GENERATION OF FAST TIMESCALE PHENOMENA IN THERMOMECHANICAL PROCESSES

We have studied thermo-mechanical mechanisms for producing fast timesc ale geological processes, A two-dimensional time-dependent convection model with the extended-Boussinesq approximation has been used in whic h both viscous and adiabatic heating are included. Both non-Newtonian and Newtonian temperature-and depth-dependent rheologies with a depth- dependent thermal expansivity have been considered. A fourth-order acc urate scheme has been used with a vertical grid spacing as fine as 3 k m being imposed in the upper portion of the mantle, and a horizontal g rid spacing of around 10 km. Non-Newtonian rheology precipitates the d evelopment of very fast upwellings with large amounts of attendant vis cous heating and high surface heat flow. Thermal instabilities with fa st timescales occur near the surface during the plume impingement. The growth times of these instabilities are found to decrease significant ly with the power-law index n and the convective vigor, and increase w ith larger surface dissipation number. For n=3 the characteristic time scales are on the order of 1 Myr. Instabilities produced with Newtonia n rheology occur over longer timescales. Non-Newtonian rheology also e nhances the production of viscous heating to a magnitude which can be 10(4) times greater than that for chondritic heating. These results su ggest that rapid geological events, less than 1 Myr, can be achieved w ith a non-Newtonian, temperature-dependent rheology by means of a posi tive thermo-mechanical feedback. (C) 1997 Elsevier Science B.V.