells, shows that cells of a definite horizontal scale are the fastest growing according to linearised theory, but that these cells are unstable to ones of larger wavelength than themselves. Thus the dominant wavelength lengthens with time. The results hold whether the heat flux is generated internally of comes from beneath the layer. These results produce flow patterns similar to those found when the heat flux is much greater than the critical value. The results have important consequences for the understanding of mantle convection.     

天山上地幔结构及其对壳内构造运动的作用

以深部地球物理资料为基础,介绍了天山地震带上地幔的基本结构,讨论了天山不同地区上地幔介质的动力学性质和可能的驱动机制。认为水平挤压形变是造成西天山和天山毗邻西昆仑附近区域上地幔岩石圈缩短和增厚的主要原因;而在中天山和东天山靠近准噶尔盆地南缘一带,除了板块运动造成的水平挤压力之外,上地幔热物质有可能上浮甚至侵入到地壳之中。它们与水平运动一样,对壳内脆性介质的构造活动起到非常重要的作用,特别是地壳底部莫霍面附近的低速滑脱层成为震源区深部构造的一个明显标志。此外,自从印度 亚洲大陆碰撞以来,天山部分地区固结冷却的山根有可能在多重挤压变形和小尺度热对流的共同作用下,脱离它们的原有的层位而沉入上地幔     

地幔对流的数值模拟及其与表面观测的关系

本文从基本的热对流方程出发,并结合地幔对流特点,特别考虑到自重及非线性影响,探讨地幔对流及其与表面观测的关系,发展了相应的数值方法.结果表明,计算得到的长波大地水准面、地表地形、板块速度场水平散度与观测值符合程度较好.上、下地幔的非绝热温度异常与由地震层析得到的地震波速异常显示一定的相关性.地幔内部的流动呈现复杂形态,反映了高瑞利数对流的特征.     

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．     

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.     

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.     

The mantle convection model with non-Newtonian rheology and phase transitions: The flow structure and stress fields

The mantle convection model with phase transitions, non-Newtonian viscosity, and internal heat sources is calculated for two-dimensional (2D) Cartesian geometry. The temperature dependence of viscosity is described by the Arrhenius law with a viscosity step of 50 at the boundary between the upper and lower mantle. The viscosity in the model ranges within 4.5 orders of magnitude. The use of the non-Newtonian rheology enabled us to model the processes of softening in the zone of bending and subduction of the oceanic plates. The yield stress in the model is assumed to be 50 MPa. Based on the obtained model, the structure of the mantle flows and the spatial fields of the stresses σ_xz and σ_xx in the Earth’s mantle are studied. The model demonstrates a stepwise migration of the subduction zones and reveals the sharp changes in the stress fields depending on the stage of the slab detachment. In contrast to the previous model (Bobrov and Baranov, 2014), the self-consistent appearance of the rigid moving lithospheric plates on the surface is observed. Here, the intense flows in the upper mantle cause the drift and bending of the top segments of the slabs and the displacement of the plumes. It is established that when the upwelling plume intersects the boundary between the lower and upper mantle, it assumes a characteristic two-level structure: in the upper mantle, the ascending jet of the mantle material gets thinner, whereas its velocity increases. This effect is caused by the jump in the viscosity at the boundary and is enhanced by the effect of the endothermic phase boundary which impedes the penetration of the plume material from the lower mantle to the upper mantle. The values and distribution of the shear stresses σ_xz and superlithostatic horizontal stresses σ_xx are calculated. In the model area of the subducting slabs the stresses are 60–80 MPa, which is by about an order of magnitude higher than in the other mantle regions. The character of the stress fields in the transition region of the phase boundaries and viscosity step by the plumes and slabs is analyzed. It is established that the viscosity step and endothermic phase boundary at a depth of 660 km induce heterogeneities in the stress fields at the upper/lower mantle boundary. With the assumed model parameters, the exothermic phase transition at 410 km barely affects the stress fields. The slab regions manifest themselves in the stress fields much stronger than the plume regions. This numerically demonstrates that it is the slabs, not the plumes that are the main drivers of the convection. The plumes partly drive the convection and are partly passively involved into the convection stirred by the sinking slabs.     

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.     

利用多种地球物理观测资料直接反演地幔对流模型

假定地幔为一个均匀的、粘滞系数为常数、同时均匀分布放射性热源的流体球层,其内部存在的对流则由流体力学3个基本方程:运动方程、能量方程和连续性方程确定.如果假定地幔处于低瑞利数的状态（临界瑞利数1.5倍左右）,那么上述方程中的非线性项可以忽略不计.作为一类可能的模型,本文计算一组用6个边界条件确定6个未知数的线性方程组.这些条件包括板块绝对运动极型场、地球大地水准面异常和地震层析结果提供的地幔密度分布横向不均匀相应的“刚性地球”水准面异常等.模型计算表明:1.地幔中流体运动格局不仅受地幔热动力学参数（瑞利数）控制,而且强烈地受边界条件的影响.2.若不限定下边界为等温边界,则上、下地幔之间并不呈现出活动性明显差异;但是在模型瑞利数加大到一定值时,核-幔边界附近将出现一些局部的小尺度对流环.3.当模型瑞利数从很小增加时,对流格局将发生变化,这些格局可能反应由地幔热动力学参数决定的地幔固有特性.4.当瑞利数为50000和80000时,核-幔边界形变与PcP波得到的结果吻合较好.     

