EXTENSIONAL AND COMPRESSIVE RATIOS AND TECTONIC EVOLUTION OF THE XIHU AND DIAOBEI SAGS

The East China Sea Shelf Basin lies in the southeast margin of the Eurasian Plate. It is characterized by the feature of east-west zonation and north-south blocking. The east belt of the depression includes the Fujiang Sag, Diaobei Sag and Xihu Sag. This paper selected 11 balanced cross-sections from the Xihu Sag and 2 from the Diaobei Sag. The extensional and compressive ratios of the Xihu and Diaobei sags in Cenozoic were calculated by using the technique of balanced cross-sections. The results showed that both the Xihu and Diaobei Sags were in a stretched state in T50-T34. However, they were different in other stages, for instance, the Xihu Sag was in a compressed state in T34-T12, while the Diaobei Sag in a compressed state in T34-T30 and in a stretched state in T30-T12. Both of them started subsiding since T12. The Xihu Sag had remained in compress since middle Eocene. The three tectonic movements:Yuquan (T30), Huagang (T20) and Longjing (T12), intensified the compression. Together with the history of basin filling, the Diaobei Sag could be divided into four evolutionary stages since Eocene:early-middle Eocene graben-type rifting, late Eocene and Oligocene depression, early-middle Miocene rifting and subsidence since late Miocene. Similarly, The Xihu Sag has also experienced four stages of evolution since Paleocene:Paleocene and early-middle Eocene rifting, late Eocene and Oligocene depression, early-middle Miocene reversion and subsidence since late Miocene. The Xihu and Diaobei sags are very different in terms of tectonic evolution that is the reason of the north-south zonation in the East China Sea Shelf Basin.