Development of the Australian‐Antarctic depth anomaly

• Published in: Geochemistry, Geophysics, Geosystems
• Main Authors: Whittaker, J, Muller, RD, Gurnis, M
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Unoin 2010
• Subjects: Earth Sciences; Geology; Marine Geoscience; Antarctic; Australian-Antarctic Discordance; Pacific; Antarc*
• Online Access: https://doi.org/10.1029/2010GC003276; http://ecite.utas.edu.au/84437

The oceanic Australian‐Antarctic Discordance (AAD) contains two unusual features: (1) NS trendinganomalously deep bathymetries and (2) rough basement morphologies in young (<∼20 Ma) crust between120E and 128E. Models generally attribute AAD formation to underlying cold and/or depleted uppermantle, but no model adequately accounts for all the anomalous attributes. We quantify anomalous basementroughness and basement depths utilizing new seismic reflection data, in combination with all available geophysicaland geological observations. We find that the interaction of negative dynamic topography andcrustal 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 NS trending residualdepth anomalies, while overprinting by westward flowing, buoyant Pacific mantle resulted in the distinctiveV‐shaped eastern boundary of the AAD. The particularly large residual depths proximal to the Australianand Antarctic margins may be due to negative dynamic topography combined with thinned oceanic crustcaused by ultraslow (<10 mm/yr) half‐spreading rates and sampling of depleted subduction wedge contaminatedmantle. 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 controlledthreshold of accretion‐related roughness had already been reached due to ultraslow spreading rates. Ouranalysis reveals that the enigmatic roughness of the Diamantina Zone is mainly related to >45 spreadingobliquities.