ORIGIN OF SYN-RIFT SUBSIDENCE DEFICIT AND RAPID POST-RIFT SUBSIDENCE IN QIONGDONGNAN BASIN

It is found by previous studies that there is a rapid subsidence event right after the rifting in the Qiongdongnan Basin. Many kinds of explanation have been proposed as the origin of the event. In order to define the relationship of the event to the whole tectonic subsidence history, a total of 39 representative points selected from five seismic profiles, which cover the major depressions of the Qiongdongnan Basin, are selected to calculate the back-stripping and theoretical tectonic subsidence with back-stripping technology and the strain rate inversion method. The results not only further confirmed the existence of the rapid post-rift subsidence starting from Late Miocene, but also found that if the crust was thinned during the syn-rift stage (45~21 Ma), the theoretical tectonic subsidence should be generally greater than the back-stripping tectonic subsidence during the syn-rift stage. Analysis of precious studies on the post-rift faulting and deep crustal structure of the basin suggested that the crust was thinned during the syn-rift stage. It means that the tectonic subsidence during the syn-rift stage is deficient. The subsidence deficit of these representative points could reach a range from 900 to 2 000 m at the end of the syn-rift stage(21 Ma). Further analysis suggests that there exists internal relationship between the syn-rift subsidence deficit and the post-rift rapid subsidence. The rapid subsidence in the later stage should be a compensation to the former subsidence deficit, probably caused by deep thermal anomaly. It is the deep thermal anomaly which caused the subsidence deficit during the syn-rift stage, and the post-rift rapid subsidence with the thermal decay.