共查询到20条相似文献，搜索用时 35 毫秒
1.
In computer simulations based on creeping motion equations, the difference in stress sensitivity of strain rate between Newtonian rheology and non-Newtonian power-law rheology does not greatly influence the types of mantled porphyroclast developed in simple shear viscous flows. Porphyroclast type is determined by the radius of an initial spherical mantle. When a thin mantle lies within the separatrix, (δ2-objects develop at small γ, then complex porphyroclasts appear with increasing γ. When the mantle is thicker and transected by the separatrix, φ-objects form at small γ and δ2-objects appear with increasing γ during the same deformation. The transition from φ- to δ2-objects is gradual, the transition strain being dependent on the initial radius of the mantle. Our simulations did not produce θ-, δ1- or σ-objects. 相似文献
2.
Earth’s lithosphere is heterogeneous in rheology on a wide range of observation scales. When subjected to a tectonic deformation, the incurred flow field can vary significantly from one rheologically distinct element to another and the flow field in an individual element is generally different from the bulk averaged flow field. Kinematic and mechanical models for high-strain zones provide the relations between prescribed tectonic boundary conditions and the resulting bulk flow field. They do not determine how structures and fabrics observed on local and small scales form. To bridge the scale gap between the bulk flow field and minor structures, Eshelby’s formalism extended for general power-law viscous materials is shown to be a powerful means. This paper first gives a complete presentation of Eshelby’s formalism, from the classic elastic inclusion problem, to Newtonian viscous materials, and to the most general case of a power-law viscous inhomogeneity embedded in a general power-law viscous medium. The formulation is then implemented numerically. The implications and potential applications of the approach are discussed. It is concluded that the general Eshelby formalism together with the self-consistent method is a powerful and physically sound means to tackle large plastic deformation of Earth’s lithosphere. 相似文献
3.
Mantle convection modeling of the supercontinent cycle： Introversion,extroversion, or a combination？
The periodic assembly and dispersal of continental fragments,referred to as the supercontinent cycle,bear close relation to the evolution of mantle convection and plate tectonics.Supercontinent formation involves complex processes of"introversion"(closure of interior oceans),"extroversion"(closure of exterior oceans),or a combination of these processes in uniting dispersed continental fragments.Recent developments in numerical modeling and advancements in computation techniques enable us to simulate Earth’s mantle convection with drifting continents under realistic convection vigor and rheology in Earth-like geometry(i.e.,3D spherical-shell).We report a numerical simulation of 3D mantle convection,incorporating drifting deformable continents,to evaluate supercontinent processes in a realistic mantle convection regime.Our results show that supercontinents are assembled by a combination of introversion and extroversion processes.Small-scale thermal heterogeneity dominates deep mantle convection during the supercontinent cycle,although large-scale upwelling plumes intermittently originate under the drifting continents and/or the supercontinent. 相似文献
4.
A formulation has been derived for the flow of non-Newtonian (power-law) fluids in deformable, fractured porous media. The formulation is enhanced with a subgrid scale model to accurately represent the flow of the power-law fluids inside the cracks. The resulting equations have been discretised using standard (Lagrangian) finite element shape functions and with non-uniform rational B-splines (NURBS), which have been cast into a standard finite element datastructure using Bézier extraction. The effect of the power-law index on the velocity inside the fracture and on the total fluid flow through the porous medium has been analysed for a typical boundary-value problem. It is shown that large differences between non-Newtonian and linearised Newtonian fluids can occur for the fluid velocity inside the fracture. This can significantly influence the total fluid transport through the domain. A mesh sensitivity study has been carried out as well and shows that markedly smaller element sizes are required in order to obtain accurate results for the local flow inside the fracture, compared with the element sizes necessary for obtaining accurate results inside the porous medium away from the fracture. Moreover, a comparison has been made between the results obtained using standard Lagrange polynomials and those obtained using NURBS. It is shown that while both discretisation methods are able to accurately simulate the deformations and pressures in the porous medium, the higher interelement continuity of NURBS is mandatory for obtaining correct values of the fluid velocities inside the fracture, especially near the tips. 相似文献
5.
Analysis and simulation of magma mixing processes in 3D 总被引：2，自引：0，他引：2
Magma mixing structures from the lava flow of Lesbos (Greece) are analyzed in three dimensions using a technique that, starting from the serial sections of rock cubes, allows the reconstruction of the spatial distribution of magmas inside rocks. Two main kinds of coexisting structures are observed: (i) “active regions” (AR) in which magmas mix intimately generating wide contact surfaces and (ii) “coherent regions” (CR) of more mafic magma that have a globular shape and do not show large deformations. The intensity of mingling is quantified by calculating both the interfacial area (IA) between interacting magmas and the fractal dimension of the reconstructed structures. Results show that the fractal dimension is linearly correlated with the logarithm of interfacial area allowing discrimination among different intensities of mingling.
The process of mingling of magmas is simulated using a three-dimensional chaotic dynamical system consisting of stretching and folding processes. The intensity of mingling is measured by calculating the interfacial area between interacting magmas and the fractal dimension, as for natural magma mixing structures. Results suggest that, as in the natural case, the fractal dimension is linearly correlated with the logarithm of the interfacial area allowing to conclude that magma mixing can be regarded as a chaotic process.
Since chemical exchange and physical dispersion of one magma inside another by stretching and folding are closely related, we performed coupled numerical simulations of chaotic advection and chemical diffusion in three dimensions. Our analysis reveals the occurrence in the same system of “active mixing regions” and “coherent regions” analogous to those observed in nature. We will show that the dynamic processes are able to generate magmas with wide spatial heterogeneity related to the occurrence of magmatic enclaves inside host rocks in both plutonic and volcanic environments. 相似文献
6.
Pascal De Caprariis 《Tectonophysics》1974,23(1-2)
The theory of folding in stratified media presented by Biot and Ramberg has been extended by considering a more general type of material response. The model consists of a viscous layer embedded in a less viscous medium, compressed parallel to the layering. A transition from Newtonian to non-Newtonian behavior is considered and an approximate solution to the governing equation is discussed. The results give the effect of local, stress-induced changes in the viscosity on the profile of the fold. The results indicate that as the transition to non-Newtonian behavior proceeds, the wavelength selection process described by Biot and Ramberg breaks down; the wavelength of the fold which develops probably will not be the “dominant” wavelength defined by Biot. 相似文献
7.
Both seismology and geochemistry show that the Earth's mantle is chemically heterogeneous on a wide range of scales. Moreover, its rheology depends strongly on temperature, pressure and chemistry. To interpret the geological data, we need a physical understanding of the forms that convection might take in such a mantle. We have therefore carried out laboratory experiments to characterize the interaction of thermal convection with stratification in viscosity and in density. Depending on the buoyancy ratio B (ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly), two regimes were found: at high B, convection remains stratified and fixed, long-lived thermochemical plumes are generated at the interface, while at low B, hot domes oscillate vertically through the whole tank, while thin tubular plumes can rise from their upper surfaces. Convection acts to destroy the stratification through mechanical entrainment and instabilities. Therefore, both regimes are transient and a given experiment can start in the stratified regime, evolve towards the doming regime, and end in well-mixed classical one-layer convection. Applied to mantle convection, thermochemical convection can therefore explain a number of observations on Earth, such as hot spots, superswells or the survival of several geochemical reservoirs in the mantle. Scaling laws derived from laboratory experiments allow predictions of a number of characteristics of those features, such as their geometry, size, thermal structure, and temporal and chemical evolution. In particular, it is shown that (1) density heterogeneities are an efficient way to anchor plumes, and therefore to create relatively fixed hot spots, (2) pulses of activity with characteristic time-scale of 50–500 Myr can be produced by thermochemical convection in the mantle, (3) because of mixing, no ‘primitive’ reservoir can have survived untouched up to now, and (4) the mantle is evolving through time and its regime has probably changed through geological times. This evolution may reconcile the survival of geochemically distinct reservoirs with the small amplitude of present-day density heterogeneities inferred from seismology and mineral physics. 相似文献
8.
