Australian Antarctic Discordance as a simple mantle boundary

Abstract

Several complex models require unique mantle conditions to explain the Australian Antarctic Discordance (AAD), an unusually deep and rugged section of the Southeast Indian Ridge (SEIR) between ∼120°–128°E. Seismic evidence suggests the AAD is instead the manifestation of two contrasting mantle domains converging along its eastern edge. Variations in axial morphology and flanking topographic relief along the SEIR arise as ridge segments to the west (Indian mantle) grade into a cooler melting regime while those to the east (Pacific mantle) are more magmatically robust. Seismic refraction data show crustal thickness decreases from the west into the AAD at a rate of 0.1 km/100 km, then rapidly increases from 4.8 ± 0.4 km to 7.3 ± 0.2 km across the eastern border. The AAD thus appears to be the terminal end of a long-wavelength reduction in melt supply at what may be the simplest global example of a mantle boundary.