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Structural Differences Between the Western and Eastern Central Depression of Qiongdongnan Basin in Northern South China Sea: New Perspectives


Located at the intersection between NW-trending slip system and NE-trending rift system in the northern South China Sea, the Central Depression of Qiongdongnan Basin (CDQB) and adjacent areas embed critical information on Indochina block extrusion and South China Sea seafloor spreading. In this study, we systematically reveal the striking structural differences between the western and eastern CDQB from fault distribution, depression form, rifting pattern, fault activities and structure evolution. A semi-quantitative analysis of the fault cut-offs identifies four stages of rifting evolution: (1) 45–32 Ma, sparsely distributed NE-trending faults formed mainly in the western and the central part of the study area; (2) 32–28.4 Ma, the area influenced by NE-trending faulting was extended into the eastern part of the study area and some NW-trending faults formed in the central and the eastern part of the study area; (3) 28.4–23Ma, the subsidence area was further enlarged but mainly extended into the flanking area of the main rift zone; and (4) 23–15.5 Ma, the old faults ended and a lot of NW-trending slip faults were activate in the eastern part of the study area. As a result, the shape of depressions is NE-trending in the west but WE-trending in the east. Composite half-grabens are distributed mainly in the western and central CDQB, composite symmetric or asymmetric grabens are focused in the eastern CDQB, and the transverse folds are observed exceptionally in the southeastern CDQB. Besides that, the deep and thermal structures are invoked to account for such deformation differences. The lithosphere of the eastern CDQB is very hot and thinned because of mantle upwelling and heating, composite symmetric grabens formed, but the lithosphere in the western sector is transitional from very hot to normal. Eventually, the tectonic development of the CDQB may be summarized into four stages with dominating influences, slab-pull of the Proto-South China Sea (45–32 Ma), rapid Indochina block extrusion together with NE-trending and uniform seafloor spreading in Northwest Sub-sea Basin (32–28.4Ma), slow Indochina block extrusion together with NS-trending Central South China Sea seafloor spreading(28.4–23Ma), and the lithospheric thermal cooling together with the NE-trending seafloor spreading and non-uniformly spreading rates in Southwest Sub-sea Basin (23–15.5Ma).