The Palawan Island: A Door Between the Opening of the South China Sea (SCS) and the Closure of the Proto South China Sea (PSCS)

Meresse, Florian¹; Savva, Dimitri¹; Pubellier, Manuel¹; Steuer, Stephan²; Franke, Dieter²; Cordey, Fabrice³; Muller, Carla⁴; Sapin, François⁵; Mouly, Benoit⁵; Auxietre, Jean-Luc^5
1Ecole Normale Supérieure, Paris, France.
²Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany.
³UMR CNRS 5276, Laboratoire de géologie de Lyon, Université Lyon 1, Villeurbanne, France.
⁴6 bis, rue Haute, Rueil-Malmaison, France.
⁵TOTAL, Paris, France.

The SE Asian region is a natural laboratory to tackle the processes of opening/closure of ocean basins. The island of Palawan illustrates the opening and closure of two adjacent basins: the southern part of the SCS and the northern part of the Sulu Sea (SS). This study combines field observations, biostratigraphy and offshore geophysical investigations: seismic lines, drill-holes and gravity modeling.

Palawan is a terrane rifted away from mainland China. It is however tectonically overlain by allochtonous material of Cretaceous/Tertiary age. The continental block consists mainly of Paleozoic-early Mesozoic units only exposed north of the island. The allochtonous series correspond to series of a deep marginal basin PSCS which has nowadays entirely disappeared. It is composed of :(i) Relics of an ophiolitic body (PSCS), Albian in age, thrusted above the accreted sediments, only present in the South.;(ii) Cretaceous deformed clastic sediments, also Albian in age, North of Palawan, they may correspond to a former accretionary complex;(iii) Eocene clastic sediments, South Palawan, known as the Palawan accretionary wedge, which extends seaward in the SCS. Well data from the Palawan wedge show that accretion was active until the middle Miocene.

The ophiolite thrusted onto the Palawan margin in the middle Miocene may extend into the Sulu Sea as imaged by seismic data. These data show a down-going reflector probably corresponding to the top of the ophiolites and acting as the basal decollement unit beneath the Cagayan wedge. Gravity modeling also shows, east of the Island, a dense body interpreted as the subducted PSCS crust. The overall multi-wedge configuration may be regarded as a single and continuous shortening zone. The identification of the sealing horizon indicates a younging of the deformation towards the SW. In addition, the sediments involved into the Palawan wedge (Cretaceous in the North and Tertiary in the South) are also getting younger to the SW, implying a migration of the deformation related to the closure of the PSCS. This may suggest that the PSCS basin was in a subduction process at the ridge axis earlier in the north, forming the Palawan wedge. Only by then started the subduction of the second panel of the PSCS and formation of the Cagayan wedge. Therefore the closing of the PSCS appeared to have evolved in a zipper manner compensating both the stretching of the SCS continental crust and propagator-like spreading of the seafloor.

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