Mantle downwelling beneath the Australian-Antarctic discordance zone: evidence from geoid height versus topography

The Austral ian-Antarct ic discordance zone (AAD) is an anomalously deep and rough segment of the Southeast Indian Ridge between 120 ° and 128°E. A large, negative (deeper than predicted) depth anomaly is centered on the discordance, and a geoid low is evident upon removal of a low-order geoid model and the geoid height-age relation. We investigate two models that may explain these anomalies: a deficiency in ridge-axis magma supply that produces thin oceanic crust (i.e. shallow Airy compensation), and a downwelling a n d / o r cooler mantle beneath the AAD that results in deeper convective-type compensation. To distinguish between these models, we have calculated the ratio of geoid height to topography from the slope of a best line fit by functional analysis (i.e. non-biased linear regression), a method that minimizes both geoid height and topography residuals. Geo id / topography ratios of 2.1 _+ 0.9 m / k m for the entire study area (380-60 ° S, 105° -140 ° E), 2.3 _+ 1.8 m / k m for a subset comprising crust _< 25 Ma, and 2.7 _+ 2.0 m / k m for a smaller area centered on the A A D were obtained. These ratios are significantly larger than predicted for thin oceanic crust (0.4 m / k m ) , and 2.7 m / k i n is consistent with downwelling convection beneath young lithosphere. Average compensation depths of 27, 29, and 34 km, respectively, est imated from these ratios suggest a mantle structure that deepens towards the AAD. The deepest compensation (34 km) of the A A D is below the average depth of the base of the young lithosphere ( 30 kin), and a downwelling of asthenospheric material is implied. The observed geoid height-age slope over the discordance is unusually gradual at -0 .133 m / m . y . We calculate that an upper mantle 170 ° C cooler and 0.02 g / c m 3 denser than normal can explain the shallow slope. Unusual ly fast shear velocities in the upper 200 km of mantle beneath the discordance, and major-element geochemical trends consistent with small amounts of melting at shallow depths, provide strong evidence for cooler temperatures beneath the AAD.