HR: 1340h
AN: T33A-1354    [Abstracts]
TI: 3-D Numerical Simulations of Mantle Flow Beneath Mid-Ocean Ridges
AU: * Morency, C
EM: christina.morency@univ-rennes1.fr
AF: G‚osciences Rennes, Equipe LithosphŠre, 263 Avenue du G‚n‚ral Leclerc, CS 74205, Rennes, 35042 France
AU: Doin, M
EM: doin@geologie.ens.fr
AF: Ecole Normale Sup‚rieure, Laboratoire de G‚ologie, 24 Rue Lhomond, Paris, 75231 France
AU: Dumoulin, C
EM: caroline.dumoulin@chimie.univ-nantes.fr
AF: Universit‚ de Nantes, Laboratoire de Plan‚tologie et de G‚odynamique, 2 Rue de la HoussiniŠre, B.P. 92208, Nantes, 44322 France
AB: Small-scale perturbations of the geoid lined up in the seafloor spreading direction have been observed in the Pacific Ocean. These anomalies may possibly originate in small-scale convection. We perform 3-D numerical simulations of the cooling of an oceanic lithosphere above a convective mantle to test this explanation. We use a Newtonian rheology with a viscosity depending strongly on temperature. The cartesian box has an aspect ratio of 6x3x1 (or 12x6x1). A constant horizontal velocity (half spreading velocity) of 2 (or 4) cm/yr is applied at the surface of the box to mimick plate motion. In all cases, due to the temperature dependence of the viscosity, a rigid conductive lithosphere develops beneath the two plates separated by the ridge. After 16 to 40~Myrs, cold downgoing instabilities develop at the base of the lithosphere. Small-scale convection can then be observed superimposed on the large-scale circulation. It produces, and then interacts with, a slowly evolving short wavelength isotherms topography at the base of the lithosphere. Three kinds of ridge geometry are modelled: (A) a ridge perpendicular to the plate motion, (B) a ridge strongly oblique to the plate motion, and (C) a ridge with transform faults along the spreading center with a mean ridge orientation strongly oblique to the plate motion. All simulations reveal complex interaction between the isotherm topography within the base of the lithosphere, the small-scale flow generated at or just below the base of the lithosphere, and the large-scale flow dominating in the mantle below. The large-scale flow appears always controlled by the mean ridge axis direction, and thus may be oblique to the imposed plate motion direction. In order to obtain a very crude quantification of the anisotropy orientation generated by the modelled mantle flow, we have evaluated the average horizontal shear direction. We thus conclude that surface wave tomography in the uppermost mantle should average small-scale, possibly complex, features associated to the geometry of the developping thermal boundary layer instabilities, plus a large-scale pattern that derives from a combination between shearing induced by plate motion and an internally driven flow.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
DE: 1213 Earth's interior--dynamics (8115, 8120)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting