The Australian-Antarctic Discordance (AAD) is the parsent-day manifestation, at a divergent plate boundary, of a region of low gravity and anomalously deep ocean floor between Australia and Antarctica. The AAD also contains the boundary between Pacific- type and Indian-type global-scale isotopic provinces. The spreading axis within the discordance displays an unusual, crenulate offset pattern and, at constant (medium) spreading rate, almost the full spectrum of axial ridge morphologies. Coupled with this morphologic and tectonic diversity is an unusually broad geochemical diversity that appears to require a broad range of primary magma compositions which require, in turn, a broad variation in conditions of melting. The principal investigator led a successful cruise to the area in 1988 and obtained 15 days of SeaMARC II side-scan coverage and 26 new dredge hauls. The objectives of this work are to: determine, in as fine detail as possible, the nature and location of the isotopic boundary; fully define the geochemical and petrologic diversity of AAD lavas and to understand the underlying petrogenetic causes for this diversity in relation to tectonics and MORB genesis in general; and define and evaluate the diversity of lavas along one section of the ridge in terms of distinctively different processes by which they evolved.
