HR: 1340h
AN: T23A-0534    [Abstracts]
TI: The Asthenosphere and the Horizontal Length-Scale of Mantle Convection
AU: * Richards, M
EM: markr@seismo.berkeley.edu
AF: University of California, Department of Earth and Planetary Science, Berkeley, CA 94720 United States
AU: Lenardic, A
EM: adrian@esci.rice.edu
AF: Rice University, Department of Earth Science, MS 126, P.O. Box 1892, Houston, TX 77251 United States
AU: Busse, F H
EM: Friedrich.Busse@uni-bayreuth.de
AF: University of Bayreuth, Institute of Physics, Bayreuth, D95440 Germany
AB: Numerical simulations show that a low viscosity asthenosphere can increase the wavelength of mantle convection. The physical mechanism behind this phenomonon and its robustness with respect to model parameters remain to be fully elucidated. Towards this end, we develop theoretical heat flow scalings for a convecting fluid layer with low viscosity channels. Bottom and internally heated end-members are considered. For the former, the viscosity structure consists of a high viscosity central region bounded from above and below by horizontal low viscosity channels. For internally heated cases, only a surface low viscosity channel is present. Theoretical scalings derived from boundary layer theory show that low viscsoity channels lower the lateral dissipation associated with steady state convective rolls, allowing longer aspect ratio cells to form as the viscosity contrast between the channels and the central region is increased. The maximum cell aspect ratio is estimated from the condition that the pressure gradients that drive lateral flow in the channels do not become so large as to inhibit vertical flow into the channels. Scaling predictions compare favorably to results of numerical simulations for steady state cells. As the Rayleigh number driving convection is increased, time-dependence sets in as small scale boundary layer instabilities. This increases lateral dissipation within the channels and the preferred cell aspect ratio decreases as a result. Internally heated simulations show that a near surface high viscosity layer, an analog to tectonic plates, can suppress these small scale instabilities. This allows a low viscosity channel to maintain large aspect ratio cells for Rayleigh numbers approaching that of the present day Earth.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Tectonophysics [T]
MN: Fall Meeting 2005