The Australian-Antarctic discordance is a region of anomalous geophysical and geochemical properties along the mid-ocean ridge system. It includes the isotopic boundary between Pacific Ocean and Indian Ocean basalts. Its lavas have compositions consistent with low mantle temperatures and a relatively low overall extent of melting. These characteristics have been attributed to downward flow in the underlying mantle. New bathymetric and side-scan sonar data show that (1) the spreading axis within the discordance is predominantly characterized by a broad rift valley and segmentation characteristics typical of slow-spreading centers, (2) the isotopic boundary appears to be associated with unusual, chaotic sea floor, and (3) the spreading axis east of the discordance is characterized by an axial ridge typical of fast-spreading centers. These extreme variations, at an essentially constant (intermediate) spreading rate are consistent with differences in melt supply and mantle properties along the spreading axis within and east of the discordance, as suggested in previous studies.

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