Abstract
The effect of lateral variations in the sub-lithospheric viscosity, i.e. in the lithosphere-asthenosphere coupling, is incorporated in dynamic earth models driven by plate velocities. If the coupling is stronger under fast-moving continents, it is shown that a geoid low will develop in their wake and a high in front of them. Thus the well-known low in the Indian ocean could be explained, at least in part, in terms of induced upper-mantle dynamics. Similarly, if the thickness of oceanic plates increases with age the models show that trenches should be associated with marked geoid highs, whereas ridges could correspond to much weaker geoid lows. This also seems to agree with some features of the observed geoid at very long wavelengths. The mathematical framework of such dynamic earth models is developed extensively here. These models are characterized by lateral viscosity variations inside an outer shell and a purely radial viscosity structure at greater depth. Their internal flow patterns are only driven by imposed surface velocities, not by internal loads. Copyright © 1988, Wiley Blackwell. All rights reserved
Highlights
The long wavelength geoid (A >4000 km) has been well known over both continents and oceans for more than two decades
It has been shown that these deep masses will induce internal flows and deflect the various density interfaces within the Earth, including its outer surface (Hager, Ricard et al 1984)
The oceanic lithospheric slabs dipping into the Earth’s interior around the Pacific Ocean give rise to a well defined belt of dense masses, not easy to resolve by present-day tomographic techniques. This deep pattern correlates strikingly with the geoid of degree 13 4 (Hager 1984). Another meaningful correlation is found between the observed geoid and the divergence of the velocity field defined by plate tectonics, in particular for degree 4 (Forte & Peltier 1987)
Summary
The long wavelength geoid (A >4000 km) has been well known over both continents and oceans for more than two decades. The oceanic lithospheric slabs dipping into the Earth’s interior around the Pacific Ocean give rise to a well defined belt of dense masses, not easy to resolve by present-day tomographic techniques This deep pattern correlates strikingly with the geoid of degree 13 4 (Hager 1984). Another meaningful correlation is found between the observed geoid and the divergence of the velocity field defined by plate tectonics, in particular for degree 4 (Forte & Peltier 1987) This observation was predicted by dynamic earth models (Ricard et al 1984). The second approach takes the observed plate velocities as its starting point It imposes these boundary conditions to the computed internal flow field, and may or may not include known internal density variations.
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