HR: 14:40h
AN: T32C-05    [PDF]
TI: Continents And Mantle Convection: Insulation And Large Scale Flow
AU: * Grigne, C
EM: grigne@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, place Jussieu, Paris Cedex 05, 75252 France
AU: * Grigne, C
EM: grigne@ipgp.jussieu.fr
AF: Department of Earth and Space Sciences, University of California, Los Angeles 595 Charles Young Drive East Box 951567, Los Angeles, CA 90095-1567 United States
AU: Labrosse, S
EM: labrosse@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, place Jussieu, Paris Cedex 05, 75252 France
AU: Tackley, P
EM: ptackley@ess.ucla.edu
AF: Department of Earth and Space Sciences, University of California, Los Angeles 595 Charles Young Drive East Box 951567, Los Angeles, CA 90095-1567 United States
AB: Continents can be seen as passive rafts at the surface of the mantle, while the oceanic lithosphere is continuously recycled through subduction and can be considered as the upper thermal boundary layer for mantle convection. Continents however participate into mantle convection by imposing specific thermal and mechanical boundary conditions. Estimates of mantle heat flow under continental shields can be as low as 10~mW.m$^{-2}$, while the mean heat flow under oceans is 100~mW.m$^{-2}$, indicating that continents are thermal insulators for mantle heat loss. Two-dimensional numerical experiments of mantle convection are carried out to study this thermal blanketing effect. Rigid conductive lids are set on top of an isoviscous fluid. These lids which represent continents induce a special pattern of convection, with a set of hot plumes under the lid feeding a large cellular circulation. We show that continental lithospheres, with an estimated thickness between 100 and 400~km, induce a very strong insulating effect that requires mantle heat flow under continents to be less than 25% of the mean oceanic heat flow. A heat flow scaling is proposed, which gives the mean heat flow on the whole mantle surface, the mean oceanic heat flow and the mantle heat flow under the continent, as a function of the Rayleigh number of the mantle, of the width of the convective cells generated by the continent, and of the continent width, thickness and thermal conductivity. The insulating effect of continents on Earth cooling can be ascribed as much on the fact that a low heat flow is the prevailing boundary condition at the base of the continent, as on the long wavelength pattern of convection induced by the continents. It is shown that mantle heat loss cannot be understood without a full description of the wavelength of mantle flow, and therefore both continents and oceanic lithospheric plates are to be considered.
DE: 5418 Heat flow
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8130 Heat generation and transport
SC: Tectonophysics [T]
MN: 2003 Fall Meeting