HR: 0830h
AN: T41C-0233    [PDF]
TI: On the Small-Scale Morphology of Asthenospheric Flow
AU: * Vidal, V
EM: vidal@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252 France
AU: Davaille, A
EM: davaille@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252 France
AU: Crambes, C
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252 France
AB: We investigated the interaction of small-scale cold instabilities dripping from a cooling lithosphere with a shear flow confined in the asthenosphere, using analog experiments. Rayleigh numbers ranged between $10^4$ and $10^8$. The fluids were either polymer solutions (constant viscosity), sugar or corn syrups (viscosity depending on temperature), or wax (phase transition). When cooling away from the ridge, the thickening lithosphere becomes thermally unstable and develops small-scale convective instabilities at its bottom. For sufficiently fast asthenospheric flow, these instabilities are sheared and remain trapped in the asthenosphere, following a helicoidal path aligned with the direction of plate motion. A phase diagram and scaling laws for the flow characteristics were determined. The observed helicoidal pattern could explain some geophysical observables such as small wavelength lineations in the terrestrial gravity field, or seismic anisotropy anomalies under the Pacific plate. Moreover, the distance from the ridge at which the small-scale instabilities form depends on the underlying mantle temperature: for a hotter mantle, they are generated closer to the ridge. Therefore, in the case of a ridge-centered plume, the hot temperature anomaly due to the plume triggers small-scale instabilities almost at the ridge. The heat transfer out of the mantle is accelerated, and the thickening of the lithosphere away from the ridge is delayed. Therefore, a groove at the bottom of the lithosphere may be expected along the track of a ridge-centred hotspot.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Tectonophysics [T]
MN: 2003 Fall Meeting