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

high opal and biogenic siliceous content of the sediments. Organic matter values in the sediments arehighest, however, along the western margin of India, facilitated by better preservation due to oxygenpoorenvironments (KOLLA et al., 1982).GEOLOGICAL HISTORY OF THE INDIAN OCEANSeafloor magnetic anomalies and paleomagnetic data from the surrounding continents provideimportant information about the age of the seafloor and the paleoposition of the continents at differenttimes. Sedimentary patterns furnish additional clues about circulation, climates, productivity and thecontinental influences on the ocean and its contained biota. Both these lines of evidence have beenvaluable in deciphering the broad picture of the geological history of the Indian Ocean.The general outline of the evolution of the Indian Ocean basin from magnetic evidence wasfirst presented by McKENZIE and SCLATER (1971). Later studies, including those by SCLATERand FISHER (1974), PIMM et al. (1974), VEEVERS and McELHINNY (1976), LARSON (1977),SCHLICH et al. (1977), LUYENDYK et al. (1980), CANDE and MUTTER (1982), CURRAY et al.(1982) and RABINOWITZ et al. (1983) have all added details on a regional or oceanwide basis.Drilling and geophysical data collected during DSDP Legs 22 through 27 have further enlarged thedatabase and a considerably detailed picture of the development of Indian Ocean has emerged. Thehistory of this youngest of the major oceans begins with the last phase of the breaking up of thesupercontinent of Gondwanaland.THE BREAKUP OF GONDWANALAND AND THE MESOZOIC DEVELOPMENT OFTHE INDIAN OCEANThe Great SlipThe major Mesozoic events in the Indian Ocean are summarized in Figure 2. The oldestidentifiable magnetic anomaly in the Indian Ocean (in the Mozambique Channel) is of late Jurassic age.Thus the actual separation of eastern Gondwanaland (Madagascar-India-Antarctica-Australia) from thewestern Gondwanaland (Africa-South America) predates anomaly M22 (ca 140 Ma), and probablyoccurred some 150 m.y. ago. The initial rifting between the two halves of the supercontinent,however, may have occurred as early as early Jurassic (ca 210 to 200 Ma) when first open marineconditions are encountered in eastern Africa (NORTON and SCLATER, 1979). The Karroo volcanicsin South Africa and the Dufek intrusion on Antarctica are both of early Jurassic age and most likelymark the timing of the initial rifting phase between East and West Gondwanaland (NORTON, per.comm., 1985). Early motion between East and West Gondwanaland was predominantly strike-slip.The large intercontinental strike-slip faults probably restricted oceanic circulation until well into theCretaceous.In the Mesozoic the area north of eastern Gondwanaland was occupied by the Tethys Sea,which extended up to the Asian mainland just north of the tropical latitudes. During the Jurassic thisseaway extended westwards into Europe, but in the early and most of the middle Jurassic, thecircum-global Tethys Current had not yet initiated as the Central American passage between North andSouth America was still blocked. This passage was breached sometime in the later part of middleJurassic, thereby connecting the eastern Tethys with the Pacific, via the proto-North Atlantic. Anepicontinental seaway existed prior to the opening of the Central American passage, butcommunication with the open ocean was restricted, except during times of high seastands.Biogeographic data suggest the establishment of a permanent connection with the Pacific in Bathoniantime (ca 175 to 170 Ma). The westward flowing Tethys Current may have been developed soonafterwards (HAQ, 1984).58