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  • GEOPHYSICS  (5)
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  • 1
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    Geoid anomalies over two South Atlantic fracture zones (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Seasat altimetry profiles across the Falkland-Agulhas and Ascension fracture zones were examined for evidence of step-like geoid offsets predicted from thermal modeling of the lithosphere. Neither the plate nor the half-space model of lithospheric thermal evolution succeeds in predicting the variations in step height with age across the fracture zone branches considered. For ages less than about 30 Ma, the step offsets decrease in a manner suggesting a plate with a thickness of 50-75 km. At greater ages, the offsets show complex behavior that may be due to bathymetric features adjacent to the fracture zones. Similar geoid patterns on opposite branches of the Falkland-Agulhas fracture zone are indicative of processes that act symmetrically on both sides of the Mid-Atlantic Ridge. This is consistent both with small-scale convection occurring beneath the lithosphere and with bathymetric features originally produced along the ridge crest and now located symmetrically on opposite sides of the ridge. The west flank of the Ascension fracture zone displays a regrowth in step height at about 40 Ma consistent with small-scale convection and in agreement with other studies of Pacific and South Atlantic fracture zones.
    Keywords: GEOPHYSICS
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 100; 3-Jan
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  • 2
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    The effect of a shallow low-viscosity zone on the mantle flow, the geoid anomalies and the geoid and depth-age relationships at fracture zones (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A two-dimensional FEM is used to investigate the flow driven by the horizontal temperature gradient at a fracture zone and to calculate the resulting geoid and topography anomalies. Using a three-layered viscosity structure for the upper mantle, results are presented for the effects of varying: (1) the viscosity contrast between the fluid layers: (2) the Rayleigh number based on the viscosity of the bottom layer; and (3) the thickness of the low-viscosity channel. Good agreement is obtained with the results of geoid anomalies over the Udintsev fracture zone when the viscosity of the top layer is greater than one order of magnitude less than post-glacial rebound values.
    Keywords: GEOPHYSICS
    Type: Geophysical Journal (ISSN 0952-4592); 93; 25-43
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  • 3
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    Dynamic topography and gravity anomalies for fluid layers whose viscosity varies exponentially with depth (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Adopting the formalism of Parsons and Daly (1983), analytical integral equations (Green's function integrals) are derived which relate gravity anomalies and dynamic boundary topography with temperature as a function of wavenumber for a fluid layer whose viscosity varies exponentially with depth. In the earth, such a viscosity profile may be found in the asthenosphere, where the large thermal gradient leads to exponential decrease of viscosity with depth, the effects of a pressure increase being small in comparison. It is shown that, when viscosity varies rapidly, topography kernels for both the surface and bottom boundaries (and hence the gravity kernel) are strongly affected at all wavelengths.
    Keywords: GEOPHYSICS
    Type: Geophysical Journal (ISSN 0016-8009); 90; 349-368
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  • 4
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    The effect of a shallow low viscosity zone on the apparent compensation of midplate swells (1987)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A model for midplate swells is assumed where the swell topography is dynamically maintained by convection beneath a conducting lid. Using an idealized viscosity structure consisting of a low viscosity layer underneath the lid and overlying a constant viscosity region, the effect of the low viscosities, the layer thickness, and the Rayleigh number on the flow and on the inference of a compensation mechanism from the associated gravity, geoid, and topography anomalies is investigated. The calculations show that a simple plume model with stratified viscosity variations of less than two orders of magnitude near the surface can produce the range of depths of compensation seen at midplate swells. Shallow depths of compensation, therefore, cannot be used to argue against dynamic models for the formation of hot spot swells.
    Keywords: GEOPHYSICS
    Type: Earth and Planetary Science Letters (ISSN 0012-821X); 82; 3-4
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  • 5
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    Effect of a shallow low-viscosity zone on small-scale instabilities under the cooling oceanic plates (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The effects of a low-viscosity zone in the uppermost mantle on instabilities in the cooling oceanic plates are discussed. The viscosity structure in the upper mantle is approximated by three layers consisting of a conducting lid overlying the low-viscosity zone, which in turn overlies a constant viscosity layer extending to the base of the upper mantle. It is found that this model can reproduce both the small-scale undulations and the flattening of the depth-age and heat flow-age curves.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 93; 3469-347
    Format: text
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