| The Qiongdongnan basin is an important extensional basin in the northwestern margin of the South China Sea due to its close link to hydrocarbon exploration. Although a lot of work related to tectonics and sequence stratigraphy has been carried out, the evolution of subsidence and thermal history remains unclear. It is difficult to unravel the basin evolution and related thermal processes in complicated situations only based on the local subsidence and thermal history analyses. Although the geological and geophysical data collection of the Qiongdongnan basin can indicate the different aspects of basin evolution, it turns out to be little comprehensive and constrained with each other. In this investigation, the detailed subsidence and thermal process of the Qiongdongnan basin since Cenozoic was modelled quantitatively by using backstripping technique, strain inverse and thermal index inverse methods. The data set was collected from regional seismic interpretation, drilling lithology, measured intrinite reflectance and other thermal indices. Based on the subsidence and thermal modelling results, the mechanism of deep dynamics was modelled to interpret the Cenozoic framework in the Qiongdongnan basin.The research comes out the following results and conclusions:1. The tectonic evolution of Qiongdongnan basin was divided into four stages, including:the initial rifting stage from early Eocene to late Eocene, the intensive rifting stage from late Eocene to Oligocene, the slow subsiding stage from early Miocene to middle Miocene and the rapid subsiding stage from late Miocene to present day. The subsidence process differs in each stage. In the initial rifting stage, the rifting process begins with weak intensity. The intensive rifting stage is the main rifting phase with much stronger tectonic intensity than the initial rifting stage. It is also the main stage that shapes the framework of subsidence and deposition in the basin. In the period of slow subsiding stage, the subsidence and deposition depend mainly on the cooling of the lithology that produces weakly tectonic activity. However, the rates of subsidence and strain reach the maximum in the rapid subsiding stage, indicating that the tectonic reactivation occurs and the framework of subsidence and deposition begins to be reshaped again.2. The differences between basin subsidence and deposition evolution, both in spatial and temporal differences, have been figured out. The basin has thicker deposition in the western part than the eastern part, and thicker deposition in the center than the side in regard to the spatial distribution. On the process of subsidence, there are two depositional summits that occurs in the period from late Eocene to Oligocene and the period of late Miocene, respectively. The western part, however, shows only one summit that appears in the period from late Eocene to Oligocene. In the respect of subsidence sequence, the basin begins to subside in the central depression zone and then the subsiding zone expands to the northern and southern sides.3. The spatial and temporal differences of thermal features have been described. The Qiongdongnan basin shows a distribution of thermal gradient, of which the south is higher than the north and the east is higher than the west. Compared with the surrounding basins, the present-day geothermal framework in the Qiongdongnan basin remains in a relatively lower level. The heat flow values in different positions differ with time. There are two periods of heat flow peaks in the Ledong-Lingshui sag and Songnan-Baodao sag, i.e., the period from the end of late Oligocene to early Miocene and from the Pliocene to the present. However, only one heat flow summit that occurs since Pliocene exists in the Changchang sag. The heat flow evolution of Qiongdongnan basin has spatial difference. The heat flow value is generally higher from shelf margin to slope area, and lower in shelf region with shallow water and continental plain with deep water. The heat flow in continental shelf area experiences four stages, i.e., slow rising, rapid rising, slow declining and rapid rising. Three stages exist in the slope area, i.e., rapid rising, rapid declining, and rapid rising. While the deep-water area experiences four stages, i.e., stable, rapid rising, stable, and rapid rising.4. The distribution of stretching factor in each tectonic stage has been compared. The lithosphere is stretched along the major depressions during the initial rifting stage with small factor from1.1to1.3. The rifting during this stage occurs locally in the direction of near SN, causing fault depression zone in the EW direction. In this stage, the southern part of the basin remains stable. The feature of lithological extension in the intensive rifting stage inherits the existed extension framework developed in the initial rifting stage. The NW-SE rifting continues in the center depression and weak crust thinning appears