The allochthonous fragments -Indosinia, Sibumasu, East Malaya, and Southwest Kalimantan rifted from Gondwanaland and drifted northward. Indosinia collided with the Yangzi-Huanan terranes in Devonian or Early Carboniferous period and became part of the East Asia continent. Sibumasu collided· with East Asia continent and East Malaya during the Indosinian Orogeny (220-200 Ma). The Southwest Kalimantan terrane probably rifted from the northeast margin of Indosinia in the Cretaceous. ·At about 50 Ma

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... the collision of the Indian continent into the Eurasian continent led to the fragmentation of Asia and was followed by the opening of the Andaman Sea, the clockwise rotation of the Indochina plate, and the rifting and the opening of the South China Sea. The Late Cretaceous alkaline intrusions in the Red River area of northern Vietnam formed during initial stage of rifting of the South China Sea. The Indian-Eurasian collision has successively pushed the Indochina Peninsula in the eastsoutheast direction. Most of the Middle Tertiary movements probably occurred along the left-lateral Red River, Tonle Sap-Mekong faults concurrent with the opening of most of the eastern South China Sea. The extensional tectonics along these predominantly strike-slip faults may be responsible for the Pliocene-Pleistocene alkaline basalts, which extend from the Mekong Delta northwestward into Thailand. Some basalt outcrops in Thailand, Kampuchea and Vietnam appear to trend north-south. The composition of basalts ranges from tholeiitic to alkalic. The basanitoid basalts contain abundant megacrysts of zircon, ruby, sapphire, and spinel. The megacrysts probably originated through assimilation of metamorphic rocks deep in the continental crust. The cores from the South China Sea indicate that the increase in subsidence rate appears in wells during the Quaternary. It may be related to the increase in Quaternary basalt and high heat flow in the western part of the South China Sea basin.