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1.
In the kinematic theory of lithospheric plate tectonics, the position and parameters of the plates are predetermined in the initial and boundary conditions. However, in the self-consistent dynamical theory, the properties of the oceanic plates (just as the structure of the mantle convection) should automatically result from the solution of differential equations for energy, mass, and momentum transfer in viscous fluid. Here, the viscosity of the mantle material as a function of temperature, pressure, shear stress, and chemical composition should be taken from the data of laboratory experiments. The aim of this study is to reproduce the generation of the ensemble of the lithospheric plates and to trace their behavior inside the mantle by numerically solving the convection equations with minimum a priori data. The models demonstrate how the rigid lithosphere can break up into the separate plates that dive into the mantle, how the sizes and the number of the plates change during the evolution of the convection, and how the ridges and subduction zones may migrate in this case. The models also demonstrate how the plates may bend and break up when passing the depth boundary of 660 km and how the plates and plumes may affect the structure of the convection. In contrast to the models of convection without lithospheric plates or regional models, the structure of the mantle flows is for the first time calculated in the entire mantle with quite a few plates. This model shows that the mantle material is transported to the mid-oceanic ridges by asthenospheric flows induced by the subducting plates rather than by the main vertical ascending flows rising from the lower mantle. 相似文献
2.
Auke Barnhoorn Martyn R. Drury Herman L.M. van Roermund 《Earth and Planetary Science Letters》2010,289(1-2):54-67
The rheological properties of upper mantle rocks play an important role in controlling the dynamics of the lithosphere and mantle convection. Experimental studies and microstructures in naturally deformed mantle rocks usually imply that olivine controls the upper mantle rheology. Here we show for the first time evidence from the geometry of folded compositional layers in mantle rocks from Western Norway that garnet-rich rocks can have lower solid-state viscosities than olivine-rich rocks. Modeling of melt-free and dry rheology of garnet and olivine confirms that the reversed viscosity contrast between garnet-rich and olivine-rich layers for this folding event can be achieved over a relatively wide range of temperatures at low stress conditions when the fine-grained garnet deforms by diffusion creep while the coarse-grained olivine deforms by dislocation creep and/or diffusion creep.In general, modeling of the fold viscosity contrast shows that in the stable subcontinental lithospheric mantle or convecting mantle such a reversed viscosity contrast can be formed due to diffusion creep processes in fine-grained garnets in a dry mantle environment or at conditions where the garnet-pyroxene layer is partially molten, i.e. close to solidus–liquidus conditions in the upper mantle. Alternatively in cold plate tectonic settings, e.g. in subduction zones, some water-weakening is a feasible mechanism to create the reversed viscosity contrast between garnet and olivine. 相似文献
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Some consequences arising from the superposition of flows of two different kinds or scales in a non-Newtonian mantle are discussed and applied to the cases mantle convection plus postglacial rebound flow as well as small- plus large-scale mantle convection. If the two flow types have similar magnitude, the apparent rheology of both flows becomes anisotropic and the apparent viscosity for one flow depends on the geometry of the other. If one flow has a magnitude significantly larger than the other, the apparent viscosity for the weak flow is linear but develops direction-dependent variations about a factorn (n being the power exponent of the rheology). For the rebound flow lateral variations of the apparent viscosity about at least 3 are predicted and changes in the flow geometry and relaxation time are possible. On the other hand, rebound flow may weaken the apparent viscosity for convection. Secondary convection under moving plates may be influenced by the apparent anisotropic rheology. Other mechanisms leading to viscous anisotropy during shearing may increase this effect. A linear stability analysis for the onset of convection with anisotropic linear rheology shows that the critical Rayleigh number decreases and the aspect ratio of the movement cells increases for decreasing horizontal shear viscosity (normal viscosity held constant). Applied to the mantle, this model weakens the preference of convection rolls along the direction of plate motion. Under slowly moving plates, rolls perpendicular to the plate motion seem to have a slight preference. These results could be useful for resolving the question of Newtonian versus non-Newtonian or isotropic versus anisotropic mantle rheology. 相似文献
5.
V. P. Trubitsyn 《Izvestiya Physics of the Solid Earth》2010,46(10):807-816
In the existing kinematic theory of the tectonics of lithospheric plates, the position and the parameters of plates are assigned a priori in the initial and boundary conditions. However, in the self-consistent dynamic theory, the properties of oceanic plates (as well as the structure of the mantle’s convection) should appear automatically as the solution of the differential equations of energy, mass, and momentum transfer for a viscous fluid. In this case, the viscosity of the mantle’s substance as a function of temperature, pressure, shear stress, and chemical composition must be taken from the data of laboratory measurements. In the present work, the results of the numerical solution of the equations of convection are presented in the problem formulation mentioned above on a simple model of heated viscous fluid with properties that correspond to the mantle’s substance. In this case, to reveal the main reason for the generation of plates and their influence on the convection, a number of simplifications are introduced; in particular, temperature variations in the viscosity in the mantle are disregarded. In spite of the undertaken simplifications, the models show how the rigid lithosphere can be split into separate plates immersed in the mantle, how in the course of evolution the sizes of plates and their number can change, and how in this case the ridges and subduction zones can be displaced. 相似文献
6.
