Development of the Australian‐Antarctic depth anomaly

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 <20 Ma is anomalously rough, a result of sampling of cool/depleted upper mantle material. Although oceanic crust older than 43 Ma may have sampled depleted mantle, the resulting oceanic basement is not anomalously rough likely because a melt volume controlled threshold of accretion‐related roughness had already been reached due to ultraslow spreading rates. Our analysis reveals that the enigmatic roughness of the Diamantina Zone is mainly related to >45° spreading obliquities.