Abstract

A lineated sequence of distinctive residual magnetic anomalies has been observed in the southern part of the Central Indian Basin. These anomalies, which strike N45°W, have been identified as anomalies 7 through 15 and were formed by a ridge spreading at a half-rate of 2.5 cm/yr. They terminate abruptly against a N45°E–trending fracture zone between 82° and 85°E. At 27°S, 84°E, the fracture zone appears to change direction abruptly to N10°E. The mean depth drops 250 m across the fracture zone. The lineations are offset right laterally to the south, and the anomalies to the east, north of 27°S, are tentatively identified as 18 through 22, trending N100°E. This change in lineation pattern can be explained only by a reorientation of the direction of the Southeast Indian Ridge from east-west to northwest-southeast between the times of anomalies 18 and 15 (∼43 to 39 m.y. B.P.). This was the same time that India collided with Eurasia; the time correlation may be evidence of a relationship between the formation of the Himalayas and an abrupt change in the direction of relative motion of the Indian and Antarctic plates at the Eocene-Oligocene boundary. Anomaly 20 abuts the Ninetyeast Ridge opposite site 253, which has a middle Eocene basal sediment age. Thus the ridge has the same age as or is slightly younger than the Indian plate to which it is attached. The magnetic data also indicate that the Southeast Indian Ridge between the Ninetyeast Ridge and 85°E jumped 11 degrees to the south between 63 and 53 m.y. B.P. This is approximately the time that Australia started to separate from Antarctica.

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