PRELIMINARY NEOTECTONIC STUDY ON SOUTHEASTERN COAST OF VIETNAM

Vietnam belongs to the southeastern Eurasian plate and surrounded by South China Sea to the east, the Gulf of Thailand to the south, Cambodia and Laos to the west and China to the north. We surveyed marine terraces and fractures related to active faults of PhoucDinh and VinhHai around the southeastern coast of Vietnam. These areas mainly consist of Cretaceous DeoCa granite complex and Quaternary sediments. Major deformation of the southern Vietnam during late Tertiary is NE and NS to NNW right lateral strike-slip faulting (Rangin et al, 1995). Linear structure analyzed by spot data is classified to three groups: NS and NNE of VinhHai, NE of Dalat and NS of PhoucDinh. It corresponds to fracture mapping data around PhoucDinh and VinhHai. The coastal area of PhoucDinh is covered by huge dune sand so that topography shows wide flat plain and low hills. Basement of this area is composed of Maviek Formation characterized by calcareous conglomerate overlying PhanThiet Formation of red and brownish-yellow sand with 4-20m thick. Marine terraces are classified into three steps: 50-40m, 20m, and <10m. The youngest step is developed well along the tidal zone and 0.5m deep below sea level at the high tide. On the basis of digital marine terrace mapping result, PhoucDinh area is divided into 4 blocks as possible products by reactivation of NE and NS faults. To clarify these active faults of PhoucDinh is urgently needed to perform the detail survey of morphotectonic approach. VinhHai is 20 km away from PhoucDinh and has a distinct geomorphic feature of coral reefs, which was registered to UNESCO as a world natural heritage. Coral platforms consist of at least four steps with different heights: 1.5m, 5m, 9m and 16m. The platforms are inclined toward the inland side (west), not toward the sea side (east). The westward dip is 15°. It reveals that the southern Vietnam has been tilted toward the west since after the latest Pleistocene or up to Holocene due to the westward tectonic compression. PhoucDinh and VinhHai show different fracture pattern and marine terrace characterization even though they locate closely. Cai river of WNW trend divide the above two areas and possibly did a role of active faulting. We interpret that the westward tilting of Vietnam is due to transpression by Philippine plate movement.