S. V. Gavrilov 《Izvestiya Physics of the Solid Earth》2009,45(5):437-443
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. 相似文献
7.
Transient creep of the lithosphere and its role in geodynamics 总被引：1，自引：0，他引：1
B. I. Birger 《Izvestiya Physics of the Solid Earth》2012,48(6):496-503
Laboratory experiments with samples of rocks show that at small strains there is transient creep, at which the strain grows with time, and the strain rate decreases. Plate tectonics allows only small strains in the lithospheric plates, so that the lithosphere creep is transient. In geodynamics, the lithosphere is regarded as a cold boundary layer formed by mantle convection. If we assume that the lithosphere has a steady-state creep, which is described by power-law non-Newtonian rheological model, the low effective viscosity of the lower layers of the lithosphere, obtained by data on small-scale postglacial flows, is possible only at high strain rates in these layers. However, the high strain rates in the lithosphere induce large strains that contradict plate tectonics. Transient creep of the lithosphere leads to its mobility at small strains, removing the discrepancy between thermal convection in the mantle and plate tectonics, which holds in the case of power-law rheological model of the lithosphere. 相似文献
8.
The Role of History-Dependent Rheology in Plate Boundary Lubrication for Generating One-Sided Subduction 总被引：1，自引：0，他引：1
Michio Tagawa Tomoeki Nakakuki Masanori Kameyama Fumiko Tajima 《Pure and Applied Geophysics》2007,164(5):879-907
We have developed a two-dimensional dynamical model of asymmetric subduction integrated into the mantle convection without
imposed plate velocities. In this model we consider that weak oceanic crust behaves as a lubricator on the thrust fault at
the plate boundary. We introduce a rheological layer that depends on the history of the past fracture to simulate the effect
of the oceanic crust. The thickness of this layer is set to be as thin as the Earth's oceanic crust. To treat 1-kilometer
scale structure at the plate boundary in the 1000-kilometer scale mantle convection calculation, we introduce a new numerical
method to solve the hydrodynamic equations using a couple of uniform and nonuniform grids of control volumes. Using our developed
models, we have systematically investigated effects of basic rheological parameters that determine the deformation strength
of the lithosphere and the oceanic crust on the development of the subducted slab, with a focus on the plate motion controlling
mechanism. In our model the plate subduction is produced when the friction coefficient (0.004–0.008) of the modeled oceanic
crust and the maximum strength (400 MPa) of the lithosphere are in plausible range inferred from the observations on the plate
driving forces and the plate deformation, and the rheology experiments. In this range of the plate strength, yielding induces
the plate bending. In this case the speed of plate motion is controlled more by viscosity layering of the underlying mantle
than by the plate strength. To examine the setting of the overriding plate, we also consider the two end-member cases in which
the overriding plate is fixed or freely-movable. In the case of the freely-movable overriding plate, the trench motion considerably
changes the dip angle of the deep slab. Especially in the case with a shallow-angle plate boundary, retrograde slab motion
occurs to generate a shallow-angle deep slab. 相似文献
9.
A numerical model describing the thermomechanical state of the “cold” upper mantle near a mid-oceanic ridge (MOR) spreading at a moderate rate is constructed in the approximation of the boundary layer theory. The condition of rift valley formation leads to a constraint on the temperature and shows what temperature distribution corresponds to the “cold” upper mantle. Taking into account the dependence of mantle rheology on the pressure, temperature, and viscous stresses, the model distributions of the pressure and normal viscous stresses at the base of the lithosphere result in a bend of the heterogeneous lithosphere near the MOR, producing a seafloor topography typical of a rift valley with a depth of a few hundred meters and a spreading rate of ～2.5 cm/yr, characteristic of the Atlantic Ocean. The model width of the rift valley (～10–15 km) agrees with observations fairly well. The model is consistent with the typical heat-flow values observed in the spreading zone. 相似文献
10.
L. Cserepes 《Physics of the Earth and Planetary Interiors》1982,30(1):49-61
Numerical model computations have been carried out to determine how the stress-dependence of non-Newtonian viscosity affects the flow structure of thermal convection. The viscosity laws have been chosen in accordance with present knowledge of upper mantle rheology, based on the diffusion and dislocation creep laws of olivine. The results show that there are important differences between the structures of Newtonian and non-Newtonian convection. While the Newtonian models are insufficient in some respects, the non-Newtonian solutions can explain the characteristics of the real mantle flow. However, this may require a faster plastic deformation than power law dislocation creep, at least in the high-stress regions of the mantle, e.g. at the active plate margins. 相似文献
11.
