GEOLOGIC AND TECTONIC CHARACTERISTICS OF THE MEKONG BASIN, SOUTH VIETNAM

The Mekong Basin is resulted from the crust extension in the northern continent margin of the South China Sea. The basin has experienced 4 stages of tectonic evolution, including formation of basement, initial and continued rifting, thermal subsiding stage Ⅰ, and thermal subsiding stage Ⅱ. A great amount of faults occur in the basement, and can be divided into 4 groups in NE, NEE, NW and NWW direction respectively. The geothermal gradient of this basin is 32℃/km. Two times of basalt eruption events have been observed in mid-Miocene about 5Ma ago. The basement of the Basin is made up of late-Mesozoic intrusive, volcanic and metamorphic rocks. Above it, there occur the deposits from Eocene to Quaternary. The basin was an open sea in early Tertiary, of which the thick sediments mainly came from the Mekong River. The basin turned to a terrestrial environment during mid-Miocene. Fluvial deposits, lacustrine turbidites, coastal and shallow lacustrine deposits and delta deposits dominated the basin with the thickness over 2000m. It is even more than 4000m thick in the depocenter of the Bach Ho oil field. The basin returned to a marine environment in later-Miocene up to Quaternary and the marine deposits are about 1500m thick, with the thickest up to 3000 m.