HR: 08:50h
AN: T41H-04    [Abstracts]
TI: Recycling the lid: The influence of subduction and stirring on convection in a fluid with a temperature-dependent viscosity with applications to planetary mantle convection
AU: * Thayalan, V
EM: vid.thayalan@utoronto.ca
AF: Universty of Toronto, Department of Physics University of Toronto 60 St. George Street, Toronto, ON M5S1A7 Canada
AU: Jellinek, M
EM: markj@physics.utoronto.ca
AF: Universty of Toronto, Department of Physics University of Toronto 60 St. George Street, Toronto, ON M5S1A7 Canada
AU: Lenardic, A
EM: adrian@geophysics.rice.edu
AF: Rice University, Department of Earth Science Rice University 6100 Main Street, Houston, TX 77005-1892 United States
AB: We use two-dimensional numerical simulations to study the dynamics and steady-state heat transfer properties of ${\rm Rayleigh-B\'enard}$ convection with additional large-scale stirring imposed externally from above in a fluid with a temperature-dependent viscosity. Our results show that the forced subduction and stirring of an otherwise stagnant cold boundary layer (i.e. "stagnant lid") influences both upper and lower boundary layer dynamics as well as the global heat transfer properties of the flow. The specific nature of the effects depends on the imposed velocity, $V$, the total viscosity ratio, $\lambda_t$, and the mechanical boundary conditions in the system. Quantitatively, Nu increases from the stagnant lid value with $V$, but the nature and magnitude of the increase depends strongly on $\lambda_t$. In addition, for a given $V > 0$ and $\lambda_t$, we find that the average thickness of the hot lower boundary layer, $\delta$, depends on the hot boundary viscosity ratio, $\lambda_h$. As $ \lambda_h$ is increased from 1 to around 10, horizontal flow in the thermal boundary layer causes $\delta$ to decrease from critical to a minimum thickness that depends on $\lambda_t$. Plumes are suppressed and heat transfer is due to large-scale flow. As $ \lambda_h$ is increased from order 10 to $10^3$, however, the hot boundary layer becomes viscously decoupled from the overlying large-scale flow and $\delta$ increases to the critical thickness for the lower boundary layer to reach local marginal stability. Low viscosity cavity plumes form as a result and are responsible for the majority of the heat transfer from the hot boundary. Our results are applied to understand mantle convection in the presence of steady or episodic plate tectonics characteristic of the Earth and possibly Venus. This application suggests that the morphology and dynamics of mantle upwellings on Earth may depend on the presence of plate subduction and that the dynamics and morphology of upwellings on Venus may have been time- and space-variable, as a result of episodic subduction.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8125 Evolution of the Earth
DE: 8130 Heat generation and transport
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting