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

Much of the old Tethys seafloor has since been lost to subduction. The only retrievable recordcomes from the margins of this seaway, from southern Eurasia and northern Africa. Evidence fromthese shores indicates that great volumes of carbonate sediments were deposited in thistropical-temperate seaway (McGOWRAN, 1978) where open marine conditions existed. Claysaccumulated in the more restricted basins (KIDD and DAVIES, 1978).The Great SplitThe late Jurassic seems to have been the time of the great split when South America separatedfrom Africa, beginning the opening of the South Atlantic. As mentioned earlier, East and WestGondwanaland had already separated by this time (NORTON and SCLATER, 1979). Madagascar, towhich India was still firmly attached, had begun moving away from Africa in the late Jurassic(EUBINOWITZ et al., 1983), some 140 m.y. ago. The spatial constraints suggest that Madagascarcould not have reached its present position relative to Africa until anomaly MO time (ca 110 Ma) assuggested by NORTON and SCLATER (1979) and SIGOUFIN and PATRIAT (1981).The breaking-up of eastern Gondwanaland predates anomaly M4 (ca 115 Ma), which is theoldest anomaly identified off western Australia (LARSON, 1977; markl, 1978). JOHNSON ET AL.(1980) date the inception of the spreading between greater India and Australia somewhat earlier, atanomaly MlON (ca 102 Ma).Prior to the breakup, “greater” India occupied the space just west and northwest of Australia.LARSON (1977) included Tibet as a part of greater India, which abutted western Australia as far northas the Exmouth Plateau. Tarim may also have been located north of Australia and east of Tibet(LARSON, 1975). Recent Chinese paleomagnetic studies in southern Tibet show, however, that Tibetlay about 8’ north of the equator and was not a part of India since at least the late Cretaceous time,when India was sttill attached to Madagascar (MOLNAR and CHEN, 1978).The separation between Australia and Antarctica had been thought to have begun in the latePaleocene (ca 55-53 Ma). A careful reinterpretation of the magnetic anomalies between the twocontinents by CANDE and MUTTER (1982) doubled the age of this event, which may have occurredsome 50 to 55 m.y. earlier, within the magnetic quiet zone preceding anomaly 34. A more up-to-datetime scale used here (Fig. 2) places the magnetic interval in question somewhere between 105 and 84Ma. It is quite likely that this event was a part of the same major breakup event that started the split ofIndia from Madagascar some 90 m.y. ago. The latter event is associated with a suite of late Cretaceouslava flows along Madagascar‘s eastern coast (NORTON and SCLATER, 1979).According to CANDE and MUTTER (1982) the first phase of spreading between Australiaand Antarctica that lasted from just before anomaly 34 to anomaly 19 (ca 43 Ma) was very slow. Thebreakup was accompanied by rapid subsidence of sedimentary basins along the southern margins ofAustralia. This breakup and spreading initiation between India and Madagascar (and between Australiaand Antarctica) marks a major reorganization of plate boundaries and a reorientation of the spreadingdirection of some 60”. At this time, or somewhat later (around 80 Ma), the spreading in the TasmanSea and between New Zealand and Antarctica was also initiated (WEISSEL et al., 1977; NORTONand SCLATER, 1979).As the pieces of the old East Gondwanaland started to drift apart, the spreading created newseafloor that would become the modem Indian Ocean. This spreading progressively constricted theTethys, and the eventual demise of the seaway had begun. Each separation event (rifting andspreading) during the breakup of East Gondwanaland was preceded by periods of uplift and erosion,and deposition of thick synrift terrigenous sediments in the rift valleys. These coarse non-marinesediments of the early phases, overlain eventually by finer, organic-rich, marine sediments, formexcellent oil reservoirs, e.g., along the southern and western margins of Australia (VEVEERS andMcELHINNY, 1976).The early sedimentation within the newly created Indian Ocean basins were characterized byaccumulation of clays in the restricted basins and the deposition of calcareous sediments in the areashigher than the CCD. Terrigenous sedimentation was confined along the margins of the continents(KIDD and DAVIES, 1978).59