Dynamics of strongly time-dependent convection with non-Newtonian temperature-dependent viscosity

Numerical simulations of time-dependent Boussinesq convection with a non-Newtonian temperature-dependent rheology have been conducted in wide aspect-ratio boxes for effective Rayleigh numbers, Ra_v, ranging from 3.5 × 10⁴ to 1.3 × 10⁶. 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 viscosity. For viscosity contrasts owing to temperature of O(10²), lateral viscosity contrasts generated by the flow are typically of O(10³) in the interior with contrasts as large as O(10⁶) in the upper boundary layer. The horizontal Fourier spectra of the temperature field and of the inverse of the viscosity field show that the asymmetry in the flow introduced by a temperature-dependent viscosity is weaker for a non-Newtonian rheology than for a Newtonian rheology. Lithospheric dynamics, such as lubrication of subduction and lithospheric thinning, are greatly facilitated by the effects of stress-softening from non-Newtonian rheology. Weak plumes are slowed down drastically in the middle of their ascent because of the stress-dependent nature of the rheology. Downgoing slabs can be found to be weakened near the middle, and detachment involving pieces of the descending material may take place. The mobility of the stiff upper boundary layer is facilitated by an increase in the vigor of convection, whereas an increase in the viscosity contrast owing to temperature retards the surface mobility.