Low Angle Normal Fault System Controls the Structure Evolution of Baiyun Deepwater Basin and Its Lithosphere Thinning, Northern South China Sea

Abstract:

The discovery of the transition area from ~30 km to weakly thinned continental crust (＜12 km) in Baiyun deepwater basin, Northern South China Sea leads to two questions: What controls extreme crustal thinning and what is the nature of Baiyun basin. The 3D seismic data newly acquired show that Baiyun basin is an asymmetric half graben mainly controlled by a set of north-dipping normal faults converging in deep. By employing the principle of back-stripping, we estimate the fault dips and slip amount would be in the absence of post-rift sediments and seawater loading. Results show these Middle Eocene faults were extremely active, with a high accumulation horizontal displacement (＞ 10 km) and an initial very low angle (＜7°), followed by a rotated into sub-horizontal. A general scenario for extension of the uppermost continental crust probably includes simultaneous operation of low angle normal fault (F1) as well as parallel arrays of step-faults (domino-faults, f2-f9). Under such a scenario, it shows no obvious extension discrepancy in Baiyun basin. Our results indicate that Baiyun sag preserves information recording the continent thinning before the seafloor spreading, and it could be an abandoned inner rifted basin.