《Russian Geology and Geophysics》2014,55(7):801-811
The structure of mantle convection and spatial fields of superlitho static pressure and vertical and horizontal stresses in the Earth’s mantle are studied in a 2D numerical model with non-Newtonian viscosity and heat sources. The model demonstrates a jump-like motion of subduction zones and reveals abrupt changes in the stress fields depending on the stage of slab detachment. The stresses decrease dramatically in the areas without slabs. The horizontal stresses oxx, superlitho static pressure, and vertical stresses ozz in the part of the mantle lacking intense near-vertical flows are approximately equal, varying within ± 6, ± 8, and ± 10 MPa, respectively. However, these fields are stronger in the areas of descending slabs, where the values of the above parameters are about an order of magnitude higher (± 50 MPa).This result agrees with the current views of the oceanic slabs as the most important gent of mantle convection. We have found significant differences among the oxx, ozz, and pressure fields. The pressure field reveals both the vertical and horizontal features of slabs and plumes, clearly showing their long thermal conduits with broader heads. The distributions of oxx are sensitive to the near-horizontal parts of the flows, whereas the ozz fields reveal mainly their vertical substructures. The model shows the presence of relatively cold remnants of slabs in the lower mantle above the thermal boundary layer. Numerous hot plumes penetrating through these high-viscosity remnants, as well as the new descending slabs, induce intense stress fields in the lower mantle, which are strongly inhomogeneous in space and time. 相似文献
9.
Stability and dynamics of the continental tectosphere 总被引：1，自引：0，他引：1
Continental cratons overlie thick, high-viscosity, thermal and chemical boundary layers, where the chemical boundary layers are less dense than they would be due to thermal effects alone, perhaps because they are depleted in basaltic constituents. If the continental tectosphere is the same age as the overlying Archaean crust, then the continental tectosphere must be able to survive for several billion years without undergoing a convective instability, despite being both cold and thick. Since platforms and shields correlate only weakly with Earth's gravity and geoid anomalies, acceptable models of the continental tectosphere must also satisfy this gravity constraint. We investigate the long-term stability of the continental tectosphere by carrying out a number of numerical convection experiments within a two-dimensional Cartesian domain. We initiate our experiments with a tectosphere (thermal and chemical boundary layers) immersed in a region of uniform composition, temperature, and viscosity, and consider the effects on the stability of the tectosphere of (1) activation energy (used to define the temperature dependence of viscosity), (2) compositional buoyancy, and (3) linear or non-linear rheology. The large lateral thermal gradients required to match oceanic and tectosphere structures initiate the dominant instability, a “drip” which develops at the side of the tectosphere and moves to beneath its center. High activation energies and high background viscosities restrict the amount and rate of entrainment. Compositional buoyancy does not significantly change the flow pattern. Rather, compositional buoyancy slows the destruction process somewhat and reduces the stress within the tectosphere. With a non-Newtonian rheology, this reduction in stress helps to stiffen the tectosphere. In these experiments, dynamical systems that adequately model the present ocean-continent structures have activation energy E*≥180 kJ mole−1 — a value about one third the estimate of activation energy for olivine, E*≈520 kJ mole−1. Although for E*≈520 kJ mole−1, compositional buoyancy is not required for the tectosphere to survive, the joint application of longevity and gravity constraints allows us to reject all models not containing compositional buoyancy, and to predict that the ratio of compositional to thermal buoyancy within the continental tectosphere is approximately unity. 相似文献
10.