in the surrounding depressions. In the late Eocene, the Lingnan high and Beijiao sag begin to develop. At the end of Eocene, the center of crust thinning remains in the Ledong sag and Baodao-Changchang sag. In the Oligocene the southern high areas begin rifting due to the further crust thinning. In the slow subsiding stage, the center of lithology extension moves from west to east and reshapes the framework formed in the INITIAL RIFTING STAGE that the extension only occurs in the NW-SW direction along the central depression. Meanwhile, the middle and south of the basin also experience intensive crust thinning. In the early Miocene, the extensional framework of lithology changes little and the spatial distribution of stretching factor keeps consistent with that in Oligocene.In the Miocene, the stretching factor in the east of Baodao-Changchang sag and in the west of Ledong sag continues to increase and the value is up to2.0. The stretching factor in the southern area is up to1.4. In the rapid subsiding stage, the crust thinning in the Ledong and Changchang sags becomes intensive. In the Pliocene, the crust thinning in the middle and western part becomes intensive, causing the occurrence of three stretching centers, i.e., the Ledong sag, the Baodao sag and the Changchang sag, with stretching factor more than2.5. In the Quaternary, these discrete centers become one continuous part in the basin center. Meanwhile, a ring-shape crust thinning center is generated in the west of Ledong sag with stretching factor about2.0and locally more than2.5. After that, the extensional framework of lithology forms.5. The variation of strain rate in different geographic settings has been analyzed. The strain rate around slope is higher than other regions and two summit could be recognized in the late Oligocene and Pliocene, which have nearly the same strain rate values. There also exists two periods of high strain rate values, corresponding to the early Oligocene and Pliocene, respectively. The first summit shows higher values and later appearing time than that of the slope. In the deep water area, only one obvious strain summit exists, corresponding to the Pliocene. The peak value increases from west to east and the two-summit mode becomes insignificant in the west.6. The post-rift anomalous subsidence has been proved to be widely developed in the Qiongdongnan basin with temporal and spatial difference, i.e., the post-rift anomalous subsidence value is generally small in the slow subsiding stage while in the rapid subsiding stage it shows a wide distribution and great magnitude. In the early Miocene, the post-rift anomalous subsidence appears. Since the middle Miocene, the post-rift anomalous subsidence in the central south and west of the basin start to increase. Following with the further development of post-rift anomalous subsidence, a large PAS zone in the nearly EW direction is formed. In the Pliocene, the post-rift anomalous subsidence extends to the sides and on the central basin it has the amount up to2000m. In the Quaternary, the post-rift anomalous subsidence increases rapidly and it is up to2500m in the central basin and3000m in the Changchang sag. Besides, the post-rift anomalous subsidence in the Yabei sag increases as well.7. The evolution of subsidence and thermal history has been proved closely linked to the deep dynamics. The initial rifting stage causes nearly NS fault depression. At the end of initial rifting stage, the lithology is stretched a little and the heat flow value is low. In the intensive rifting stage, the extension enhances and generate large stretching factor, causing the widening of rifting depression, accompanying with higher heat flow value than initial rifting stage. The faulting depression controls the depositional framework in this stage. In the post-rifting stage, subsiding continues. In the slow subsiding stage, stretching factor stop rapid increase with relatively small deposition. The post-rift anomalous subsidence is not obvious in this stage, but heat flow increases towards the deep water area. At the ending of slow subsiding stage, the lithological extension center coincides with the sag center. The stretching factor in this stage is high and the peak of heat flow is located in the slope and deep water regions. The post-rift anomalous subsidence center is inconsistent with the sag center and it is located in the deep water area with trend increasing towards the sea. In the rapid subsiding stage, the lithology is re-stretched intensively and the post-rifting deposits increase significantly. The heat flow and post-rift anomalous subsidence also increases. The present-day lithology thinning center is located in the Ledong sag center and continental margin break area where the deposition has reached a thickness up to12kilometers. The heat flow increases towards the deep water area from the continental margin break and in the slope it is up to80mW/m2. The post-rift anomalous subsidence in the continental shelf is more than1km and the amount is up to1.7km in the deep water area.8. The relationship among the stretching factor, heat flow and post-rift anomalous subsidence has been revealed. In the slow subsiding stage, the stretching center is located in the central depression zone. This center is arranged along the interpolating direction of the northwestern sub-sea basin and the present marginal break. The heat flow peak occurs in areas that has large stretching factor. The post-rift anomalous subsidence increases from shelf to deep water area and has large values in the southern high and southern Changchang sag. In the rapid subsiding stage, the stretching center remains in the central depression zone where the distribution of heat flow and stretching factor coincide well. The post-rift anomalous subsidence becomes intensive in this stage, and its center is also in the southern high and southern Changchang sag which is consistent the distribution of present water depth, i.e., the eastern center is located in the areas with water depth of1500-3000m and the southern center is located in the areas with water depth of1500-2000m. The fact that the distribution of post-rift anomalous subsidence is not same with that of stretching factor and heat flow indicates the post-rift anomalous in this region cannot be simply explained by McKenzie’s post-rifting subsidence mechanism. A main reason for the Quaternary post-rift anomalous subsidence increase in the west is the reverse of Red River fault zone from left lateral to right lateral. The reverse and reactivation of the slipping provide huge unfilled space and in the same time numbers of base fault release the stress caused during subsiding, resulting in the subsidence increasing towards the Red River fault zone. The No.2fault zone blocks the stress delivering to the Ledong sag although it has been little active since the Quaternary, leading to the rapid subsidence concentrated in a triangular zone. Therefore, it is a complex subsiding process that is caused by the Iithology thinning and deep faulting activity with base faults as media and boundary.9. The evolution of Qiongdongnan basin has been considered to be associated with the tectonic activities of the South China Sea during spreading. In the initial rifting stage, the southern and northern margins keep together and both belong the South China plate. Because of the near NS stretching, the Qiongdongnan basin generates a series of fault depressions in the EW direction. Since the Qiongdongnan basin is in its initial rifting stage before the South China Sea spreading, there is small strain rate and stretching amount. The asthenosphere upwells a small amount, causing a distribution of low heat flow. The subsidence of this stage is bounded in the fault depression. In the rapid subsiding stage, the South China Sea begins two episodes of spreading, including northwestern sub-sea spreading in the NS direction and the southwestern sub-sea spreading in the NW-SE direction. In the late Oligocene when the northwestern sub-sea breaks to the Xisha trough, the strain rate reaches its peak along the interpolating direction and the asthenosphere quickly upwells, causing the heat flow increases rapidly. The tectonic subsidence does not increase sharply due to the small accumulation of strain rate that just lasts a short time since nearly the end of rifting, but the subsidence area expands. The basin begins in a post-rifting stage when the South China Sea stop spreading at the end of Oligocene and the surrounding tectonic activities stop. In the slow subsiding stage, the basin continues to subside due to lithology cooling, causing small strain rate and heat flow. The increment of tectonic subsidence is small in this stage. The post-rift anomalous subsidence appears only in the northwestern sub-sea spreading axis near the Xisha trough. In the rapid subsiding stage, the activity of Red River fault zone becomes weak and starts the slipping reverse from right lateral to left lateral. In the Pliocene, the basin becomes tectonic-active when slipping reverse ends, causing huge subsidence. The strata in the west makes a counterclockwise rotate to the NW-SE direction due to the right lateral slag, and then forms the present-day framework where the strata and tectonic units distribute in the NNE and EW direction. Besides, the tectonic activation causes the further increase of strain rate. The asthenosphere upwells intensively and the heat flow increases rapidly. The tectonic subsidence increases as well and along the No.1fault (a branch of the Red River fault zone) the post-rift anomalous subsidence is huge. To be conclude, the present-day subsidence and thermal features of the Qiongdongnan basin are caused by the joint events of South China Sea spreading, lithology cooling and tectonic activation. |
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