琼东南盆地张裂期沉降亏损与裂后期快速沉降成因

前人研究发现琼东南盆地裂后期存在大规模快速沉降事件,并提出了多种成因机制。为了考察该事件与整个沉降历史的关系,选取跨越琼东南盆地主要凹陷的5条地震剖面上的39个代表点位,利用回剥技术和应变速率反演方法,计算了这些点位的回剥构造沉降及理论构造沉降。计算结果不仅确认了前人的认识即琼东南盆地在裂后期存在快速的构造沉降现象,而且还发现,如果假定地壳减薄发生在张裂期(45~21 Ma),那么,依据地壳减薄计算的理论构造沉降在张裂期明显大于回剥构造沉降量。结合琼东南盆地裂后期断裂活动特点和深部结构探测结果,认为地壳减薄发生在古近纪张裂期,表明琼东南盆地张裂期存在明显的沉降亏损现象,在张裂期结束时(21 Ma)各代表点处的回剥构造沉降量,比由地壳减薄计算得到的理论构造沉降量小约900~2 000 m。进一步分析认为,裂陷期的沉降亏损与裂后期的快速沉降很可能存在内在成因联系,后期快速沉降是早期沉降亏损的补偿,其成因可能和该区深部热异常的演变有关。深部热异常可导致张裂期的沉降不足,随着热异常热源的消退,导致晚期快速沉降现象的发生。

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.