Abstract

Following the cessation of seafloor spreading in the Late Miocene, the Pliocene-Quaternary tectonics in the southwest sub-basin of the East Vietnam Sea was mainly controlled by the Late Miocene-Early Pliocene sea-bottom subsidence. Using the 1D back-stripping method and analytic results of composition, thickness, age and porosity of sediments in the log 1143, site 184 of Ocean Drilling Program (ODP), the Pliocene-Quaternary seafloor subsidence in the southwest sub-basin of the East Vietnam Sea is determined from 150 to 180m in which tectonic subsidence contributed from 100 to 120m for the past 5 million years. The tectonic subsidence rate caused by deep tectonic movement in the basement was limited within the range of 20-24m per million years or 2-2,4cm per thousand years. The Pliocene-Quaternary tectonic subsidence rate in the study area experienced three times of variation. The first change happened in-between 4,5 and 4,4 million years, the tectonic subsidence rate decreased in comparison with the earlier Late Miocene period. The second one occurred in-between 4,1 and 3,8 million years, tectonic subsidence rate increased faster than the previous period. The third change occurred between 2,6 and 2,5 million years ago, i.e. at the beginning of Quaternary, the tectonic subsidence rate increased faster than the Late Pliocene period.