Structural Variability and Rifting Process of the Segmented Cenozoic Pearl River Mouth Basin,Northern Continental Margin of the South China Sea

The Pearl River Mouth Basin(PRMB)is an important area for studying the evolution of continental marginal basins in the northern South China Sea(SCS),but the structural variability and spatiotemporal rifting process remains poorly understood.This study investigates the differential structural features of the eastern,middle and western PRMB,as well as the extensional deformation laws in operation during the rifting stage,according to an integrated analysis of geometric characteristics and kinematic parameters,i.e.,horizontal displacement and stretching factors of basin and crust.The PRMB underwent at least three phases of intense extension,which varied in time and space.(1)During the middle Eocene,most sags in the PRMB were intensely stretched and high-angle planar to listric boundary faults controlled the wedge-shaped stratigraphic geometry.(2)During the late Eocene-to-early Oligocene,the stratigraphic geometry of the sags was slightly wedge-shaped and continuously controlled by boundary faults,however,the extensional strength decreased relatively in the Northern depression zone,but increased in the Southern depression zone.(3)During the late Oligocene,the extension was extremely weak in the northeast PRMB,but relatively strong in the southwest PRMB,leading to tabular stratigraphic geometry in the northeast PRMB,but localized slightly wedge-shaped stratigraphic geometry in the southwest.The southwest PRMB still underwent relatively strong extension during the early Miocene.The southwest PRMB that was induced by a small-scale localized mantle convection system constantly rifted during the late Oligocene,controlled by the weak lithosphere,westward(southwestward)diachronous opening and southward jump of the ocean ridge.The applied quantitative parameters and spatiotemporal rifting process may be used as a reference with which to study the segmented continental margin rifts.

Initial Rifting Process and Dynamics Mechanism of Huaguang Sag: Evidence from a Numerical Modeling Method

Huaguang Sag is located in the deep seawater area of Qiongdongnan Basin, and its tectonic position belongs to the intersection of NE-trending, SN-trending and NW-trending tectonic systems in the continental margin of the Northwest South China Sea. To investigate the initial rifting process and further more the dynamics mechanism of Huaguang Sag, this paper sets up the structure model of basement which mainly makes up with several depression-controlling faults, and simulates the initial rifting process of Huaguang Sag by the FLAC software. The simulation results show that only affected by the S-N trending extensional stress, the rifting center appears in northern boundary basement faults（two NEE-trending and NWW-trending faults） of Huaguang Sag while does not take place at the NNE-trending and NE-trending basement fault zone in the middle sag, and doesn’t match the current pattern that the basement fault plays a main role in controlling the sediment. In the other case, affected by the S-N trending and E-W trending extensional stress at the same time, the areas of the northern boundary faults zone and internal NNE-trending basement faults zone come to be rifting center quickly, the sedimentary is controlled by the main basement faults to different degrees, and is consistent with the tectonic-sedimentary framework of Huaguang Sag which obtained by the data of geophysical interpretation. In combination with the analysis of regional tectonic background, the paper proposes that two remote tectonic effects occurred by the collision of India-Eurasian Plate： One remote effect was the rotational extrusion of Indo China Block, which led to form a series of NE-trending and NNE-trending basement faults, as well as the E-W trending tensile stress field in Huaguang Sag. The other remote effect was that the deep mantle material of South China Block flowed southward, which resulted in the S-N trending extensional rifting of the lithosphere in northern South China Sea, and finally formed a series of EW-trending and NEE-trending basement faults and the S-N trending tensile stress field in Huaguang Sag. Affected by the above tensile stress fields and the basement faults, the initial rifting occurred in E-W and nearly S-N directions along the pre-existed basement faults（the weak structural zones） in Huaguang Sag.