Parameterized thermal model of a mixed mantle convection

IntroductionTectonicevolutionisinfluencedbythermalhistoryoftheEarth.TheEarthhasabout4.6Gahistory.ThermalenergyfromtheinterioroftheEachprovidesthemainpowerfortectonicevolution.ItnotonlycontrolstheformationofthelayeredstructuresinsidetheEarth,butalsopromotesthetectonicmovementsofthesurfaceplatesduringthegeologicalera.ThestudyofthethermalhistoryoftheEarthhaspassedseveralstages.Inearlystudies,onlyconductivemechanism(Lubimova,1958)isdiscussedinthethermalevolution.However,theimpotalceofthermalco…     

A model for the layered upper mantle

Numerical modeling of mantle convection by Liu (1994, Science, 264: 1904–1907) favors a two-layer convection, if the results are reinterpreted for the correct phase relations in (Mg,Fe)₂SiO₄. The resulting chemical isolation of the upper and lower mantle suggests a highly differentiated and layered upper mantle to account for the discrepancy between the observed compositions of mantle xenoliths and the cosmic abundances of elements. It is shown that a layered upper mantle with a hidden reservoir can have a structure consistent with the observed seismic velocity profiles and an average bulk composition corresponding to the cosmic abundances. The evolution of the upper mantle and the origin of komatiites are discussed in the context of the proposed model.     

大陆岩石圈、地幔底部异常体与地幔对流相互作用的数值模拟

在地球表层存在着占地表面积约30%的具有低固有密度、高黏度的大陆岩石圈.由于其特殊的物理化学性质,大陆岩石圈通常不直接参与下方的地幔对流,但其与地幔对流格局有着重要的相互影响.大量研究显示,在中太平洋和非洲的下地幔底部,存在着两块占核幔边界（CMB）面积约20%的高密度热化学异常体（由于其剪切波速度较低,常称作低剪切波速度省（LSVPs））.LSVPs的演化既受地幔对流的影响,同时也影响地幔物质运动的格局和动力学过程.本文系统研究了存在大陆岩石圈,下地幔LSVPs的地幔对流模型.模拟结果显示：（1）当大陆体积较小时,其边缘常伴随着俯冲,大陆区域地幔常处于下涌状态,其上地幔温度较低,大陆岩石圈在水平方向处于压应力状态.随着大陆体积的增大,大陆边缘的俯冲逐渐减弱,大陆区域地幔由下涌转为上涌,其上地幔温度较高,大陆岩石圈水平方向处于拉应力状态.（2） 岩石圈与软流圈边界（LAB）在大陆下方较深,温度较低;在海洋区域较浅,温度较高.随着大陆体积的增大,陆洋之间LAB深度、温度的差异逐渐减小.（3）大陆区域地幔底部LSVPs物质的丰度与大陆的体积呈正相关.当大陆体积较小时,大陆下方的LSVPs丰度比海洋区域少.随着大陆体积的增大,大陆下方LSVPs的丰度逐渐增大.（4）海洋地区地表热流高,且随时间波动大,大陆地区地表热流低,随时间波动较小;LSVPs区域的核幔边界热流低.     

Upper mantle flow beneath the Northwest of China and its lithospheric dynamics

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全球地幔垂直流动速度研究

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.     

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.     

剥蚀及地幔作用下青藏高原隆升过程的数值模拟

修改了England和Mckenzie的黏性薄层流变模型中控制大陆形变的连续性方程,将剥蚀作用对高原隆升演化的影响直接引入该方程,并考虑下伏地幔小尺度对流对增厚岩石层的搬离作用对高原隆升演化后期的影响,用有限差分法直接模拟青藏高原隆升过程. 数值模拟结果所显示的高原隆升演化过程与实际观测资料吻合较好,揭示了高原隆升演化过程的非平稳和多阶段的特性;同时还表明上地幔小尺度对流对岩石层底部的搬离作用可能是最近8Ma以来高原快速隆升的主导机制.     

上地幔密度异常驱动小尺度对流及实验模型

建立了由密度异常驱动上地幔小尺度对流的数学 物理模型, 发展了利用地震层析成像数据反演上地幔小尺度对流的基本理论和方法. 该模型建立在三维直角坐标系框架上, 假设地震层析成像所显示的地震波速度异常对应于上地幔物质密度异常, 而该密度异常反映了上地幔小尺度热对流系统的温度异常场. 模型首先将地震层析成像确定的地震波速度异常转换为密度异常, 并视其为对流的驱动力; 进而利用三维傅立叶变换, 在波数域内, 在给定的边界条件下, 求解控制流体行为的运动方程和连续性方程, 最后求得对流的流场. 为检验本研究提出的理论和方法的有效性, 本文使用了两个简单的实验模型: 热体和冷体模型; 俯冲断离( break off)板片模型, 计算了其驱动的地幔流场. 结果表明, 本文提供的理论和方法, 可以直接应用于与区域岩石层构造动力学相关的上地幔小尺度对流的研究.      

蒙古-贝加尔地区上地幔小尺度对流及 地球动力学意义

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.