3-DIMENSIONAL STRUCTURE OF ASTHENOSPHERIC FLOW BENEATH THE SOUTHEAST INDIAN RIDGE

Both geophysical and geochemical evidence suggests the presence of alo ng-axis asthenospheric flow toward the Australian-Antarctic Discordanc e (AAD) beneath the Southeast Indian Ridge (SEIR). We use a three-dime nsional, 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-v iscosity mantle without horizontal temperature gradients is unable to reproduce the inferred asthenospheric flow velocities and observed geo graphic distribution of the ''Indian'' and ''Pacific'' upper mantle is otopic provinces. The presence of a cooler, more viscous mantle direct ly beneath the AAD is necessary to reproduce observed constraints. Hig h viscosities beneath the AAD induce significant along-axis flow benea th 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 in clude the effects of buoyancy forces reduce the required temperature a nomaly to 100 degrees-200 degrees C, a result in good agreement with o ther estimates of the regional temperature anomaly. These models also match observed near-axis variations in residual depth and crustal thic kness. In bath passive and buoyant simulations, the presence of high-v iscosity (cooler) upper mantle beneath the AAD results in reduced upwe lling, 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 t he deeper mantle and results in a temperature inversion in the subaxia l asthenosphere. This effect may also reduce the variations in geochem ical parameters such as Na-8.0 and Fe-8.0 with axial depth below those observed in global correlations.