HR: 0800h
AN: U41B-0417    [Abstracts]
TI: Formation of Steep-Sided Topography From Compositionally Distinct Dense Material at the Base of the Mantle
AU: Houseman, G A
EM: g.houseman@see.leeds.ac.uk
AF: Earth Sciences, School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, United Kingdom
AU: * Youngs, B A
EM: bryony.youngs@yale.edu
AF: Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, CT 06520-8109, United States
AB: In this study we use 3D, numerical 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 convection in the mantle. Previous authors have found it necessary to introduce compressibility into calculations or employ a large viscosity increase in the lower layer in order for discrete, steep-sided piles of material to form. Our calculations are based on incompressible convection with layers of constant viscosity and we find that, as long as the lower layer is thin compared with the convection wavelength, it is pushed away beneath downwellings leaving steep-sided, flat- topped structures beneath upwelling regions. Experiments involving simple convection planforms allow determination of the factors which encourage or inhibit this topography style. Comparison of the results with previous 2D analytic predictions for thin layer boundary topography enables detailed physical understanding of the deformation process. When convection within the lower layer is coupled viscously with the overlying mantle convection, deformation in the form of discrete, steep- sided piles is observed. Whereas, when the convection systems are coupled thermally, deformation tends to take the form of cusp-like peaks beneath upwelling regions with a relatively small deflection beneath downwelling regions. The dependence of the interface topography style on various system parameters (Rayleigh number, density contrast, initial layer thickness, convection wavelength) can be largely explained by a scaling derived from a balance of buoyancy derived stress with viscous flow stress. The results also explain why the addition of compressibility or a significant viscosity increase in the lower layer has enabled this topography style in previous studies. Calculations with plausible convection planforms are used to estimate parameter ranges which generate interface topography consistent with that observed in seismic studies.
DE: 0545 Modeling (4255)
DE: 3225 Numerical approximations and analysis (4260)
SC: Union [U]
MN: 2007 Fall Meeting