On the dynamic mechanism of formation of rift valleys

A dynamic mechanism that accounts for the sinking of a lithospheric plate near an accretion zone in the vicinity of a passive rift is revealed. It is shown that the influence of the underlying “cold” mantle can be described in terms of a concentrated vertical force applied to the rift axis. At a moderate spreading rate, the value of this force is an order of magnitude smaller than the characteristic values of forces acting in the plate tectonics. The average viscosity coefficient of the cold upper mantle is estimated at ～10²¹ P. The concentrated force at the rift axis produces a characteristic topography of the rift valley type of mid-ocean ridges.     

南海西南海盆构造演化的热模拟研究

南海西南海盆的张裂和海底扩张是白垩纪末至中始新世南海形成过程中最重要的构造事件.本文采用三维有限单元法对该区的热演化过程进行了模拟计算.通过对变形、温度结构的计算,研究了西南海盆张裂变形、海底扩张持续时间、地幔物质上升、地壳岩墙沿扩张中心的挤入扩张活力、岩浆活动等.计算结果表明：由于其深部动力学条件不足,海盆一次扩张持续时间在10～15Ma之间,其后地幔物质的上升活动逐渐停止,地壳失去扩张动力,使得扩张中心成为残留扩张中心的死亡裂谷,而未构成中脊或中隆带.虽然该处地幔物质上升的潜力不足,但伴随局部的断裂,尤其是盆、缘边界的拆离拉张,仍能产生相当强烈的岩浆喷溢活动,导致此区海盆成型之后的海山崛起.     

Horizontal stresses in the mantle and in the moving continent for the model of two-dimensional convection with varying viscosity

Numerical experiments on studying the spatial fields and evolution of viscous overlithostatic horizontal stresses and pressure in the mantle and in the moving continent are carried out. The continent moves consistently with time-dependent forces, which act from the viscous mantle. By introducing the varying viscosity, we gain the possibility for taking into account the oceanic lithosphere and the difference between the viscosity of the upper and the lower mantle in the context of a purely viscous model. The typical overlithostatic horizontal stresses in the main part of the mantle are ±(7–9) MPa (70–90 bar); in the highly viscous regions and, particularly, in the subduction zones they are at least three times larger. The descending mantle flows in the depth interval from approximately 50 km to about 300 km are more sharply pronounced in the pressure field than in the field of horizontal stresses. At the considered stages of motion and in different parts, the continent is characterized by the following typical values of stresses: the overlithostatic pressure ranges from ?5 to +15 MPa; the horizontal overlithostatic tensile stress amounts up to ?4MPa (?40 bar); and the compressive stress in case of the overriding of the subduction zone attains +35 MPa (350 bar).     

The structure of 2D mantle convection and stress fields: Effects of viscosity distribution

Spatial fields of temperature, velocity, overlithostatic pressure, and horizontal stresses in the Earth’s mantle are studied in two-dimensional (2D) numerical Cartesian models of mantle convection with variable viscosity. The calculations are carried out for three different patterns of the viscosity distribution in the mantle: (a) an isoviscous model, (b) a four-layer viscosity model, and (c) a temperature- and pressure-dependent viscosity model. The pattern of flows, the stresses, and the surface heat flow are strongly controlled by the viscosity distribution. This is connected with the formation of a cold highly viscous layer on the surface, which is analogous to the oceanic lithosphere and impedes the heat transfer. For the Rayleigh number Ra = 10⁷, the Nusselt number, which characterizes the heat transfer, is Nu = 34, 28, and 15 in models with constant, four-layered, and p, T-dependent viscosity, respectively. In all three models, the values of overlithostatic pressure and horizontal stresses σ _xx in a vast central region of the mantle, which occupies the bulk of the entire volume of the computation domain, are approximately similar, varying within ±5 MPa (±50 bar). This follows from the fact that the dimensionless mantle viscosity averaged over volume is almost similar in all these models. In the case of temperature- and pressure-dependent viscosity, the overlithostatic pressure and stress σ _xx fields exhibit much stronger concentration towards the horizontal boundaries of the computation domain compared to the isoviscous model. This effect occurs because the upwellings and downwellings in a highly viscous region experience strong variations in both amplitude and direction of flow velocity near the horizontal boundaries. In the models considered with the parameters used, the stresses in the upper and lower mantle are approximately identical, that is, there is no denser concentration of stresses in the upper or lower mantle. In contrast to the overlithostatic pressure field, the fields of horizontal stresses σ _xx in all models do not exhibit deep roots of highly viscous downwelling flows.     

Dynamics of the mid-ocean ridge plate boundary: Recent observations and theory

The global mid-ocean ridge system is one of the most active plate boundaries on the earth and understanding the dynamic processes at this plate boundary is one of the most important problems in geodynamics. In this paper I present recent results of several aspects of mid-ocean ridge studies concerning the dynamics of oceanic lithosphere at these diverging plate boundaries. I show that the observed rift valley to no-rift valley transition (globally due to the increase of spreading rate or locally due to the crustal thickness variations and/or thermal anomalies) can be explained by the strong temperature dependence of the power law rheology of the oceanic lithosphere, and most importantly, by the difference in the rheological behavior of the oceanic crust from the underlying mantle. The effect of this weaker lower crust on ridge dynamics is mainly influenced by spreading rate and crustal thickness variations. The accumulated strain pattern from a recently developed lens model, based on recent seismic observations, was proposed as an appealing mechanism for the observed gabbro layering sequence in the Oman Ophiolite. It is now known that the mid-ocean ridges at all spreading rates are offset into individual spreading segments by both transform and nontransform discontinuities. The tectonics of ridge segmentation are also spreading-rate dependent: the slow-spreading Mid-Atlantic Ridge is characterized by distinct bulls-eye shaped gravity lows, suggesting large along-axis variations in melt production and crustal thickness, whereas the fast-spreading East-Pacific Rise is associated with much smaller along-axis variations. These spreading-rate dependent changes have been attributed to a fundamental differences in ridge segmentation mechanisms and mantle upwelling at mid-ocean ridges: the mantle upwelling may be intrinsically plume-like (3-D) beneath a slow-spreading ridge but more sheet-like (2-D) beneath a fast-spreading ridge.     

Rheological properties of deep subducted oceanic lithosphere and their geodynamic implications

According to the experimental studies on the rheology of two important mantle rocks (eclogite and harzburgite), the rheological properties of the deep subducted oceanic lithosphere are investigated by assuming a simplified harzburgite type slab model with moderate thickness of basaltic layer. When the mantle convergence rate is small or the subducting slab has been trapped in the mantle for an enough long time, the strength profile of the slab is characterized by a strong subducting crustal component lying on a weak subducting upper mantle. However, if the convergence rate is large enough, the subducting slab will be featured only by a rigid cold center. Our study suggests that the detachment of the subducting crust component from the underlying upper mantle is only likely to happen in hot slow subducting slabs, but not the cold fast subducting lithosphere. Rheological properties of the harzburgitic and the eclogitic upper mantle vary with depths. The eclogitic upper mantle is stronger than the peridotitic upper mantle across the upper mantle. Transition zone is the high strength and high viscosity layer in the upper mantle except the lithosphere.     

Thermal anomalies and magmatism due to lithospheric doubling and shifting

We present some thermal and magmatic consequences of the processes of lithospheric doubling and lithospheric shifting.Lithospheric doubling concerns the obduction of a cold continental or old oceanic lithospheric plate over a young and hot oceanic lithosphere/upper mantle system, including an oceanic ridge.Lithospheric shifting concerns the translation and rotation of a lithospheric plate relative to the upper mantle.In both cases the resulting thermal state of the upper mantle below the obducting or shifting lithosphere may be perturbed relative to a “normal” continental or oceanic geothermal situation.The perturbed geothermal state gives rise to a density inversion at depth and thus induces a vertical gravitational instability which favours magmatism.We speculate about the magmatic consequences of this situation and infer that in the case of lithospheric doubling our model may account for the petrology and geochemistry of the resulting magma.The original layering and composition of the overridden young oceanic lithosphere may strongly influence magmatic processes.We dwell shortly on the genesis of kimberlites within the framework of our lithospheric doubling model and on magmatism in general. Lithospheric recycling is inherent to the mechanism of lithospheric doubling.     

