Lower Mantle (lower + mantle)

Distribution by Scientific Domains

Selected Abstracts

Subducted slabs and lateral viscosity variations: effects on the long-wavelength geoid

Nicola Tosi
SUMMARY The characteristic broad local maxima exhibited by the long-wavelength geoid over subduction zones are investigated with a numerical model of mantle flow. In a spherical axisymmetric geometry, a synthetic model of buoyancy driven subduction is used to test the effects on the geoid caused by the depth of penetration of the lithosphere into the mantle, by the viscosity stratification and by lateral viscosity variations (LVV) in the lithosphere, upper and lower mantle. The presence of anomalous slab density in the lower mantle guarantees geoid amplitudes comparable with the observations, favouring the picture of slabs that penetrate the transition zone and sink into the deep mantle. The viscosity of the lower mantle controls the long-wavelength geoid to the first order, ensuring a clear positive signal when it is at least 30-times greater than the upper-mantle viscosity. The presence of LVV in the lithosphere, in the form of weak plate margins, helps to increase the contribution of the surface topography, causing a pronounced reduction of the geoid. Localized LVV associated with the cold slab play a secondary role if they are in the upper mantle. On the other hand, highly viscous slabs in the lower mantle exert a large influence on the geoid. They cause its amplitude to increase dramatically, way beyond the values typically observed over subduction zones. Long-wavelength flow becomes less vigorous as the slab viscosity increases. Deformation in the upper mantle becomes more localized and power is transferred to short wavelengths, causing the long-wavelength surface topography to diminish and the total geoid to increase. Slabs may be then weakened in the lower mantle or retain their high viscosity while other mechanisms act to lower the geoid. It is shown that a phase change from perovskite to post-perovskite above the core,mantle boundary can cause the geoid to reduce significantly, thereby helping to reconcile models and observations. [source]

Seismic evidence for a sharp lithospheric base persisting to the lowermost mantle beneath the Caribbean

Tadashi Kito
SUMMARY Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method,the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P -wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core,mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10,15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection. [source]

Inference of mantle viscosity from GRACE and relative sea level data

Archie Paulson
SUMMARY Gravity Recovery And Climate Experiment (GRACE) satellite observations of secular changes in gravity near Hudson Bay, and geological measurements of relative sea level (RSL) changes over the last 10 000 yr in the same region, are used in a Monte Carlo inversion to infer-mantle viscosity structure. The GRACE secular change in gravity shows a significant positive anomaly over a broad region (>3000 km) near Hudson Bay with a maximum of ,2.5 ,Gal yr,1 slightly west of Hudson Bay. The pattern of this anomaly is remarkably consistent with that predicted for postglacial rebound using the ICE-5G deglaciation history, strongly suggesting a postglacial rebound origin for the gravity change. We find that the GRACE and RSL data are insensitive to mantle viscosity below 1800 km depth, a conclusion similar to that from previous studies that used only RSL data. For a mantle with homogeneous viscosity, the GRACE and RSL data require a viscosity between 1.4 × 1021 and 2.3 × 1021 Pa s. An inversion for two mantle viscosity layers separated at a depth of 670 km, shows an ensemble of viscosity structures compatible with the data. While the lowest misfit occurs for upper- and lower-mantle viscosities of 5.3 × 1020 and 2.3 × 1021 Pa s, respectively, a weaker upper mantle may be compensated by a stronger lower mantle, such that there exist other models that also provide a reasonable fit to the data. We find that the GRACE and RSL data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle. [source]

Radial resolving power of far-field differential sea-level highstands in the inference of mantle viscosity

Roblyn A. Kendall
SUMMARY For two decades leading to the late 1980s, the prevailing view from studies of glacial isostatic adjustment (GIA) data was that the viscosity of the Earth's mantle increased moderately, if at all, from the base of the lithosphere to the core,mantle boundary. This view was first questioned by Nakada & Lambeck, who argued that differential sea-level (DSL) highstands between pairs of sites in the Australian region preferred an increase of approximately two orders of magnitude from the mean viscosity of the upper to the lower mantle, in accord with independent inferences from observables related to mantle convection. We use non-linear Bayesian inference to provide the first formal resolving power analysis of the Australian DSL data set. We identify three radial regions, two within the upper mantle (110,270 km and 320,570 km depth) and one in the lower mantle (1225,2265 km depth), over which the average of viscosity is well constrained by the data. We conclude that: (1) the DSL data provide a resolution in the inference of upper mantle viscosity that is better than implied by forward analyses based on isoviscous regions above and below the 670 km depth discontinuity and (2) the data do not strongly constrain viscosity at either the base or top of the lower mantle. Finally, our inversions also quantify the significant bias that may be introduced in inversions of the DSL highstands that do not simultaneously estimate the thickness of the elastic lithosphere. [source]

Post-seismic relaxation following the great 2004 Sumatra-Andaman earthquake on a compressible self-gravitating Earth

Fred F. Pollitz
SUMMARY The Mw, 9.0 2004 December 26 Sumatra-Andaman and Mw= 8.7 2005 March 28 Nias earthquakes, which collectively ruptured approximately 1800 km of the Andaman and Sunda subduction zones, are expected to be followed by vigorous viscoelastic relaxation involving both the upper and lower mantle. Because of these large spatial dimensions it is desirable to fully account for gravitational coupling effects in the relaxation process. We present a stable method of computing relaxation of a spherically-stratified, compressible and self-gravitating viscoelastic Earth following an impulsive moment release event. The solution is cast in terms of a spherical harmonic expansion of viscoelastic normal modes. For simple layered viscoelastic models, which include a low-viscosity oceanic asthenosphere, we predict substantial post-seismic effects over a region several 100s of km wide surrounding the eastern Indian Ocean. We compare observed GPS time-series from ten regional sites (mostly in Thailand and Indonesia), beginning in 2004 December, with synthetic time-series that include the coseismic and post-seismic effects of the 2004 December 26 and 2005 March 28 earthquakes. A viscosity structure involving a biviscous (Burgers body) rheology in the asthenosphere explains the pattern and amplitude of post-seismic offsets remarkably well. [source]

Compositional constraints on the equation of state and thermal properties of the lower mantle

Frank D. Stacey
SUMMARY By extrapolating the lower mantle equation of state (EoS) to P=0, T=290 K, we determine the EoS parameters that are compatible with a mixture of (Mg, Fe)SiO3 perovskite (with a small admixture of Al2O3), (Mg, Fe)O magnesiowüstite and CaSiO3 perovskite in arbitrary proportions and with arbitrary Fe/(Fe+Mg) ratio. The parameters fitted are density, ,, adiabatic incompressibility, KS, and its pressure derivative, K,S,(,KS/,P)S. The first stage is adiabatic extrapolation to P=0, T=T0, that is, to the foot of the lower mantle adiabat, at which K,0(T0) is allowed to have any value between 3.8 and 4.6, and 1500 K,T0,2000 K. It is important to use an equation for which the lower mantle fitting does not prescribe K,0(T0) and this rules out the third-order Birch theory, which gives a seriously wrong value. The further extrapolation to 290 K at P=0 uses thermodynamic relationships with maximum generality, allowing all of the following thermoelastic parameters to be arbitrary functions of temperature: K;,; Grüneisen parameter, ,;q=(, ln ,/, ln V)T, where V is volume; volume coefficient of thermal expansion, ,; adiabatic Anderson,Grüneisen parameter, ,S=(1/,) (, ln KS/,T)P; and the mixed P, T derivative (,K,S/,T)P. The heat capacity at constant volume, CV, is assumed to follow the Debye function, so , is controlled by that. The temperature dependences of the dimensionless parameters ,, q and ,S at P=0 are slight. We find , to be precisely independent of T at constant V. The parameter dK,0/dT increases strongly with T, as well as with the assumed value of K,0(T0), where K,0 is K,S at P=0. The fitting disallows significant parameter ranges. In particular, we find solutions only if K,0(T0),4.2 and the 290 K value of K,0 for Mg perovskite is less than 3.8. Conclusions about composition are less secure, partly because of doubt about individual mineral properties. The volume of magnesiowüstite is found to be between 10 and 25 per cent for respective T0 values of 2000 and 1500 K, but the Ca-perovskite volume is no more than 6 per cent and has little influence on the other conclusions. The resulting overall Fe/(Fe+Mg) ratio is 0.12 to 0.15. Although this ratio is higher than expected for a pyrolite composition, the ratio depends critically on the assumed mineral densities; some adjustment of the mineral mix may need to be considered. [source]

