Confirmation and genesis of the Middle Eocene tectonic transformation in northwestern margin of Xihu Depression, East China Sea Basin

Comprehensive analyses of faulting systems, basin tectonic framework and igneous rock evolution were conducted to confirm the existence of the Middle Eocene tectonic transformation in northwestern margin of Xihu Depression, East China Sea Basin.The results show that the tectonic transformation has formed remarkable unconformities in the bottom of Pinghu Formation, and the main performances of these unconformities are flattening and overshooting. The more westward the adjacent Yangtze Depression, the more obvious the unconformity is. In addition, the tectonic framework of Xihu Depression has changed remarkably before and after the tectonic transformation, and the main performance of the change is the trend of the large scale fracture from the early difference to the later one in the Hangzhou Slope Belt. This eventually led to the unification of early multi-segregated sedimentary centers in later stages. Accordingly, we suggest that the occurrence of tectonic transformation during the Middle Eocene is related to the adjustment of the subduction of the surrounding plates. The period of tectonic transformation is synchronous with the uplift of the plateau causing by the subduction of Indian plate in the west, and it is also synchronous with the diversion and retreat of the Pacific plate subduction in the east. Therefore, under the background of increasing extrusion in the west, the tectonic transformation responses to differential development transition from the whole tension to western extrusion and eastern tension. In Xihu Depression, this tectonic transformation is widespread, and as a result, the characteristics of source rocks in Pinghu Formation and underlying Baoshi Formation are different. The contribution of this difference to differential enrichment of oil and gas resources needs to be further studied.