DYNAMICS OF STRONGLY TIME-DEPENDENT CONVECTION WITH NON-NEWTONIAN TEMPERATURE-DEPENDENT VISCOSITY

Numerical simulations of time-dependent Boussinesq convection with a n on-Newtonian temperature-dependent rheology have been conducted in wid e aspect-ratio boxes for effective Rayleigh numbers, Ra-v, ranging fro m 3.5 x 10(4) to 1.3 x 10(6). The transition to the chaotic regime for non-Newtonian temperature-dependent viscosity takes place at Ra-v an order of magnitude lower than for purely temperature-dependent viscosi ty. For viscosity contrasts owing to temperature of O(10(2)), lateral viscosity contrasts generated by the flow are typically of O(10(3)) in the interior with contrasts as large as O(10(6)) in the upper boundar y layer. The horizontal Fourier spectra of the temperature field and o f the inverse of the viscosity field show that the asymmetry in the fl ow introduced by a temperature-dependent viscosity is weaker for a non -Newtonian rheology than for a Newtonian theology. Lithospheric dynami cs, such as lubrication of subduction and lithospheric thinning, are g reatly facilitated by the effects of stress-softening from non-Newtoni an theology. Weak plumes are slowed down drastically in the middle of their ascent because of the stress-dependent nature of the theology. D owngoing slabs can be found to be weakened near the middle, and detach ment involving pieces of the descending material may take place. The m obility of the stiff upper boundary layer is facilitated by an increas e in the vigor of convection, whereas an increase in the viscosity con trast owing to temperature retards the surface mobility.