Determining the possible building blocks of the Earth and Mars

Thomas H. Burbine
Only a very small percentage of the combinations match the oxygen isotopic composition, the assumed bulk FeO concentration, and the assumed Fe/Al weight ratio for the Earth. Since chondrites are enriched in silicon relative to estimates of the bulk Earth, none of the combinations fall near the terrestrial magmatic fractionation trend line in Mg/Si-Al/Si space. More combinations match the oxygen isotopic composition and the assumed bulk FeO concentration for Mars. These combinations fall near the trend for shergottite meteorites in Mg/Si-Al/Si space. One explanation for the difficulty in forming Earth out of known chondrites is that the Earth may be composed predominately of material that did not survive to the present day as meteorites. Another explanation could be that significant amounts of silicon are sequestered in the core and/or lower mantle of the Earth. [source]

Pressure induced high spin to low spin transition in magnesiowüstite

Taku Tsuchiya
Abstract Using a rotationally invariant formulation of LDA + U, we report a successful study of the high spin (HS)/low spin (LS) transition in low solute concentration magnesiowüstite (Mw), (Mg1,xFex )O, (x < 20%), the second most abundant phase in Earth's lower mantle. The HS state crosses over smoothly to the LS state passing through an insulating mixed spins state where properties change continuously, as seen experimentally. These encouraging results indicate this method should enable first principles studies of strongly correlated iron-bearing minerals, a major class of mineral physics problems. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Analytical approach for the toroidal relaxation of viscoelastic earth

Hansheng Wang
SUMMARY This paper is concerned with post-seismic toroidal deformation in a spherically symmetric, non-rotating, linear-viscoelastic, isotropic Maxwell earth model. Analytical expressions for characteristic relaxation times and relaxation strengths are found for viscoelastic toroidal deformation, associated with surface tangential stress, when there are two to five layers between the core,mantle boundary and Earth's surface. The multilayered models can include lithosphere, asthenosphere, upper and lower mantles and even low-viscosity ductile layer in the lithosphere. The analytical approach is self-consistent in that the Heaviside isostatic solution agrees with fluid limit. The analytical solution can be used for high-precision simulation of the toroidal relaxation in five-layer earths and the results can also be considered as a benchmark for numerical methods. Analytical solution gives only stable decaying modes,unstable mode, conjugate complex mode and modes of relevant poles with orders larger than 1, are all excluded, and the total number of modes is found to be just the number of viscoelastic layers between the core,mantle boundary and Earth's surface,however, any elastic layer between two viscoelastic layers is also counted. This confirms previous finding where numerical method (i.e. propagator matrix method) is used. We have studied the relaxation times of a lot of models and found the propagator matrix method to agree very well with those from analytical results. In addition, the asthenosphere and lithospheric ductile layer are found to have large effects on the amplitude of post-seismic deformation. This also confirms the findings of previous works. [source]

The seismic anomaly beneath Iceland extends down to the mantle transition zone and no deeper

G. R. Foulger
A 3-D teleseismic tomography image of the upper mantle beneath Iceland of unprecedented resolution reveals a subvertical low wave speed anomaly that is cylindrical in the upper 250 km but tabular below this. Such a morphological transition is expected towards the bottom of a buoyant upwelling. Our observations thus suggest that magmatism at the Iceland hotspot is fed by flow rising from the mantle transition zone. This result contributes to the ongoing debate about whether the upper and lower mantles convect separately or as one. The image also suggests that material flows outwards from Iceland along the Reykjanes Ridge in the upper 200 km, but is blocked in the upper 150 km beneath the Tjornes Fracture Zone. This provides direct observational support for the theory that fracture zones dam lateral flow along ridges. [source]