| Newly acquired, basin-wide 3D seismic data show that the southern boundary of the Baiyun is a large-scale detachment fault system (DFS). A qualitative and quantitative analysis of the basin structure and its evolution indicate that the Baiyun sag is a supradetachment basin. Also, the process of lithospheric stretching, thinning and breakup within the Northern South China Sea (NSCS) is discussed in the thesis. The main results are as follows:1. A north-dipping DFS has been found in the deep region of the Baiyun sag. From seismic sections, the DFS dip becomes gentler with increasing depth, converging on the Moho at its maximum depth. The DFS gradually changes from a step-fault combination, a single-fault style, to a complex of ramp-slop-ramp style, from west to east, and formed a northwest-facing arc-shape in map view. The discovery of the DFS provides key evidence to the Baiyun sag as a supradetachment basin.2. Different orders of sequence boundaries have been identified in the Baiyun sag, which lead to the development of a structural stratigraphic framework. Specifically, SD and PD separately represent the onset and cessation of the detachment faulting and subclassified a detachment sequence in the syn-rift system, which is absent in the shallow-water shelf basins. This sequence divides the syn-rift stage into a high-angled normal faulting episode, a detachment-faulting episode and a rifting-sagging transitional episode.3. By using paleo-fall methods and back-striping techniques, an analysis of the geometric features, main fault activity, and basin subsidence rates of the Baiyun sag have been carried out. The main detachment fault was active during the late stage of the Mid-Eocene, which makes the early faulted-basin structure unclear. Nevertheless, the basin subsidence analysis demonstrates that the dispersed subsidence centers migrated into a cluster area. A basin structural evolution has been restored based on this analysis.4. The Baiyun sag characteristics support the theory that the breakup unconformity is due to breakup of the continental lithosphere in the rift basin as opposed to being a transitional boundary between the faulted and sagging basin. Such transitional boundaries are diachronous in the basin over different structural units.5. Quantification of the extent of lithospheric stretching confirms that the crust beneath the Baiyun sag was extremely thinned, with a development of the exhumed mantle zone. The main area of the sag shows typical depth-dependent extension, while in the hyper-extended areas, the detachment faulting could meet the whole crust extension. Therefore, the Baiyun sag has mainly experienced simple shear and formed a deepwater basin in the necking zone of the NSCS. |
