越南南部湄公盆地地质构造与沉积特征

GEOLOGIC AND TECTONIC CHARACTERISTICS OF THE MEKONG BASIN, SOUTH VIETNAM

湄公盆地的形成是南海北部陆缘地壳发生伸展作用的结果。其构造演化上总体可以分为基底形成、裂谷初期及其裂谷期、热沉降阶段Ⅰ和热沉降阶段Ⅱ共4个阶段;盆地基底中发育大量断裂构造,从断裂的走向来看可以分为4组:NE、NEE、NW和NWW走向;盆地的地温梯度值为32℃/km;发育中中新世和大约5Ma前后两次玄武质火山活动。湄公盆地基底由晚中生代侵入岩、火山岩和变质沉积岩组成,基底之上从始新统一直到第四系均有发育。盆地古近纪期间的沉积作用处于开阔海的状态,沉积物主要通过湄公河供给,沉积物厚度较大;中中新世期间,以河流-湖相浊积岩-滨、浅湖相-三角洲沉积体系为主,沉积物厚度较大,主体超过2000m,沉积中心——白虎油田附近沉积厚度超过4000m;晚中新世-第四纪期间,以海相沉积体系为主,沉积物厚度明显减薄,主体在1500m左右,最大沉积厚度为3000m。

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.