Polyphased Rifting in the South China Sea Leading to Crustal Hyper-Thinning Crust and Moho Exhumation

Savva, Dimitri¹; Meresse, Florian¹; Pubellier, Manuel¹; Chamot-Rooke, Nicolas¹; Franke, Dieter²; Steuer, Stephan²; Mouly, Benoit³; Auxietre, Jean-Luc³
¹Ecole Normale Supérieure, Paris, France.
²Geology, Ecole Normale Supérieure, Paris, France.
³Total, La Défense, Paris, France.

The South China Sea is a V-shape basin which opened by rifting during Paleogene and underwent sea floor spreading from Oligocene to the middle Miocene; the last stage being a propagator opening. If the two steps history of the sea floor opening with a change of spreading direction from N-S to NW-SE has been demonstrated for a long by magnetic anomalies, the rifting history is not well constrained.

The study of a large seismic dataset combined with well calibrations over the entire basin allows us to identify two main directions of extensional faults; E-W and NE-SW. These may be well or poorly expressed depending on the sector, with dominant E-W fault trends in the NW of the basin and less in the southern part. Contrarily, the second phase is characterized by a dense network of faults to the South, whereas these directions are rare in the northern part.

The two sets of faults and half-grabens are described and intersected in some areas where the crust has been extremely stretched, like in the Phu Kanh Basin (PKB) offshore Vietnam. This hyper-extension led to Moho uplift and intense crustal boudinage.

The two directions of rifting and spreading, coeval during the Miocene, raise questions about the mechanisms of crustal thinning and the influence of transfer faults acting at the transition between the area where the oceanic crust was forming and the area where the same amount of extension had to be accommodated by crustal thinning.

These two directions of rifting happened during at least 3 stages which are well illustrated in the PKB.

The oldest stage during the early Paleogene (Late Cretaceous to Eocene) was receiving continental material probably coarse with reference to the basins observed onshore. It is followed by a second phase characterized by a seismic facies exhibiting occasional reef and pinnacles edifices. The seismic facies change from rough and poorly stratified horizons (conglomeratic?) along the normal fault plane, to well stratified with high amplitudes near the apex of the tilted block. This suggests that the environment remained shallow and confined within small basin inherited from tilted blocks. They evolve from continental with coarse sandy deposit and lacustrine shales to restricted marine environment, without deep troughs over a large platformic area.

Although, the faults strike imaging varies according to the sector, we identify a last stage between 15.5 and 10.5 My in the southern part of the basin on either sides of the propagator.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012