Three-dimensional structure of asthenospheric flow beneath the Southeast Indian Ridge

Both geophysical and geochemical evidence suggests the presence of along-axis asthenospheric flow toward the Australian-Antarctic Discordance (AAD) beneath the Southeast Indian Ridge (SEIR). We use a three-dimensional, finite-volume formulation of viscous flow to investigate the structure of asthenospheric motion beneath the SEIR. Our results show that simple continental separation in either a constant- or variable-viscosity mantle without horizontal temperature gradients is unable to reproduce the inferred asthenospheric flow velocities and observed geographic distribution of the ''Indian'' and ''Pacific'' upper mantle isotopic provinces. The presence of a cooler, more viscous mantle directly beneath the AAD is necessary to reproduce observed constraints. High viscosities beneath the AAD induce significant along-axis flow beneath the neighboring SEIR that advects warmer material over the cooler, more viscous mantle. In passive flow models, a temperature anomaly of about 300 degrees C at a 400-km depth is required. Simulations that include the effects of buoyancy forces reduce the required temperature anomaly to 100 degrees-200 degrees C, a result in good agreement with other estimates of the regional temperature anomaly. These models also match observed near-axis variations in residual depth and crustal thickness. In bath passive and buoyant simulations, the presence of high-viscosity (cooler) upper mantle beneath the AAD results in reduced upwelling, consistent with low extents of decompressional melting inferred from geochemical and geophysical constraints. Along-axis flow acts to subdue temperature variations within the melting region relative to the deeper mantle and results in a temperature inversion in the subaxial asthenosphere. This effect may also reduce the variations in geochemical parameters such as Na-8.0 and Fe-8.0 with axial depth below those observed in global correlations.