The Xisha Trough, located in the northwest of the South China Sea (SCS) mainly rifted 30 Ma ago, has been a failed rift since the cessation of the seafloor spreading of the NW subbasin. Based on the velocity–depth model along Profile OBH-4 across the Xisha Trough, a seven-layer density–depth model is used to estimate density structure for the profile. The relationship between seismic velocity and radiogenic heat production is used to estimate the vertical distribution of heat sources in the lower crust. The 2-D temperature field is calculated by applying a 2-D numerical solution of the heat conduction equation and the thermal lithosphere thickness is obtained from the basalt dry solidus (BDS). The rheology of the profile is estimated on the basis of frictional failure in the brittle regime and power-law steady-state creep in the ductile regime. Rheological model is constructed for a three-layer model involving a granitic upper crust, a quartz diorite lower crust and an olivine upper mantle. Gravity modeling supports basically the velocity–depth model. The Moho along Profile OBH-4 is of relatively high heat flow ranging from 46 to 60 mW/m2 and the Moho heat flow is higher in the trough than on the flanks. The depth of the “thermal” lithospheric lower boundary is about 54 km in the center, deepens toward two sides, and is about 75 km at the northern slope area and about 70 km at the southern Xisha–Zhongsha Block. Rheological calculation indicates that the two thinnest ductile layers in the crust and the thickest brittle layer in the uppermost mantle lie in the central region, showing that the Xisha Trough has been rheologically strengthened, which are mainly due to later thermal relaxation. In addition, the strengthening in rheology during rifting was not the main factor in hampering the breakup of the Xisha Trough. 相似文献
11.
Yu. L. Rebetskii 《Doklady Earth Sciences》2011,440(1):1328-1333
The set of equations for the problem of thermal-gravitational convection accounts for compressibility of solid bodies, which
changes for elementary volumes moving during convection process in fields of the initial temperature and the initial gravitational
stress pattern for rheology of an elastic-viscous Maxwell body. It was shown that equations of momentum conservation in the
vertical direction and heat transfer for steady convection differ from the equation for incompressible liquid by terms containing
the rate of elastic volume change and the connected rate of heat change. It was established that an additional term in the
momentum conservation equation defines a new class of the instable state of a solid body, which is able to form huge deformations
at the expense of plasticity and creep at large segments of time—flow in the field of gravity force—instability of the gravitational
stress pattern of elastic-plastic body. Analysis of different boundary conditions for which this instability can be realized
in the form of convective cells showed that the convection rate is totally defined by reconstruction processes of vertical
stresses on horizontal boundaries close to the initial gravitation pattern. Alignment process of these stresses can be provided
not only by erosion and denudation processes occurring on the Earth’s surface, but also by processes on the inner boundaries
of the tectonosphere which provide isostasy in the mantle. 相似文献
12.
Transient surface deflections associated with non-continuous subduction are studied through the use of a sequence of idealized numerical models of time dependent mantle convection. The major concerns of this study are the magnitude and duration of dynamically supported topographic fluctuations resulting from either the initiation or termination of a subduction zone at the Earth's surface. The former is modelled by prescribing an initial temperature structure which mimics the presence of a cold slab of lithosphere immersed in a warm uniform-temperature mantle, while the latter is induced by thermally detaching this slab from the upper surface of the convection cell. The model “slabs” are thus defined thermally, rather than mechanically, and their negative buoyancy induces convective flow in the neighbouring mantle. The full hydrodynamic equations governing natural convection are solved numerically in order to follow the evolution of the temperature and velocity fields with time. The resulting model flows are, therefore, dynamically self-consistent, and differ from previous kinematic models in that the flow velocities are determined at each instant from the evolving thermal field, rather than prescribed as boundary conditions. As the lithospheric slab sinks into the mantle, the induced flow produces normal stresses, which in turn result in a broad topographic depression. Subsequently, as the slab-surface distance increases, the topography rebounds at a rate which is strongly dependent upon the imposed temperature contrast between the slab and the surrounding mantle material. Assuming that these depressions are filled with sediments, if a subsequent episode of subduction were to begin before the initial depression was eliminated, new sediments would be superimposed upon the old. Repeated episodes of subduction at a continental margin may, therefore, be an important factor contributing to repeated cycles of platform sedimentation. 相似文献
13.
H. Pascal 《国际地质力学数值与分析法杂志》1983,7(3):289-303
This paper describes the flow behaviour of certain non-Newtonian fluids through a porous medium. A generalized Bingham rheological model of power-law in the presence of a yield stress has been considered. Several problems of fluid mechanics, which appear currently in oil reservoir engineering, have been investigated and the rheological behaviour effect has been emphasized. The short time solutions have been formulated in terms of a moving boundary problem. The approximate solutions in a closed formwere obtained by means of the integral method. Several dimensionless groups have been found to be relevant in evaluating the rheological effect on the steady and unsteady and unsteady flow behaviour. The deviation from Newtonian flow behaviour has been illustrated using several numerical examples. 相似文献
14.
