Tectonics of the Ninetyeast Ridge derived from spreading records in adjacent oceanic basins and age constraints of the ridge
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Analysis of new and existing geophysical data for the Central Indian and Wharton Basins of the Indian Ocean were used to understand the formation and evolution of the Ninetyeast Ridge (NER), especially its relationship to the Kerguelen hot spot and the Wharton spreading ridge. Satellite gravity data and magnetic anomalies 34 through 19 define crustal isochrons and show fracture zones striking approx. N5 degrees E. One of these, at 89 degrees E, crosses the approx. N10 degrees E trending NER, impacting the NER morphology. From 77 to 43 Ma the NER lengthened at a rate of approx. 118 km/Myr, twice that of the approx. 48–58 km/Myr accretion rate of adjacent oceanic crust. This difference can be explained by southward jumps of the Wharton spreading ridge toward the hot spot, which transferred portions of crust from the Antarctic plate to the Indian plate, lengthening the NER. Magnetic anomalies document a small number of large spreading ridge jumps in the ocean crust immediately to the west of the NER, especially two leaving observable 65 and 42 Ma fossil spreading ridges. In contrast, complex magnetic anomaly progressions and morphology imply that smaller spreading ridge jumps occurred at more frequent intervals beneath the NER. Comparison of the NER dates and magnetic anomaly ages implies that the hot spot first emplaced NER volcanoes on the Indian plate at a distance from the Wharton Ridge, but as the northward drifting spreading ridge approached the hot spot, the two interacted, keeping later NER volcanism near the spreading ridge crest by spreading center jumps.
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