Numerical model of the supercontinental cycle stages: Integral transfer of the oceanic crust material and mantle viscous shear stresses

We compute the transfer of oceanic lithosphere material from the surface of the model to the inner convective mantle at successive stages of the supercontinental cycle, in the time interval from the beginning of convergence of the continents to their complete dispersal. The sequence of stages of a supercontinental cycle (Wilson cycle) is calculated with a two-dimensional numerical model of assembling and dispersing continents driven by mantle flows; in turn, the flows themselves are forming under thermal and mechanical influence of continents. We obtain that during the time of the order of 300 Myr the complete stirring of oceanic lithosphere through whole mantle does not occur. This agrees with current ideas on the circulation of oceanic crust material. Former oceanic crust material appears again at the Earth’s surface in the areas of mantle upstreams. The numerical simulation demonstrates that the supercontinental cycle is a factor which intensifies stirring of the material, especially in the region beneath the supercontinent. The reasons are a recurring formation of plumes in that region as well as a global restructuring of mantle flow pattern due to the process of joining and separation of continents. The computations of viscous shear stresses are also carried out in the mantle as a function of spatial coordinates and time. With a simplified model of uniform mantle viscosity, the numerical experiment shows that the typical maximal shear stresses in the major portion of the mantle measure about 5 MPa (50 bar). The typical maximal shear stresses located in the uppermost part of mantle downgoing streams (in a zone that measures roughly 200 × 200 km) are approximately 8 times greater and equal to 40 MPa (400 bar).     

3-D spherical shell modeling of mantle flow and its implication for global tectogenesis

In order to study the relationship between mantle flow and global tectogenesis, we present a 3-D spherical shell model with incompressible Newtonian fluid medium to simulate mantle flow which fits the global tectogenesis quite well. The governing equations are derived in spherical coordinates. Both the thermal buoyancy force and the self-gravitation are taken into account. The velocity and pressure coupled with temperature are computed, using the finite-element method with a punitive factor. The results show that the lithosphere, as the boundary layer of the earth's thermodynamic system, moves with the entire mantle. Both its horizontal and vertical movements are the results of the earth's thermal motion. The orogenesis occurs not only in the collision zones at the plates' boundaries, but also occurs within the plates. If the core-mantle boundary is impermeable and the viscosity of the lower mantle is considerable, the vertical movement is mostly confined to the upper mantle. The directions of the asthenospheric movements are not fully consistent with those of the lithospheric movements. The depths of spreading movements beneath all ridges are less than 220 km. In some regions, the shear stresses, acting on the base of the lithosphere by the asthenosphere, are the main driving force; but in other regions, the shear stresses are the resisting force.     

Dynamics of continental rift propagation: the end-member modes

An important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation. In regions of rift propagation, specifically transition zones from continental rifting to seafloor spreading, it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones. The focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere. More specifically, this study explores how modeled rift behavior depends on the value of rheological parameters of the crust. A three-dimensional finite element model is used to simulate lithosphere deformation in an extensional regime. The chosen approach emphasizes understanding the tectonic forces involved in rift propagation. Dependent on plate strength, two end-member modes are distinguished. The stalled rift phase is characterized by absence of rift propagation for a certain amount of time. Extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone, which requires a buildup of shear stresses near the rift tip and significant intra-plate deformation. This stage represents a situation in which a rift meets a locked zone. Localized deformation changes to distributed deformation in the locked zone, and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend. In the alternative rift propagation mode, rift propagation is a continuous process when the initial crust is weak. The extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses. Model predictions address aspects of previously unexplained rift evolution in the Laptev Sea, and its contrast with the tectonic evolution of, for example, the Gulf of Aden and Woodlark Basin.     

