Convective Instability of a Thickened Convecting Boundary Layer (and Thickened Lithosphere)

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9725648 Molnar The P. I. and collaborators will carry out an investigation of convective instability of a mechanically thickened boundary layer, with the goal of understanding processes that might occur beneath mountain belts when crust, and presumably mantle lithosphere too, is thickened during mountain building. They will perform numerical experiments with fluids in 2 and 3 dimensions, with depth- and temperature-dependent viscosity and with non- linear viscosity. They seek simple scaling laws that will allow a transfer of experimental results with temperature-dependent and strain-rate-dependent viscosity to the range of plausible conditions in the earth. Three-dimensional calculations will concentrate on determining how planforms depend on magnitudes and distributions of perturbations to the boundary layers of fluids with depth-dependent viscosity and later temperature-dependent viscosity, both with and without moving top boundaries. Two- dimensional calculations will focus on the effect of non-linear viscosity on growth rates of instabilities and on magnitudes of thinning of the boundary layer. It is anticipated that they will develop scaling laws that relate magnitudes of perturbations and rheological structure to growth rates and magnitudes of lithospheric thinning that can relate observed geologic structures and measured timing of processes beneath mountain belts. ***