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  • 1
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    Geophysical inferences of thermal-chemical structures in the lower mantle (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Lateral variations of the temperature field in the lower mantle have been reconstructed using new results in mineral physics and seismic tomographic data. We show that, with the application of high-pressure experimental values of thermal expansivity and of sound velocities, the slow seismic anomalies in the lower mantle under the Pacific and Africa can be converted into realistic-looking plume structures with large dimensions of 0(1000 km). The outer fringes of the plumes have an excess temperature of around 400 K. In the core of the plumes are found tonguelike structures with extremely high thermal anomalies. These values can exceed 1200 K and are too high to be explained on the basis of thermal anomalies alone. We suggest that these major plumes in the deep mantle may be driven by both thermal and chemical buoyancies or that enhanced conductive heat-transfer may be important there.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 10; p. 899-902.
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  • 2
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    Three-dimensional instabilities of mantle convection with multiple phase transitions (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The effects of multiple phase transitions on mantle convection are investigated by numerical simulations that are based on three-dimensional models. These simulations show that cold sheets of mantle material collide at junctions, merge, and form a strong downflow that is stopped temporarily by the transition zone. The accumulated cold material gives rise to a strong gravitational instability that causes the cold mass to sink rapidly into the lower mantle. This process promotes a massive exchange between the lower and upper mantles and triggers a global instability in the adjacent plume system. This mechanism may be cyclic in nature and may be linked to the generation of superplumes.
    Keywords: GEOPHYSICS
    Type: Science (ISSN 0036-8075); 259; 5099; p. 1308-1311.
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  • 3
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    Time-domain approach for the transient responses in stratified viscoelastic Earth models (1995)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We have developed the numerical algorithm for the computation of transient viscoelastic responses in the time domain for a radially stratified Earth model. Stratifications in both the elastic parameters and the viscosity profile have been considered. The particular viscosity profile employed has a viscosity maximum with a constrast of O(100) in the mid lower mantle. The distribution of relaxation times reveals the presence of a continuous spectrum situated between O(100) and O(exp 4) years. The principal mode is embedded within this continuous spectrum. From this initial-value approach we have found that for the low degree harmonics the non-modal contributions are comparable to the modal contributions. For this viscosity model the differences between the time-domain and normal-mode results are found to decrease strongly with increasing angular order. These calculations also show that a time-dependent effective relaxation time can be defined, which can be bounded by the relaxation times of the principal modes.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 22; 10; p. 1285-1288
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  • 4
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    Lower mantle thermal structure deduced from seismic tomography, mineral physics and numerical modelling (1994)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The long-wavelength thermal anomalies in the lower mantle have been mapped out using several seismic tomographic models in conjunction with thermodynamic parameters derived from high-pressure mineral physics experiments. These parameters are the depth variations of thermal expansivity and of the proportionality factor between changes in density and seismic velocity. The giant plume-like structures in the lower mantle under the Pacific Ocean and Africa have outer fringes with thermal anomalies around 300-400 K, but very high temperatures are found in the center of the plumes near the base of the core-mantle boundary. These extreme values can exceed +1500 K and may reflect large hot thermal anomalies in the lower mantle, which are supported by recent measurements of high melting temperatures of perovskite and iron. Extremely cold anomalies, around -1500 K, are found for anomalies in the deep mantle around the Pacific rim and under South America. Numerical simulations show that large negative thermal anomalies in the mid-lower mantle have modest magnitudes of around -500 K. correlation pattern exists between the present-day locations of cold masses in the lower mantle and the sites of past subduction since the Cretaceous. Results from correlation analysis show that the slab mass-flux in the lower mantle did not conform to a steady-state nature but exhibited time-dependent behavior.
    Keywords: GEOPHYSICS
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 121; 3/4; p. 385-402
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  • 5
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    Lower-mantle viscosity constrained by seismicity around deglaciated regions (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: It is shown here that seismicity around the margins of deglaciated areas provides a constraint on the viscosity of the lower mantle. Calculations using a spherical, viscoelastic earth model show that the present-day magnitude of the stress fields induced in the lithosphere beneath the Laurentide and Fennoscandian ice sheets is very sensitive to the value of the lower-mantle viscosity. Stress of about 100 bar, sufficient to cause seismicity, can still remain in the lithosphere for lower-mantle viscosities greater than about 10 to the 22nd Pa-s; for lower-mantle viscosities of about 10 to the 21st Pa-s, only a few tens of bars of stress persist in the lithosphere today. This influence of lower-mantle viscosity on the state of stress in the lithosphere also has implications for the migration of stress from earthquakes, and hence for earthquake recurrence times.
