东海陆架盆地西湖凹陷新生代反转构造样式及其形成机制初探

通过对东海陆架盆地西湖凹陷十多条地震剖面的精细解释,发现西湖凹陷自晚白垩世以来,经历了早期的拉张和晚期的断续挤压两个构造演化阶段.晚期的断续挤压在凹陷内产生构造反转的样式复杂多样,可划分为上下均正型、下正上逆型、下凹上凸型、上下均凸型、下正上隆型和下隆上逆型等6种单个的反转构造样式及y形、帚状、"似花状"等7种反转构造组合样式.对反转断层生长指数（G_i）并结合平衡剖面的研究,表明西湖凹陷新生代经历了多期构造反转.通过计算反转率（R_i）,发现凹陷内反转程度存在明显的不均一性,具有中带强和东、西弱的特征.结合晚白垩世以来东亚大陆边缘的区域构造演化及新生代以来太平洋板块相对欧亚板块的俯冲速度讨论了西湖凹陷构造反转的机制,并探讨了它对油气勘探的意义.

Styles of inversion structures and their mechanisms of the Cenozoic Xihu Sag,East China Sea Shelf Basin

The detailed interpretation and analysis of seismic section revealed that evolution of the Xihu Sag, a suborder unit of the East China Sea Shelf Basin,experienced two phases during the Cenozoic,namely,extension from Paleocene to Eocene and fluctuated contraction from ear1y Oligocene to the present.The structural inversion occurred when basin-controlling faults reversed and became compressional ones.There occurred six single inversion structural styles and seven combinational modes.There were two styles of single inversion structures for a fault:both of the upper and lower parts being normal fault,the lower part normal and the upper part reversed fault.Also,there were two styles of single inversion structures for a fold:both of the upper and lower parts being convex,the lower concave and the upper convex.The combinational inversion structures included y-shaped inversion structure,λ-shaped inversion structure, flower-shaped inversion structure,brush structure,etc.The mechanisms of each inversion style were analyzed by calculating the growth index of fault and study of balanced sections.The Xihu Sag, with the thickness of Cenozoic sediments over 10 000 m,experienced multi-phase tectonic inversion,which was closely related to the tectonic evolution of the East Asian continental margin,and influenced by velocity changing between the Pacific plate to the Eurasia.The inversion ratio indicated that the inversion degree might be varied in each belt of the Xihu Sag, the inversion seemed stronger in the Central Uplift Belt, and the West Marginal and the East Marginal Belts being relatively gentle.The study on inversion structures in the Xihu had an important meaning for oil-gas exploration in the shelf of East China Sea.