EVOLUTION OF THE INDIAN-OCEAN TRIPLE JUNCTION BETWEEN 65 AND 49 MA (ANOMALIES 28 TO 21)

Reinterpretation of newly published geophysical data (Kamesh-Raju and Ramprasad, 1989) and older profiles of the Central Indian Basin associated with similar studies of the Madagascar and Crozet basins, shows that the Indian Ocean Triple Junction trace on the Indian plate corresponds, at anomalies 23 and 22, to a N38-degrees-E offset of the magnetic lineations, oblique to both the Southeast Indian Ridge (SEIR) and Central Indian Ridge (CIR) spreading directions. The conjugate Triple Junction trace on the African plate identified in the Madagascar Basin is associated with a roughly north-south offset, parallel to the Southwest Indian Ridge (SWIR) fracture zones. In order to account for these observations and the velocity triangle of the Indian, African, and Antarctic plates close to the Triple Junction, a ridge-fault-fault mode is proposed, with a propagatorlike SEIR-CIR offset. The Triple Junction jumped between anomalies 24 and 23 and between anomalies 22 and 21, restoring a ridge-ridge-ridge configuration which immediately turned to a pseudo-ridge-ridge-fault and later to a true ridge-fault-fault configuration. After the Triple Junction jump at anomaly 21, the former SEIR-CIR offset was accommodated by a new CIR fracture zone. The lack of such a fracture zone prior to anomaly 21 suggests that either a pseudo-ridge-ridge-fault or an unstable ridge-ridge-ridge configuration prevailed before anomaly 24, in agreement with the velocity triangles which predict more unstable Triple Junction modes. Both modes support the creation of numerous SWIR fracture zones, presently observed between 52-degrees-30'E and 59-degrees-30'E. as a consequence of the Triple Junction evolution between anomalies 29 and 24. This result suggests that the physiography of the SWIR records the history of the Triple Junction.