Evolution of the East: Central Indian Ocean, with Emphasis on the Tectonic Setting of the Ninetyeast Ridge

Abstract

The meridional Ninetyeast Ridge in the eastern Indian Ocean separates the deep Central Indian Basin from the deeper Wharton Basin (or Cocos Basin–West Australian Basin) to the east. The flattish-summited ridge extends slightly east of north from near 32° S. directly to 7° S. where it appears segmented as a series of en echelon northeast-southwest–trending highs, then in a northerly direction disappears beneath the sediments of the Bengal Fan system near 9° N. Linear parallel to subparallel troughs border this linear ridge on the east side; on the west, from results of magnetic observations and preliminary deep drilling information, Ninetyeast Ridge apparently is bonded to the Indian plate. A second extensive north-south topographic rise and magnetic boundary zone, herein named the Investigator Fracture Zone, lies near 98° E. in the Wharton Basin. Easterly trending magnetic-anomaly lineations identified as numbers 5 through 16 and numbers 21 through 33b, increasing in age northward and with spreading rates variable through time, have been recognized in the Central Indian Basin. East of Ninetyeast Ridge in the Wharton Basin, anomalies 19 through 27, with spreading rates varying in concert with those of comparable age west of the ridge, have been found to increase in age toward the south. Older anomalies 28 through 33 have been identified in both basins; their divergent trends provide evidence that spreading rates decrease markedly westward during the time span they cover in the Late Cretaceous. From deep-sea drilling information supplementing and supporting magnetic, topographic, and gravity data obtained principally by research ships and PROJECT MAGNET since 1962, we interpret Ninetyeast Ridge to be an extrusive pile with a low-density shallow root, rather than a horst or an uplift resulting from the convergence of plates. The trough system that is partially buried with sediment east of the ridge and the north-south Investigator Fracture Zone several hundred kilometers farther to the east are remnants of formerly active transform faults that marked huge relative offsets between the spreading centers separating the Indian and Antarctic-Australian plates from anomaly 33b (Late Cretaceous) to anomaly 19 (Eocene) time. During the Late Cretaceous, Ninetyeast Ridge and Chagos-Laccadive Ridge had similar settings, marking paired offsets of an active spreading center around the southern tip of India. Both features terminated as active transform faults with the cessation of north-south spreading and the commencement of northeast-southwest spreading close to the time of anomaly 11 (Oligocene). The here-interpreted oceanic data is strong but not conclusive support for fitting India to Enderby Land in Antarctica in the Early Cretaceous. With presently available information, we have been unable to establish the precise time at which the spreading center in the Wharton Basin ceased to function.