A nonstationary model of the thermal regime of axial zones of mid-ocean ridges: Formation of crustal and mantle magma chambers

A nonstationary model of spreading with periodic intrusions of a molten material into an axial zone of a mid-ocean ridge (MOR) is applied to numerical analysis of the thermal state in MOR axial zones and the formation of crustal and mantle magma chambers in them. The model satisfactorily explains the positions, dimensions, and shapes of magma chambers, as well as variations in these parameters depending on the spreading rate, temperature, and composition of crustal and mantle rocks. The release and absorption of the latent heat of rock melting, hydrothermal heating of the crust, and variations in the solidus and liquidus temperatures of crustal and mantle rocks as a function of their composition are factors controlling the shape and position of crustal magma chambers.     

Upwelling flow in Earth's mantle and sea-floor spreading

Upwelling flows in the Earth's mantle are accompanied by mass, momentum and energy transports from deep to upper layers. Those flows beneath the mid-ocean ridges give rise to sea-floor spreading. Mantle plumes, on the other hand, cause hot spots to be formed on the Earth's surface. Using the basic equations of fluid dynamics, temperature and velocity distributions in two-dimensional upwelling and cylindrical plumes can be obtained by an integral-relation method. Then the mass, momentum and energy transported to the lithosphere by these upwelling flows can readily be calculated. Based on those results we can more thoroughly discuss problems of plate dynamics, such as the driving mechanism of plate motion, the causes of formation of rift valleys over mid-ocean ridges, and the effect of mantle plumes on sea-floor spreading.     

南海深部地球动力学特征及其演化机制

利用地热学、流变学和重力学方法，计算了南海岩石层温度结构、流变特征及地幔对流格局.南海莫霍面温度在600-1000℃之间.岩石层底界面温度在1150-1300℃之间，有效粘滞系数为1020-1021Pa·s，与冰期回弹资料确定的地幔粘度吻合，表明南海深部具备产生地幔热对流的物理条件.研究认为地幔物质由北西向南东方向的运移以及印澳-欧亚板块的碰撞，导致南海北部大陆边缘向洋扩张、离散和断裂解体.在向洋离散过程中，陆-洋岩石层底部地幔局部对流使中央海盆扩张和北部陆缘发生差异性块断运动.     

Implications of melting for stabilisation of the lithosphere and heat loss in the Archaean

The increased depth and volume of melting induced in a higher temperature Archaean mantle controls the stability of the lithosphere, heat loss rates and the thickness of the oceanic crust. The relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions. The density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200°C or more hotter than modern upper mantle. If Archaean continental crust formed initially above oceanic lithosphere, the compositional density gradients may be sufficient to preserve a thick Archaean continental lithosphere within which the Archaean age diamonds are preserved. The amount of heat advected by melts at mid-ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases. Archaean tectonics could have been dominated by spreading rates 2–3 times greater than now and with mantle temperatures between ca. 1600°C and 1800°C at the depth of the solidus. Mid-ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form, protected from copious volcanism and high mantle temperatures.     

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.     

The spread of subducted lithospheric material along the mid-mantle boundary

Studies of phase transitions in silicate minerals at high temperatures and pressures suggest that the bulk density of subducted lithosphere at the mid-mantle boundary is intermediate between the densities of the upper and lower mantle. We argue that, if this is the case, then the lithospheric material will intrude along the mid-mantle boundary driven by buoyancy forces resulting from the compositional density differences between the intrusion and its surroundings. The rate of spread of the intrusion is given by a balance between these buoyancy forces and the viscous resistance of the mantle to motion. Using results from our recent studies of the fluid mechanics of such viscous gravity currents, we find that lithospheric material can propagate between one thousand and two thoudand kilometres in a hundred million years and can cover the entire boundary in one to six billion years. This spreading may be reflected in the global distribution of the isotopic characteristics of oceanic basalts.     

Heat flow and thickness of the oceanic lithosphere

Existing thermal models of the oceanic lithosphere predict too sharp an increase of heat flow towards the ridge axis. A new mathematical model of a thickening lithosphere is presented. The temperature distribution is computed by the use of observed surface heat flow as a boundary condition. If observed heat flow values represent flow of heat from the mantle, the model predicts a rather rapid growth of the lithosphere within the first 30 m.y. and a nearly steady state after 100 m.y. Heat flow from the asthenosphere to the lithosphere shows a minimum near the ridge axis, suggesting a down-going convective flow in the asthenosphere at both sides of a spreading center.     

