| Summary: | The axial morphology of the Southeast Indian Ridge (SEIR) between Australia and Antarctica changes dramatically along 120 o E to 127 o E (Fig. 1). At approximately 127 o E the ridge changes character along with a difference in water depth of about 1 km. Eastward it changes from a Mid-Atlantic Ridge (MAR) type cross-section to an East-Pacific Rise (EPR) type cross-section (Fig. 2). The MAR profiles a bathymetric low (5- to 20-km wide, 500- to 1500-m-deep rift valley), while the EPR profiles a bathymetric high (10-km-wide, 500-m-high ridge). The MAR type extends westward to approximately 100 o E and is called the Australian-Antarctic Discordance (AAD). This geomorphology is unique globally. The MAR is considered a slow-spreading center and like the AAD has similar segmentation characteristics of non-transform (small, <10 km) and transform discontinuous partitions of the ridge at 40 to 60 km. In contrast, the SEIR east of the AAD has segmentation characteristics of a fast-spreading center like the EPR. There are no transforms until 138 o E, and westward propagating rifts are the only non-transform discontinuities.
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