# Tectonic Subsidence and Its Response to Geological Evolution in the Xisha Area, South China Sea
## 3 Data and Methods
### Restoration Principle of Tectonic Subsidence  
Basement subsidence consists of loading subsidence and tectonic subsidence. In order to acquire the tectonic subsidence, it is necessary to remove the loading subsidence from the total subsidence. Presently, the back-stripping technique is widely used to calculate the amounts and rates of tectonic subsidence of the basement. Based on the back-stripping technique of [PERSON] and [PERSON] [30], we used Thermodel for Windows 2004, which has  
Figure 4: Interpreted seismic section showing seismic reflectors in the Xisha area. Line location is shown in Figure 2. MTDs: mass transport deposits. Figure 5: Major faults and relating syn-faulting sedimentary sequences in the Xisha area. Line location is shown in Figure 2.  
been widely used in the north margin of the South China Sea with certain success [9; 10; 31]. Three parameters--decompaction of stratum, paleo-water depths, and fluctuation in sea level--should be corrected in this system. The lithology of the stratigraphic units is an important influential factor in porosity restoration for decompaction. The systematic analysis of 33 wells in the Xisha area and its adjacent areas, including the newest wells--YL19-1-1, CD26-1-1, XK-1, and 120-CX-1X [32; 33]--was used to calculate the lithologic thickness percentages of different stratigraphic units and their generalized formation. Mudstone, shale, sandstone, limestone, and coral are the main ingredients of the Cenozoic strata in the study area. Moreover, their porosity-depth relationships have been analyzed in the literature [30; 34; 35]. This study adapted the modified compaction parameters from [PERSON] and Christie [30] and ODP Site 1148, as shown in Table 1. Although numerous volcanoes are distributed in this area, there is no distinct relationship between porosity and depth obtained. Furthermore, the synthesized pseudo-wells by extracted common depth points (CDPs) in the center of basins used to calculate the tectonic subsidence probably represented regional subsidence. The largest uncertainty originates from estimating paleo-water depths. In former research, sedimentary environment analysis from ODP site 1148 provided some useful information to estimate the paleo-water depths in the present abyssal area of the Qiong-dongnan Basin since the Oligocene [21; 29; 36]. The increment in wells in the abyssal area surrounding the Xisha area, such as LS33-1-1, YC35-1-2, and CC26-1-1, is helpful to estimate paleo-water depths. The sedimentary environments were lacustrine and the water depths less than 100 m during the Eocene. From the Early to Late Oligocene, it gradually changed from the initial marine-continental transitional face into the neritic face, which indicated that the water depths rose from approximately 100 m to over 200 m. From the Early Miocene to the present, the water depths in each center of those basins speculated by successive changes in benthic combination was over 200 m in the earliest Miocene, and ascended to 500 m at the beginning of the Middle Miocene, reaching 1000-1500 m subsequently during the Later Miocene. Greatly benefiting from the retrogression of carbonate platform and the change in sedimentary environment [21; 29; 36], the estimation of the paleo-water depths on the slope was less than 100 m in the Early Miocene, ranged from 200-500 m during the Middle Miocene, and exceeded 500 m and rose to about 1000 m at the end of the Late Miocene. Sea-level change is also an influential factor in tectonic subsidence calculation. In this study, the sea-level curve is extracted from [PERSON] et al. [22]. It is clear that the restoration of tectonic subsidence should be tied on strata. However, the basement age of each tectonic unit is not always parallel with the age of its initial sedimentation. Therefore, the restoration of tectonic subsidence represents subsidence process since receiving deposits.