东海盆地西湖凹陷西北缘中始新世构造变革厘定及成因探讨

通过西湖凹陷西北缘断裂体系、构造格局、火成岩演变等构造条件综合分析,首次明确了区内中始新世存在构造变革。研究结果表明,构造变革在平湖组底界形成了显著的不整合,主要表现为削平、削超,愈向西邻近长江坳陷不整合愈发显著;构造变革前后西湖凹陷格局发生了显著变化,在杭州斜坡带主要表现为大断裂走向由早期差异到后期逐步趋同,最终导致早期分隔、多沉积中心在后期连片统一。认为中始新世这期构造变革作用的发生与周缘板块俯冲调整有关,构造变革期在时间上与西部印度板块俯冲引发高原隆升和东部太平洋板块俯冲转向、后撤作用相同步,是在西部挤压作用逐步增强背景下,盆地由整体拉张背景到西压、东张的差异化发育转变作用的主要响应。在西湖凹陷内这种构造变革作用具有广泛性,在平湖组与下伏宝石组地层的烃源岩特征上形成差异,对油气资源差异富集的贡献需要做更深入的研究。

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.