# Tectonic Subsidence and Its Response to Geological Evolution in the Xisha Area, South China Sea
## 4 Results  
Tectonic subsidence curves and rates in the Xisha area have been calculated based on the multi-channel seismic and wells data, shown in Figures 6 and 7. Cenozoic tectonic subsidence varies dramatically in the different units in the Xisha area, and three groups of tectonic units can be clearly identified: Group 1, the Guangle Uplift and Xisha Uplift; Group \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Lithology** & **Shale** & **Sand-Stone** & **Limestone** & **Coal** \\ \hline Initial porosity & 0.6 & 0.52 & 0.6 & 0.9 \\ \hline Compaction & 0.515 & 0.217 & 0.22 & 0.7 \\ \hline Skeletal density (g/cm\({}^{3}\)) & 2.402 & 2.8 & 2.72 & 1.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Petrophysical parameters of variable lithology used in this study. The subsidence of Group 1 in the structural highs is much smaller than that of Group 3 in the center of the basins. The minimum tectonic subsidence of Group 1 is close to 0.8 km and 1.5 km on the Guangle Uplift and Xisha Uplift, respectively. The tectonic subsidence of Group 2 is between 3.2 km and 3.8 km on the Zhongsha Trough and Zhongjian Depression, respectively. The maximum tectonic subsidence of Group 3 in the Changchang Depression is up to 5.4 km, which is approximate to the Qiongdongnan Basin, as shown in Figure 6.  
Five phases of tectonic subsidence history in the Xisha area can be identified according to the thickness and rates of tectonic subsidence, as shown in Figures 6-8. First phase--the slow subsidence phase in the Eocene (53.5-32 Ma). The tectonic subsidence was 1.0-1.5 km in the slow subsidence stage, with subsidence rate of 0.05-0.07 m/ky in Figures 6 and 7. Second phase--the rapid subsidence phase in the Oligocene (32-23.3 Ma). The first rapid subsidence emerged in the Oligocene and the subsidence rate was up to 0.45 m/ky in the Changchang Depression. The subsidence rates in other tectonic units also exceeded 0.2 m/ky (Figure 7). The tectonic subsidence was widespread up to 2 km in the basins  
Figure 8: Structural evolution of the main sections in the Xisha area (see Figure 4 for the location). CD: Changchang Depression, XU: Xisha Uplift, ZT: Zhongsha Trough, SCS: South China Sea. around the Xisha area in the rapid subsidence phase and could be further divided into two portions by the boundary of T\({}_{7}\) (29.3 Ma), as shown in Figures 6 and 7. The rate of tectonic subsidence of 32-29.3 Ma was higher than that of 29.3-23.3 Ma, as shown in Figure 7. The tectonic subsidence in the Zhongsha Trough was also restored with 0.15 m/ky in 29.3-23.3 Ma, as shown in Figure 7.  
Third phase--the interim phase in the Early Miocene (23.3-15.5 Ma). Relatively slow subsidence followed the first rapid subsidence that occurred in the Early Miocene. The rate of subsidence was about 0.05 m/ky or even lower in most of the tectonic units in Figure 7. During the interim stage (23.3-15.5 Ma), the tectonic subsidence was about 0.6 km in basins and 0.4 km on the uplifts, and the average rate of tectonic subsidence, much smaller than that in the rapid subsidence phase, was about 0.05 m/ky, even in basins, as shown in Figure 6.  
Fourth phase--the accelerated rise phase from the Middle to Late Miocene (15.5-5.5 Ma). From the Middle Miocene, the rate of tectonic subsidence gradually rose and reached its peak at the end of the Late Miocene. The accelerated rise phase spanned from the Middle Miocene (15.5 Ma) to the Late Miocene (5.5 Ma) and the tectonic subsidence was about 1.6 km and 0.5 km in basins and 0.5 km on the uplifts, respectively. The rate of tectonic subsidence gradually increased and reached its peak at 0.25 m/ky in 8.2-5.5 Ma, while there was a slight variation among different tectonic units. The most typical representatives of this rising process are in Changchang Depression and Huaguang Depression, with a maximum rate of about 0.3 m/ky, as shown in Figures 6 and 7. Fifth phase--the transitional phase since the Pliocene (5.5-1.9 Ma) to the present.