Recent multichannel seismic reflection data from the Labuan Basin, in the Southern Indian Ocean, are used to reevaluate older, single channel data from this region. Together, they throw light on the structure and evolution of this basin, situated between the older than 100 Ma Southern Kerguelen Plateau and the younger than 43 Ma Australian-Antarctic Basin. The Labuan Basin is a deep, extensive basement depression, more than 350,000 km2 in area, located adjacent to the eastern margin of the Southern Kerguelen Plateau. In contrast to the nearby Kerguelen Plateau, no prominent reflectors are observed within the basement of the Labuan Basin. This suggests a different mechanism of formation from that of the Kerguelen Plateau, which is believed to have been formed by thick sequences of large lava flows. The basement surface of most of the Labuan Basin is presently quite rough, as the result of a tectonic event which created prominent tilted block structures and turned it into a large northwest-southeast trending syncline. There is a 1--1.5 km elevation difference between the Labuan Basin and the Australian-Antarctic Basin created at the Southeast Indian Ridge, as well as a large difference in sedimentary thickness between them, indicating that the Labuan Basin is significantly older. The boundary of the Labuan Basin with the Kerguelen Plateau is generally a steep and somewhat linear feature which appears to be of tectonic origin; it seems to result from two extensive tectonic eposides, dated at 96 Ma and 75--68 Ma, associated with the prerift phase of plate breakup between Australia and Antarctica. The basement of the Labuan Basin was created 130--100 Ma ago at about the same time as the Kerguelen Plateau.

An extensive tectonic episode, to the south of the Labuan Basin, initiated the formation of the boundary between the Southern Kerguelen Plateau and the Labuan Basin. This tectonic episode could be linked to the beginning of seafloor spreading between Australia and Antarctica 96 Ma ago. Sediments were regularly deposited in the basin until 75--68 Ma, at which time an important extensional tectonic event occurred. It involved two large northwest-southeast uplifts; one, centered on the Kerguelen Plateau, affected the western part of the Labuan Basin while the other, of which only the western half is observed, affected the eastern part of the Labuan Basin. This tectonic eposide seems to correspond to the prerifting episode leading to the breakup between the Kerguelen Plateau-Labuan Basin and Broken Ridge-Diamantina Zone at 43 Ma. After this tectonic eposode, the sediments of the postuplift megasequence filled topographic lows formed by the normal faults and the tilted blocks. The Diamantina Zone, which prior to 43 Ma was continuous with the Labuan Basin, can be expected to have a similar tectonic history. In particular, the rough basement of the Diamantina Zone can be due to postformation tectonism rather than to slow spreading as previously suggested. ¿ American Geophysical Union 1991