HR: 1340h
AN: T33A-1134    [Abstracts]
TI: The growth of Rayleigh-Taylor instability under a shear-stress free top boundary condition
AU: * Harig, C
EM: christopher.harig@colorado.edu
AF: University of Colorado Department of Geological Sciences, Campus Box 399, Boulder, CO 80309, United States
AU: Molnar, P
EM: Peter.Molnar@colorado.edu
AF: University of Colorado Department of Geological Sciences and Cooperative Institute for Research in Environmental Sciences (CIRES), Campus Box 399, Boulder, CO 80309,
AU: Houseman, G
EM: g.houseman@see.leeds.ac.uk
AF: University of Leeds, School of Earth and Environment, Leeds, LS2 9JT, United Kingdom
AB: The separation of zones of apparent downwelling flow at the ends of the Sierra Nevada suggest a relatively large wavelength (~500km), but Rayleigh-Taylor instability for plausible rheological structures with a fixed top boundary condition require much shorter wavelength (<100km). To understand this difference we perform analytical and numerical plane-strain experiments on the stability of a dense layer overlying a less dense substratum, representing the Rayleigh-Taylor instability between the mantle lithosphere and the underlying asthenosphere, focusing on the effects of a shear-stress free boundary condition at the top. The overall effect of this condition is an enhancement of growth rate factors at long wavelengths. The enhancement depends greatly on the viscosity variation of the layer, and less so on the density distribution and viscosity of the substratum. With large variation, the viscosity at the top of the layer will be great enough that the solution approximates that with a fixed top boundary condition. With little variation, the solution approximates that for constant viscosity, which has also been shown to stifle growth at long wavelengths. The intermediate zone showing the enhancement includes ratios of layer thickness to viscosity e-folding length, h/L, of ~ 1 - 8 in Newtonian viscosity experiments, and ~ 1 - 4 in non-linear viscosity cases. The free top condition is likely to be applicable to many geologic situations where the lower crust is weak. Olivine flow laws and a gentle temperature gradient (≈ 5° C/km) place the Sierra Nevada viscosity scaling length, L, at the upper limit of this range of h/L values. Thus longer wavelengths than commonly assumed for Rayleigh-Taylor instabilities seems permissible when viscosity decreases with depth and the top surface of the layer is only weakly constrained.
DE: 8110 Continental tectonics: general (0905)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting