--> --> Basin Structure and Tectonic Evolution of a Detachment Rift Basin at Magma Poor Rifted Margin: A New Perspective on Basin Structure of Baiyun Sag, Northern South China Sea

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Basin Structure and Tectonic Evolution of a Detachment Rift Basin at Magma Poor Rifted Margin: A New Perspective on Basin Structure of Baiyun Sag, Northern South China Sea

Abstract

The Baiyun Sag is the largest deep-water basin located at the northern continental slope of the South China Sea. With LW 3–1 gas field discovered in Baiyun sag in 2006, it shows an excellent prospectiveness of hydrocarbon exploration. Reconstruction of tectonic-stratigraphic evolution could provide guidance for further petroleum exploration, as well as an analogue for other rift basin analysis with similar geodynamics and background. Based on drill data and high quality three-dimension seismic data, four tectono-stratigraphic units are recognized, together with seven sequence boundaries. The results revealed that the Baiyun Sag is a superimpose basin which deposited Cenozoic sediments. A detailed analysis of extensional features showed that it has experienced two major phases of extension: unit A, the first rifting phase, was controlled by high-angle normal fault, while unit B, the second rifting phase, was controlled by low-angle faults (detachment faults) with heaves on the order of 20–30 km. In the most highly thinned areas, a set of deep-seated detachments which converge on the Moho (crust-mantle boundary) cut through the whole extended crust and, probably, the exhumed mantle, leaving the thinned continental, which is extremely reduced by the detachment faults for embrittlement, to be preserved under thick synrift and postrift deposits. The detachment faults were the boundary between weakly thinned crust and hyper-extended crust. And also they played the role of boundary faults of the Baiyun Sag, and resulted in formation the necking zone of rifted margin. After strong crustal thinning(unit B), due to an oceanward migration of tectonic activity, the following unit C suggests the ending of rifting phase predates T70 (32Ma), the break-up unconformity, corresponding with the beginning of sea-floor spreading in the South China Sea. After the opening of the South China Sea, mainly thermal subsidence controlled the evolution of the Baiyun Sag. Combining with other boundaries identified in the basin filling and subsidence analysis, the Cenozoic tectonic evolution of the study area could be divided into four stages: rifting stage including stretching (unit A), thinning (unit B), and rift-drift transition (unit C), fault-sag stage which corresponded to ridge jump, post-rifted thermal subsidence stage and post-rifted accelerating subsidence stage.