HR: 0830h
AN: T41C-0234    [PDF]
TI: 3-D numerical simulations of mantle flow beneath mid-ocean ridges
AU: * morency, c
EM: morency@geologie.ens.fr
AF: ENS-Dpt T.A.O. Labo de Geologie, 24 rue Lhomond, Paris, 75231 France
AU: Doin, M
EM: doin@geologie.ens.fr
AF: ENS-Dpt T.A.O. Labo de Geologie, 24 rue Lhomond, Paris, 75231 France
AU: Dumoulin, C
EM: caroline.dumoulin@chimie.univ-nantes.fr
AF: Universite de Nantes, Labo de Planetologie et Geodynamique, 2 rue de la Houssiniere, Nantes, 44322 France
AB: Small-scale perturbations in the oceanic geoid aligned with the seafloor spreading direction have been observed in the Pacific Ocean. One explanation is to attribute these anomalies to small-scale convection.\\ 3-D analogical and 2-D numerical experiments of the cooling of an oceanic lithosphere show that, at young ages, the material flows at the base of the lithosphere along the isotherms topography of the lower part of the lithosphere. After 20 to 70~Myrs, cold downgoing instabilities develop at the base of the lithosphere. Small-scale convection can then be observed superimposed on the large-scale circulation. With a strongly temperature and pressure dependent viscosity, large wavelength topography, thermally stable, is observed at the base of the lithosphere.\\ We perform 3-D numerical simulations using a Newtonian rheology with a viscosity depending strongly on temperature in a cartesian box. To avoid any influence of the boundary conditions on the convective cells evolution, we use an aspect ratio of 6x3x1. A constant horizontal velocity of 2 or 4~cm/yr is applied at the surface of the box to mimick the plate motion. Two kinds of ridge geometry have been modelled: (A) a ridge perpendicular to the plate motion, and (B) a ridge with transform faults along the spreading center with a mean ridge orientation strongly oblique to the plate motion.\\ Different 3-D flow geometries have been observed. For case (A), we first observed a series of downgoing instabilities parallel to the ridge axis at young ages and then a series of downgoing instabilities nearly perpendicular to the ridge axis. At old ages, an irregular small-scale flow dominates. But this flow pattern is time-dependent, since for a caracteristic time greater than 200~Myrs, depending on the surface velocity value, ridge-perpendicular downgoing instabilities become the preferred form. For case (B), the flow at the base of the lithosphere appears nearly perpendicular to the mean orientation ridge and is oblique to the plate velocity direction. Morever downgoing instabilities initiating at each transform fault then become perpendicular to the mean ridge axis. The thermal state does not evolve with time, contrarily to case (A).\\ We then compare some variables issued from cases (A) and (B) like the heat flow evolution with time, the onset time of instabilities at the base of the lithosphere, and the main strain direction generated by the flow. The latter could explain oceanic surface-wave velocities anisotropy showing fast velocity directions perpendicular to the ridge axis, which does not correlate simply with plate motion direction.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Tectonophysics [T]
MN: 2003 Fall Meeting