Cenozoic Structural Features And Evolution Of A Sag In The Northwestern Slope,South China Sea

To be focused globally, deepwater oil and gas exploration is the most potential field recently. The sag, which is located in the northwestern slope of the deep water area in the South China Sea, is selected as the research object of the paper. Based on ample seismic profiles and terrestrial heat flow data, using the methods of structural analysis and quantitative calculation, and combined with tectonic setting, the structural features, the subsidence history and the tectonic evolution of the sag were analyzed, and the following progress have been made.Open slope and separated slope types have been proposed, and the sag is located in the separated slope. It is speculated that the development of the separated slope is related to the existence of the granitic rock base with greater tensile and shear strength.Eight seismic sequence boundaries have been recognized. They are Tg, T80, T70, T60, T50, T40, T30 and T20, and they divided Cenozoic to three tectonic sequences and eight tectonic sub-sequences.Horizontally, there mainly developed the NE-, NW-and nearly EW trending faults with a converse 'L' shape in the sag, and vertically, there are three main activity time:Tg to T60 is for the strong fault activity, T60 to T40 is for the weak fault activity, and T40 till now, poor fault activity, only a turtle carapace rift is developed in T40 interface.Extensional tectonic styles take primary in the sag. Composite rift structures are mostly developed in the main area, and there have some half grabens on the edge of the sag. Some inverted extrusion structures are developed from the late Oligocene to Miocene. Reverse folds accompanied with shearing action are also developed in the sag.The sag is mainly divided into four major tectonic zones:the western sub-sag, the western horst, the eastern sub-sag, and the southern slope. In the eastern sub-sag, there have more magmatic activities from Eocene to early Oligocene, mainly show as the bedding lined volcanic activity. Magmatic activities distribute widely after middle Miocene, mainly along the major fracture with the NW strike. During basin expansion period, magmatic activity is absent.The tectonic evolution of the sag is mainly divided into three stages:Tg to T60 is the rift tectonic epoch, there have more magmatic activities during Eocene and Oligocene in the central of the sag. Local thermal anomalies and crustal strength reduce were caused by magmatic activities, thus the earliest deformation occurred, and the deformation effect is strong. T60 to T40 is the post-rift thermal subsidence stage. Neritic deposit was developed in the sag, and this sag was dipped below the shelf edge until the speed up of sedimentation after T40.