Cenozoic tectonic and stratigraphic development of the Central Vietnamese continental margin

Abstract

The East Vietnam Boundary Fault Zone (EVBFZ) forms the seaward extension of the Red River Shear Zone and interacted with the extensional rift systems in basins along the Central Vietnamese continental margin. The structural outline of the central Vietnamese margin and the timing of deformation are therefore fundamental to understanding the development of the South China Sea and its relation to Indochinese escape tectonism and the India-Eurasia collision. This study investigates the structural and stratigraphic evolution of the Central Vietnamese margin in a regional tectonic perspective based on new 2-D seismic and well data. The basin fill is divided into five major Oligocene to Recent sequences separated by unconformities. Deposition and the formation of unconformities were closely linked with transtension, rifting, the opening of the South China Sea and Late Neogene uplift and denudation of the eastern flank of Indochina. The structural outline of the Central Vietnamese margin favors a hybrid tectonic model involving both escape and slab-pull tectonics. Paleogene left-lateral transtension over the NNW-striking EVBFZ, occurred within the Song Hong Basin and the Quang Ngai Graben and over the Da Nang Shelf/western Phu Khanh Basin, related to the escape of Indochina. East of the EVBFZ, Paleogene NE-striking rifting prevailed in the outer Phu Khanh Basin and the Hoang Sa Graben fitting best with a prevailing stress derived from a coeval slab-pull from a subducting proto-South China Sea beneath the southwest Borneo – Palawan region. Major rifting terminated near the end of the Oligocene. However, late stage rifting lasted to the Early Miocene when continental break-up and seafloor spreading commenced along the edge of the outer Phu Khanh Basin. The resulting transgression promoted Lower and Middle Miocene carbonate platform growth on the Da Nang Shelf and the Tri Ton High whereas deeper marine conditions prevailed in the central part of the basins. Partial drowning and platform retreat occurred after the Middle Miocene due to increased siliciclastic input from the Vietnamese mainland. As a result, siliciclastic, marine deposition prevailed offshore Central Vietnam during the Pliocene and Pleistocene.