Short-wavelength isostasy over the South-west Indian Ridge and implications for mid-ocean ridges

Current geophysical analyses of the mechanical behaviour of surface topographic features over the Mid-Atlantic Ridge and East Pacific Rise suggest that they are isostatically compensated. We used shipborne gravity and topography data to investigate the short-wavelength isostasy over the South-west Indian Ridge. From a straight comparison between individual gravity profiles and model predictions, we found elastic thickness values of 0.55 plus or minus 0.40 to 4.45 plus or minus 1.50 km; the Airy crustal thickness was between 7.0 plus or minus 2.2 and 13.0 plus or minus 2.3 km. The very low elastic thickness, together with an Airy crustal thickness several times greater than that determined seismically, suggests that short-wavelength isostasy does not prevail over the ridge. When applied to the same data, cross-spectral techniques led to thicknesses at least one order of magnitude greater than those obtained by fitting predicted to observed profiles. From isostatic studies over mid-ocean ridges and based on the latter techniques, it is shown that the elastic thickness is, in general, independent of spreading rate but biased by the seafloor roughness. A simple direct two-dimensional model of gravity may explain the observations. We conclude that, over mid-ocean ridges, the small-scale topography is probably not in isostatic equilibrium.