太平洋板块、印度板块和欧亚板块的演化对中国近海沉积盆地的沉降及充填具有控制作用。根据地幔对流及地壳拉伸特征可将中国近海沉积盆地沉降类型划分为被动、主动和组合热沉降型3种。不同沉降类型分别具有不同的盆地结构,其中被动热沉降型以断陷为主,主动热沉降型以坳陷为主,组合热沉降型则是两种盆地结构的叠加或侧加。中国近海北部板内沉积盆地沉降类型以被动热沉降为主,远离海洋,受海侵影响较小,以陆相沉积体系为主;中部板缘沉积盆地沉降类型为被动侧加主动热沉降,水体整体较浅,坡折及三角洲发育规模小;南部板缘沉积盆地沉降类型也为被动侧加主动热沉降,水体整体较深,坡折及三角洲发育规模大。 相似文献
15.
In order to identify the dominant non-Newtonian effects which occur during the injection of a new Newtonian magma through a partially crystallized magma chamber, we have performed some preliminary analogue experiments which enable us to point out several features induced by the non-Newtonian properties of the host fluid during injection processes. These experiments were performed in a three-dimensional device and involve complex non-Newtonian fluids—clay suspensions in which rheological properties such as bulk strength, yield strength and rheofluidification exponent may vary. Forced injection takes place through a slot which in the case of a Newtonian host fluid is the geometry that provides planar structures. Depending both on the density contrast and on the rheological contrast between the injected dyed water and the host fluid three kind of structures were observed: (1) permanent plumes when the injected water is lighter than the suspension exhibiting rheological properties close to Newtonian fluids; (2) pseudo-fountains and spreading at the bottom of the tank with a destabilizing density contrast and in high yield strength/more viscous suspensions; (3) fountains with a slightly stablizing density contrast. The implications for magma chamber evolution are briefly discussed. In particular it seems that homogeneous non-Newtonian media inhibit the formation of planar structures and partially crystallized magma may induce the spreading of the new magma at the bottom or at the top of the chamber regardless of the density contrast between the magmas. 相似文献
16.
板块构造登陆导致大陆动力学的兴起,但大陆动力学迄今在理论上尚无长足发展,究其原因是大陆构造的复杂性,这一复杂性取决于大陆岩石圈复杂的流变学结构(纵向分层和横向不均一)。因此,大陆岩石圈流变学研究是大陆动力学的核心,并成为发展大陆动力学、完善板块构造理论的关键研究领域,同时对构造地质学学科发展具有推动作用,并在资源形成和地震机理方面具有实际意义。本文在阐述了岩石圈流变学的发展与现状的基础上,较详细分析了大陆流变学的核心内容:大陆岩石圈流变学结构特征,大陆物质的本构方程及大陆流变学特性对大陆变形与构造演化的影响。本文还总结了我国开展大陆流变学的基础和条件,对未来研究进行了展望:1)形成常态的多尺度-多手段-多学科-多部门的联合研究;2)缩小量子力学和岩石/矿物流变结构研究间的差距; 3)从全球尺度物质流动着手,构建以流变学为基础的大陆动力学模型。 相似文献
17.
Small-scale heterogeneity in the deep mantle is concentrated in the upper-mantle transi-tion zone(TZ),in the depth range 410-660 km and also at the bottom 250 km D region.This encour-ages a more detailed investigation of the potential for seismic reflectivity imaging by modelling hetero-geneous structures in mantle convection models including phase transitions of the TZ and D regions.We applied finite elements with variable spacing near the boundary layers in 2-D cylindrical geometry that allow for sufficie... 相似文献
18.
