东海陆架盆地西湖凹陷天台斜坡构造变换带发育特征及形成机理

Characteristics and forming mechanisms of transform zone in the Tiantai slope,Xihu Sag,East China Sea Shelf Basin

东海陆架盆地西湖凹陷天台斜坡构造位置特殊,深层NW向断裂带从斜坡区通过,并对斜坡南、北部新生界的构造特征起到了明显的调节和变换作用。通过三维地震资料精细解释和平衡剖面复原,建立了天台斜坡深层NW向断裂和浅层断裂的发育模式,厘定了NW向变换带的构造属性,同时结合区域地质背景,探讨了构造变换带的演化过程和形成机理。结果表明:天台斜坡中生界发育包括天台西断裂和孤山断裂在内的NW向伸展滑脱断层,并对斜坡南、北部新生界地质结构、断裂发育和伸展强度等起到了明显的调节和变换作用。天台斜坡北部和南部分别表现为缓坡和陡坡结构特征,北部断裂密集发育,而且孤山断裂上盘发育弧形断裂,北部伸展强度也要明显大于南部。发育于天台西断裂和孤山断裂之间的NW向构造变换带明显不同于伸展背景下的变换构造,属于离散构造变换带。受秦岭-大别造山后碰撞伸展作用与欧亚板块和太平洋板块的碰撞、俯冲过程影响,天台斜坡构造变换带主要经历了晚侏罗世—早白垩世初期变换带孕育期、早白垩世中期—早始新世变换带雏形期、中晚始新世—渐新世早期变换带成熟期和渐新世晚期—现今变换带静止期4个演化阶段。

The Tiantai slope in the Xihu Sag,East China Sea Shelf Basin,has a special tectonic background with a deep NW-trending fault zone cutting through and adjusting/transforming the tectonic features of the Cenozoic in the southern and northern parts of the slope.Based on the detailed interpretation of 3D seismic data and restored balanced cross sections,this study establishes models for the development of deep and shallow faults,determines the tectonic attribute of the NW-trending transform zone and discusses the evolution history and genetic mechanisms of the transform zone against regional geological settings.The results show that the NW-trending extensional detachment faults,including the Tiantaixi fault and the Gushan fault,were developed in the Mesozoic of the Tiantai slope,and have played an accommodating and transforming role in shaping the geological framework and controlling fault development and extension intensity of the Cenozoic,resulting in a gentle and a steep slope respectively on the south and more stretched north parts of the slope.The faults of the Cenozoic in the northern slope obviously outnumber those in the southern slope.Arc fault systems are found to be developed in the hanging wall of the Gushan fault.The NW-trending tectonic transform zone bounded by the Tiantaixi fault and the Gushan fault differs greatly from that formed in the extensional setting,indicating a discrete type of tectonic transform zone.Influenced by the post-orogenic collision and extension between the Qinling and Dabie orogenic belts,and the collision and subduction of the Eurasian and Pacific plates,the transform zone in the Tiantai slope has experienced four evolutionary stages,including the gestation period from the Late Jurassic to the Early Cretaceous,the embryonic period from the middle Early Cretaceous to the Early Eocene,the maturation period from the middle Late Eocene to the Early Oligocene,and the quiescence stage from the Late Oligocene to present.