# Tectonic Subsidence and Its Response to Geological Evolution in the Xisha Area, South China Sea
## 5 Discussion
### Tectonic Subsidence of Miocene (23.5-5.5 Ma)
#### 5.2.2 Fourth Phase--Accelerated Rise Phase from Middle to Late Miocene (15.5-5.5 Ma)  
The relatively rapid tectonic subsidence in the Late Miocene was consistent with the value restored in the Qiongdongnan Basin, which was considered to be associated with the dynamic topography [9]. In this period, the evolution of the Red River Fault Zone entered the reversal stage and terminated at the end of the Late Miocene [4]. Based on the information from the stress-field analysis, as shown in Figure 10A, there was an extrusive component to the Xisha Block generated by the sinistral movement of the Red River Fault Zone, which compelled the Xisha Block to split anticlockwise of the South China continent in the Paleogene [47]. Although this splitting process stopped in 23.3 Ma, the extrusive component remained and the extrusive intensity was interrelated with the displacement rate of sinistral movement. During the reversal stage of the Red River Fault Zone, the gradual weakening of extrusive intensity with the fall in displacement rate led to stress release of the Xisha area, which further resulted in tectonic subsidence. The accelerating leakage of stress might have caused the rise in tectonic subsidence rate from the early Middle Miocene (15.5 Ma) to the end of the Late Miocene (5.5 Ma).  
Figure 10: Diagrams of stress-field analyses in different tectonic evolution stages around the Xisha area. (**A**) During the Oligocene, there was an extrusive component to the stud area generated by the Red River Fault Zone. (**B**) The component in the Xisha area was also changed to extensive, with the reversion of the Red River Fault Zone from left lateral to right lateral after 5.5 Ma. The distribution of faults and magnetic anomalies and wells were modified from [4,18,47]. XS: Xisha Islands, ZS: Zhongsha Island, NS: Nansha Islands, RB: Reed Block, EVBF: East Vietnam Boundary Fault, ZF: Zhongnan Fault.  
Two sedimentary recordings in the Xisha area coming from the carbonate platform and the mass transport deposits (MTDs) responded well to the tectonic subsidence, as shown in Figure 3[21; 28; 36]. Since the Middle Miocene, the spatiotemporal distribution of carbonate platform was identified in six periods and could be further divided into three stages (Table 2). The scale of carbonate platform reduced slowly in the Middle Miocene and shrank quickly in the Late Miocene, which reflected an accelerating process of the tectonic subsidence in accordance with the restoration in this stage. The development of the Huaguang MTDs in the upper Huaguaguang Formation (8.2-5.5 Ma) provided evidence of coeval rapid tectonic subsidence [28]. The instability of sediment caused by rapid subsidence was considered to be the dominant trigger mechanism for MTDs, which were related to the reversal of the Red River Fault Zone from sinistral to dextral movement. It gradually weakened from west to east in the Xisha area, inferred from the effects of the heating event [52]. Additionally, the tectonic subsidence in the north slope of the Qiongdongnan Basin also gradually weakened from west to east in the Late Miocene, which corresponds to the effect that it gradually declined with the increased distance to the Red River Fault Zone [10].