Development of the Australian‐Antarctic depth anomaly

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Abstract
The oceanic Australian‐Antarctic Discordance (AAD) contains two unusual features: (1) N–S trending anomalously deep bathymetries and (2) rough basement morphologies in young (<∼20 Ma) crust between 120°E and 128°E. Models generally attribute AAD formation to underlying cold and/or depleted upper mantle, but no model adequately accounts for all the anomalous attributes. We quantify anomalous basement roughness and basement depths utilizing new seismic reflection data, in combination with all available geophysical and geological observations. We find that the interaction of negative dynamic topography and crustal thickness variations results in the observed complex patterns of residual basement depths. Downwelling, caused by a sinking Mesozoic slab, is the most likely cause of the broad N–S trending residual depth anomalies, while overprinting by westward flowing, buoyant Pacific mantle resulted in the distinctive V‐shaped eastern boundary of the AAD. The particularly large residual depths proximal to the Australian and Antarctic margins may be due to negative dynamic topography combined with thinned oceanic crust caused by ultraslow (<10 mm/yr) half‐spreading rates and sampling of depleted subduction wedge contaminated mantle. Only oceanic basement aged 45° spreading obliquities.

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