    Keywords: GEOPHYSICS
    Type: Nature (ISSN 0028-0836); 351; 53-55
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  • 6
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    Characteristics-based methods applied to infinite Prandtl number thermal convection in the hard turbulent regime (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Characteristics-based methods for the advection-diffusion equation are presented and directly applied to study thermal convection with extremely large Rayleigh number (Ra). It is shown that the operator-splitting method for advection-diffusion problems is very accurate for determining the advected field at extremely high Peclet number (Pe). The technique presented is considered to have great potential for solving advection-dominated problems, while the Langrangian method is more accurate for lower Pe. It is noted that the accuracy of these characteristics-based methods strongly depends on the quality of interpolation. The computational time for the operator-splitting method grows with the number of time steps employed. The Langrangian method was used for simulations of convection at very high Ra, up to 3 x 10 to the 9th, and time-dependent, thermal convection solutions were obtained for infinite Prandtl number.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Physics of Fluids A (ISSN 0899-8213); 3; 2105-211
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  • 7
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    The role of a pressure-dependent rheology in the dynamics of mantle circulation (1985)
    In:  CASI
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    Publication Date: 2016-06-07
    Description: A thermomechanical model for upper mantle convection was constructed such that the thickness and the structure of the lithosphere are determined self consistently by the heat transported by convection. In this study of the interaction between the lithosphere and upper mantle, strongly temperature and pressure dependent rheologies for both Newtonian and non-Newtonian creep mechanisms are employed. For a strictly temperature dependent rheology an insignificant amount of heat, less than 12.5 mW/sq m, can be transported convectively for an interior viscosity, 0(10 sup 21 Pas), compatible with post glacial rebound. On the other hand, for similar values of the interior viscosity, steady heat fluxes between 20 and 40 mW/sq m are produced by introducing pressure dependence into the rheology. For the temperature and pressure dependent flow law the horizontally averaged interior temperature displays very little variation with the amount of heat evacuated, once all of the rheological parameters are fixed. This finding may have important ramifications for parameterized convection.
    Keywords: GEOPHYSICS
    Type: NASA, Washington Geopotential Res. Mission (GRM); p 66-67
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  • 8
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    The effects of Venusian mantle convection with multiple phase transitions (1992)
    In:  CASI
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    Publication Date: 2013-08-31
    Description: Recently there was a flurry of activities in studying the effects of phase transitions in the Earth's mantle. From petrological and geophysical considerations, phase-transitions would also play an important role in venusian dynamics. The basic differences between the two planets are the surface boundary conditions, both thermally and mechanically. In this vein we have studied time-dependent mantle convection with multiple phase transitions and depth-dependent thermal expansivity (alpha is approximately rho(exp -6)), based on high-pressure and temperature measurements. Both the olivine-spinel and spinel-perovskite transitions were simulated by introducing an effective thermal expansivity, as described. Used together with the extended Boussinesq Approximation this method serves as a powerful tool to examine the effects of phase transitions on convection at relatively low computational costs.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Papers Presented to the International Colloquium on Venus; p 121-122
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  • 9
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    Development of diapiric structures in the upper mantle due to phase transitions (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Solid-state phase transition in time-dependent mantle convection can induce diapiric flows in the upper mantle. When a deep mantle plume rises toward phase boundaries in the upper mantle, the changes in the local thermal buoyancy, local heat capacity, and latent heat associated with the phase change at a depth of 670 kilometers tend to pinch off the plume head from the feeding stem and form a diapir. This mechanism may explain episodic hot spot volcanism. The nature of the multiple phase boundaries at the boundary between the upper and lower mantle may control the fate of deep mantle plumes, allowing hot plumes to go through and retarding the tepid ones.
    Keywords: GEOPHYSICS
    Type: Science (ISSN 0036-8075); 252; 1836-183
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  • 10
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    Internal heating and thermal constraints on the mantle (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Numerical simulations have been carried out to study the average temperature and thermal structure of an internally heated mantle. It is found that in an incompressible mantle the hottest parts are localized in regions of slow flow in the middle and upper mantle, whereas in a compressible mantle they occur over laterally extensive regions near the core-mantle boundary (CMB). For chondritic concentrations of heat-producing elements (hpe's), the average thermal conductivity in the mantle must be high to avoid heating the core, and the temperature of the CMB is low to avoid large-scale melting. The mantle may have been extensively molten in the Archaean.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 16; 1407-141
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