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Abstract: Tectonic Patterns and Cenozoic Basalts in the Western Margin of the South China Sea

Pow-Foong Fan

Three Precambrian micro-continents (Shan, western Thailand, and Malaysia) rifted from Gondwanaland in Carboniferous times and drifted northward and attached to the China and Indochina plates. They welded together during the Indosinian movement as a result of a plate collision. The Indochina plate is an amalgamation of the Shan-West Malay, Tak, Ultaradit, and Khorat-Kontum terranes. About 50 Ma, the collision of the Indian continent into the Asian 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 opening of the South China Sea. The Late Cretaceous alkaline intrusions in the Red

River area of northern Vietnam formed during the initial stage of rifting of the South China Sea. The Indian-Asian collision has successively pushed Sundaland and South China in the east-southeast 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 zi con, 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.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994