The East Pacific Rise near 21°N, 13°N and 20°S: inferences for along-strike variability of axial processes of the Mid-Ocean Ridge

Data obtained along various segments of the Mid-Ocean Ridge (MOR) are used to construct an idealized model for crustal accretion. The model seeks to predict the topographic, volcanic, tectonic, and hydrothermal characteristics of any given spreading segment of the MOR as a function of distance away from the bounding transform faults. This model is based on a series of detailed mapping efforts carried out on segments of the MOR having a broad range of spreading rates between 2 and 16 cm/yr. This paper includes the results of the French SEARISE program carried out during the summer of 1980 aboard the N/O “Jean Charcot” and two American cruises conducted in 1981 aboard the R/V “Melville”.     

Rheology of the mantle and tectonics of the oceanic lithospheric plates

The modern concepts of the rheology of viscous mantle and brittle lithosphere, as well as the results of the numerical experiments on the processes in a heated layer with a viscosity dependent on pressure, temperature, and shear stress, are reviewed. These dependences are inferred from the laboratory studies of olivine and measurements of postglacial rebound (glacial isostatic adjustment) and geoid anomalies. The numerical solution of classical conservation equations for mass, heat, and momentum shows that thermal convection with a highly viscous rigid lithosphere develops in the layer with the parameters of the mantle with the considered rheology under a temperature difference of 3500 K, without any special additional conditions due to the self-organization of the material. If the viscosity parameters of the lithosphere correspond to dry olivine, the lithosphere remains monolithic (unbroken). At a lower strength (probably due to the effects of water), the lithosphere splits into a set of separate rigid plates divided by the ridges and subduction zones. The plates submerge into the mantle, and their material is involved in the convective circulation. The results of the numerical experiment may serve as direct empirical evidence to validate the basic concepts of the theory of plate tectonics; these experiments also reveal some new features of the mantle convection. The probable structure of the flows in the upper and lower mantle (including the asthenosphere), which shows the primary role of the lithospheric plates, is demonstrated for the first time.     

Study of seismic tomography in Panxi paleorift area of southwestern China--Structural features of crust and mantle and their evolution

Structural features of the typical continental paleorift in Panxiarea are revealed by seismic tomography. (1) In the profile along the minor axis of Panxi paleorift, we found alternating high and low-velocity strips existing at different depths in the crust, presenting itself as a "sandwich" structure. The existence of these high and low-velocity anomaly strips is related to the basal lithology in the rift area. (2) An addition layer with velocity values of 7.1-7.5 km/s and 7.8 km/s exists from the base of lower crust to uppermost mantle and its thickness is about 20 km. Some study results indicate that the addition layer results from the invasion of mantle material. (3) A lens-shaped high-velocity body surrounded by relatively low-velocity material is observed at depths of 110-160 km between Huaping and Huidong in the axis of the paleorift. This is the first time to discover it in the upper mantle of the paleorift. Based on the results of geology, petrology and geochemistry, we infer that the formation of the addition layer and the lens-shaped high-velocity body in the upper mantle are related to the deep geodynamic process of generation, development and termination of the rift. On the one hand, the upwelling of asthenosphere mantle caused partial melting, and then the basaltic magma from the partial melted material further resulted in underplating and formed the crustal addition layer. On the other hand, the high-density content of mineral facies was increased in the residual melted mass of intensely depleted upper mantle, formed by basalt withdrawing. The solid-melt medium in the depleted upper mantle was mainly an accumulation of garnet and peridotite because the heating effect of lithosphere was relatively weakened in the later riftogenesis, so that a lens-shaped high-density and high-velocity zone was produced in the upper mantle. The results indicate that the energy and material exchange between asthenosphere and lithosphere and remarkable underplating would have an important effect on the material state and propagation of seismic wave in the lower crust, crust-mantle interface, asthenosphere and lithosphere. This process possibly is an important mechanism on the growth of continental crust and the evolution of deep mantle.