Evidence for variable upper mantle temperature and crustal thickness in and near the Australian-Antarctic Discordance

Abstract

The Southeast Indian Ridge (SEIR) in and near the Australian-Antarctic Discordance (AAD) exhibits, at a constant spreading rate, almost the full range of the many geophysical and geochemical parameters characteristic of the ‘slow’ Mid-Atlantic Ridge and ‘fast’ East Pacific Rise. We used satellite-derived gravity data, in combination with SeaMARC II bathymetry in and near the AAD, to examine regional density variations in the upper mantle beneath the AAD. Through three-dimensional gravity analysis, we found that at least two end-member models satisfy the gravity observations: regional crustal thickness variations of at least 3 km along the SEIR near the AAD or a temperature anomaly of the order of 150°C in the upper mantle beneath the SEIR. These new observations, combined with other geophysical and geochemical characteristics of the Australian-Antarctic Discordance, provide further evidence that the temperature structure of a mid-ocean ridge is a controlling factor, in addition to spreading rate, in the crustal accretionary process. Numerical models of mantle flow beneath mid-ocean ridges offer one means of investigating the dynamic effect of a variable upper mantle temperature on the accretionary process. Our results indicate that temperature is important, especially at intermediate and slower spreading rates, where thermal effects can dominate mantle flow beneath a mid-ocean ridge and result in increasing crustal production with decreasing spreading rate. At the constant, intermediate spreading rate of 37 mm/yr, characteristic of the SEIR in and near the AAD, our numerical models show that significant crustal thinning (2–4 km) can occur with relatively small variations in upper mantle temperature, all else being equal. Thus, combined with our end-member gravity models, these observations and results suggest that both anomalously cool upper mantle and thin crust exist beneath the AAD.