The Australian-Antarctic Discordance (AAD) is an anomalously deep region centered on the Southeast Indian Ridge (SEIR) between Australia and Antarctica. Among its unique features is an unusually sharp boundary between the ocean-basin scale upper mantle isotopic domains of the Pacific and Indian Oceans. This boundary has migrated westward into and across the easternmost segment of the AAD at a rate of 25-40 mm/yr during the last 4 m.y., yet the long-term relationship of this important boundary to the AAD remains unclear. There is limited evidence to suggest that the boundary has been migrating westward for ~40 m.y., since the separation of the South Tasman Rise from Antarctica. However, it seems likely, perhaps even probable, that the isotopic boundary is genetically linked to the mantle processes that have maintained the existence of the AAD for >90 m.y., since Australia and Antarctica first rifted apart.

The long-term configuration and dynamic history of the isotopic boundary can be determined by systematic off-axis sampling beyond the limit of effective dredging (~7 Ma). During Leg 187, we will extend the sampling program to older crust (10-30 Ma). An array of 19 drill sites has been designed to determine the configuration of the isotopic boundary and to distinguish among competing hypotheses concerning the nature and extent of mantle migration beneath the SEIR. Approximately 10-12 single-bit holes will sample 20-100 m (ideally about 50 m) into igneous basement. A reactive drilling strategy will allow the selection of later sites within a few hours of core recovery on the basis of trace element data obtained from the earlier sites.

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