Areas adjacent to rifts, or rift shoulders, are often observed to be uplifted as much as a kilometer or more. In some of these regions geologic data indicate a passive origin for the rifting itself (i.e. there was no anomalous heating of the regions before rifting). Purely conductive heat transport between the rift, where the lithosphere has been thinned, and the rift flanks cannot account for the magnitude of the uplift. Small-scale convection will be induced in the mantle beneath a rift due to the lateral temperature gradients there. Numerical experiments show that convection increases the amount of heat transported vertically into the rift and laterally out of it. In these calculations, the viscosity is taken to be dependent on temperature and pressure and, in some cases, stress. The mantle flow results in thinning of the adjacent lithosphere causing flanking uplift as well as slowing of the subsidence of the middle of the rift. The magnitude of the uplift is dependent on the geometry of the rift and the importance of stress-dependence in the rheology of the mantle. For viscosity parameters which are consistent with the pre-rift temperature structure small-scale convection can produce uplift at least twice as great as would be produced by lateral conduction alone. 相似文献
12.
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. 相似文献
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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. 相似文献
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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. 相似文献
17.
Preferred aspect ratios of convection in a strongly temperature and pressure dependent viscous fluid
A.C. Fowler 《Physics of the Earth and Planetary Interiors》1983,31(1):83-90
If it is assumed that mantle convection is shallow, i.e. limited to an upper layer of ～700 km, then the extent of surface plates must be such that aspect ratios are noticeably large, in contrast to many laboratory and numerical experiments. A rheology which depends strongly on temperature and pressure is provided as an explanation of why this may occur. The aspect ratio, it is suggested, is preferentially large because of the unwillingness of the stiff plates to subduct, which raises the problem of how subduction occurs. A hypothesis is proposed that this is caused by viscous heating in the asthenosphere, and the preferred aspect ratio is large enough such that partial melting takes place underneath the sinking slab, causing it to sink by releasing the sub-lithospheric pressure so that a transverse buckling may occur. 相似文献
18.
V. P. Trubitsyn A. N. Evseev M. N. Evseev E. V. Kharybin 《Izvestiya Physics of the Solid Earth》2011,47(12):1027-1033
The process of multiple self-nucleation and ascent of mantle plumes is studied in the numerical models of thermal convection.
The plumes are observed even in the simplest isoviscous models of thermal convection that leave aside the more complex rheology
of the material, thermochemical effects, phase transformations, etc., which, although controlling the features of plumes,
are not necessary for their formation. The origin of plumes is mainly due to the instability of the mantle flows at highly
intense (low-viscous) thermal convection. At high viscosity, convective flows form regular cells. As viscosity decreases,
the ascending and descending flows become narrower and unsteady. At a further decrease in viscosity, the ascending plumes
assume a mushroom-like shape and occasionally change their position in the mantle. The lifetime of each flow can attain 100
Ma. Using markers allows visualizing the evolution of the shape of the mantle plumes. 相似文献
19.
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. 相似文献
20.
V. P. Trubitsyn 《Izvestiya Physics of the Solid Earth》2016,52(5):627-636
Viscosity is a fundamental property of the mantle which determines the global geodynamical processes. According to the microscopic theory of defects and laboratory experiments, viscosity exponentially depends on temperature and pressure, with activation energy and activation volume being the parameters. The existing laboratory measurements are conducted with much higher strain rates than in the mantle and have significant uncertainty. The data on postglacial rebound only allow the depth distributions of viscosity to be reconstructed. Therefore, spatial distributions (along the depth and lateral) are as of now determined from the models of mantle convection which are calculated by the numerical solution of the convection equations, together with the viscosity dependences on pressure and temperature (PT-dependences). The PT-dependences of viscosity which are presently used in the numerical modeling of convection give a large scatter in the estimates for the lower mantle, which reaches several orders of magnitude. In this paper, it is shown that it is possible to achieve agreement between the calculated depth distributions of viscosity throughout the entire mantle and the postglacial rebound data. For this purpose, the values of the volume and energy of activation for the upper mantle can be taken from the laboratory experiments, and for the lower mantle, the activation volume should be reduced twice at the 660-km phase transition boundary. Next, the reduction in viscosity by an order of magnitude revealed at the depths below 2000 km by the postglacial rebound data can be accounted for by the presence of heavy hot material at the mantle bottom in the LLSVP zones. The models of viscosity spatial distribution throughout the entire mantle with the lithospheric plates are presented. 相似文献