HR: 14:55h
AN: T13D-06    [Abstracts]
TI: Effects of a Low Viscosity Zone on Plate Tectonics and Mantle Convection in the Terrestrial Planets: A Physical Theory and Numerical Experiments
AU: * Richards, M A
EM: markr@seismo.berkeley.edu
AF: Dept. of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Lenardic, A
EM: adrian@esci.rice.edu
AF: Dept. of Earth Science, Rice University, Houston, TX 77005 United States
AU: Busse, F H
EM: Friedrich.Busse@uni-bayreuth.de
AF: Institue of Physics, University of Bayreuth, Bayreuth, 95440 Germany
AB: A variety of geophysical evidence suggests that there may be a relatively thin zone of low viscosity at the base of the Earth's lithosphere, at least in oceanic and active continental regions. Numerical experiments on mantle convection and, more recently, on the self-consistent generation of plates show that the effects of a low viscosity zone (LVZ) are probably very important. An LVZ strongly increases the horizontal lengthscale of convection, and Earth's LVZ may be a key ingredient to the occurrence of plate tectonics. We have developed a simple boundary layer theory that yields considerable physical insight into these effects, and have tested the theory using 2-D numerical simulations. The theory successfully predicts how the preferred horizontal length scale of convection is controlled by the thickness and viscosity contrast of the LVZ (i.e., how Nusselt number varies with length scale.) Surprisingly, the degree of influence an LVZ exerts on the wavelength of convection actually increases with decreasing LVZ thickness, provided that the viscosity contrast is sufficiently strong according to a straightforward criterion that is also derived. An extended version of the theory further predicts that the effects of an LVZ are particularly pronounced if the LVZ is "submerged" beneath a strong lithosphere, a result initially discovered in 2-D numerical simulations and consistent with the apparently profound influence of an LVZ in promoting plate tectonics.
DE: 8147 Planetary interiors (5430, 5724)
DE: 8149 Planetary tectonics (5475)
DE: 8155 Plate motions--general
DE: 8162 Rheology--mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting