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when there was little or no movement along the 90E Ridge (McKENZIE and SCLATER, 1971). Theevent most certainly marks the first encounter of the Indian Plate with Eurasia. Up to this time theAsian mainlands marginal subduction zone may have been actively consuming oceanic lithosphere,until the more dense continental crust of the northern Indian Plate was encountered. Magnetic data inFigure 5 narrow the date of this event to around anomaly 22 time (ca 50 Ma). This still ongoingcollision of India with Asia is a natural laboratory in which the implications of a continent- continenttype conjunction can be studied.As mentioned earlier, the 90E Ridge is a long (4500 km) and narrow (50-100 km) aseismicfeature with an average height of about 2 km above the seafloor. Along its eastern edge is a sharpescarpment. The Ninetyeast Ridge in the east and Chagos-Laccadive Ridge in the west were bothformed during the rapid movement of India northwards. MORGAN (1972) was first to propose thatNinetyeast Ridge was formed as the Indian Plate moved over a fixed mantle plume-hotspot of theKerguelen Islands. SCLATER and FISHER (1974), on the other hand, preferred to think of it asoriginating from excessive volcanism at a migrating spreading center-transform fault junction. Theymodeled the Ninetyeast and Chagos-Laccadive Ridges as transform faults, marking a paired offset ofan active spreading center. The transform faults terminated as active faults when the spreadingdirection changed from N-S to NE-SW, around anomaly 11 time (SCLATER and FISHER, 1974).LUYENDYK and RENNICK (1977) suggested that Ninetyeast Ridge was laid down mainlyfrom the Amsterdam - St. Paul hotspot. According to these authors the Broken Ridge and itsNaturaliste Plateau extension, as well as the Kerguelen Plateau, were once joined and were formedfrom the Kerguelen hotspot. Kerguelen and Broken Ridge rifted and separated once spreading beganbetween Australia and Antarctica.PIERCE'S (1979) study of the paleolatitudes of basement rocks from the Ninetyeast Ridgeand from the Deccan Traps from India suggests that the source of volcanic rocks on the Ridge hadremained constant for some 60 m.y. near 50" S latitude, the present location of the Kerguelen hotspot.Thus the genesis of the Ninetyeast Ridge seems to be intimately linked to the Kerguelen hotspot. TheAmsterdam - St. Paul hotspot may, in fact, also be a younger and a northward leaking expression ofthe Kerguelen, emanating from the same magma chamber.CENOZOIC EVOLUTION OF THE INDIAN OCEANThe chronology of major Cenozoic events in the Indian Oceanhas been summarized inFigure 6. The Cenozoic development of this ocean is characterized by the continued rapid northwardflight of India and its eventual collision with Eurasia. In the early Paleocene (some 60 m.y. ago) thespreading ridge between Madagascar and India jumped northeast towards India. This initiated thespreading between India and the Seychelles Platform and the formation of the Chagos-Laccadivetransform (NORTON and SCLATER, 1979).The most significant event of the Cenozoic was the collision between India and Eurasia. Thetiming of this event also marks the acceleration of the motion between Africa and Antarctica (NORTONand SCLATER, op. cit.). This event and its repercussions for the Asian mainland are discussedbelow.The Grand CollisionThe first encounter of the northward migrating Indian plate with the Asian mainland occurredbetween magnetic anomaly 23 and 22 time, around 50 m.y. ago in the early Eocene (Fig. 6). Thisevent caused a major reorganization of the geohedron that initiated spreading on the Central Ir-&anRidge at this time, (NORTON and SCLATER, 1979). The average half-spreading rates changed fromabout 18-19 cm/yr before to less than 2 cm/yr after the collision, literally stopping the Indian Plate inits tracks (Fig. 5). A period of about 10 m.y. of stasis followed in which little or no motion tookplace. The spreading pace quickened somewhat between anomalies 17 and 19 (about 40 m.y. ago),but then slowed to about 2 cm/yr after anomaly 11 time (ca 32 Ma). The present convergence ratebetween India and Asia is calculated at about 1/2 cm/yr.61