AAD EXPERIMENT

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The AAD experiment was a seismic refraction experiment conducted in November & December of 1994 on the R/V Melville.  It was designed to measure on and off-axis crustal thickness/structure within the Australian Antarctic Discordance (AAD), and the neighboring South East Indian Ridge (SEIR).  The AAD and the SEIR are spreading at the same rate, and yet have contrasting axial morphology.  The AAD has a deep axial valley, while the SEIR has an axial high. The AAD is more than a kilometer deeper than the SEIR, posing many interesting questions.  Four seismic refraction lines were shot - one on axis and one off axis in both areas.  Preliminary analysis indicates that the crust within the AAD is unusually thin, and considerably thinner than at the SEIR.  There appears to be a magma chamber underlying the axis of the SEIR, but not the AAD.