EVIDENCE FOR VARIABLE UPPER-MANTLE TEMPERATURE AND CRUSTAL THICKNESS IN AND NEAR THE AUSTRALIAN ANTARCTIC DISCORDANCE

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 characte ristic 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 variation s in the upper mantle beneath the AAD. Through three-dimensional gravi ty analysis, we found that at least two end-member models satisfy the gravity observations: regional crustal thickness variations of at leas t 3 km along the SEIR near the AAD or a temperature anomaly of the ord er of 150 degrees C in the upper mantle beneath the SEIR. These new ob servations, combined with other geophysical and geochemical characteri stics of the Australian-Antarctic Discordance, provide further evidenc e that the temperature structure of a mid-ocean ridge is a controlling factor, in addition to spreading rate, in the crustal accretionary pr ocess. Numerical models of mantle flow beneath mid-ocean ridges offer one means of investigating the dynamic effect of a variable upper mant le temperature on the accretionary process. Our results indicate that temperature is important, especially at intermediate and slower spread ing rates, where thermal effects can dominate mantle flow beneath a mi d-ocean ridge and result in increasing crustal production with decreas ing 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 bein g equal. Thus, combined with our end-member gravity models, these obse rvations and results suggest that both anomalously cool upper mantle a nd thin crust exist beneath the AAD.