Crustal studies within the Japanese islands have provided important constraints on the physical properties and deformation styles of the island arc crust. The upper crust in the Japanese islands has a significant heterogeneity characterized by large velocity variation (5.5–6.1 km/s) and high seismic attenuation (Qp=100–400 for 5–15 Hz). The lateral velocity change sometimes occurs at major tectonic lines. In many cases of recent refraction/wide-angle reflection profiles, a “middle crust” with a velocity of 6.2–6.5 km/s is found in a depth range of 5–15 km. Most shallow microearthquakes are concentrated in the upper/middle crust. The velocity in the lower crust is estimated to be 6.6–7.0 km/s. The lower crust often involves a highly reflective zone with less seismicity, indicating its ductile rheology. The uppermost mantle is characterized by a low Pn velocity of 7.5–7.9 km/s. Several observations on PmP phase indicate that the Moho is not a sharp boundary with a distinct velocity contrast, but forms a transition zone from the upper mantle to the lower crust. Recent seismic reflection experiments revealed ongoing crustal deformations within the Japanese islands. A clear image of crustal delamination obtained for an arc–arc collision zone in central Hokkaido provides an important key for the evolution process from island arc to more felsic continental crust. In northern Honshu, a major fault system with listric geometry, which was formed by Miocene back arc spreading, was successfully mapped down to 12–15 km. 相似文献
19.
We present a thermophysical model for interaction between the conduit of a thermochemical plume and horizontal free convection flows in the mantle: The mantle flow incident on the plume conduit melts at the conduit boundary (front part) and crystallizes at its back. Geological data on the intensity of plume magmatism over the last 150 Myr are used to estimate the total thermal power of mantle plumes. A possible scenario for plume-related mantle recrystallization is proposed. Over the lifespan of a thermochemical plume, mantle melts and recrystallizes owing to the motion of the plume source and interaction between the plume conduit and horizontal free convection flows. The plume conduits can melt and recrystallize the entire mantle over a certain period of time. The model for the interaction of drifting plume conduits with mantle flows and the estimated total thermal power of mantle plumes are used to estimate the duration of plume-related melting and recrystallization of the entire mantle. The influence of mantle plumes on the convective structure of the mantle through melting is judged from the model for plume interaction with horizontal mantle flows. 相似文献
20.
At the transition from the Permian to the Triassic, Eurasia was the site of voluminous flood-basalt extrusion and rifting. Major flood-basalt provinces occur in the Tunguska, Taymyr, Kuznetsk, Verkhoyansk–Vilyuy and Pechora areas, as well as in the South Chinese Emeishen area. Contemporaneous rift systems developed in the West Siberian, South Kara Sea and Pyasina–Khatanga areas, on the Scythian platform and in the West European and Arctic–North Atlantic domain. At the Permo–Triassic transition, major extensional stresses affected apparently Eurasia, and possibly also Pangea, as evidenced by the development of new rift systems. Contemporaneous flood-basalt activity, inducing a global environmental crisis, is interpreted as related to the impingement of major mantle plumes on the base of the Eurasian lithosphere. Moreover, the Permo–Triassic transition coincided with a period of regional uplift and erosion and a low-stand in sea level. Permo–Triassic rifting and mantle plume activity occurred together with a major reorganization of plate boundaries and plate kinematics that marked the transition from the assembly of Pangea to its break-up. This plate reorganization was possibly associated with a reorganization of the global mantle convection system. On the base of the geological record, we recognize short-lived and long-lived plumes with a duration of magmatic activity of some 10–20 million years and 100–150 million years, respectively. The Permo–Triassic Siberian and Emeishan flood-basalt provinces are good examples of “short-lived” plumes, which contrast with such “long lived” plumes as those of Iceland and Hawaii. The global record indicates that mantle plume activity occurred episodically. Purely empirical considerations indicate that times of major mantle plume activity are associated with periods of global mantle convection reorganization during which thermally driven mantle convection is not fully able to facilitate the necessary heat transfer from the core of the Earth to its surface. In this respect, we distinguish between two geodynamically different scenarios for major plume activity. The major Permo–Triassic plume event followed the assembly Pangea and the detachment of deep-seated subduction slabs from the lithosphere. The Early–Middle Cretaceous major plume event, as well as the terminal–Cretaceous–Paleocene plume event, followed a sharp acceleration of global sea-floor spreading rates and the insertion of new subduction zone slabs deep into the mantle. We conclude that global plate kinematics, driven by mantle convection, have a bearing on the development of major mantle plumes and, to a degree, also on the pattern of related flood-basalt magmatism. 相似文献