Topography on the D′′ region from analysis of a thin dense layer beneath a convecting cell

Youngs, Bryony; Houseman, Gregory A.

doi:10.1016/j.pepi.2006.09.003

Cited by 3 publications

(6 citation statements)

References 36 publications

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“…As the topography develops into a steep‐sided, flat‐topped structure, the direction of flow in the lower layer reverses, suggesting that the two flow fields are now coupled by viscous shear. This behavior is consistent with conclusions of Youngs and Houseman [2007] that thermal coupling between layers leads to cusp‐like deformation of the interface and viscous coupling leads to steep‐sided, flat‐topped topography. The differences between the zero stream function contour and the trace of the interface suggest that an equilibrium shape has still not been reached after t = 0.00763 (2000 steps) in this experiment.…”

Section: Resultssupporting

confidence: 91%

“…The primary effect of increasing the convection wavelength is to produce more stable, better defined, steep‐sided structures. This increase in stability with convection wavelength agrees with earlier predictions based on 2‐D analytical studies [ Youngs and Houseman , 2007] that the domain of stability for steep‐sided topography increases as the ratio of convection wavelength to dense layer thickness increases.…”

Section: Resultssupporting

confidence: 90%

“…Our experiments demonstrate a link between the way in which the circulations in the two layers are coupled and the deformation style on the interface: thermal coupling (circulation in the same sense above and below the interface) leads to cusp‐like peaks; viscous coupling (circulation in opposite senses) to steep‐sided, flat‐topped islands. This observation is consistent with the conclusions of Youngs and Houseman [2007], who used a simplified analytical model to predict the boundary topography on a 2‐D thin layer whose internal flow field is driven partly by internal density gradients resulting from temperature variation and partly by upper boundary tractions resulting from an overlying convecting cell. Beneath an upwelling region, thermal coupling causes both the upper and lower layers to flow upward, leading to significant upward deflection of the density interface.…”

Section: Discussionsupporting

confidence: 90%

“…A significant characteristic of the tomographically imaged low‐velocity anomalies is their shape which, in cross section, has a flat top and steep side boundaries [ Ni et al , 2002; Wen , 2001; Wen et al , 2001]. Previous studies of two‐layer thermochemical convection have indicated that two distinct styles of deformation of the density interface may occur: (1) the lower layer remains ubiquitous with cusp‐like peaks beneath upwellings or (2) the lower layer is pushed aside beneath downwellings leaving discrete ‘islands’ of dense material beneath upwellings [ Tackley , 1998; McNamara and Zhong , 2004; Tan and Gurnis , 2005; Youngs and Houseman , 2007].…”

Section: Introductionmentioning

confidence: 99%

“… Tackley [2002] and McNamara and Zhong [2005] also described formation of discrete islands in mantle models with depth‐ and temperature‐dependent viscosity. Youngs and Houseman [2007] established, however, that the countercirculation established in the dense layer by tractions on the interface, tends to produce flat‐topped, steep‐sided topography even in the absence of factors such as compressibility or viscosity variation.…”

Section: Introductionmentioning

confidence: 99%

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Formation of steep‐sided topography from compositionally distinct dense material at the base of the mantle

Youngs¹,

Houseman²

2009

J. Geophys. Res.

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[1] In this study we use 3-D, two-layer, thermochemical convection experiments to explore the development of a thin, compositionally distinct, dense layer at the base of the mantle. We investigate the conditions under which steep-sided, flat-topped topography will form on an initially ubiquitous layer as a result of the convecting mantle. Previous authors found that discrete, steep-sided piles of material form when compressibility, temperature-dependent viscosity, or a large viscosity increase in the lower layer is assumed. We observe, however, that this topographic style develops in calculations of incompressible convection with layers of constant viscosity. Experiments involving simple convection planforms are used to systematically investigate the factors which encourage or inhibit this topographic style. The shape of the topography that develops on the density interface can be linked to the convection coupling style between the two layers: viscous coupling leads to discrete, steep-sided piles and thermal coupling to a ubiquitous layer with cusp-like peaks. The dependence of the topographic style on the Rayleigh number, density ratio, and volume ratio can largely be explained by considering their effect on the convection coupling style. Consequently, we find that it is crucial for the lower layer to be thin compared with the overlying convection wavelength and have a density contrast of the order 1% in order for discrete, steep-sided piles to develop.

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Section: Resultssupporting

confidence: 91%

Section: Resultssupporting

confidence: 90%

Section: Discussionsupporting

confidence: 90%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Formation of steep‐sided topography from compositionally distinct dense material at the base of the mantle

Youngs¹,

Houseman²

2009

J. Geophys. Res.

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Deep Earth Structure: Lower Mantle and D″

Lay

2015

Treatise on Geophysics

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Detection of a tall ridge at the core–mantle boundary from scattered PKP energy

Frost

Rost

Selby

et al. 2013

Geophysical Journal International

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The seismic wavefield mainly contains reflected, refracted and direct waves but energy related to elastic scattering can also be identified at frequencies of 1 Hz and higher. The scattered, highfrequency seismic wavefield contains information on the small-scale structure of the Earth's crust, mantle and core. Due to the high thermal conductivity of mantle materials causing rapid dissipation of thermal anomalies, the Earth's small-scale structure most likely reveals details of the composition of the interior, and, is therefore essential for our understanding of the dynamics and evolution of the Earth. Using specific ray configurations we can identify scattered energy originating in the lower mantle and under certain circumstances locate its point of origin in the Earth allowing further insight into the structure of the lowermost mantle. Here we present evidence, from scattered PKP waves, for a heterogeneous structure at the core-mantle boundary (CMB) beneath southern Africa. The structure rises approximately 80 km above the CMB and is located at the eastern edge of the African LLSVP. Mining-related and tectonic seismic events in South Africa, with m b from 3.2 to 6.0 recorded at epicentral distances of 119.3 • to 138.8 • from Yellowknife Array (YKA) (Canada), show large amplitude precursors to PKP df arriving 3-15 s prior to the main phase. We use array processing to measure slowness and backazimuth of the scattered energy and determine the scatterer location in the deep Earth. To improve the resolution of the slowness vector at the medium aperture YKA we present a new application of the F-statistic. The high-resolution slowness and backazimuth measurements indicate scattering from a structure up to 80 km tall at the CMB with lateral dimensions of at least 1200 km by 300 km, at the edge of the African Large Low Shear Velocity Province. The forward scattering nature of the PKP probe indicates that this is velocity-type scattering resulting primarily from changes in elastic parameters. The PKP scattering data are in agreement with dynamically supported dense material related to the Large Low Shear Velocity Province.

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Topography on the D′′ region from analysis of a thin dense layer beneath a convecting cell

Cited by 3 publications

References 36 publications

Formation of steep‐sided topography from compositionally distinct dense material at the base of the mantle

Formation of steep‐sided topography from compositionally distinct dense material at the base of the mantle

Deep Earth Structure: Lower Mantle and D″

Detection of a tall ridge at the core–mantle boundary from scattered PKP energy

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