Use of a viscous model of till rheology to describe gravitational loading instabilities in glacial sediments
Published:January 01, 2000
Richard C. A. Hindmarsh, Kenneth F. Rijsdijk, 2000. "Use of a viscous model of till rheology to describe gravitational loading instabilities in glacial sediments", Deformation of Glacial Materials, Alex J. Maltman, Bryn Hubbard, Michael J. Hambrey
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This paper models the operation of loading (Rayleigh–Taylor) instabilities in sediments using an effective-pressure-dependent viscosity such has been used to model the deformation of sediment beneath glaciers. A particular feature is a strong increase of viscosity with depth, resulting from the fact that the effective pressure increases with depth.
Observations suggest that more than one wavelength is generally present (e.g. diapirism and loadcasting) which requires at least three layers with uniform properties to be present. Three layers permit wavelength growth maxima at two distinct wavelengths. We investigate whether an effective-pressure dependent rheology is consistent with RT instabilities, and whether the non-uniformity it produces is able to increase the number of growth-rate maxima.
The investigation starts from the point where sediment in an underlying layer is less dense than the overlying sediment, and the Rayleigh–Taylor instability starts to operate. The mechanics of two layers of finite thickness but infinite extent are modelled by the Stokes equations. The equation set is linearized, and the Fourier transform taken in order to describe the periodic horizontal variation of flow fields at a specified wavelength.
The influences of layer thickness and viscosity ratio on the flow fields are considered. It is found that, for a given wavelength, layer thickness has a far stronger influence on flow fields than does viscosity ratio. For all configurations inspected, the dependence of growth rate on wavenumber exhibited one maximum, meaning that a variable viscosity model does not produce multiple wavelengths. Maximum growth rates occur at wavelengths corresponding to the layer thicknesses.
We infer that loading instabilities occurring at wavelengths around the layer thicknesses are consistent with the effective-pressure-dependent viscous model.
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Deformation of Glacial Materials
The flow of glacier ice can produce structures that are striking and beautiful. Associated sediments, too, can develop spectacular deformation structures, and examples are remarkbly well preserved in Quaternary deposits. Although such features have long been recognized, they are now the subject of new attention from glaciologists and glacial geologists.
This collection of papers addresses how the methods for unravelling deformation structures evolved in recent years by structural geologists can be used for glacial materials, and the opportunities offered to structural geologists by glacial materials for studying deformation in rocks.