Abstract
Interpretations of seismic, gravity and magnetic anomalies and structural data on the coastal zone of southern part of Central Viet Nam (SCVN) and the adjacent Tertiary basins suggest several phases in the tectonic evolution of the study region since the Late Cretaceous to Quaternary. In this paper, we try to clarify the tectonic evolution of SCVN and the adjacent continental margin. The Cretaceous – Paleocene tectonic phase commenced after cessation of the West Pacific plutonic magmatic activity that produced numerous diabases and aplite dykes of mainly sub-meridian orientation. It was characterized by N–S compression and E–W extension. The geomorphology and geology ofSE Asiawere considerably changed during the Neotectonic phases caused by collision between the Indian plate and the Eurasian continent. Two tectonic phases – Early and Late Neotectonic – are separated by a regional unconformity represented by a boundary surface between below strongly deformed strata (synrift) and above less deformed formations (post-rift). The Early Neotectonic phase was related to the left-lateral movement of the Red River Fault Zone (RRFZ) and includes two tectonic sub-phases: Eocene – Oligocene (NW–SE compression), and Oligocene – Miocene (E–W compression). Activity in the Oligocene-Miocene sub-phase gave birth to rift basins in the continental margin of the SCVN. The Late Neotectonic phase began since the RRFZ stopped left-lateral movement and the East Viet Nam (orSouth China) Sea stopped spreading. The Late Neotectonic phase is also divided into two tectonic sub-phases: Late Early Miocene (sub-meridian compression), and Late Miocene – Pliocene (NE–SW compression). The Late Miocene – Pliocene sub-phase is characterized by vertical movements that caused episodic uplifting of the onland terrains, and subsidence of the offshore Phu Khanh basin. Besides, Miocene – Pliocene-Quarternary basaltic eruptions were widespread all over the southern Indosinian terrain and the continental margin.
Highlights
The geological history of South East Asia landmass is closely related to ocean spreading and transfor‐ mation of the geoblocks that belonged to Gondwana
The tectonic phases were established from the onland data and correlated with the events detected from the seismic sections of the conti‐ nental margin
The tectonic events in the Tertiary basins of the Viet Nam continental margin are well recorded in seismic sections that are widely available for this region
Summary
An alkaline volca‐ no‐plutonic island‐arc belt, known as the Zhejiang – Da Lat belt, was formed along the coastal area of Vietnam and China It resulted from subduction of the Pacific plate under the Eurasian continent. The Early Cenozoic equilibrium in SE Asia was dis‐ turbed by commencement of the Neotectonic phases This was evidenced by the following: (i) strike‐slip tec‐ tonics was widely observed in Viet Nam and the adja‐ cent area; there were hundreds kilometers of horizon‐ tal displacement along the NW‐SE fault system; and the Indosinian landmass moved considerably to the SE; (ii) formation of numerous extended centers in the peri‐ pheral margin caused opening of the East Viet Nam Sea (South China Sea) (EVS). Studies of SCVN and the adja‐ cent area can clarify the tectonic evolution of the re‐ gion, considering in particular the processes of for‐ mation and development of EVS
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