139736eo.pdf (20MB) - Japan Oceanographic Data Center

helped in the piecing-together of the sedimentary, hydrographic and climatic histories of this basin.The purpose of this paper is to outline the geological and oceanographic-climatic evolution ofthe Indian Ocean since 150 Ma (Ma = m. yrs. B.P.), from the time of the breakup of easternGondwanaland to the development of modem day patterns. Sedimentary and climatic history of thebasin are outlined and inferred global paleoceanographic patterns, in which the Indian and SouthernOcean play a major role, are also presented. In this paper the paleogeographic events that led to thedestruction of the Tethys Seaway and the development of the Indian Ocean, and the oceanographicclimaticevolution of the region will be reviewed. The sources for this review are varied and based ongeophysical, sedimentological and paleontological evidence from the deep sea, as well as stratigraphicdata from the continents surrounding the Indian Ocean.In order to discern the sequence of events that led to the development of this complex basin, itmay be helpful to first examine its present-day geologic setting and sedimentary patterns.MODERN GEOLOGIC SETTINGThe Ocean floor in the Indian Ocean is dominated by the mid-Ocean ridge system where theseafloor is actively spreading at the present time (Fig. 1). This spreading ridge system forms aninverted Y, whose three limbs are offset by extensive fracture zones (see paper in this volume bySIDDIQUE et al.). The limbs extend: 1) first north and then northwest towards Gulf of Aden,forming the Central Indian and Carlsberg Ridges, 2) southwest between Africa and Antarctica,extending into South Atlantic and forming the Southwest Indian Ridge, and 3) southeast betweenAustralia and Antarctica forming the Southeast Indian Ridge. The present-day half spreading ratesrange from about 1 cdyr on the Southwest Ridge, and between Africa and Antarctica, to about 2cdyr on Central and Southeast Ridges. These rates are comparable to rates in other Ocean basins.Also prominent on the Indian Ocean floor are two aseismic ridges and several topographichighs or plateaus with little or no seismic activity at the present time. The Ninetyeast Ridge (90E) isthe most prominent of these features (Fig. 1). This presently "quiet" ridge is the longest aseismicfeature in the world Ocean and extends from the Bay of Bengal to 32's along 90"E meridian. It hasbeen shown to have a mixed tectonic and volcanic origin, being a paleo-transform fault with volcanicleakage, and forming a trace of the northward movement of Indian Plate. The age of 90E Ridgedecreases progressively away from its northern end, ranging from late Cretaceous (ca 85-65 Ma) in thenorth to Oligocene (ca 30-35 Ma) in the south.The second aseismic ridge - the Chagos-Laccadive Ridge - extends from the westernmargin of India to about loos, where it is truncated by the Central Indian Ridge. Like the 90E Ridge,the Chagos-Laccadive Ridge is also a former transform fault and a leaky western trace of the IndianPlate's northward movement (McKENZIE and SCLATER, 197 1).The more prominent oceanic highs in the eastern and southern Indian Ocean include theBroken Ridge, and the Naturaliste and Exmouth Plateaus in the east, off Australia, and the KerguelenPlateau south of the inverted Y-shaped spreading ridge. Naturaliste Plateau has been in existence atleast since the mid Cretaceous (ca 100 Ma) and was a shallow platfonn receiving terrigenous sedimentsfrom surrounding continents in its early history. Later on pelagic carbonate sedimentation replacedterrigenous input. Much of the early Tertiary record, however, has been removed by subsequenterosion (LUYENDYK and DAVIES, 1974).Broken Ridge maybe as old as the Naturaliste Plateau. In the late Cretaceous it was a shallowcarbonate platform that probably split from the Kerguelen Plateau during Eocene when the SoutheastIndian Ridge originated. Since then subsidence and pelagic carbonate sedimentation have characterizedthe Broken Ridge. The Kerguelen Plateau most probably had an oceanic origin as the rocks fromKerguelen Island show similarities to other oceanic islands (UPTON, 1982). Seismic data from theExmouth Plateau suggests that its basement is at least as old as Triassic. Although some authors haveargued in favor of an Oceanic origin for this plateau (see e.g., VEEVERS and COTTERILL, 1978), thequestion whether it is of oceanic or continental origin has not been fully resolved.In the western Indian Ocean the major topographic features include the Mascarene (orSeychelles-Mascarene) Plateau and the Madagascar Ridge, northeast and south of Madagascar,respectively; and the Mozambique Ridge, off the eastern coast of South Africa. At least the northern56